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Purpose: To examine the effect of freediving depth on risk for hypoxic blackout by recording arterial oxygen saturation (SpO 2 ) and heart rate (HR) during deep and shallow dives in the sea., Methods: Fourteen competitive freedivers conducted open-water training dives wearing a water-/pressure proof pulse oximeter continuously recording HR and SpO 2 . Dives were divided into deep (> 35 m) and shallow (10-25 m) post-hoc and data from one deep and one shallow dive from 10 divers were compared., Results: Mean ± SD depth was 53 ± 14 m for deep and 17 ± 4 m for shallow dives. Respective dive durations (120 ± 18 s and 116 ± 43 s) did not differ. Deep dives resulted in lower minimum SpO 2 (58 ± 17%) compared with shallow dives (74 ± 17%; P = 0.029). Overall diving HR was 7 bpm higher in deep dives (P = 0.002) although minimum HR was similar in both types of dives (39 bpm). Three divers desaturated early at depth, of which two exhibited severe hypoxia (SpO 2  ≤ 65%) upon resurfacing. Additionally, four divers developed severe hypoxia after dives., Conclusions: Despite similar dive durations, oxygen desaturation was greater during deep dives, confirming increased risk of hypoxic blackout with increased depth. In addition to the rapid drop in alveolar pressure and oxygen uptake during ascent, several other risk factors associated with deep freediving were identified, including higher swimming effort and oxygen consumption, a compromised diving response, an autonomic conflict possibly causing arrhythmias, and compromised oxygen uptake at depth by lung compression possibly leading to atelectasis or pulmonary edema in some individuals. Individuals with elevated risk could likely be identified using wearable technology., (© 2023. The Author(s).)

This article aimed to synthesize the various triggers of the diving response and to perform a meta-analysis assessing their effects on cardiac vagal activity. The protocol was preregistered on PROSPERO (CRD42021231419; 01.07.2021). A systematic and meta-analytic review of cardiac vagal activity was conducted, indexed with the root mean square of successive differences (RMSSD) in the context of the diving response. The search on MEDLINE (via PubMed), Web of Science, ProQuest and PsycNet was finalized on November 6th, 2021. Studies with human participants were considered, measuring RMSSD pre- and during and/or post-exposure to at least one trigger of the diving response. Seventeen papers (n = 311) met inclusion criteria. Triggers examined include face immersion or cooling, SCUBA diving, and total body immersion into water. Compared to resting conditions, a significant moderate to large positive effect was found for RMSSD during exposure (Hedges' g = 0.59, 95% CI 0.36 to 0.82, p < .001), but not post-exposure (g = 0.11, 95% CI -0.14 to 0.36, p = .34). Among the considered moderators, total body immersion had a significantly larger effect than forehead cooling (Q M  = 23.46, df = 1, p < .001). No further differences were detected. Limitations were the small number of studies included, heterogenous triggers, few participants and low quality of evidence. Further research is needed to investigate the role of cardiac sympathetic activity and of the moderators., (© 2022 The Authors. Psychophysiology published by Wiley Periodicals LLC on behalf of Society for Psychophysiological Research.)

Static apneas performed after an overnight fast as opposed to postprandially have been evinced to improve apneic performance. However, no study has explored the effect of dietary intake on apneic performance, cardiovascular or splenic responses over a series of repeated apneas. Ten healthy adults attended the laboratory on three separate occasions (≥48-h apart): after a 14-h fast (F14), 1 h postconsumption of a high-calorie, high-carbohydrate (HCHC) meal, or 1 h postconsumption of a low-calorie, low-carbohydrate (LCLC)-based meal. During each visit, the subjects performed a hyperoxic rebreathing trial and a series of three repeated maximal static apneas. Heart rate, peripheral oxyhemoglobin saturation ([Formula: see text]), and gas exchange were monitored continuously, whereas splenic volume (SV) and hematology were assessed after the rebreathing and apneas. At rest, after HCHC, the respiratory exchange ratio (0.87 ± 0.17, P ≤ 0.043), expired minute volume of carbon dioxide (CO 2 ; HCHC, 0.35 ± 0.09 L/min, P ≤ 0.014), and SV (227 ± 45 mL, P ≤ 0.031) were higher compared with F14 (0.71 ± 0.08; 0.23 ± 0.04 L/min; 204 ± 49 mL) and LCLC (0.72 ± 0.07; 0.25 ± 0.03 L/min; 199 ± 49 mL). A faster CO 2 accumulation was recorded during the HCHC (96 ± 35 s) rebreathing trial (F14, 162 ± 42 s, P = 0.001; LCLC, 151 ± 23 s, P = 0.002). Longer apneas were reported in F14 compared with HCHC (apneas 1-3, P ≤ 0.046) and LCLC (apneas 2-3, P ≤ 0.006). After the first apnea, SV was lower in F14 (141 ± 43 mL, P = 0.015) compared with HCHC (180 ± 34 mL). Moreover, after the third apnea, end-tidal partial pressure of oxygen and nadir [Formula: see text] were lower in F14 (8.6 ± 2.2 kPa, P = 0.028; 77 ± 13%, P = 0.009) compared with HCHC (10.1 ± 1.7 kPa; 84 ± 9%). No differences were measured in end-apneic end-tidal partial pressure of CO 2 , heart rate nor hematology across diets. Fasting improved apneic performance with apneas being terminated at lower oxygen levels through altering the rate of CO 2 accumulation but without affecting the cardiovascular responses.

