Sympathetic activation and deactivation modulated the cerebrovascular response to CO2, with regression slopes of 1.3 during tilt and 2.3 during blockade vs 1.6 cm/(s·mm Hg) supine (P<0.05).
How does sympathetic nervous system activity interact with carbon dioxide to regulate cerebral perfusion in healthy humans?
The sympathetic nervous system attenuates CO2-induced increases in cerebral blood flow, indicating a moderate direct effect on the cerebral vasculature.
p-value: p=<0.05
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. PaCO(2) was manipulated through hyperventilation and CO(2) breathing (5%). With subjects in the supine position and during HUT, mean arterial pressure was not influenced by PaCO(2). 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 PaCO(2) 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 PaCO(2) 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 PaCO(2) levels. However, the sympathetic nervous system seems to attenuate the CO(2)-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.
Jordan et al. (Fri,) conducted a other in Healthy subjects (n=6). Sympathetic activation (head-up tilt) and deactivation (ganglionic blockade) vs. Supine position was evaluated on Slope of the regression between PaCO2 and mean cerebral blood flow velocity (p=<0.05). Sympathetic activation and deactivation modulated the cerebrovascular response to CO2, with regression slopes of 1.3 during tilt and 2.3 during blockade vs 1.6 cm/(s·mm Hg) supine (P<0.05).