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We used transcranial Doppler ultrasonography to examine the cerebral blood flow response to orthostatic stress in the middle and posterior cerebral circulations and to determine the effects of healthy aging on regional cerebral blood flow regulation.Continuous simultaneous middle (MCA) and posterior (PCA) cerebral artery blood flow velocities (BFV) and mean arterial pressure (MAP) were measured in response to standing from a sitting position in 13 younger (30 +/- 7 years) and 13 older (73 +/- 4 years) healthy participants.The older participants had a significantly larger decline in MAP (-31% +/- 3 in the older and -21% +/- 2 in the younger) and a smaller increase in heart rate (HR) (15 bpm +/- 1 in the older, 24 bpm +/- 2 in the younger) during the posture change. Despite a larger decline in MAP, the older participants had a decline very similar to that of the younger participants in BFVs in both vascular territories. This was associated with a significantly larger vasodilatory response in the MCA and PCA vascular territories of the older participants. There were no regional differences of the cerebrovascular resistance and BFV responses to orthostasis in the younger participants. However, in the older participants, there was a significantly larger BFV decline and a smaller vasodilatory response in the PCA as compared to the MCA territory.Healthy aging is associated with preserved cerebrovascular adaptation to orthostatic hypotension. However, in older persons, the PCA territory blood flow may be more vulnerable to reduced perfusion during orthostatic stress.

Reductions in end-tidal Pco2 (PetCO2) during upright posture have been suggested to be the result of hyperventilation and the cause of decreases in cerebral blood flow (CBF). The goal of this study was to determine whether decreases in PetCO2 reflected decreases in arterial Pco2 (PaCO2) and their relation to increases in alveolar ventilation (V̇a) and decreases in CBF. Fifteen healthy subjects (10 women and 5 men) were subjected to a 10-min head-up tilt (HUT) protocol. PaCO2, V̇a, and cerebral flow velocity (CFV) in the middle and anterior cerebral arteries were examined. In 12 subjects who completed the protocol, reductions in PetCO2 and PaCO2 (−1.7 ± 0.5 and −1.1 ± 0.4 mmHg, P < 0.05) during minute 1 of HUT were associated with a significant increase in V̇a (+0.7 ± 0.3 l/min, P < 0.05). However, further decreases in PaCO2 (−0.5 ± 0.5 mmHg, P < 0.05), from minute 1 to the last minute of HUT, occurred even though V̇a did not change significantly (−0.2 ± 0.3 l/min, P = not significant). Similarly, CFV in the middle and anterior cerebral arteries decreased (−7 ± 2 and −8 ± 2%, P < 0.05) from minute 1 to the last minute of HUT, despite minimal changes in PaCO2. These data suggest that decreases in PetCO2 and PaCO2 during upright posture are not solely due to increased V̇a but could be due to ventilation-perfusion mismatch or a redistribution of CO2 stores. Furthermore, the reduction in PaCO2 did not fully explain the decrease in CFV throughout HUT. These data suggest that factors in addition to a reduction in PaCO2 play a role in the CBF response to orthostatic stress.

Aging is associated with frontal subcortical microangiopathy and executive cognitive dysfunction, suggesting that elderly individuals may have impaired metabolic activation of cerebral blood flow to the frontal lobes. We used transcranial Doppler (TCD) ultrasound to examine the cerebral blood flow response to executive control and visual tasks in the anterior and posterior cerebral circulations and to determine the effects of healthy aging on cerebral blood flow regulation during cognitive tasks. Continuous simultaneous anterior cerebral artery (ACA) and posterior cerebral artery (PCA) blood flow velocities (BFVs) and mean arterial pressure (MAP) were measured in response to word stem completion (WSC) and a visual search (VS) task in 29 healthy subjects (14 young, 30 ± 1.5 years; 15 old, 74 ± 1.4 years). We found that: (1) ACA and PCA blood flow velocities are both significantly increased during WSC and VS cognitive tasks, (2) ACA and PCA activations were task specific in our young volunteers, with ACA > PCA BFV during the WSC task and PCA > ACA BFV during the VS task, (3) while healthy elderly subjects also had PCA > ACA BFV during the VS task, they did not have ACA > PCA activation during the WSC task, and (4) healthy elderly subjects tend to have overall greater increases in BFV during both cognitive tasks. We conclude that TCD can be used to monitor cerebrovascular hemodynamics during the performance of cognitive tasks. Our data suggest that there is differential blood flow increase in the ACA and PCA in young versus elderly subjects during cognitive tasks.

