(Hypertension. 2000;36:383.)
© 2000 American Heart Association, Inc.
Scientific Contributions |
Interaction of Carbon Dioxide and Sympathetic Nervous System Activity in the Regulation of Cerebral Perfusion in Humans
From the Clinical Research Center, Franz-Volhard Clinic, Humboldt University, Berlin, Germany (J.J.), and the Autonomic Dysfunction Center, Vanderbilt University, Nashville, Tenn.
Correspondence to Jens Jordan, MD, Clinical Research Center, Franz-Volhard Clinic, Humboldt University, Wiltbergstr 50, 13125 Berlin, Germany. E-mail jordan{at}fvk-berlin.de
Abstract—Recent studies suggest that activation of the sympathetic nervous system either directly or indirectly influences cerebrovascular tone in humans even within the autoregulatory range. In 6 healthy subjects (aged 29±4 years), we used transcranial Doppler sonography to determine cerebral blood flow velocity during sympathetic activation elicited through head-up tilt (HUT) and sympathetic deactivation through ganglionic blockade. PaCO2 was manipulated through hyperventilation and CO2 breathing (5%). With subjects in the supine position and during HUT, mean arterial pressure was not influenced by PaCO2. During ganglionic blockade, mean arterial pressure decreased markedly with hyperventilation (-13±1.9 mm Hg). Manipulation of sympathetic tone elicited only mild changes in cerebral blood flow (64±5.8 cm/s supine, 58±4.9 cm/s upright, and 66±6.2 cm/s during ganglionic blockade; P=0.07 by ANOVA). The slope of the regression between PaCO2 and mean velocity was 1.6±0.18 cm/(s · mm Hg) supine, 1.3±0.14 cm/(s · mm Hg) during HUT, and 2.3±0.36 cm/(s · mm Hg) during ganglionic blockade (P<0.05). Spontaneous PaCO2 and ventilatory response to hypercapnia were also modulated by the level of sympathetic activity. Changes in sympathetic tone have a limited effect on cerebral blood flow at normal PaCO2 levels. However, the sympathetic nervous system seems to attenuate the CO2-induced increase in cerebral blood flow. This phenomenon may indicate a moderate direct effect of the sympathetic nervous system on the cerebral vasculature. Furthermore, sympathetic activation tends to increase ventilation and thus can indirectly increase cerebrovascular tone.
Key Words: carbon dioxide • baroreflex • receptors, adrenergic • phenylephrine • sympathetic nervous system
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