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The basic model of SCD physiology states that vaso-occlusion occurs when hemoglobin S-containing red blood cells (RBC) undergo sickling before they escape the capillary into a larger vessel. We have shown that mental stress, pain and cold, and events reported by patients to trigger SCD vaso-occlusive crisis (VOC), cause rapid and significant decrease in blood flow, reducing the likelihood that RBC could transit the microvasculature before sickling occurs. However, the critical link between decrease in microvascular blood flow and the incidence of future sickle VOC has never been established experimentally in humans. Using data from centrally adjudicated, overnight polysomnograms (PSG), previously collected in a prospective multi-center cohort sleep study, we analyzed the beat-to-beat amplitudes of vasoconstriction reported by the fingertip photoplethysmogram in 212 children and adolescents with SCD and developed an algorithm that detects vasoconstriction events and quantifies the magnitude (Mvasoc ), duration, and frequency of vasoconstriction that reflect the individual's inherent peripheral vasoreactivity. The propensity to vasoconstrict, quantified by median Mvasoc , predicted the incidence rate of post-PSG severe acute vaso-occlusive pain events (P = .006) after accounting for age and hemoglobin. Indices of sleep-disordered breathing contributed to median Mvasoc but did not predict future pain rate. Median Mvasoc was not associated with vaso-occlusive pain events that occurred prior to each PSG. These results show that SCD individuals with high inherent propensity to vasoconstrict have more frequent severe acute pain events. Our empirical findings are consistent with the fundamental SCD hypothesis that decreased microvascular flow promotes microvascular occlusion.

The optimal length of spontaneous breathing trials (SBTs) in children is unknown.What are the most common reasons for SBT failure in children, and when do they occur? Can clinical parameters at the 30-min mark of a 120-min SBT predict outcome?We performed a secondary analysis of a clinical trial in pediatric ARDS, in which 2-h SBTs are conducted daily. SBT failure is based on objective criteria, including esophageal manometry for effort of breathing, categorized as passage, early failure (≤ 30 min), or late failure (30-120 min). Spirometry was used to calculate respiratory rate (RR), tidal volume (Vt), and rapid shallow breathing index (RSBI), in addition to pulse oximetry and capnography. Predictive models evaluated parameters at 30 min against SBT outcome, using receiver operating characteristic plots and area under the curve.We included 100 children and 305 SBTs, with 42% of SBTs being successful, 32% failing within 30 min, and 25% failing between 30 and 120 min. Of the patients passing SBTs at 30 min, 40% went on to fail by 120 min. High respiratory effort (esophageal manometry) was present in80% of failed SBTs. At the 30-min mark, there were no clear thresholds for RR, Vt, RSBI, FioA 30-min SBT may be too short in children recovering from pediatric ARDS because many go on to fail between 30 and 120 min. Reassuring values of Vt, RR, and gas exchange at 30 min do not reliably predict SBT passage at 2 h, likely because they do not capture the effort of breathing.ClinicalTrials.gov; No.: NCT03266016; URL: www.gov.

Recent studies have shown that individuals with sickle cell disease (SCD) exhibit greater vasoconstriction responses to physical autonomic stressors, such as heat pain and cold pain than normal individuals, but this is not the case for mental stress. We sought to determine whether this anomalous finding for mental stress is related to inter-group differences in baseline cardiac and vascular autonomic function. Fifteen subjects with SCD and 15 healthy volunteers participated in 3 mental stress tasks: N-back, Stroop, and pain anticipation. R-R interval, arterial blood pressure and finger photoplethysmogram (PPG) were continuously monitored before and during these mental stress tasks. The magnitude of vasoconstriction was quantified using change in PPG amplitude from the baseline period. To represent basal autonomic function, we assessed both cardiac and vascular arms of the baroreflex during the baseline period. Cardiac baroreflex sensitivity (BRSc) was estimated by applying both the “sequence” and “spectral” techniques to beat-to-beat measurements of systolic blood pressure and R-R intervals. The vascular baroreflex sensitivity (BRSv) was quantified using the same approaches, modified for application to beat-to-beat diastolic blood pressure and PPG amplitude (PPGa) measurements. Baseline BRSc was not different between SCD and non-SCD subjects, was not correlated with BRSv, and was not associated with the vasoconstriction responses to mental stress tasks. BRSv in both groups was correlated with mean PPGa, and since both baseline PPGa and BRSv were lower in SCD, these results suggested that the SCD subjects were in a basal state of higher sympathetically-mediated vascular tone. In both groups, baseline BRSv was positively correlated with the vasoconstriction responses to N-back, Stroop and pain anticipation. After adjusting for differences in BRSv within and between groups, we found no difference in the vasoconstriction responses to all 3 mental tasks between SCD and non-SCD subjects. The implications of these findings are significant in subjects with SCD since vasoconstriction reduces microvascular flow and prolongs capillary transit time, increasing the likelihood for VOC to be triggered by exposure to stressful events.

Transient increases in peripheral vasoconstriction frequently occur in obstructive sleep apnea and periodic leg movement disorder, both of which are common in sickle cell disease (SCD). These events reduce microvascular blood flow and increase the likelihood of triggering painful vaso-occlusive crises (VOC) that are the hallmark of SCD. We recently reported a significant association between the magnitude of vasoconstriction, inferred from the finger photoplethysmogram (PPG) during sleep, and the frequency of future VOC in 212 children with SCD. In this study, we present an improved predictive model of VOC frequency by employing a two-level stacking machine learning (ML) model that incorporates detailed features extracted from the PPG signals in the same database. The first level contains seven different base ML algorithms predicting each subject's pain category based on the input PPG characteristics and other clinical information, while the second level is a meta model which uses the inputs to the first-level model along with the outputs of the base models to produce the final prediction. Model performance in predicting future VOC was significantly higher than in predicting VOC prior to each sleep study (F1-score of 0.43 vs. 0.35, p-value

Vaso-occlusive crisis (VOC) is a hallmark of sickle cell disease (SCD) and occurs when deoxygenated sickled red blood cells occlude the microvasculature. Any stimulus, such as mental stress, which decreases microvascular blood flow will increase the likelihood of red cell entrapment resulting in local vaso-occlusion and progression to VOC. Neurally mediated vasoconstriction might be the physiological link between crisis triggers and vaso-occlusion. In this study, we determined the effect of mental stress on microvascular blood flow and autonomic nervous system reactivity. Sickle cell patients and controls performed mentally stressful tasks, including a memory task, conflict test and pain anticipation test. Blood flow was measured using photoplethysmography, autonomic reactivity was derived from electrocardiography and perceived stress was measured by the State-Trait Anxiety Inventory questionnaire. Stress tasks induced a significant decrease in microvascular blood flow, parasympathetic withdrawal and sympathetic activation in all subjects. Of the various tests, pain anticipation caused the highest degree of vasoconstriction. The magnitude of vasoconstriction, sympathetic activation and perceived stress was greater during the Stroop conflict test than during the N-back memory test, indicating the relationship between magnitude of experimental stress and degree of regional vasoconstriction. Baseline anxiety had a significant effect on the vasoconstrictive response in sickle cell subjects but not in controls. In conclusion, mental stress caused vasoconstriction and autonomic nervous system reactivity in all subjects. Although the pattern of responses was not significantly different between the two groups, the consequences of vasoconstriction can be quite significant in SCD because of the resultant entrapment of sickle cells in the microvasculature. This suggests that mental stress can precipitate a VOC in SCD by causing neural-mediated vasoconstriction.

Although respiratory sinus arrhythmia and blood pressure variability have been investigated extensively, there have been far fewer studies of the respiratory modulation of peripheral blood flow in humans. Existing studies have been based primarily on noninvasive measurements using digit photoplethysmography and laser-Doppler flowmetry. The cumulative knowledge derived from these studies suggests that respiration can contribute to fluctuations in peripheral blood flow and volume through a combination of mechanical, hemodynamic, and neural mechanisms. However, the most convincing evidence suggests that the sympathetic nervous system plays the predominant role under normal, resting conditions. This mini-review provides a consolidation and interpretation of the key findings reported in this topical area. Given the need to extract dynamic information from noninvasive measurements under largely “closed-loop” conditions, we propose that the application of analytical tools based on systems theory and mathematical modeling can be of great utility in future studies. In particular, we present an example of how the transfer relation linking respiration to peripheral vascular conductance can be derived using measurements recorded during spontaneous breathing, spontaneous sighs, and ventilator-induced sighs.