Many physicians are reluctant to lower blood pressure to recommended levels in elderly hypertensive patients because of concern about producing cerebral hypoperfusion. Because hypertension is associated with potentially reversible structural and functional alterations in the cerebral circulation that may improve with treatment, we investigated whether long-term pharmacological reduction of systolic blood pressure will improve, rather than worsen, cerebral blood flow and its regulation. Three groups of elderly subjects 65 years of age or older were studied prospectively: normotensive subjects (N=19), treated hypertensive subjects with systolic pressure 160 mm Hg at entry into the study (N=14). We measured beat-to-beat blood flow velocity in the middle cerebral artery (transcranial Doppler ultrasonography), finger arterial pressure (photoplethysmography), and pulsatile distensibility of the carotid artery (duplex Doppler ultrasonography) at baseline and after 6 months of observation or antihypertensive therapy. After baseline hemodynamic measurements, uncontrolled hypertensive subjects underwent aggressive treatment with lisinopril with or without hydroclorothiazide or, if not tolerated, nifedipine or an angiotensin receptor blocker to bring their systolic pressure

Previous experiments have shown that the vestibular system participates in cardiovascular control. However, the effects of vestibular activation on cerebrovascular regulation are not known. Therefore, the present experiment tested the hypothesis that specific vestibular activations may be beneficial to cerebral circulation during simulated orthostatic stress. Middle cerebral artery blood flow velocity (CBV; Doppler ultrasound) was measured to examine the effects of head-down neck flexion (HDNF) compared to head-down neck extension (HDNE) with and without lower body negative pressure (LBNP; -40 mmHg) (n=9). The change in CBV (DeltaCBV) during HDNF and HDNE were not different during baseline conditions, however, during LBNP, DeltaCBV was greater in HDNE compared to HDNF (-5.5+/-3.2 cm/s, -11+/-4.6%) vs. (-0.7+/-1.0 cm/s, -1.9+/-1.9%), respectively (P

Item does not contain fulltext Spinal cord-injured (SCI) individuals, having a sympathetic nervous system lesion, experience hypotension during sitting and standing. Surprisingly, they experience few syncopal events. This suggests adaptations in cerebrovascular regulation. Therefore, changes in systemic circulation, cerebral blood flow, and oxygenation in eight SCI individuals were compared with eight able-bodied (AB) individuals. Systemic circulation was manipulated by lower body negative pressure at several levels down to -60 mmHg. At each level, we measured steady-state blood pressure, changes in cerebral blood velocity with transcranial Doppler, and cerebral oxygenation using near-infrared spectroscopy. We found that mean arterial pressure decreased significantly in SCI but not in AB individuals, in accordance with the sympathetic impairment in the SCI group. Cerebral blood flow velocity decreased during orthostatic stress in both groups, but this decrease was significantly greater in SCI individuals. Cerebral oxygenation decreased in both groups, with a tendency to a greater decrease in SCI individuals. Thus present data do not support an advantageous mechanism during orthostatic stress in the cerebrovascular regulation of SCI individuals.

Humans have several adaptive mechanisms to deal with the effect of gravity during upright postures. The effect of passive upright tilt (HUT) on respiration has been shown to reduce the end tidal partial pressure of carbon dioxide (PETCO2) and increase the end tidal partial pressure of oxygen (PETO2) (1,4,5,9,15–19). The typical 4 mmHg drop in PETCO2 has been shown to correspond with a decrease in arterial CO2 (PaCO2) of approximately 2 mmHg (1,5). McHenry et al. (17) found that PaCO2 decreased by ~1 mmHg during 30° HUT. Boutellier and coworkers (6) demonstrated that as subjects went from +1 Gz force to +2 and +3, PaCO2 continued to decrease.