Purpose: This study investigated the effects of lung volume and trigeminal nerve stimulation (TS) on diving responses in breath-hold divers (BHDs) and non-divers (NDs)., Methods: Eight BHDs and nine NDs performed four breath-hold trials at different lung volumes, with or without TS, and one trial of TS. Haemodynamic parameters and electrocardiograms were measured for each trial., Results: During the TS trial, the total peripheral resistance increased more in BHDs. Breath-hold performed at total lung capacity showed a more pronounced decrease in stroke volume and cardiac output in BHDs. The decrease in heart rate and increase in total peripheral resistance were more pronounced in BHDs when breath-holding was performed with TS., Conclusion: The more pronounced diving response in BHDs was attributed to the greater increase in total peripheral resistance caused by TS. Furthermore, the lower stroke volume and cardiac output in BH performed at total lung capacity could also cause a more pronounced diving response in BHDs., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Heart rates of air-breathing diving animals can change on a short time scale due to the diving response during submergence. Heart rate is used frequently as a proxy for indirectly estimating metabolic rates on a fine time scale. However, most studies to date have been conducted on endothermic diving animals, and the relationships between metabolic rates and heart rates in ectothermic diving animals have not been well studied. Sea turtles are unique model organisms of diving ectotherms because they spend most of their life in the ocean and perform deep and/or long dives. In this study, we examined the relationship between heart rates and metabolic rates in captive loggerhead turtles, Caretta caretta, to estimate oxygen consumption rates during each dive based on heart rates. The oxygen consumption rates (V̇O2: mlO2 min-1 kg-1) and average heart rates (fH: beats min-1) were measured simultaneously in indoor tanks at water temperatures of 15-25°C. Our results showed that oxygen consumption rate was affected by heart rate and water temperature in loggerhead turtles. Based on the collected data, we formulated the model equation as V̇O2=0.0124fH+0.0047Tw - 0.0791. The equation can be used for estimating fine-scaled field metabolic rates in free-ranging loggerhead turtles. The results of this study will contribute to future comparative studies of the physiological states of ectothermic diving animals., Competing Interests: Competing interests The authors declare no competing or financial interests., (© 2022. Published by The Company of Biologists Ltd.)

Purpose: The present study aimed to measure diving response, CO 2 sensitivity and forced vital capacity in male and female breath-hold divers (BHDs), and to determine their effect on breath-hold diving performance., Methods: This study included 8 non-divers (NDs, 4 males and 4 females) and 15 BHDs (7 males and 8 females). For NDs, diving response was measured during breath-holding with facial immersion, whereas for BHDs CO 2 sensitivity was also measured., Results: Compared to NDs, BHDs showed a prominent diving response. In BHDs, no statistically significant sex differences were observed in diving response and CO 2 sensitivity. Furthermore, a positive correlation was found between performance and the % forced vital capacity in BHDs., Conclusion: It was suggested that % forced vital capacity contributed more significantly to performance than diving response and CO 2 sensitivity. Furthermore, the higher performance of male divers compared to female divers may be due to the % forced vital capacity rather than the diving response and CO 2 sensitivity., (Copyright © 2021 Elsevier B.V. All rights reserved.)

New Findings: What is the central question of this study? Splenic contractions occur in response to apnoea-induced hypoxia with and without face immersion in water. However, the splenic responses to a series of static or dynamic apnoeas with whole-body water immersion in non-divers and elite breath-hold divers are unknown. What is the main finding and its importance? Static and dynamic apnoeas were equally effective in stimulating splenic contractions across non-divers and elite breath-hold divers. These findings demonstrate that the magnitude of the splenic response is largely dictated by the degree of the hypoxemic stress encountered during voluntary apnoeic epochs., Abstract: Splenic contractions occur in response to apnoea-induced hypoxia with and without facial water immersion. However, the splenic responses to a series of static (STA) or dynamic (DYN) apnoeas with whole-body water immersion in non-divers (NDs) and elite breath-hold divers (EBHDs) are unknown. EBHD (n = 8), ND (n = 10) and control participants (n = 8) were recruited. EBHD and ND performed a series of five maximal DYN or STA on separate occasions. Control performed a static eupnoeic (STE) protocol to control against any effects of water immersion and diurnal variation on splenic volume and haematology. Heart rate (HR) and peripheral oxygen saturation (SpO 2 ) were monitored for 30 s after each apnoea. Pre- and post-apnoeic splenic volumes were quantified ultrasonically, and blood samples were drawn for haematology. For EBHD and ND end-apnoeic HR was higher (P < 0.001) and SpO 2 was lower in DYN (P = 0.024) versus STA. EBHD attained lower end-apnoeic SpO 2 during DYN and STA than NDs (P < 0.001). Splenic contractions occurred following DYN (EBHD, -47 ± 6%; ND, -37 ± 4%; P < 0.001) and STA (EBHD, -26 ± 4%; ND, -26 ± 8%; P < 0.01). DYN-associated splenic contractions were greater than STA in EBHD only (P = 0.042). Haemoglobin concentrations were higher following DYN only (EBHD, +5 ± 8g/L  , +4 ± 2%; ND, +8 ± 3 g/L , +4.9 ± 3%; P = 0.019). Haematocrit remained unchanged after each protocol. There were no between group differences in post-apnoeic splenic volume or haematology. In both groups, splenic contractions occurred in response to STA and DYN when combined with whole-body immersion. DYN apnoeas, were effective at increasing haemoglobin concentrations but not STA apnoeas. Thus, the magnitude of the splenic response relates to the hypoxemic stress encountered during apnoeic epochs., (© 2020 The Authors. Experimental Physiology published by John Wiley & Sons Ltd on behalf of The Physiological Society.)