Sickle cell disease (SCD) is a monogenetic disease that results in the formation of hemoglobin S. Due to more rapid oxidation of hemoglobin S due to intracellular heme and adventitious iron in SCD, it has been thought that an inherent property of SCD red cells would be an imbalance in antioxidant defenses and oxidant production. Less deformable and fragile RBC in SCD results in intravascular hemolysis and release of free hemoglobin (PFHb) in the plasma, which might be expected to produce oxidative stress in the plasma. Thus, we aimed to characterize intracellular and vascular oxidative stress in whole blood and plasma samples from adult SCD patients and controls recruited into a large study of SCD at Children's Hospital of Los Angeles. We evaluated the cellular content of metHb and several components of the antioxidant system in RBC as well as oxidation of GSH and Prx-2 oxidation in RBC after challenge with hydroperoxides. Plasma markers included PFHb, low molecular weight protein bound heme (freed heme), hemopexin, isoprostanes, and protein carbonyls. While GSH was slightly lower in SCD RBC, protein carbonyls, NADH, NAD+ and total NADP++NADPH were not different. Furthermore, GSH or Prx-2 oxidation was not different after oxidative challenge in SCD vs. Control. Elevated freed heme and PFHb had a significant negative, non-linear association with hemopexin. There appeared to be a threshold effect for hemopexin (200μg/ml), under which the freed heme rose acutely. Plasma F2-isoprostanes were not significantly elevated in SCD. Despite significant release of Hb and elevation of freed heme in SCD when hemopexin was apparently saturated, there was no clear indication of uncompensated vascular oxidative stress. This somewhat surprising result, suggests that oxidative stress is well compensated in RBCs and plasma during a period of relative health.

Alpha thalassemia is a hemoglobinopathy due to decreased production of the α-globin protein from loss of up to four α-globin genes, with one or two missing in the trait phenotype. Individuals with sickle cell disease who co-inherit the loss of one or two α-globin genes have been known to have reduced risk of morbid outcomes, but the underlying mechanism is unknown. While α-globin gene deletions affect sickle red cell deformability, the α-globin genes and protein are also present in the endothelial wall of human arterioles and participate in nitric oxide scavenging during vasoconstriction. Decreased production of α-globin due to α-thalassemia trait may thereby limit nitric oxide scavenging and promote vasodilation. To evaluate this potential mechanism, we performed flow-mediated dilation and microvascular post-occlusive reactive hyperemia in 27 human subjects (15 missing one or two α-globin genes and 12 healthy controls). Flow-mediated dilation was significantly higher in subjects with α-trait after controlling for age (P = .0357), but microvascular perfusion was not different between groups. As none of the subjects had anemia or hemolysis, the improvement in vascular function could be attributed to the difference in α-globin gene status. This may explain the beneficial effect of α-globin gene loss in sickle cell disease and suggests that α-globin gene status may play a role in other vascular diseases.

Plant gain quantifies the extent and rapidity with which arterial blood gases change following hypopneic or hyperpneic events. High plant gain, acting in concert with a highly collapsible upper airway and low arousal threshold, may contribute significantly towards increasing the severity of obstructive sleep apnea (OSA), even when controller gain is low. Elevated plant gain may be a manifestation of abnormal gas exchange resulting from ventilation-perfusion mismatch in the lungs. Using a mathematical model, we explore in this paper how ventilation-perfusion mismatch can affect plant gain, as well as the severity of OSA.

Sickle cell disease (SCD) is a monogenic hemoglobinopathy associated with significant morbidity and mortality. Cardiopulmonary, vascular and sudden death are the reasons for the majority of young adult mortality in SCD. To better understand the clinical importance of multi-level vascular dysfunction, in 2009 we assessed cardiac function including tricuspid regurgitant jet velocity (TRV), tissue velocity in systole(S') and diastole (E'), inflammatory, rheologic and hemolytic biomarkers as predictors of mortality in patients with SCD. With up to 9 years of follow up, we determined survival in 95 children, adolescents and adults with SCD. Thirty-eight patients (40%) were less than 21 years old at initial evaluation. Survival and Cox proportional-hazards analysis were performed. There was 19% mortality in our cohort, with median age at death of 35 years. In the pediatric subset, there was 11% mortality during the follow up period. The causes of death included cardiovascular and pulmonary complications in addition to other end-organ failure. On Cox proportional-hazards analysis, our model predicts that a 0.1 m/s increase in TRV increases risk of mortality 3%, 1 cm/s increase in S' results in a 91% increase, and 1 cm/s decrease in E' results in a 43% increase in mortality. While excluding cardiac parameters, higher plasma free hemoglobin was significantly associated with risk of mortality (p=.049). In conclusion, elevated TRV and altered markers of cardiac systolic and diastolic function predict mortality in a cohort of adolescents and young adult patients with SCD. These predictors should be considered when counseling cardiovascular risk and therapeutic optimization at transition to adult providers.

Sickle cell disease (SCD) is a genetic disorder of hemoglobin producing hemoglobin-S (HbS) and resulting in recurrent severe episodes of pain, organ damage and premature death due to vaso- occlusion. Deoxy HbS polymerizes, causing red cells to become rigid and lodge in the microvasculature if they do not escape into larger vessels before this transformation occurs. The mechanism that triggers this transition from steady state to vaso-occlusive crisis (VOC) is not known. Patients state that cold, emotional stress, and pain itself can trigger these events. In spite of the connection between these symptoms and the autonomic nervous system (ANS), and the fact that the ANS regulates regional microvascular blood flow, the role of the ANS in sickle pathophysiology has not been significantly investigated. We will briefly review the mechanism of SCD vaso-occlusion, the dysautonomia associated with SCD and sickle trait, and the role that the ANS may play in the genesis of sickle vaso-occlusive crisis.

Introduction Absence of alpha globin genes has long been known to influence the physiology of sickle cell disease (SCD). Individuals with SCD who are missing one or two alpha globin genes have decreased rates of cerebral vasculopathy, stroke, acute chest syndrome, and leg ulcers (Bernaudin, Blood 2008; Flanagan, Blood 2011; Nolan, Br J Haematol 2006). Although there is laboratory evidence of decreased hemolytic rate in these patients (Higgs, N Engl J Med 1982), the mechanism for their improved clinical outcomes has not been identified. Recently, the alpha globin protein has been shown to be present in the endothelial wall of human arterioles, where it modulates nitric oxide (NO) scavenging during vasoconstriction (Straub, Nature 2012). In mice, pharmacological inhibition of alpha globin leads to increased endothelial NO activity, independently of NO production, and results in increased blood perfusion, reduced systemic hypertension, and increased pulmonary artery vasodilation (Keller, Hypertension 2016; Alvarez, Am J Respir Cell Mol Biol 2017). The relationship between absence of alpha globin and arterial vasodilation, and the role of alpha globin in NO-mediated vascular signaling are potential mechanisms that could explain the beneficial effect of missing alpha globin genes in SCD. Using alpha thalassemia as a naturally occurring human model of alpha globin gene knockout, we hypothesized that loss of alpha globin genes leads to improvement in microvascular blood flow in thalassemia trait subjects without hemolysis. Methods Alpha thalassemia trait subjects missing one or two alpha globin genes, and healthy controls were recruited to the study, which was approved by the Children's Hospital Los Angeles Institutional Review Board. Blood samples were obtained from all subjects to test for hemoglobin, mean corpuscular volume (MCV), reticulocyte count, plasma hemoglobin, lactate dehydrogenase, and alpha globin genotype. We assessed flow-mediated dilation (FMD) of the brachial artery following distal forearm occlusion (Detterich, Blood 2015) simultaneously with laser Doppler flowmetry (LDF) and photoplethysmography (PPG) in the fingertip. We also measured the increase in microvascular perfusion with a thermal stimulus. The maximal change in vascular perfusion after provocation indicates vasodilatory capacity. Statistical analysis was performed in JMP® version 14 (SAS Institute Inc., USA). Results Twenty-seven subjects were enrolled, including 12 controls (4 alpha globin genes), 10 patients with 3 alpha globin genes and 5 with 2. The mean MCV was lower in subjects missing alpha globin genes than in controls (p=0.0099). Importantly, hemoglobin levels and markers of hemolysis were normal in both groups. There was no detectable difference in FMD between individuals missing one and two alpha globin genes; thus, these groups were combined and labeled as alpha trait for further analyses. FMD was significantly higher in alpha trait subjects after adjusting for age (Figure 1, p=0.0357). Missing alpha globin genes had no effect on microvascular flow by LDF or PPG (data not shown). Discussion FMD is an established and specific predictor of NO bioavailability (Thijssen, Am J Physiol Heart Circ Physiol 2011), and, in addition to shear-mediated NO circulation in conduit vessels, it reflects the sum of flow in multiple arteriolar networks downstream of the conduit artery. Using this method, a difference in endothelial function between control and alpha thalassemia trait was easily detected (Figure 1). Because endothelial alpha globin is present in arterioles rather than conduit vessels (Butcher, Free Radic Biol Med 2014), we measured microvascular flow in a 1-mm3 volume in the skin using a laser Doppler sensor, and in the fingertip by PPG, but were unable to detect an effect of alpha trait. As none of the subjects had anemia or evidence of hemolysis, the significantly increased FMD associated with loss of alpha globin genes is most likely due to increased NO as a result of decreased scavenging by alpha globin. The finding reported here that lower alpha globin gene number is associated with increased NO-related perfusion in humans may explain the beneficial effect of alpha thalassemia trait in SCD and suggests that the presence of alpha thalassemia trait may also play a role in other types of vascular disease. Disclosures Wood: BiomedInformatics: Consultancy; Imago Biosciences: Consultancy; BluebirdBio: Consultancy; Celgene: Consultancy; WorldcareClinical: Consultancy; Philips Medical Systems: Research Funding. Coates:apo pharma (Chiesi Pharma): Consultancy, Honoraria; Sangamo: Honoraria, Membership on an entity's Board of Directors or advisory committees; Agios pharma: Consultancy, Honoraria; Vifor Pharma: Consultancy, Honoraria; Celgene, BMS: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Bluebird Pharma: Honoraria, Membership on an entity's Board of Directors or advisory committees.