Background Dynamic cerebral autoregulation after intracerebral hemorrhage (ICH) remains poorly understood. We performed a case-control study to compare dynamic autoregulation between ICH patients and healthy controls. Methods Twenty-one patients (66 ± 15 years) with early (< 72 hours) lobar or basal ganglia ICH were prospectively studied and compared to twenty-three age-matched controls (65 ± 9 years). Continuous measures of mean flow velocity (MFV) in the middle cerebral artery and mean arterial blood pressure (MAP) were obtained over 5 min. Cerebrovascular resistance index (CVRi) was calculated as the ratio of MAP to MFV. Dynamic cerebral autoregulation was assessed using transfer function analysis of spontaneous MAP and MFV oscillations in the low (0.03-0.15 Hz) and high (0.15-0.5 Hz) frequency ranges. Results The ICH group demonstrated higher CVRi compared to controls (ipsilateral: 1.91 ± 1.01 mmHg·s·cm-1, p = 0.04; contralateral: 2.01 ± 1.24 mmHg·s·cm-1, p = 0.04; vs. control: 1.42 ± 0.45 mmHg·s·cm-1). The ICH group had higher gains than controls in the low (ipsilateral: 1.33 ± 0.58%/mmHg, p = 0.0005; contralateral: 1.47 ± 0.98%/mmHg, p = 0.004; vs. control: 0.82 ± 0.30%/mmHg) and high (ipsilateral: 2.11 ± 1.31%/mmHg, p < 0.0001; contralateral: 2.14 ± 1.49%/mmHg, p < 0.0001; vs. control: 0.66 ± 0.26%/mmHg) frequency ranges. The ICH group also had higher coherence in the contralateral hemisphere than the control (ICH contralateral: 0.53 ± 0.38, p = 0.02; vs. control: 0.38 ± 0.15) in the high frequency range. Conclusions Patients with ICH had higher gains in a wide range of frequency ranges compared to controls. These findings suggest that dynamic cerebral autoregulation may be less effective in the early days after ICH. Further study is needed to determine the relationship between hematoma size and severity of autoregulation impairment.

Cerebral blood flow (CBF) is critical for proper neuronal function. Since flow is dependent on perfusion pressure, the cerebrovasculature dilates/constricts in response to driving pressure to maintain flow relatively constant. This process is termed autoregulation and essential to maintaining flow in the upright posture when the effect of gravity results in an 20 mmHg reduction in perfusion pressure to the brain due to the hydrostatic gradient. Up regulation of CBF has been used extensively to localize cognitive activation, i.e. functional imaging. However, due to technical constraints the majority of this work has been done with subjects in the supine position. The recent development of transcranial Doppler (TCD) allows the continuous measurement of cerebral flow velocity (CFV) in the main cerebral arteries. By measuring flow velocity during cognitive tasks, we are able to perform functional TCD. To assess whether gravity had an effect on the functional response to a cognitive task (Trails A & B), we performed functional TCD on subjects in both the upright and supine positions. In addition to cerebral flow velocity in the anterior (ACA) and middle cerebral arteries (MCA) (TCD), we measured beat-by-beat blood pressure (Finapres) and breath-by-breath end tidal CO2 (PETCO2) via nasal cannula (Datex-Ohmeda). Subjects were randomly assigned to either the supine or upright position, performed the task, and then moved to the other position, and performed the same task with different number and letter patterns. Upon going from the supine to upright position, subjects demonstrated a non-significant reduction in both ACA (-44%) and MCA (-32%) flow. In contrast mean arterial pressure increased (817 to 914 mmHg) although non-significant, while PETCO2 did not change (40.81.3 vs 40.01.7 mmHg). During cognitive activation with either the Trails A or B there was a significant increase in flow associated with reductions in cerebrovascular resistance (CVR). Interestingly, this functional activation was attenuated in the upright posture (only significant in the MCA). Surprisingly, the functional activation was also reduced in Trails B compared to Trails A. This was likely the consequence of the hypocapnia that was only present during the Trails B assessment. These data highlight the possibility that functional response to cognitive activation may be affected by body position. Since our healthy control subjects demonstrate an attenuated response, one might postulate that patients with an impaired cerebrovasculature and already attenuated response in the supine position may be at even more risk when upright. Further work is necessary to determine what role perfusion pressure may play in functional cerebral blood flow response and how this attenuated response may impact on functional cognitive performance (See Figure 1).

OBJECTIVE—To investigate the link between changes in level of physical activity and the pattern of heart rate variability during long term ambulatory monitoring. DESIGN—Heart rate variability was measured simultaneously with a quantitative indicator of muscle activity by electromyography (EMG) in five men and five women while they did activities typical of daily life or while they rested for 2-3 hours. Spectral and cross spectral analyses were performed on both variables with standard fast Fourier transform. RESULTS—There was a marked reduction in spectral power in the ultra low frequency band (