New Findings: What is the central question of this study? Does facial cooling-mediated stimulation of cutaneous trigeminal afferents associated with the diving response increase cerebral blood flow or are factors associated with breath-holding (e.g. arterial carbon dioxide accumulation, pressor response) more important in humans? What is the main finding and its importance? Physiological factors associated with breath-holding such as arterial carbon dioxide accumulation and the pressor response, but not facial cooling (trigeminal nerve stimulation), make the predominant contribution to diving response-mediated increases in cerebral blood flow in humans., Abstract: Diving evokes a pattern of physiological responses purported to preserve oxygenated blood delivery to vital organs such as the brain. We sought to uncouple the effects of trigeminal nerve stimulation on cerebral blood flow (CBF) from other modifiers associated with the diving response, such as apnoea and changes in arterial carbon dioxide tension. Thirty-seven young healthy individuals participated in separate trials of facial cooling (FC, 3 min) and cold pressor test (CPT, 3 min) under poikilocapnic (Protocol 1) and isocapnic conditions (Protocol 2), facial cooling while either performing a breath-hold (FC +BH) or breathing spontaneously for a matched duration (FC -BH) (Protocol 3), and BH during facial cooling (BH +FC) or without facial cooling (BH -FC) (Protocol 4). Under poikilocapnic conditions neither facial cooling nor CPT evoked a change in middle cerebral artery blood flow velocity (MCA v mean ; transcranial Doppler) (P > 0.05 vs. baseline). Under isocapnic conditions, facial cooling did not change MCA v mean (P > 0.05), whereas CPT increased MCA v mean by 13% (P < 0.05). Facial cooling with a concurrent BH markedly increased MCA v mean (Δ23%) and internal carotid artery blood flow (ICA Q ; duplex Doppler ultrasound) (Δ26%) (P < 0.001), but no change in MCA v mean and ICA Q was observed when facial cooling was accompanied by spontaneous breathing (P > 0.05). Finally, MCA v mean and ICA Q were similarly increased by BH either with or without facial cooling. These findings suggest that physiological factors associated with BH, and not facial cooling (i.e. trigeminal nerve stimulation) per se, make the predominant contribution to increases in CBF during diving in humans., (© 2020 The Authors. Experimental Physiology © 2020 The Physiological Society.)

Purpose: Serum erythropoietin (EPO) concentration is increased following static apnoea-induced hypoxia. However, the acute erythropoietic responses to a series of dynamic apnoeas in non-divers (ND) or elite breath-hold divers (EBHD) are unknown., Methods: Participants were stratified into EBHD (n = 8), ND (n = 10) and control (n = 8) groups. On two separate occasions, EBHD and ND performed a series of five maximal dynamic apnoeas (DYN) or two sets of five maximal static apnoeas (STA). Control performed a static eupnoeic (STE) protocol to control against any effects of water immersion and diurnal variation on EPO. Peripheral oxygen saturation (SpO 2 ) levels were monitored up to 30 s post each maximal effort. Blood samples were collected at 30, 90, and 180 min after each protocol for EPO, haemoglobin and haematocrit concentrations., Results: No between group differences were observed at baseline (p > 0.05). For EBHD and ND, mean end-apnoea SpO 2 was lower in DYN (EBHD, 62 ± 10%, p = 0.024; ND, 85 ± 6%; p = 0.020) than STA (EBHD, 76 ± 7%; ND, 96 ± 1%) and control (98 ± 1%) protocols. EBHD attained lower end-apnoeic SpO 2 during DYN and STA than ND (p < 0.001). Serum EPO increased from baseline following the DYN protocol in EBHD only (EBHD, p < 0.001; ND, p = 0.622). EBHD EPO increased from baseline (6.85 ± 0.9mlU/mL) by 60% at 30 min (10.82 ± 2.5mlU/mL, p = 0.017) and 63% at 180 min (10.87 ± 2.1mlU/mL, p = 0.024). Serum EPO did not change after the STA (EBHD, p = 0.534; ND, p = 0.850) and STE (p = 0.056) protocols. There was a significant negative correlation (r = - 0.49, p = 0.003) between end-apnoeic SpO 2 and peak post-apnoeic serum EPO concentrations., Conclusions: The novel findings demonstrate that circulating EPO is only increased after DYN in EBHD. This may relate to the greater hypoxemia achieved by EBHD during the DYN.