Persons with sickle cell disease (SCD) exhibit subjective hypersensitivity to cold and heat perception in experimental settings, and triggers such as cold exposure are known to precipitate vaso-occlusive crises by still unclear mechanisms. Decreased microvascular blood flow (MBF) increases the likelihood of vaso-occlusion by increasing entrapment of sickled red blood cells in the microvasculature. Because those with SCD have dysautonomia, we anticipated that thermal exposure would induce autonomic hypersensitivity of their microvasculature with an increased propensity toward vasoconstriction. We exposed 17 patients with SCD and 16 control participants to a sequence of predetermined threshold temperatures for cold and heat detection and cold and heat pain via a thermode placed on the right hand. MBF was measured on the contralateral hand by photoplethysmography, and cardiac autonomic balance was assessed by determining heart rate variability. Thermal stimuli at both detection and pain thresholds caused a significant decrease in MBF in the contralateral hand within seconds of stimulus application, with patients with SCD showing significantly stronger vasoconstriction (P = .019). Furthermore, patients with SCD showed a greater progressive decrease in blood flow than did the controls, with poor recovery between episodes of thermal stimulation (P = .042). They had faster vasoconstriction than the controls (P = .033), especially with cold detection stimulus. Individuals with higher anxiety also experienced more rapid vasoconstriction (P = .007). Augmented vasoconstriction responses and progressive decreases in perfusion with repeated thermal stimulation in SCD are indicative of autonomic hypersensitivity in the microvasculature. These effects are likely to increase red cell entrapment in response to clinical triggers such as cold or stress, which have been associated with vaso-occlusive crises in SCD.

Sickle cell disease (SCD) is an inherited hemoglobinopathy characterized by polymerization of hemoglobin S upon deoxygenation that results in the formation of rigid sickled-shaped red blood cells that can occlude the microvasculature, which leads to sudden onsets of pain. The severity of vaso-occlusive crises (VOC) is quite variable among patients, which is not fully explained by their genetic and biological profiles. The mechanism that initiates the transition from steady state to VOC remains unknown, as is the role of clinically reported triggers such as stress, cold and pain. The rate of hemoglobin S polymerization after deoxygenation is an important determinant of vaso-occlusion. Similarly, the microvascular blood flow rate plays a critical role as fast-moving red blood cells are better able to escape the microvasculature before polymerization of deoxy-hemoglobin S causes the red cells to become rigid and lodge in small vessels. The role of the autonomic nervous system (ANS) activity in VOC initiation and propagation has been underestimated considering that the ANS is the major regulator of microvascular blood flow and that most triggers of VOC can alter the autonomic balance. Here, we will briefly review the evidence supporting the presence of ANS dysfunction in SCD, its implications in the onset of VOC, and how differences in autonomic vasoreactivity might potentially contribute to variability in VOC severity.

In sickle cell disease (SCD), prolonged capillary transit times, resulting from reduced peripheral blood flow, increase the likelihood of rigid red cells entrapment in the microvasculature, predisposing to vaso-occlusive crisis. Since changes in peripheral flow are mediated by the autonomic nervous system (ANS), we tested the hypothesis that the cardiac and peripheral vascular responses to head-up tilt (HUT) are abnormal in SCD. Heart rate, respiration, non-invasive continuous blood pressure and finger photoplethysmogram (PPG) were monitored before, during, and after HUT in SCD, anemic controls and healthy subjects. Percent increase in heart rate from baseline was used to quantify cardiac ANS response, while percent decrease in PPG amplitude represented degree of peripheral vasoconstriction. After employing cluster analysis to determine threshold levels, the HUT responses were classified into four phenotypes: (CP) increased heart rate and peripheral vasoconstriction; (C) increased heart rate only; (P) peripheral vasoconstriction only; and (ST) subthreshold cardiac and peripheral vascular responses. Multinomial logistic regression (MLR) was used to relate these phenotypic responses to various parameters representing blood properties and baseline cardiovascular activity. The most common phenotypic response, CP, was found in 82% of non-SCD subjects, including those with chronic anemia. In contrast, 70% of SCD subjects responded abnormally to HUT: C-phenotype = 22%, P-phenotype = 37%, or ST-phenotype = 11%. MLR revealed that the HUT phenotypes were significantly associated with baseline cardiac parasympathetic activity, baseline peripheral vascular variability, hemoglobin level and SCD diagnosis. Low parasympathetic activity at baseline dramatically increased the probability of belonging to the P-phenotype in SCD subjects, even after adjusting for hemoglobin level, suggesting a characteristic autonomic dysfunction that is independent of anemia. Further analysis using a mathematical model of heart rate variability revealed that the low parasympathetic activity in P-phenotype SCD subjects was due to impaired respiratory-cardiac coupling rather than reduced cardiac baroreflex sensitivity. By having strong peripheral vasoconstriction without compensatory cardiac responses, P-phenotype subjects may be at increased risk for vaso-occlusive crisis. The classification of autonomic phenotypes based on HUT response may have potential use for guiding therapeutic interventions to alleviate the risk of adverse outcomes in SCD.

We have previously demonstrated that sickle cell disease (SCD) patients maintain normal global systemic and cerebral oxygen delivery by increasing cardiac output. However, ischemic end-organ injury remains common suggesting that tissue oxygen delivery may be impaired by microvascular dysregulation or damage. To test this hypothesis, we performed fingertip laser Doppler flowmetry measurements at the base of the nailbed and regional oxygen saturation (rSO2 ) on the dorsal surface of the same hand. This was done during flow mediated dilation (FMD) studies in 26 chronically transfused SCD, 75 non-transfused SCD, and 18 control subjects. Chronically transfused SCD patients were studied prior to and following a single transfusion and there was no acute change in rSO2 or perfusion. Laser Doppler estimates of resting perfusion were 76% higher in non-transfused and 110% higher in transfused SCD patients, compared to control subjects. In contrast, rSO2 was 12 saturation points lower in non-transfused SCD patients, but normal in the transfused SCD patients. During cuff occlusion, rSO2 declined at the same rate in all subjects suggesting similar intrinsic oxygen consumption rates. Upon cuff release, laser doppler post occlusive hyperemia was blunted in SCD patients in proportion to their resting perfusion values. Transfusion therapy did not improve the hyperemia response. FMD was impaired in SCD subjects but partially ameliorated in transfused SCD subjects. Taken together, non-transfused SCD subjects demonstrate impaired conduit artery FMD, impaired microcirculatory post-occlusive hyperemia, and resting hypoxia in the hand despite compensated oxygen delivery, suggesting impaired oxygen supply-demand matching. Transfusion improves FMD and oxygen supply-demand matching but not microcirculation hyperemic response.

