Cerebral blood flow velocity in humans exposed to 24 h of head-down tilt

Kawai, Yasuaki; Murthy, G. G. Krishna; Watenpaugh, Donald E.; Breit, G.; Deroshia, C. W.; Hargens, Alan R.

doi:10.1152/jappl.1993.74.6.3046

Cited by 76 publications

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“…In the present study, controlled breathing maintained end-tidal CO 2 concentrations, and so we conclude that acute Ϫ10°head-down tilting does not affect cerebral blood flow velocity. This conclusion is in agreement with Satake et al (17), who found no change in middle cerebral blood flow velocity with Ϫ6°head-down tilt by using single-photon-emission computed tomography, and at odds with Kawai et al (11), who documented increases of blood flow velocities with a similar Ϫ6°head-down tilt using transcra- Although loading of various baroreceptor populations should conceivably increase parasympathetic activity, we were unable to confirm such an increase. R-R intervals, R-R interval spectral power at the high frequency, and cardiovagal baroreflex sensitivity [assessed by the transfer function magnitude between systolic pressure and R-R interval (Table 2)] were unaffected by head-down tilt.…”

Section: Discussionsupporting

confidence: 86%

Dynamic cerebral autoregulation is preserved during acute head-down tilt

Cooke

Pellegrini

Kovalenko

2003

Journal of Applied Physiology

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Complete ganglion blockade alters dynamic cerebral autoregulation, suggesting links between systemic autonomic traffic and regulation of cerebral blood flow velocity. We tested the hypothesis that acute head-down tilt, a physiological maneuver that decreases systemic sympathetic activity, would similarly disrupt normal dynamic cerebral autoregulation. We studied 10 healthy young subjects (5 men and 5 women; age 21 Ϯ 0.88 yr, height 169 Ϯ 3.1 cm, and weight 76 Ϯ 6.1 kg). ECG, beat-by-beat arterial pressure, respiratory rate, endtidal CO2 concentration, and middle cerebral blood flow velocity were recorded continuously while subjects breathed to a metronome. We recorded data during 5-min periods and averaged responses from three Valsalva maneuvers with subjects in both the supine and Ϫ10°head-down tilt positions (randomized). Controlled-breathing data were analyzed in the frequency domain with power spectral analysis. The magnitude of input-output relations were determined with cross-spectral techniques. Head-down tilt significantly reduced Valsalva phase IV systolic pressure overshoot from 36 Ϯ 4.0 (supine position) to 25 Ϯ 4.0 mmHg (head down) (P ϭ 0.03). Systolic arterial pressure spectral power at the low frequency decreased from 5.7 Ϯ 1.6 (supine) to 4.4 Ϯ 1.6 mmHg 2 (head down) (P ϭ 0.02), and mean arterial pressure spectral power at the low frequency decreased from 3.3 Ϯ 0.79 (supine) to 2.0 Ϯ 0.38 mmHg 2 (head down) (P ϭ 0.05). Head-down tilt did not affect cerebral blood flow velocity or the transfer function magnitude and phase angle between arterial pressure and cerebral blood flow velocity. Our results show that in healthy humans, mild physiological manipulation of autonomic activity with acute head-down tilt has no effect on the ability of the cerebral vasculature to regulate flow velocity.transcranial Doppler; spectral analysis; transfer function THE CEREBRAL CIRCULATION MAINTAINS relatively constant blood flow in the face of changes in pressure and other imposed demands through normal autoregulation. Determination of factors that may modulate or disrupt normal cerebral autoregulation is important for the diagnosis and treatment of patients suffering from autonomic dysfunction. Zhang and coworkers (24) recently demonstrated that the transfer function magnitude between oscillations of mean arterial pressure and mean cerebral blood flow velocity is increased by ganglion blockade, suggesting a role for autonomic neural activity in the modulation of cerebral autoregulation. Nonpharmacological, physiological modulation of autonomic neural activity may similarly provide insight into mechanisms underlying normal cerebral autoregulation.Acute head-down tilting decreases leg volume (9) and increases central venous pressure (13). Consequent loading of baroreceptors decreases muscle sympathetic nerve activity (5, 19) and could affect regulation of cerebral blood flow velocity, although such associations have not been evaluated.The purpose of the present study was to determine the effects of acute head-down tilt...