To check whether the evolution of alveolar pressures of O 2 (P A O 2 ) and CO 2 (P A CO 2 ) explains the cardiovascular responses to apnoea, eight divers performed resting apnoeas of increasing duration in air and in O 2 . We measured heart rate (f H ), arterial pressure (AP), and peripheral resistances (TPR) beat-by-beat, P A O 2 and P A CO 2 at the end of each apnoea. The three phases of the cardiovascular response to apnoea were observed. In O 2 , TPR increase (9 ± 4 mmHg min l -1 ) and f H decrease (-11 ± 8 bpm) were lower than in air (15 ± 5 mmHg min l -1 and -28 ± 13 bpm, respectively). At end of maximal apnoeas in air, P A O 2 and P A CO 2 were 50 ± 9 and 48 ± 5 mmHg, respectively; corresponding values in O 2 were 653 ± 8 mmHg and 55 ± 5 mmHg. At end of phase II, P A O 2 and P A CO 2 in air were 90 ± 13 mmHg and 42 ± 4 mmHg respectively; corresponding values in O 2 were 669 ± 7 mmHg and 47 ± 6 mmHg. The P A CO 2 increase may trigger the AP rise in phase III., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Understanding the physiology and genetics of human hypoxia tolerance has important medical implications, but this phenomenon has thus far only been investigated in high-altitude human populations. Another system, yet to be explored, is humans who engage in breath-hold diving. The indigenous Bajau people ("Sea Nomads") of Southeast Asia live a subsistence lifestyle based on breath-hold diving and are renowned for their extraordinary breath-holding abilities. However, it is unknown whether this has a genetic basis. Using a comparative genomic study, we show that natural selection on genetic variants in the PDE10A gene have increased spleen size in the Bajau, providing them with a larger reservoir of oxygenated red blood cells. We also find evidence of strong selection specific to the Bajau on BDKRB2, a gene affecting the human diving reflex. Thus, the Bajau, and possibly other diving populations, provide a new opportunity to study human adaptation to hypoxia tolerance. VIDEO ABSTRACT., (Copyright © 2018 Elsevier Inc. All rights reserved.)

In cetaceans, diving behavior immediately induces a change in blood circulation to favor flow to the brain and heart; this is achieved by intense vasoconstriction of the blood vessels that serve other organs. This blood circulation response is allied to a decrease in heart rate in order to optimize oxygen usage during diving. Vasoconstrictors are present in all mammals and stimulate the contraction of the smooth muscle in the walls of blood vessels. The most important of these vasoconstrictors are the hormones adrenaline (A), noradrenaline (NA), and angiotensin II (ANG II). At present, the contribution of these hormones to vasoconstriction during diving in cetaceans is unclear. To elucidate their possible roles, changes in serum levels of A, NA and ANG II were monitored together with heart rate in the Indo-Pacific bottlenose dolphin Tursiops aduncus during 90 and 180s dives. Both brief diving periods induced an increase in serum NA concentration and a decrease in heart rate; however, no changes were detected in serum levels of A or ANG II. These data indicate that NA may play a role in diving-induced vasoconstriction., (Copyright © 2017 Elsevier Inc. All rights reserved.)

Introduction: Technical diving is very popular in Finland throughout the year despite diving conditions being challenging, especially due to arctic water and poor visibility. Cold water, immersion, submersion, hyperoxia, as well as psychological and physiological stress, all have an effect on the autonomic nervous system (ANS). Materials and methods: To evaluate divers' ANS responses, short-term (5 min) heart rate variability (HRV) during dives in 2–4°C water was measured. HRV resting values were evaluated from separate measurements before and after the dives. Twenty-six experienced closed circuit rebreather (CCR) divers performed an identical 45-meter decompression dive with a non-physical task requiring concentration at the bottom depth. Results: Activity of the ANS branches was evaluated with the parasympathetic (PNS) and sympathetic (SNS) indexes of the Kubios HRV Standard program. Compared to resting values, PNS activity decreased significantly on immersion with face out of water. From immersion, it increased significantly with facial immersion, just before decompression and just before surfacing. Compared to resting values, SNS activity increased significantly on immersion with face out of water. Face in water and submersion measures did not differ from the immersion measure. After these measurements, SNS activity decreased significantly over time. Conclusion: Our study indicates that the trigeminocardiac part of the diving reflex causes the strong initial PNS activation at the beginning of the dive but the reaction seems to decrease quickly. After this initial activation, cold seemed to be the most prominent promoter of PNS activity – not pressure. Also, our study showed a concurrent increase in both SNS and PNS branches, which has been associated with an elevated risk for arrhythmia. Therefore, we recommend a short adaptation phase at the beginning of cold-water diving before physical activity. [ABSTRACT FROM AUTHOR]

Essential hypertension (EH) is a widespread disease and might be prevalent in apnea divers and master athletes. Little is known about the influence of EH and the antihypertensive drugs (AHD) on cardiovascular reactions to combined breath hold (BH) and exercise. In this pilot study, healthy divers (HCON) were compared with treated hypertensive divers with regard to heart rate (HR) and mean blood-pressure (MAP) responses to BH, exercise and the combination of both. Ten subjects with EH and ten healthy divers were tested. 3 different 20s stimuli were applied: BH combined with 30W or 150W and 150W without BH. The time-charts during the stress intervals and during recovery were compared. Subjects treated with an angiotensin-converting enzyme (ACE) inhibitor showed higher changes for MAP values if breath hold was performed. HR responses were obviously changed if a β-blocker was part of the medication. One subject showed extreme MAP responses to all stimuli and conspicuous HR if BH was involved. The modulation of HR-/MAP-response in EH subjects depends on the mechanisms of antihypertensive agents. The combination of an ACE inhibitor and a β-blocker may give the best protection. It is recommended to include short apnea tests in the fitness-to-dive examination to individually predict potential endangerment. [ABSTRACT FROM AUTHOR]