Pain and vaso-occlusive crises (VOC) are hallmark complications of sickle cell disease (SCD) and result in significant physical and psychosocial impairment. The variability in SCD pain frequency and triggers for the transition from steady state to VOC are not well understood. This paper summarizes the harmful physiological effects of pain and emotional stressors on autonomically-mediated vascular function in individuals with SCD and the effects of a cognitive, neuromodulatory intervention (i.e. hypnosis) on microvascular blood flow. We reviewed recent studies from the authors’ vascular physiology laboratory that assessed microvascular responses to laboratory stressors in individuals with SCD. Results indicate that participants with SCD exhibit marked neurally mediated vascular reactivity in response to pain, pain-related fear, and mental stress. Further, pilot study results show that engagement in hypnosis may attenuate harmful microvascular responses to pain. The collective results demonstrate that autonomically-mediated vascular responses to pain and mental stress represent an important SCD intervention target. This ongoing work provides physiological justification for the inclusion of cognitive, neuromodulatory and complementary treatments in SCD disease management and may inform the development of targeted, integrative interventions that prevent the enhancement of autonomic vascular dysfunction in SCD.

The painful vaso-occlusive crises (VOC) that characterize sickle cell disease (SCD) progress over hours from the asymptomatic steady-state. SCD patients report that VOC can be triggered by stress, cold exposure and pain itself. We anticipated that pain could cause neural-mediated vasoconstriction, decreasing regional blood flow and promoting entrapment of sickle cells in the microvasculature. Therefore, we measured microvascular blood flow in the fingers of both hands using plethysmography and laser-Doppler flowmetry while applying a series of painful thermal stimuli on the right forearm in 23 SCD patients and 25 controls. Heat pain applied to one arm caused bilateral decrease in microvascular perfusion. The vasoconstriction response started before administration of the thermal pain stimulus in all subjects, suggesting that pain anticipation also causes significant vasoconstriction. The time delay between thermal pain application and global vasoconstriction ranged from 5 to 15.5 seconds and increased with age (p

The articles in this special section explore new technologies, treatments, and medical services in the field of sleep disorders.

Sickle cell anemia (SCA) is genetic disease associated with chronic hemolysis and severe vaso-occlusive pain. Triggers for vaso-occlusive pain include both extreme temperatures (hot and cold), and stress. Our prior work demonstrates that anticipatory pain, experimental pain, experimental mental stress, and cold exposure cause significant global vasoconstriction in SCA and vasoconstriction responses differ among subjects.Anything that decreases microvascular flow will increase the likelihood that the red blood cells will not escape the microvasculature before deoxyhemoglobin S (HbS) polymerizes, leading to vaso-occlusion. However, this theoretical model has not been proven at a macro scale in patients. We showed that hypoxia induced in SCA subjects causes loss of parasympathetic modulation of heart rate but not decreased microvascular flow. Consistent with this, a prospective cohort study of SCA subjects showed no significant association between low mean nocturnal SpO2 and the incidence of VOC. However, surges in sympathetic activity accompany transient arousals and respiratory events during sleep, and lead to varying levels of peripheral vasoconstriction. In a prospective NIH funded cohort study of children with SCA, we tested the hypothesis that children with SCA and high levels when compared to low levels of intrinsic microvascular vasoreactivity (based on vasoconstriction events detected by polysomnography (PSG)) have a higher incidence rate of severe vaso-occlusive pain that requires hospitalization within 6 months of the PSG. We analyzed the time-series of R-R intervals (RRI) derived from the electrocardiogram and beat-to-beat amplitudes of finger photoplethysmography reflecting peripheral blood flow (PPGa) derived from the scored PSGs and clinical records of 212 children, who participated in the multi-center prospective SCA cohort study reported by Willen et al. (Am J Hematol 2018; 93:478-485). Vasoconstriction events were detected by applying an algorithm that found reductions in PPGa from local baseline that exceeded a threshold level of change. The changes in RRI from the pre-vasoconstriction baseline were also computed (Fig 1). The duration, average magnitude (Mvasoc) and area under the curve of PPGa were extracted from each significant vasoconstriction. The median values of these compact descriptors were taken to represent the nocturnal vasoconstriction characteristics of each participant. A participant was classified to be in the high- or low-pain event category if they were hospitalized for severe pain at least two times or less than 2 times within a 1-year window from 6 months before to 6 months after the PSG, respectively. Multiple logistic regression analysis was applied, using pain category as the outcome variable, and vasoconstriction descriptors, arousal index, obstructive apnea-hypopnea index, age, sex and hemoglobin level as predictor variables. Pain category was strongly associated with magnitude of vasoconstriction (Mvasoc; p=0.0058), with larger Mvasoc predicting high-pain, after accounting for age, sex and hemoglobin level (Hgb). Figure 2 displays the adjusted Mvasoc (mean ± SE) in the high-pain and low/no-pain groups. Mvasoc was strongly correlated with arousal index (p=0.0073). However, none of the traditional measures of sleep-disordered breathing, including arousal index, were associated with pain category. Age was the only other predictor of severe pain. These findings, showing that propensity for a high degree of microvascular vasoconstriction is associated with the temporal relationship of severe pain requiring hospitalization, validate on a macro scale the model that increased microvascular transit time promotes entrapment of sickled red blood cells, elevating the risk of severe vaso-occlusive pain. Our data reveals for the first time, a mechanistic link between the patient's symptoms and the triggering of transition from steady state to acute severe pain. The fact that individuals have intrinsic vasoreactivity phenotypes, that can be identified by analysis of the photoplethysmogram, provides yet another factor in acute vaso-occlusive pain. Autonomic balance and vasoreactivity likely contribute to known variability of the incidence rate of severe pain and represent another plausible target for therapeutic intervention. Disclosures Coates: apo pharma: Consultancy, Honoraria, Speakers Bureau; vifor: Consultancy, Honoraria; agios pharma: Consultancy, Honoraria; celgene: Consultancy, Honoraria, Other: steering committee of clinical study.

Background Chronic transfusion therapy (CTT) is a mainstay for stroke prophylaxis in sickle cell anemia, but its effects on hemodynamics are poorly characterized. Transfusion improves oxygen-carrying capacity, reducing demands for high cardiac output, while decreasing hemoglobin (Hb)S%, reticulocyte count, and hemolysis. We hypothesized that transfusion would improve oxygen-carrying capacity, but that would be counteracted by a decrease in cardiac output due to increased hematocrit (Hct) and vascular resistance, leaving oxygen delivery unchanged. Study Design and Methods To test this hypothesis, we examined patients on CTT immediately before transfusion and again 12 to 120 hours after transfusion, using echocardiography and near infrared spectroscopy. Results Comparable increases in Hb and Hct and decreases in reticulocyte count and HbS with transfusion were observed in all patients, but males had a larger rebound of HbS%, reticulocyte count, and free Hb levels between transfusions. In males, transfusion decreased heart rate by 12%, stroke volume by 15%, and cardiac index by 24% while estimates for pulmonary and systemic vascular resistance increased, culminating in 6% decrease in oxygen delivery. In contrast, stroke volume and cardiac index and systemic and pulmonary vascular resistance did not change in women after transfusion, such that oxygen delivery improved 17%. Conclusion In our sample population, males exhibit a paradoxical reduction in oxygen delivery in response to transfusion because the increase in vascular resistance is larger than the increase in oxygen capacity. This may result from an inability to adequately suppress their HbS% between transfusion cycles.