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Section: Discussionsupporting

confidence: 86%

Dynamic cerebral autoregulation is preserved during acute head-down tilt

Cooke

Pellegrini

Kovalenko

2003

Journal of Applied Physiology

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“…Second, the alternative mechanism could be autoregulation of cerebral circulation. Exposure to actual or simulated microgravity elevates blood pressures in cerebral arteries [6] and veins [24,25], and increases the blood flow velocity in the middle cerebral artery [6,26,27]. These changes in hemodynamics may cause a constriction of cerebral vessels through myogenic responses [28] or changes in endothelial functions [28,29].…”

Section: Discussionmentioning

confidence: 99%

Does Edema Formation Occur in the Rabbit Brain Exposed to Head-Down Tilt?

Shimoyama

Miyata²,

Ohama³

et al. 2000

JJP

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“…The factor ␣ was used to correct for the height reduction caused by the semisupine position of our bicycle ergometer with a backrest inclination of 55°(␣ ϭ sin 55°) (26).…”

Section: Subjectsmentioning

confidence: 99%

Dynamic cerebral autoregulation remains stable during physical challenge in healthy persons

Bryś

Brown²,

Marthol³

et al. 2003

American Journal of Physiology-Heart and Circulatory Physiology

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. Dynamic cerebral autoregulation remains stable during physical challenge in healthy persons. Am J Physiol Heart Circ Physiol 285: H1048-H1054, 2003; 10.1152/ajpheart.00062.2003.-The effects of physical activity on cerebral blood flow (CBF) and cerebral autoregulation (CA) have not yet been fully evaluated. There is controversy as to whether increasing heart rate (HR), blood pressure (BP), and sympathetic and metabolic activity with altered levels of CO2 might compromise CBF and CA. To evaluate these effects, we studied middle cerebral artery blood flow velocity (CBFV) and CA in 40 healthy young adults at rest and during increasing levels of physical exercise. We continuously monitored HR, BP, endexpiratory CO 2, and CBFV with transcranial Doppler sonography at rest and during stepwise ergometric challenge at 50, 100, and 150 W. The modulation of BP and CBFV in the low-frequency (LF) range (0.04-0.14 Hz) was calculated with an autoregression algorithm. CA was evaluated by calculating the phase shift angle and gain between BP and CBFV oscillations in the LF range. The LF BP-CBFV gain was then normalized by conductance. Cerebrovascular resistance (CVR) was calculated as mean BP adjusted to brain level divided by mean CBFV. HR, BP, CO 2, and CBFV increased significantly with exercise. Phase shift angle, absolute and normalized LF BP-CBFV gain, and CVR, however, remained stable. Stable phase shift, LF BP-CBFV gain, and CVR demonstrate that progressive physical exercise does not alter CA despite increasing HR, BP, and CO 2. CA seems to compensate for the hemodynamic effects and increasing CO2 levels during exercise.cross-spectral analysis; low-frequency blood pressure-cerebral blood flow velocity gain; phase shift angle; cerebrovascular resistance CEREBRAL AUTOREGULATION normally ensures that cerebral blood flow (CBF) remains relatively constant despite fluctuations in blood pressure (BP), provided that mean BP remains within the range of autoregulation, usually from 50 to 170 mmHg (6,21,37). If BP exceeds these limits, CBF increases, initially in a curvilinear relation, and then follows BP passively in a linear relation (21, 31). During increasing levels of physical effort, cerebral perfusion is not only driven by increasing BP and heart rate (HR) but also has to adjust to high levels of sympathetic activity and to altered metabolism with changes in PO 2 and PCO 2 . With increasing muscle metabolism, PO 2 in the blood decreases, whereas PCO 2 increases, unless augmented respiration compensates for the higher O 2 demand and production of CO 2 (17,35,54). Cerebral autoregulation has to counterbalance all the cardiovascular and metabolic responses to physical effort. Although some studies suggest that CBF velocity (CBFV) and CBF remain stable despite physical effort (15), several others found an increase in CBFV (14, 17, 30) as well as regional CBF (10, 55) and global CBF during exercise (24, 51).During cardiovascular changes as they occur with increasing physical challenge, cerebral autoregulation can be evaluated b...

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Cerebral blood flow velocity in humans exposed to 24 h of head-down tilt

Cited by 76 publications

References 0 publications

Dynamic cerebral autoregulation is preserved during acute head-down tilt

Dynamic cerebral autoregulation is preserved during acute head-down tilt

Does Edema Formation Occur in the Rabbit Brain Exposed to Head-Down Tilt?

Dynamic cerebral autoregulation remains stable during physical challenge in healthy persons

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