Cardiac output has never been assessed during free-diving diving in the sea. Knowledge of human diving response in this setting is therefore scarce. 3 immersions were performed by 7 divers: at depths of 10 m, 20 m and 30 m. Each test consisted of 3 apnea phases: descent, static and ascent. An impedance cardiograph provided data on stroke volume, heart rate and cardiac output. Mean blood pressure, arterial O2 saturation and blood lactate values were also collected. Starting from a resting value of 4.5±1.6 L·min-1, cardiac output at 10 m showed an increase up to 7.1±2.2 L·min-1 (p < 0.01) during the descent, while conditions during the static and ascent phases remained unchanged. At 20 m cardiac output values were 7.3±2.4 L·min-1 and 6.7±1.2L·min-1 during ascent and descent, respectively (p < 0.01), and 4.3±0.9 L·min-1 during static phase. At 30 m cardiac output values were 6.5±1.8 L·min-1 and 7.5±2 L·min-1 during descent and ascent, respectively (p < 0.01), and 4.7±2.1 L·min-1 during static phase. Arterial O2 saturation decreased with increasing dive depth, reaching 91.1±3.4% (p < 0.001 vs. rest) upon emergence from a depth of 30 m. Blood lactate values increased to 4.1±1.2 mmol·L-1 at the end of the 30 m dive (p < 0.001 vs. rest). Results seem to suggest that simultaneous activation of exercise and diving response could lead to an absence of cardiac output reduction aimed at an oxygen-conserving effect. [ABSTRACT FROM AUTHOR]

Swimming involves muscular activity and submersion, creating a conflict of autonomic reflexes elicited by the trigeminal receptors and skeletal muscle afferents. We sought to determine the autonomic cardiovascular responses to separate and concurrent stimulation of the trigeminal cutaneous receptors and metabolically sensitive skeletal muscle afferents (muscle metaboreflex). In eight healthy men (30 ± 2 yr) muscle sympathetic nerve activity (MSNA; microneurography), mean arterial pressure (MAP; Finometer), femoral artery blood flow (duplex Doppler ultrasonography), and femoral vascular conductance (femoral artery blood flow/MAP) were assessed during the following three experimental conditions: 1) facial cooling (trigeminal nerve stimulation), 2) postexercise ischemia (PEI; muscle metaboreflex activation) following isometric handgrip, and 3) trigeminal nerve stimulation with concurrent PEI. Trigeminal nerve stimulation produced significant increases in MSNA total activity (Δ347 ± 167%) and MAP (Δ21 ± 5%) and a reduction in femoral artery vascular conductance (Δ-17 ± 9%). PEI also evoked significant increases in MSNA total activity (Δ234 ± 83%) and MAP (Δ36 ± 4%) and a slight nonsignificant reduction in femoral artery vascular conductance (Δ-9 ± 12%). Trigeminal nerve stimulation with concurrent PEI evoked changes in MSNA total activity (Δ341 ± 96%), MAP (Δ39 ± 4%), and femoral artery vascular conductance (Δ-20 ± 9%) that were similar to those evoked by either separate trigeminal nerve stimulation or separate PEI. Thus, excitatory inputs from the trigeminal nerve and metabolically sensitive skeletal muscle afferents do not summate algebraically in eliciting a MSNA and cardiovascular response but rather exhibit synaptic occlusion, suggesting a high degree of convergent inputs on output neurons. [ABSTRACT FROM AUTHOR]

Humans freedive for recreation, food, and for records, displaying an ability superior to any other known terrestrial mammal. Harvest divers may spend up to 5 h daily submerged, and competitive deep divers may swim down to depths beyond 100 m - and up again - on one breath. This is paralleled only by aquatic and semi-aquatic mammals. In all air breathers, the ability to breath-hold dive is set by a number of physiological factors. Some of these have been extensively studied, like the 'diving response,' which diverts blood to the brain, heart and working muscles, while less sensitive organs can rely on anaerobic metabolism. The diving response is effectively triggered in trained human divers and may reduce the heart rate by 50% - similar to the response in semi-aquatic mammals, and it has been shown to conserve oxygen and prolong apneic duration. More recently, a spleen response during diving was described in humans resembling that found in seals. The spleen ejects its stored reserve supply of red blood cells into the circulation, elevating hematocrit, and thereby the oxygen carrying capacity, which also increases apneic duration. Other features not typical of terrestrial mammals, such as: naked skin with a continuous layer of subcutaneous fat; under water vision; effective locomotion in water; large flexible lungs; an ability to equalize ears and sinuses; efficient voluntary respiratory control and vocal communication; a long lifespan; and a large brain, are shared by many species with a semi-aquatic evolutionary history and are also shared by Man. This would seem to suggest a period of evolutionary pressure from life involving swimming and diving in human prehistory. [ABSTRACT FROM AUTHOR]