INTRODUCTION Chronic red blood cell (RBC) transfusion therapy can prevent new and progressive stroke in Sickle Cell Disease (SCD), as shown by multiple clinical trials in SCD including the STOP 1&2, SIT, and SWITCH trials. Middle cerebral artery time-averaged mean velocity (MCA velocity), measured by Trans-cranial Doppler Ultrasound (TCD), is a clinical biomarker utilized to initiate chronic RBC transfusion to reduce stroke risk in children with SCD. However, MCA velocity measured by TCD lacks sensitivity and specificity in predicting stroke. Most SCD patients with high MCA velocity do not experience overt stroke, while 10-20% of SCD children will have new strokes on chronic RBC transfusion in spite of normalized MCA velocity. Prior studies showed an association with high MCA velocity and MCA narrowing when TCD and angiography were performed after RBC transfusion to target a hematocrit (HCT) > 30%. HCT adjustments addressed effects of anemia and higher cardiac output (Adams et al, 1992; Stroke PMID 1636180). Nevertheless, HCT adjustments may not always be used for routine TCD. Thus, despite the substantial decrease in strokes with TCD screening, there may be significant lack of specificity that leads to unnecessary RBC transfusion resulting from TCD screening in non-optimal conditions. Due to these concerns, we undertook a detailed study assessing MCA velocity by TCD immediately pre- and post-transfusion in SCD patients. METHODS The study was approved by the Children's Hospital Los Angeles Institutional Review Board. SCD and thalassemia patients who receive chronic transfusion had pre- and post-transfusion blood laboratory tests and MCA velocity measurements by TCD (GE Vivid I, FE Medical Systems Ultrasound Israel Ltd., Israel). The TCD ultrasound probe was placed on left and right trans-temporal window, landmarks were visualized, and MCA velocity data were collected in 1.7mm depth increments from 65mm to 35mm depth. Magnetic resonance angiography was not done at the time of these measurements. Data was recorded and analyzed in JMP Pro Version 13.0. RESULTS SCD patients (n=24) had a median and interquartile range of 15 years & 14-18 years, with 18 females & 6 males, received 90 transfusions, and 70% of the patients had 3 transfusion events with repeat studies. Seventeen patients received simple RBC transfusion and 7 received red cell exchange. The thalassemia control population received simple transfusion (n=11; 48 transfusion events). Figure 1 shows the relation of MCA velocity compared to HCT in all subjects. There is a highly significant negative correlation showing lower HCT is associated with higher MCA velocity (Dots = TAMV measurements; lines = average TAMV measurements at different TCD depths; R2=0.12, F(1,4270)=556, p Discussion These data show a striking relationship between anemia and TCD velocity, and that RBC transfusion can alter MCA velocity. Even an 11% increase in hemoglobin such as that seen in the low-responder in Figure 3 could change a patient classification from high-risk to conditional TCD. These data also show the significant sensitivity of MCA velocity to the depth of measurement (Figure 2 & 3). The investigators in the STOP study went to extreme ends to standardize these measurements. Nonetheless, TCD measures may not be done with such rigor in clinical settings. It is clear that standard TCD screening has been of tremendous benefit to SCD patients, and many enjoy a good quality of life as a result. Our data suggest that the specificity of TCD screening could be improved if testing was done at an HCT> 30%. Quantifiable blood, patient, and measurement factors can impact TCD MCA velocity and clinical decisions that commit children to years of RBC transfusions. Disclosures Coates: ApoPharma: Consultancy, Honoraria; Celgene Corp.: Consultancy; Vifor Pharma: Consultancy; Sangamo: Consultancy, Honoraria.

Introduction: Vaso-occlusive pain crisis (VOC) is a significant contributor to the morbidity of Sickle Cell Disease (SCD) and cold exposure has long been associated with increased frequency and intensity of VOC. However, the mechanism by which cold exposure causes the transition from steady state to vaso-occlusion has not been well elucidated. Decreased regional blood flow results in red blood cells spending a longer period of time in the deoxygenated state in the capillaries, increasing the likelihood of hemoglobin S polymerization in the microvasculature and subsequent vaso-occlusion. Regional blood flow is primarily regulated by the autonomic nervous system (ANS) and recent literature shows evidence of cardiovascular autonomic dysfunction in SCD and of its role as a disease modulating factor. We hypothesized that cold exposure triggers a central autonomic response leading to vasoconstriction in the microvasculature and that SCD subjects will have a stronger response when compared to controls. Methods: 17 SCD and 16 control (healthy or sickle trait) subjects aged 13 to 39 years were exposed to thermal stimuli via a computer-controlled thermode (TSA-II) placed on the thenar eminence of the right hand. Predetermined individual threshold temperatures for heat detection, cold detection, heat pain and cold pain were applied, and changes in the microvascular blood flow (MBF) were measured at a sub-second sampling rate on the contralateral thumb using photo-plethysmography (PPG). Mean MBF was derived from the amplitude of the PPG signal, a greater decrease in mean MBF from baseline signifies stronger vasoconstriction. The vasoconstriction response within the PPG signal, and the time (in sec) to vasoconstriction were determined from the cross-correlation function of the pain stimulus signal and the vascular response. The R-to-R interval (RRI) derived from electrocardiogram was used to evaluate cardiac autonomic balance. Standard deviation of RRI represents total heart rate variability, Spectral indices of the RRI represent parasympathetic activity (high frequency power; HFP) and sympatho-vagal balance (Low to high Ratio; LHR = low frequency power(LFP)/HFP). Prior to the study, subjects were administered measures of current anxiety and general level of anxiety (STAI Y1 and Y2, respectively) as well as a measure of pain related anxiety (PASS). Results: All the thermal stimulation tasks caused a significant decrease in MBF from baseline (p values for cold detection, heat detection, heat pain and cold pain=0.027, At baseline and across all tasks, SCD subjects showed a significantly lower heart rate variability suggestive of autonomic dysfunction (standard deviation of RRI- 41.2 Vs 50 msec, p=0.007). Both cold and heat pain caused a significant increase in the mean LFP and LHR from baseline with no significant differences in HFP thus suggesting an overall increase in the cardiac sympathetic output with thermal pain. Conclusion: Using MBF as an objective marker of pain responses, cold pain causes significant decrease in MBF followed by other thermal stimuli. The rapid vasoconstriction responses to the thermal stimuli occurred in the contralateral hand, suggesting a neural mechanism. SCD subjects demonstrated an augmented peripheral vasoconstriction response with evidence of cardiac autonomic imbalance. Our findings imply that global autonomic mediated vasoconstriction and subsequent vaso-occlusion is the likely mechanistic link between cold exposure and VOC in SCD. Thus, vascular autonomic reactivity is a biomarker of basic SCD pathophysiology and serves as a potential target for prevention and treatment of pain in SCD. Disclosures Coates: ApoPharma: Consultancy, Honoraria; Sangamo: Consultancy, Honoraria; Vifor Pharma: Consultancy; Celgene Corp.: Consultancy.

Introduction Over the last two decades, sickle cell disease has become a model of diffuse endotheliopathy. In 2004, Gladwin and colleagues demonstrated that elevated tricuspid regurgitation jet velocity (TRJ) was an independent predictor of mortality and that chronic red blood cell hemolysis underlies pulmonary vascular disease (Gladwin et. al. NEJM, PMID:14985486). Belhassen and colleagues demonstrated that flow-mediated dilation of the brachial artery (FMD) was decreased in patients with sickle cell disease, specifically shear-mediated vasodilation (Belhassen et. al. Blood, PMID:11238095). Our group demonstrated that plasma free hemoglobin links decreased FMD and elevated TRJ in patients with sickle cell disease and that chronic transfusion therapy improves FMD (Detterich et. al. Blood, PMID:26036801). We aimed to determine whether markers of vascular function predict the risk of mortality in a cohort of patients with sickle cell disease. Methods This was a single center, prospective cohort study of non-transfused and chronically transfused sickle cell anemia patients at Children's Hospital of Los Angeles between 2009 and 2012 aiming to assess vascular function in sickle cell disease. This study was approved by the Children's Hospital Los Angeles Institutional Review Board. We used FMD, a test of shear-mediated endothelial nitric oxide release as the model for vascular function. We simultaneously measured the regional hemoglobin oxygen saturation in the hand using near infrared spectroscopy (NIRS). We also measured microcirculatory perfusion using laser doppler flowmetry (LDF) over the skin of the nailbed. Echocardiography was performed on the same day to assess tricuspid regurgitation jet velocity and cardiac function. Markers of inflammation, cell count and hemolysis were also measured on the day of the study. We prospectively followed them until 2018 to assess our primary outcome, survival. All of the measurements were made pre-transfusion for chronically transfused subjects. Kaplan Meier and Cox-Proportional Hazards multivariate model was used to determine independent predictors of survival. All statistics were performed using Stata/IC version 14.0. Results We enrolled 26 chronically transfused patients with SCD and 63 non-transfused SCD patients in a study assessing the effects of hemolysis on endothelial function. Overall mortality in our cohort was 20% with a median follow up time of 7.8 years (7.1-8.3 IQR). Median age was 24 years at the time of study (17-36 IQR). Median age at the time of death was 36 years(28-51 IQR) and at the last follow up for the alive patients was 30 years (25-41 IQR), p=0.01. In the patients who died, they had higher TRJ (median 270 vs 235, p=0.03) and had elevated plasma Hgb, median 26 vs 14, p=0.01. FMD, regional oxygen saturation and microcirculatory perfusion were not different between alive and deceased patients. There was no difference for inflammatory markers (white blood cell count, platelet count, high sensitivity C-reactive protein), hemoglobin S%/transfusion, or anemia (hemoglobin/hematocrit/reticulocytes) between alive or deceased patients. By Kaplan Meier analysis individual factors associated with mortality were TRJ>250 (p=0.04) and age>21 (p=0.02) (Fig 1 and Fig 2). By Cox-Proportional Hazards Model, both TR jet velocity and plasma hemoglobin remain significant after correcting for age (Table 1). Conclusions In our cohort, there are multiple abnormal markers of vascular function and elevated markers of hemolysis; however, the strongest predictors of mortality were age, plasma free hemoglobin and tricuspid regurgitation jet velocity. These findings support the association between hemolysis, pulmonary vascular disease and mortality, a controversial topic in sickle cell disease. Disclosures Coates: Vifor Pharma: Consultancy; Sangamo: Consultancy, Honoraria; Celgene Corp.: Consultancy; ApoPharma: Consultancy, Honoraria.

