Published Online
on March 1, 2007

A more recent version of this article appeared on April 1, 2007

This Article Full Text (PDF) All Versions of this Article: 38/4/1238    most recent 01.STR.0000260095.94175.d0v1 Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Zhang, R. Articles by Levine, B. D. Search for Related Content PubMed PubMed Citation Articles by Zhang, R. Articles by Levine, B. D. Pubmed/NCBI databases Compound via MeSH Substance via MeSH Hazardous Substances DB PHENYLEPHRINE Related Collections Brain Circulation and Metabolism Doppler ultrasound, Transcranial Doppler etc. Autonomic, reflex, and neurohumoral control of circulation

Submitted on October 3, 2006
Accepted on November 24, 2006

Autonomic Ganglionic Blockade Does Not Prevent Reduction in Cerebral Blood Flow Velocity During Orthostasis in Humans

Rong Zhang PhD^* and Benjamin D. Levine MD

From Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas, Tex; and the University of Texas Southwestern Medical Center at Dallas, Tex.

^* To whom correspondence should be addressed. E-mail: rongzhang{at}TexasHealth.org.

Background and Purpose--The underlying mechanisms for reductions in cerebral blood flow (CBF) during orthostasis are not completely understood. This study tested the hypothesis that sympathetic activation causes cerebral vasoconstriction leading to reductions in CBF during lower body negative pressure (LBNP).

Methods--CBF velocity, arterial pressure, and end-tidal CO₂ were measured during LBNP (-30 to -50 mm Hg) in 11 healthy subjects before and after autonomic ganglionic blockade with trimethaphan. Arterial partial pressure of CO₂ also was measured in a subgroup of 5 subjects. Mean arterial pressure during LBNP after blockade was maintained by infusion of phenylephrine.

Results--Before blockade, mean arterial pressure did not change during LBNP. However, CBF velocity was reduced in all subjects by 14% (P<0.05). Systolic and pulsatile (systolic-diastolic) CBF velocity were reduced by 18% and 28%, respectively, associated with significant reductions in pulse arterial pressure and end-tidal CO₂ (all P<0.05). After blockade, mean arterial pressure during LBNP was well-maintained and even increased slightly with infusion of phenylephrine. However, reductions in mean, systolic, and pulsatile CBF velocity, pulse arterial pressure, and ETCO₂ were similar to those before blockade. In contrast to reductions in end-tidal CO₂, arterial partial pressure of CO₂ did not change during LBNP.

Conclusions--These data, contrary to our hypothesis, demonstrate that sympathetic vasoconstriction is not the primary mechanism underlying reductions in CBF during moderate LBNP. We speculate that diminished pulse arterial pressure or pulsatile blood flow may reduce cerebral vessel wall shear stress and contribute to reductions in CBF during orthostasis through flow mediated regulatory mechanisms.

Key words: cerebral hemodynamics • orthostasis • sympathetic nervous system • transcranial Doppler

This article has been cited by other articles:

				S. Strandgaard and S. T. Sigurdsson Counterpoint: Sympathetic nerve activity does not influence cerebral blood flow J Appl Physiol, October 1, 2008; 105(4): 1366 - 1367. [Full Text] [PDF]

				B. D. Levine, R. Zhang, M. Visocchi, P. N. Ainslie, S. Ogoh, L. Edvinsson, M. Yildiz, O. B. Paulson, G. M. Knudsen, P. A. Cassaglia, et al. Comments on Point:Counterpoint: Sympathetic activity does/does not influence cerebral blood flow J Appl Physiol, October 1, 2008; 105(4): 1369 - 1373. [Full Text] [PDF]

				S. Strandgaard and S. T. Sigurdsson Last Word on Point:Counterpoint: Sympathetic nervous activity does/does not influence cerebral blood flow J Appl Physiol, October 1, 2008; 105(4): 1375 - 1375. [Full Text] [PDF]

				Rebuttal from Strandgaard and Sigurdsson J Appl Physiol, October 1, 2008; 105(4): 1367 - 1368. [Full Text] [PDF]

				J. J. van Lieshout and N. H. Secher Point:Counterpoint: Sympathetic activity does/does not influence cerebral blood flow J Appl Physiol, October 1, 2008; 105(4): 1364 - 1366. [Full Text] [PDF]