A dramatic bradycardia is induced by underwater submersion in vertebrates. The location of parasympathetic preganglionic cardiac motor neurons driving this aspect of the diving response was investigated using cFos immunohistochemistry combined with retrograde transport of cholera toxin subunit B (CTB) to double-label neurons. After pericardial injections of CTB, trained rats voluntarily dove underwater, and their heart rates (HR) dropped immediately to 95 ± 2bpm, an 80% reduction. After immunohistochemical processing, the vast majority of CTB labeled neurons were located in the reticular formation from the rostral cervical spinal cord to the facial motor nucleus, confirming previous studies. Labeled neurons caudal to the rostral ventrolateral medulla were usually spindle-shaped aligned along an oblique line running from the dorsal vagal nucleus to the ventrolateral reticular formation, while those more rostrally were multipolar with extended dendrites. Nine percent of retrogradely-labeled neurons were positive for both cFos and CTB after diving and 74% of these were found rostral to the obex. CTB also was transported transganglionically in primary afferent fibers, resulting in large granular deposits in dorsolateral, ventrolateral, and commissural subnuclei of the nucleus tractus solitarii (NTS) and finer deposits in lamina I and IV-V of the trigeminocervical complex. The overlap of parasympathetic preganglionic cardiac motor neurons activated by diving with those activated by baro- and chemoreceptors in the rostral ventrolateral medulla is discussed. Thus, the profound bradycardia seen with underwater submersion reinforces the notion that the mammalian diving response is the most powerful autonomic reflex known. [ABSTRACT FROM AUTHOR]

The mammalian diving response is a dramatic autonomic adjustment to underwater submersion affecting heart rate, arterial blood pressure, and ventilation. The bradycardia is known to be modulated by the parasympathetic nervous system, arterial blood pressure is modulated via the sympathetic system, and still other circuits modulate the respiratory changes. In the present study, we investigate the submergence of rats brought past their aerobic dive limit, defined as the diving duration beyond which blood lactate concentration increases above resting levels. Hemodynamic measurements were made during underwater submergence with biotelemetric transmitters, and blood was drawn from cannulas previously implanted in the rats' carotid arteries. Such prolonged submersion induces radical changes in blood chemistry; mean arterial Pco2 rose to 62.4 Torr, while mean arterial Po2 and pH reached nadirs of 21.8 Ton" and 7.18, respectively. Despite these radical changes in blood chemistry, the rats neither attempted to gasp nor breathe while underwater. Immunohistochemistry for Fos protein done on their brains revealed numerous Fos-positive profiles. Especially noteworthy were the large number of immunopositive profiles in loci where presumptive chemoreceptors are found. Despite the activation of these presumptive chemoreceptors, the rats did not attempt to breathe. Injections of biotinylated dextran amine were made into ventral parts of the medullary dorsal horn, where central fibers of the anterior ethmoidal nerve terminate. Labeled fibers coursed caudal, ventral, and medial from the injection to neurons on the ventral surface of the medulla, where numerous Fos-labeled profiles were seen in the rats brought past their aerobic dive limit. We propose that this projection inhibits the homeostatic chemoreceptor reflex, despite the gross activation of chemoreceptors. [ABSTRACT FROM AUTHOR]

To define the dynamics of cardiovascular adjustments to apnoea during immersion, beat-to-beat heart rate (HR) and systolic (SBP) and diastolic (DBP) blood pressures were recorded in six divers during and after prolonged apnoeas while resting fully immersed in 27 degrees C water. Apnoeas lasted 215 +/- 35 s. Compared to control values, HR decreased by 20 beats min(-1) and SBP and DBP increased by 23 and 17 mmHg, respectively, in the initial 20 +/- 3 s (phase I). Both HR and BP remained stable during the following 92 +/- 15 s (phase II). Subsequently, during the final 103 +/- 29 s, SBP and DBP increased linearly to values about 60% higher than control, whereas HR remained unchanged (phase III). Cardiac output (Q') decreased by 35% in phase I and did not further change in phases II and III. Compared to control, total peripheral resistances were twice and three times higher than control, respectively, at the end of phases I and III. After resumption of breathing, HR and BP returned to control values in 5 and 30 s, respectively. The time courses of cardiovascular adjustments to immersed breath-holding indicated that cardiac response took place only at the beginning of apnoea. In contrast, vascular responses showed two distinct adjustments. This pattern suggests that the chronotropic control via the baroreflex is modified during apnoea. These cardiovascular changes during immersed static apnoea are in agreement with those already reported for static dry apnoeas. [ABSTRACT FROM AUTHOR]

Hallmarks of the mammalian diving response are protective apnea and bradycardia. These cardiorespiratory adaptations can be mimicked by stimulation of the trigeminal ethmoidal nerve (ENS) and reflect oxygen-conserving mechanisms during breath-hold dives. Increasing drive from peripheral chemoreceptors during sustained dives was reported to enhance the diving bradycardia. The underlying neuronal mechanisms, however, are unknown. In the present study, expression and plasticity of EN5-bradycardias after paired stimulation of the ENS and peripheral chemoreceptors was investigated in the in situ working heart-brain stem preparation. Paired stimulations enhanced significantly the bradycardic responses compared with EN5-evoked bradycardia using submaximal stimulation intensity. Alternating stimulations of the ENS followed by paired stimulation of the ENS and chemoreceptors (10 trials, 3-mm interval) caused a progressive and significant potentiation of ENS-evoked diving bradycardia. In contrast, bradycardias during paired stimulation remained unchanged during repetitive stimulation. The progressive potentiation of ENS- bradycardias was significantly enhanced after microinjection of the S-HT3 receptor agonist (CPBG hydrochloride) into the nucleus tractus solitarii (NTS), while the S-HT3 receptor antagonist (zacopride hydrochloride) attenuated the progressive potentiation. These results suggest an integrative function of the NTS for the multimodal mediation of the diving response. The potentiation or training of a submaximal diving bradycardia requires peripheral chemoreceptor drive and involves neurotransmission via 5-HT3 receptor within the NTS. [ABSTRACT FROM AUTHOR]