Sickle cell disease (SCD), a genetically-determined pathology due to an amino acid substitution (i.e., valine for glutamic acid) on the beta-chain of hemoglobin, is characterized by abnormal blood rheology and periods of painful vascular occlusive crises. Sickle cell trait (SCT) is a typically benign variant in which only one beta chain is affected by the mutation. Although both SCD and SCT have been the subject of numerous studies, information related to neurological function and transfusion therapy is still incomplete: an overview of these areas is presented. An initial section provides pertinent background information on the pathology and clinical significance of these diseases. The roles of three factors in the clinical manifestations of the diseases are then discussed: hypoxia, autonomic nervous system regulation and blood rheology. The possibility of a causal relationship between these three factors and sudden death is also examined. It is concluded that further studies in these specific areas are warranted. It is anticipated that the outcome of such research is likely to provide valuable insights into the pathophysiology of SCD and SCT and will lead to improved clinical management and enhanced quality of life.

The objective of this study was to non-invasively assess cardiac autonomic control in subjects with sickle cell anemia (SCA) by tracking the changes in heart rate variability (HRV) that occur following brief exposure to a hypoxic stimulus. Five African–American SCA patients and seven healthy control subjects were recruited to participate in this study. Each subject was exposed to a controlled hypoxic stimulus consisting of five breaths of nitrogen. Time-varying spectral analysis of HRV was applied to estimate the cardiac autonomic response to the transient episode of hypoxia. The confounding effects of changes in respiration on the HRV spectral indices were reduced by using a computational model. A significant decrease in the parameters related to parasympathetic control was detected in the post-hypoxic responses of the SCA subjects relative to normal controls. The spectral index related to sympathetic activity, on the other hand, showed a tendency to increase the following hypoxic stimulation, but the change was not significant. This study suggests that there is some degree of cardiovascular autonomic dysfunction in SCA that is revealed by the response to transient hypoxia.

Dynamic modeling has played an important role in advancing and integrating the fields of pharmacokinetics and pharmacodynamics. However, the vast majority of models in the literature do not take into account the fact that pharmacological responses are frequently affected by the homeostatic mechanisms inherent in physiological control systems. This article provides a short tutorial presenting examples that illustrate the basic properties of closed-loop control and how these can influence model predictions of drug responses in both the steady-state and under dynamic conditions. Physiological control systems can be modeled using two basic approaches: (a) “minimal modeling”, in which all model parameters for individuals can be estimated from experiment; and (b) “structured modeling ”, in which the model parameters are isomorphic to key physiological entities, but not all can be identified from the measurements. A discussion of these two approaches is presented, along with a case study of how minimal modeling can be applied to extend a larger structured model. Finally, the importance of modeling functional linkages and interactions across organ systems and across scales is highlighted through a brief exposition of a recently developed structured model of cardiorespiratory, sleep-wake state and metabolic control .

Autonomic dysfunction is known to be associated with sleep apnea and insulin resistance. With the growing prevalence of obesity, the common factor among subjects with sleep apnea and insulin resistance, we sought to determine how autonomic cardiovascular function is affected by the independent and combined effects of sleep apnea and insulin resistance at the early stage of these syndromes. In this study, we investigated the autonomic function of overweight-to-obese children with either sleep apnea or insulin resistance or both during supine and standing postures. Using computational models of heart rate and peripheral vascular resistance variability, we found that children with sleep apnea, regardless of the presence of insulin resistance, had higher peripheral vascular baroreflex gain. Furthermore, children with insulin resistance, regardless of the presence of sleep apnea, had decreased heart rate baroreflex gain from supine to standing, which was opposite to those without insulin resistance. In conclusion, we were able to detect altered autonomic function by sleep apnea and insulin resistance using markers derived from our computational models that based only on non-invasive measurements.

Deep breaths akin to sighs have been reported to cause peripheral vasoconstriction. Our previous simulation studies have shown that this phenomenon cannot be reproduced in existing circulatory control models without inclusion of a respiratory-vascular coupling mechanism. To better understand this "sigh-vasoconstriction reflex", we investigated the effect of spontaneous and passively induced sighs as well as spontaneous breathing on peripheral vasoconstriction during wakefulness and non-rapid eye movement sleep in human subjects. We found that both spontaneous and induced sighs caused vasoconstriction during wakefulness and sleep. The coupling between respiration and vasoconstriction is also present even in an absence of deep breaths. The coupling mechanism is largely linear with increased nonlinearity during induced sighs. Since peripheral vascular resistance modulation is known to be sympathetically mediated, investigation of this coupling could potentially allow us to assess sympathetic function through non-invasive measurements and simple interventions.

Painful vaso-occlusive crisis (VOC), a complication of sickle cell disease (SCD), occurs when sickled red blood cells obstruct flow in the microvasculature. We postulated that exaggerated sympathetically mediated vasoconstriction, endothelial dysfunction and the synergistic interaction between these two factors act together to reduce microvascular flow, promoting regional vaso-occlusions, setting the stage for VOC. We previously found that SCD subjects had stronger vasoconstriction response to pulses of heat-induced pain compared to controls but the relative degrees to which autonomic dysregulation, peripheral vascular dysfunction and their interaction are present in SCD remain unknown. In the present study, we employed a mathematical model to decompose the total vasoconstriction response to pain into: 1) the neurogenic component, 2) the vascular response to blood pressure, 3) respiratory coupling and 4) neurogenic-vascular interaction. The model allowed us to quantify the contribution of each component to the total vasoconstriction response. The most salient features of the components were extracted to represent biophysical markers of autonomic and vascular impairment in SCD and controls. These markers provide a means of phenotyping severity of disease in sickle-cell anemia that is based more on underlying physiology than on genotype. The marker of the vascular component (BMv) showed stronger contribution to vasoconstriction in SCD than controls (p = 0.0409), suggesting a dominant myogenic response in the SCD subjects as a consequence of endothelial dysfunction. The marker of neurogenic-vascular interaction (BMn-v) revealed that the interaction reinforced vasoconstriction in SCD but produced vasodilatory response in controls (p = 0.0167). This marked difference in BMn-v suggests that it is the most sensitive marker for quantifying combined alterations in autonomic and vascular function in SCD in response to heat-induced pain.

The peripheral vascular resistance (RPV) control is known to be largely sympathetically-mediated; thus assessment of the RPV control would allow us to infer valuable information regarding sympathetic nervous activity. The linear and 2nd-order nonlinear minimal models were used to capture the influences of blood pressure (baroreflex) and respiration (respiratory-coupling) on fluctuations of RPV. To validate the minimal models, they were applied on the "data" generated by the simulation model developed in our previous study. This study demonstrated that the linear minimal model was able to recover the "true" (simulated) kernels. The nonlinear model was able to detect the increase in nonlinearity in the system. The system gains derived from the estimated kernels showed strong relationship with the simulation gains, suggesting that the system gains could be employed as potential biomarkers of autonomic function. These results also showed that the nonlinear model had sufficient sensitivity to detect the difference in autonomic reactivity between subjects with mild and severe metabolic syndrome and obstructive sleep apnea syndrome exposed to orthostatic stress.

Short-term blood pressure variability is generally attributed to the baroreflex feedback control on heart rate and systemic vascular resistance (SVR), and the mechanical effect of respiration on stroke volume. Although it is known that respiration affects sympathetic outflow and deep breaths can lead to peripheral vasoconstriction, the respiratory modulation of SVR has been little studied. In the present study, we investigated the dynamics resulting from the respiratory modulation of SVR and its effect on blood pressure variability by employing structured and minimal modeling approaches. Using peripheral arterial tonometry as a noninvasive measure of SVR, we were able to estimate the respiratory-vascular conductance coupling mechanism. We found that the dynamics of the sigh-vasoconstriction reflex could be reproduced only when the respiratory modulation of SVR was incorporated into the closed-loop model. Lastly, we demonstrated that taking this respiratory modulation effect into account is essential for accurately estimating the dynamics of the SVR baroreflex.

The polysomnographic and ventilatory patterns of nine normal adults were measured during non-rapid-eye-movement (NREM) stage 2 sleep before and after repeated administrations of a tone (40-72 dB) lasting 5 s. The ventilatory response to arousal (VRA) was determined in data sections showing electrocortical arousal following the start of the tone. Mean inspiratory flow and tidal volume increased significantly above control levels in the first seven breaths after the start of arousal, with peak increases (64.2% > control) occurring on the second breath. Breath-to-breath occlusion pressure 100 ms after the start of inspiration showed significant increases only on the second and third postarousal breaths, whereas upper airway resistance declined immediately and remained below control for > or = 7 consecutive breaths. These results suggest that the first breath and latter portion of the VRA are determined more by upper airway dynamics than by changes in the neural drive to breathe. Computer model simulations comparing different VRA time courses show that sustained periodic apnea is more likely to occur when the fall in the postarousal increase in ventilation is more abrupt.

Introduction: Sickle cell disease (SCD), an inherited blood disorder, is characterized by episodes of painful vaso-occlusive crises (VOC). Peripheral vasoconstriction may contribute to VOC by prolonging the transit time of red blood cells through the microvasculature. We recently demonstrated that heat-induced pain produces stronger vasoconstriction in SCD subjects versus controls, suggesting abnormal autonomic regulation of regional blood flow in SCD. Thus, although pain is generally thought to be a consequence of VOC, it is possible that pain itself may trigger a cascade of events that leads to large-scale VOC by promoting regional peripheral vasoconstriction. Objective: To determine if the underlying functional mechanisms of the vasoconstriction response to heat-induced pain in SCD differ from normal controls. Experimental Protocols: 22 SCD and 23 control (healthy and sickle cell trait) subjects were recruited at Children's Hospital of Los Angeles. Quasi-periodic pulses of pain were delivered on the right forearm using TSA-II neuro analyzer heating thermode. Electrocardiogram, respiration, continuous blood pressure and photoplethysmogram (PPG) were recorded. Both blood pressure and PPG were measured on the ring finger and thumb on the contralateral hand. Reductions in PPG amplitude were taken to represent vasoconstriction. Analysis: A mathematical model was applied to the data to extract the key parameters relating the thermal (pain) pulses, blood pressure and respiration to vasoconstriction. The model enabled the vasoconstriction response to pain to be decomposed into: 1) a neurogenic component, measuring the direct effect of the thermal pain pulse on vasoconstriction; 2) a local vascular component, relating fluctuations in blood pressure, induced by pain, to vasoconstriction; 3) a neurogenic-vascular interaction component, reflecting the modulation of the vascular component by the pain stimulus; and 4) a respiratory coupling component. Results: The model predicted stronger vasoconstriction responses to heat pain in SCD than controls (p = 0.002), consistent with the previous results reported by Khaleel et al. (Blood 126 (23):67, 2015). The pattern of the neurogenic response in both groups was similar but stronger in SCD (Fig 1a). The time-course of the local vascular component was quite pronounced in SCD (Fig 1b) - increases in blood pressure promoted stronger and more rapid vasoconstriction in SCD relative to controls. The neurogenic-vascular interaction (not shown) caused further peripheral vasoconstriction in SCD, but tended to reduce vasoconstriction in controls. The respiratory contribution was small in both groups. Discussion & Conclusions: Heat-induced pain produces peripheral vasoconstriction via direct autonomic nervous system activation, as well as indirectly through the local vascular response to pain-triggered blood pressure fluctuations. The complex dynamics of the vasoconstriction response can only be understood by application of a mathematical model incorporating several relevant simultaneously measured and frequently sampled physiological signals. The stronger neurogenic response in SCD suggests elevated sympathetic activity compared to controls. The local vascular component in the SCD subjects exhibits a higher reactivity to increases in local blood pressure that promotes vasoconstriction, consistent with underlying endothelial dysfunction. The contribution of neurogenic-vascular interaction derived from the model likely represents the degree to which sympathetic overactivity leads to vascular dysfunction in SCD. These results suggest that dysautonomia and its interaction with peripheral vascular dysregulation participate in the genesis of vaso-occlusive crisis. Figure 1 (a) average neurogenic response (% change from baseline) to a heat pain pulse (20 °C, 10-sec long) in SCD (N = 22) and controls (N = 23); (b) average vascular response (% change from baseline) to a blood pressure pulse induced by pain (10 mmHg, 10-sec long) in SCD and controls. Figure 1. (a) average neurogenic response (% change from baseline) to a heat pain pulse (20 °C, 10-sec long) in SCD (N = 22) and controls (N = 23); (b) average vascular response (% change from baseline) to a blood pressure pulse induced by pain (10 mmHg, 10-sec long) in SCD and controls. Disclosures Wood: Biomed Informatics: Consultancy; AMAG: Consultancy; Vifor: Consultancy; Ionis Pharmaceuticals: Consultancy; Vifor: Consultancy; World Care Clinical: Consultancy; Celgene: Consultancy; Biomed Informatics: Consultancy; AMAG: Consultancy; Apopharma: Consultancy; Apopharma: Consultancy; World Care Clinical: Consultancy; Celgene: Consultancy; Ionis Pharmaceuticals: Consultancy.

Introduction: Sickle cell disease (SCD) is a genetic disorder characterized by sudden onset of painful vaso-occlusive episodes (VOC) which can be triggered by stress as reported by sickle cell patients. The exact mechanism of VOC origin is not well understood; however, it could result as a progression of microvascular blockade with rigid sickled red blood cells (RBC). Anything that decreases regional blood flow (RBF) will increase the transit time that it takes the RBC to escape the microvasculature before it gets rigid and entrapped, thus increasing the chances of vaso-occlusion that may progress to VOC. The RBF is controlled by the autonomic nervous system (ANS) which is modulated by stress, and as we have previously shown, SCD patients have augmented autonomic mediated vasoconstriction response (VCR) to sigh and pain. Our studies showed significant VCR when subjects were told they were about to experience pain, suggesting that VCR might be the physiologic link between common VOC triggers like stress and sickled RBC retention in the microvasculature. Objectives: To study the effect of mental stress on autonomic parameters peripheral blood flow (PBF) and heart rate variability (HRV) in SCD. Methods: 19 SCD and 16 control (healthy and sickle cell trait) subjects were studied. Two standard mental stress tasks with graded levels of difficulty (N-back test and Stroop test) were presented to subjects using E-prime (Psychological software). Subjects were also exposed to a novel pain anticipation task, previously shown to induce VCR. PBF was measured using photo-plethysmography (PPG) on the left thumb. Reduction in PPG amplitude indicates vasoconstriction. Cardiac beat-to-beat variability (R-to-R interval;RRI) was extracted from electrocardiogram. ANS balance was derived from the following spectral indices of the RRI: high frequency power (HFP) ≈ parasympathetic activity, low frequency power (LFP), and the low/high ratio (LFP/HFP) representing sympathovagal balance. Average changes in PBF and HRV from the baseline were taken as the responses to the mental tasks. Results: There was a significant decrease in mean PBF, RRI and HFP during all the mental stressors (N-back test, Stroop test and pain fear) compared to the baseline (p Conclusions: Graded mental stress and fear of pain causes significant decrease in regional blood flow and parasympathetic withdrawal in SCD subjects and normal controls. The vasoconstriction response to "fear of pain" was significantly greater than the VRC caused by N-back and to a lesser extent in Stroop. The pattern of responses were not significantly different between SCD and controls however the consequences of decreased blood flow could be quite different because of the resulted entrapment of sickle cells in the microvasculature. This could explain how stress may play a role in the initiation of VOC in SCD by enhancing ANS mediated vasoconstriction, prolonging microvascular transit time and increasing the likelihood of vaso-occlusion. The 1st signal is mental tasks event marker, 2nd signal is photo-plethysmography (PPG) and the 3rd signal is heart rate variability derived from the electrocardiogram signal. Peripheral blood flow and heart rate variability during baseline and various mental tasks Peripheral blood flow and heart rate variability during baseline and various mental tasks Box plot of mean peripheral blood flow (PBF) during baseline, mental tasks and fear of pain. Box plot of mean peripheral blood flow (PBF) during baseline, mental tasks and fear of pain. Disclosures Wood: Celgene: Consultancy; AMAG: Consultancy; Ionis Pharmaceuticals: Consultancy; Vifor: Consultancy; Ionis Pharmaceuticals: Consultancy; Biomed Informatics: Consultancy; Vifor: Consultancy; World Care Clinical: Consultancy; World Care Clinical: Consultancy; Celgene: Consultancy; AMAG: Consultancy; Apopharma: Consultancy; Apopharma: Consultancy; Biomed Informatics: Consultancy.