To evaluate the separate cardiovascular response to body immersion and increased environmental pressure during diving, 12 healthy male subjects (mean age 35.2 ± 6.5 yr) underwent two-dimensional Doppler echocardiography in five different conditions: out of water (basal); head-out immersion while breathing (condition A); fully immersed at the surface while breathing (condition B) and breath holding (condition C); and breath-hold diving at 5-m depth (condition D). Heart rate, left ventricular volumes, stroke volume, and cardiac output were obtained by underwater echocardiography. Early (E) and late (A) transmitral flow velocities, their ratio (E/A), and deceleration time of E (DTE) were also obtained from pulsed-wave Doppler, as left ventricular diastolic function indexes. The experimental protocol induced significant reductions in left ventricular volumes, left ventricular stroke volume (P < 0.05), cardiac output (P < 0.00 1), and heart rate (P < 0.05). A significant increase in E peak (P < 0.01) and E/A (P < 0.01) and a significant reduction of DTE (P < 0.01) were also observed. Changes occurring during diving (condition D) accounted for most of the changes observed in the experimental series. In particular, cardiac output at condition D was significantly lower compared with each of the other experimental conditions, E/A was significantly higher during condition D than in conditions A and C. Finally, DTE was significantly shorter at condition D than in basal and condition C. This study confirms a reduction of cardiac output in diving humans. Since most of the changes were observed during diving, the increased environmental pressure seems responsible for this hemodynamic rearrangement. Left ventricular diastolic function changes suggest a constrictive effect on the heart, possibly accounting for cardiac output reduction. [ABSTRACT FROM AUTHOR]

To define the dynamics of cardiovascular adjustments to apnoea, beat-to-beat heart rate (HR) and blood pressure and arterial oxygen saturation (SaO(2)) were recorded during prolonged breath-holding in air in 20 divers. Apnoea had a mean duration of 210 +/- 70 s. In all subjects, HR attained a value 14 beats min(-1) lower than control within the initial 30 s (phase I). HR did not change for the following 2-2.5 min (phase II). Then, nine subjects interrupted the apnoea (group A), whereas 11 subjects (group B) could prolong the breath-holding for about 100 s, during which HR continuously decreased (phase III). In both groups, mean blood pressure was 8 mmHg above control at the end of phase I; it then further increased by additional 12 mmHg at the end of the apnoea. In both groups, SaO(2) did not change in the initial 100-140 s of apnoea; then, it decreased to 95% at the end of phase II. In group B, SaO(2) further diminished to 84% at the end of phase III. A typical pattern of cardiovascular readjustments was identified during dry apnoea. This pattern was not compatible with a role for baroreflexes in phase I and phase II. Further readjustment in group B may imply a role for both baroreflexes and chemoreflexes. Hypothesis has been made that the end of phase II corresponds to physiological breakpoint. [ABSTRACT FROM AUTHOR]

This study was to elucidate the physiological effects of dynamic apneas, as performed as a discipline in breath-hold diving for recreational or competitive purposes. Therefore, cardiovascular and respiratory effects of apneas with simultaneously initiated exercise were investigated in ten trained breath-hold divers. They performed maximum duration apneas with face immersion (26 degrees C) under rest and exercise (40 W, 80 W and 120 W) on a cycle ergometer in the laboratory. Apneic time, heart rate (HR), mean arterial pressure (MAP), arterial oxygen saturation and O2 (.VO2) and CO2 exchange were measured. All end-apnea heart rates were lower than corresponding control values. Higher workloads increased the initial rise in HR and delayed the onset of bradycardia. After an initial drop, MAP rose to 150% of control towards the end of apnea. Apneic .VO2 was reduced by 25% during exercise and by 40% during resting apneas compared to eupneic control values. It was concluded that magnitude and time course of diving bradycardia depend on work intensity. Higher workloads delay the onset and attenuate HR reduction, presumably due to increased sympathetic activity. It was also found that apnea with simultaneously initiated exercise has an O2 conserving effect compared to eupneic exercise. Although aimed to be a realistic approach to breath-hold diving, the study has certain methodological limitations in terms of body-immersion (hydrostatic pressure effects) and body-cooling effects due to conduction of the experiments in a laboratory set-up. [ABSTRACT FROM AUTHOR]

Diving mammals may enhance dive duration by injecting extra erythrocytes into the circulation by spleen contraction. This mechanism may also be important for apneic duration in humans. We studied the speed and magnitude of spleen volume changes evoked by serial apneas, and the associated changes in hematocrit (Hct) and hemoglobin (Hb) concentration, diving response and apneic duration. Three maximal apneas separated by 2 min rest elicited spleen contraction in all ten subjects, by a mean of 49 (27) ml (18%; P<0.001). During the same period, Hct and Hb rose by 2.2 and 2.4% respectively (P<0.01 and P<0.001), and apneic duration rose by 20 s (22% P<0.05). The mean heart rate reduction of the diving response was 15%, which remained the same throughout the apnea series. While the diving response was completely reversed between the apneas, spleen size was not recovered until 8-9 min after the final apnea corresponding with recovery of Hct and Hb. Thus, although the spleen contraction may be associated with the cardiovascular diving response, it is likely to be triggered by different mechanisms, and it may remain activated between dives spaced by short pauses. The two adjustments may provide a fast, quickly reversed, and a slow, but long-lasting, way of shifting to a diving mode in humans. [ABSTRACT FROM AUTHOR]