It is my great pleasure to welcome you to the 34th Annual International Conference of the IEEE Engineering in Medicine and Biology Society (EMBS). Our Annual Conference (known as EMBC) is a premier international conference in biomedical engineering, and has been strategically held in different locations around the world, for example, in Boston (USA), Buenos Aires (Argentina), Minneapolis (USA), Vancouver (Canada), Lyon (France), New York City (USA), and Shanghai (China) over the last 7 years.

Spectral analysis of heart rate variability (HRV) is commonly employed to track changes in autonomic nervous system and respiratory activity during sleep. However, conventional HRV spectral indices can be seriously confounded by inter-subject differences or intra-individual changes in ventilation and ventilatory pattern, especially in subjects with obstructive sleep apnea syndrome (OSAS). We highlight the approach we have undertaken to circumvent this problem by introducing “respiration-adjusted” spectral indices of HRV. Since fluctuations in sleep state also affect HRV considerably, we describe a method for combining the information derived from sleep staging and the information derived from cardiorespiratory measurements. We also introduce a new complementary index of autonomic function, BRS PTT , based on measurements of heart period and pulse transit time. We demonstrate that this surrogate measure of baroreflex gain correlates well with the corresponding measures of baroreflex sensitivity based on noninvasive blood pressure measurements. Our experience to date suggests that BRS PTT , along with respiration-adjusted spectral measures of HRV, are useful as clinical tools for assessing autonomic dysfunction in OSAS.

Sickle cell anemia (SCA) is a genetic disease which is characterized by painful episodes of vascular occlusions. In the present study, we investigated a potential contribution of the autonomic nervous system (ANS) in initiating such episodes. We observed prominent decreases in microvascular perfusion in response to spontaneous sighs, which may increase the likelihood of red blood cell polymerization followed by vascular occlusions in SCA patients. Time-varying spectral analysis of heart rate variability (HRV), based on recursive least squares estimation, was employed to study the modulation of the ANS in response to sighs. To improve robustness of the spectral estimation while retaining its ability to track rapid changes, we propose a time-varying parameter estimate variability reduction (TV-PEVR) technique. Because respiration patterns can vary considerably across subjects, we employed a time-varying autoregressive with exogenous input (TV-ARX) model to adjust for the effect of respiration patterns on the HRV indices. Results from 8 SCA subjects and 9 normal controls suggested that the cardiac ANS responses to sighs are not different between the two groups, after adjusting for the effect of post-sigh respiration. However, the peripheral sympathetic response in SCA appeared to be enhanced in this group relative to normals, and sighs may play a role in initiation of vaso-occlusive events in this group of patients.

To elucidate the mechanisms that lead to sleep-disordered breathing, we have developed a mathematical model that allows for dynamic interactions among the chemical control of respiration, changes in sleep-waking state, and changes in upper airway patency. The increase in steady-state arterial PCO2 accompanying sleep is shown to be inversely related to the ventilatory response to CO2. Chemical control of respiration becomes less stable during the light stage of sleep, despite a reduction in chemoresponsiveness, due to a concomitant increase in "plant gain" (i.e., responsiveness of blood gases to ventilatory changes). The withdrawal of the "wakefulness drive" during sleep onset represents a strong perturbation to respiratory control: higher magnitudes and rates of withdrawal of this drive favor instability. These results may account for the higher incidence of periodic breathing observed during light sleep and sleep onset. Periodic ventilation can also result from repetitive alternations between sleep onset and arousal. The potential for instability is further compounded if the possibility of upper airway occlusion is also included. In systems with high controller gains, instability is mediated primarily through chemoreflex overcompensation. However, in systems with depressed chemoresponsiveness, rapid sleep onset and large blood gas fluctuations trigger repetitive episodes of arousal and hyperpnea alternating with apneas that may or may not be obstructive. Between these extremes, more complex patterns can arise from the interaction between chemoreflex-mediated oscillations of shorter-cycle-duration (approximately 36 s) and longer-wavelength (approximately 60-80 s) state-driven oscillations.

Abnormalities in autonomic control in sickle cell anemia (SCA) patients have been reported by multiple researchers. However their potential causal association with sickle cell crisis remains unknown. We employed hypoxia, a known trigger to sickle cell crisis, to perturb the autonomic systems of the subjects. Cardiac autonomic control was non-invasively assessed by tracking the changes in heart rate variability (HRV) that occur following brief exposure to a hypoxia stimulus. Time varying spectral analysis of HRV was applied to estimate the cardiac autonomic response to the transient episode of hypoxia. The results demonstrate that cardiovascular autonomic response to hypoxia is substantially more sensitive in SCA than in normal controls. We also developed a model to compensate for the confounding effects of respiration on the HRV spectral indices by using the corresponding respiration signal to compensate for the respiratory correlated part of the HRV. This technique improved the resolution with which the effect of hypoxia on changes in HRV could be measured.

The accuracy of the single-breath CO2 inhalation test as a method for determining peripheral chemoreflex gain (Gp) is evaluated through computer simulations using a mathematical model of the closed-loop respiratory control system. Estimates of Gp (G'p) are based on “corrected” changes in end-tidal PCO2, because the uncorrected end-tidal values do not accurately reflect changes in alveolar PCO2. The influence of the central chemoreflex on G'p is generally less than 10% but can become disproportionally more significant as the relative contribution of the peripheral chemoreflex diminishes. G'p tends to overestimate Gp with the inclusion of peripheral chemoreceptor rate sensitivity, but this effect is offset by the time constant for adaptation. The spontaneous variability of breathing can seriously impair the resolution of G'p. Averaging of G'p deduced from individual single-breath tests can lead to erroneous estimates of Gp even when a large number of repetitions are performed. This problem can be minimized by first ensemble averaging the data from individual single-breath tests and, then, computing G'p from the resulting mean changes.

Adults with obstructive sleep apnea syndrome (OSAS) are known to have impaired autonomic function but the corresponding effects in children appear to be more subtle. Model-based analysis of the cardiovascular response to cold face test (CFT) was used to quantify daytime autonomic dysfunction. The increase in transfer gain between respiration and RRI was not different between controls and OSAS. However, the transfer gain between "surrogate cardiac output" (pulse pressure ÷ R-R interval) and systolic blood pressure (SBP) and the transfer gain between cardiac output and SBP both increased significantly in controls but not in OSAS during CFT. These findings suggest that the parasympathetic function remains relatively normal in pediatric OSAS, but cardiovascular sympathetic reactivity is impaired.

We searched for possible associations between various measures of severity of sleep-disordered breathing (SDB) and indices of cardiac autonomic function in older subjects (60 years). Twenty four overnight unattended homebased based polysomnograms obtained from the Sleep Heart Health Study were analyzed using spectral analysis. For each subject, six autonomic indices reflecting heart rate variability were quantitatively determined during wakefulness, REM sleep and non-REM sleep. Each individual autonomic marker was regressed against each of 4 measures of SDB, including the respiratory disturbance index (RDI), respiratory oscillation index, cumulative oxygen desaturation, and arousal index. In general, we found no correlation between any of these measures of SDB severity and each of the autonomic indices. However, mean heart rate was found to decrease as RDI increased. As well, the ratio of low-frequency to high-frequency power (LHR) decreased with increasing RDI. Contrary to previous reports, our preliminary findings suggest that sympathetic activity decreases with increasing severity of SDB. This paradoxical association between SDB and cardiac autonomic function may be the result of natural compensatory mechanisms at work, allowing some subjects with SDB to be protected from systemic hypertension or other cardiovascular diseases. Supported in part by NIH Grants HL076375, EB001978, HL63463 and HL53941.