Several features of potential adaptation to breath-hold diving in diving populations and extreme divers are reviewed. Thermal adaptation consists of an improvement in cold tolerance, as witnessed by a decrease in critical water temperature, and implies an elevation of the shivering threshold associated with a greater body insulation. This is indicative of either a strong peripheral vasoconstriction or a more effective countercurrent heat exchange. Respiratory adaptation consists of a blunted ventilatory response to carbon dioxide and an enlargement of lung volumes. Finally, the occurrence of a diving response has been demonstrated. An extreme peripheral vasoconstriction is associated with a dramatic increase in arterial blood pressure. The consequent stimulation of arterial baroreceptors causes an extreme drop of heart rate. Bradycardia is not compensated by a higher stroke volume, with consequent decrease in cardiac output. This decrease, however, is not such as to undermine perfusion to vital organs. Redistribution of blood flow occurs, and some organs such as skeletal muscle may become unperfused, as indicated by the high blood lactate concentrations at low metabolic rate. It is not possible to state, however, whether these changes reflect genetic adaptations or an adaptive response to a prolonged environmental stress. [Copyright &y& Elsevier]

In this paper, the respiratory, circulatory and metabolic adjustments to human extreme breath-hold diving are reviewed. A survey of the literature reveals that in extreme divers, adaptive mechanisms take place that allow prolongation of apnoea beyond the limits attained by non-diving subjects, and preservation of oxygen stores during the dives. The occurrence of a diving response, including peripheral vasoconstriction, increased arterial blood pressure, bradycardia and lowered cardiac output, is strongly implicated. Some peripheral regions may be excluded from perfusion, with consequent reliance on anaerobic metabolism. In addition, extreme breath-hold divers show a blunted ventilatory response to carbon dioxide breathing, possibly as a consequence of frequent exposure to high carbon dioxide partial pressures during the dives. These mechanisms allow the attainment of particularly low alveolar oxygen (<30 mmHg) and high alveolar carbon dioxide (>50 mmHg) partial pressures at the end of maximal dry breath-holds, and reduce oxygen consumption during the dive at the expense of increased anaerobic glycolysis (rate of blood lactate accumulation >0.04 mM·s–1). The current absolute world record for depth in breath-hold diving is 150 m. Its further improvement depends upon how far the equilibrium between starting oxygen stores, the overall rate of energy expenditure, the fraction of energy provided by anaerobic metabolism and the diving speed can be pushed, with consciousness upon emersion. The ultimate limit to breath-hold diving records may indeed be imposed by an energetic constraint. [ABSTRACT FROM AUTHOR]

In competitive apnea divers, the nutritional demands may be essentially different from those of, for example, endurance athletes, where energy resources need to be maximised for successful performance. In competitive apnea, the goal is instead to limit metabolism, as the length of the sustainable apneic period will depend to a great extent on minimising oxygen consumption. Many but not all elite divers fast before performing static apnea in competition. This may increase oxygen consumption as mainly lipid stores are metabolised but could also have beneficial effects on apneic duration. Our aim was to determine the effect of over-night fasting on apnea performance. Six female and seven male divers performed a series of three apneas after eating and fasting, respectively. The series consisted of two 2-min apneas spaced by 3 min rest and, after 5 min rest, one maximal effort apnea. Apneas were performed at supine rest and preceded by normal respiration and maximal inspiration. Mean (±SD) time since eating was 13 h (±2 h 43 min) for the fasting and 1 h 34 min (±33 min) for the eating condition (P< 0.001). Mean blood glucose was 5.1 (±0.4) mmol/L after fasting and 5.9 (±0.7) mmol/L after eating (P<0.01). Lung volumes were similar in both conditions (NS). For the 2-min apneas, nadir SaO2during fasting was 95 (±1)% and 92 (±2)% (P< 0.001) on eating and ETCO2was lower in the fasting condition (P< 0.01) while heart rate (HR) during apnea was 74 (±10) bpm for fasting and 80 (±10) bpm for eating conditions (P< 0.01). Maximal apnea durations were 4 min 41 s (±43 s) during fasting and 3 min 51 s (±37 s) after eating (P< 0.001), and time without respiratory contractions was 31 s (25%) longer after fasting (P< 0.01). At maximal apnea termination, SaO2and ETCO2were similar in both conditions (NS) and apneic HR was 63 (±9) bpm for fasting and 70 (±10) bpm for eating (P< 0.01). The 22% longer apnea duration after fasting with analogous end apnea SaO2levels suggests that fasting is beneficial for static apnea performance in elite divers, likely via metabolism-limiting mechanisms. The oxygen-conserving effect of the more pronounced diving response and possibly other metabolism-limiting mechanisms related to fasting apparently outweigh the enhanced oxygen consumption caused by lipid metabolism. [ABSTRACT FROM AUTHOR]