Middle cerebral artery blood velocity during exercise with β‐1 adrenergic and unilateral stellate ganglion blockade in humans

Abstract

A reduced ability to increase cardiac output (CO) during exercise limits blood flow by vasoconstriction even in active skeletal muscle. Such a flow limitation may also take place in the brain as an increase in the transcranial Doppler determined middle cerebral artery blood velocity (MCA V(mean)) is attenuated during cycling with beta-1 adrenergic blockade and in patients with heart insufficiency. We studied whether sympathetic blockade at the level of the neck (0.1% lidocaine; 8 mL; n=8) affects the attenuated exercise - MCA V(mean following cardio-selective beta-1 adrenergic blockade (0.15 mg kg(-1) metoprolol i.v.) during cycling. Cardiac output determined by indocyanine green dye dilution, heart rate (HR), mean arterial pressure (MAP) and MCA V(mean) were obtained during moderate intensity cycling before and after pharmacological intervention. During control cycling the right and left MCA V(mean) increased to the same extent (11.4 +/- 1.9 vs. 11.1 +/- 1.9 cm s(-1)). With the pharmacological intervention the exercise CO (10 +/- 1 vs. 12 +/- 1 L min(-1); n=5), HR (115 +/- 4 vs. 134 +/- 4 beats min(-1)) and delta MCA V(mean) (8.7 +/- 2.2 vs. 11.4 +/- 1.9 cm s(-1) were reduced, and MAP was increased (100 +/- 5 vs. 86 +/- 2 mmHg; P < 0.05). However, sympathetic blockade at the level of the neck eliminated the beta-1 blockade induced attenuation in delta MCA V(mean) (10.2 +/- 2.5 cm s(-1)). These results indicate that a reduced ability to increase CO during exercise limits blood flow to a vital organ like the brain and that this flow limitation is likely to be by way of the sympathetic nervous system.

Cerebrovascular responses to incremental exercise during hypobaric hypoxia: effect of oxygenation on maximal performance

“…Similar findings in cerebrovascular reactivity have been reported (1,27) and may be explained by the direct influence of increased cardiac output on cerebral blood flow (25,26,38). At moderate and high work rates, the PET CO 2 and MCA V mean relationship was restored to a linear relationship as PET CO 2 began to fall.…”

“…1), associated with a switch from rising to falling PET CO 2 and MCA V mean , has not been described during continuous incremental exercise. Support for a relative independence between Pa CO 2 and cerebral blood flow, which may explain the weaker PET CO 2 -to-MCA V mean correlations at low work rates observed in the present study, has been provided by studies demonstrating a strong association between cerebral blood flow and cardiac output during submaximal exercise (25,26,39). Specifically, Ogoh et al (38) showed that increases in cardiac output were largely responsible for augmented cerebral blood flow during low-intensity exercise (50% V O 2max ).…”

Blood Flow Distribution during Heat Stress: Cerebral and Systemic Blood Flow

“…8,9,[24][25][26] However, heat stress rather reduced than augmented MCAV mean despite an increase in Q. 10 Recently, Schlader et al 27 demonstrated that reduced MCAV mean during heat stress was unaffected by acute volume expansion (i.e., additional increase of Q) and suggested that the further decreased systemic vascular resistance resulted in no change of cerebrovascular resistance.…”

Maintained consciousness during witnessed asystole after spinal anesthesia for Cesarean section

“…During central blood volume depletion, the increase in sympathetic nerve activity shifts the cerebral autoregulation curve to the right 17 and the vasoconstrictor sympathetic nerve activity overrides vasodilatation 18 . Ide et al also showed that sympathetic blockade at the level of the neck eliminated the limitation to the increase in cerebral blood flow following cardio-selective β-1 adrenergic blockade 16 . Although a theoretical hypothesis, a T4 level blockade in our patient may have blunted the effect of reduced cardiac output on cerebral vasoconstriction, and may have helped maintain cerebral perfusion prior to the asystolic events.…”

“…Whether aortic compression by a gravid uterus enhances cerebral blood flow at the expense of lower extremity blood flow remains unknown. Through their experiments on healthy controls Ide et al showed that an impaired ability to increase cardiac output during exercise with a large muscle mass appears to limit blood flow distribution not only to active muscle 15 but also to such a vital organ as the brain, and that this flow restriction was by way of the sympathetic nervous system 16 . During central blood volume depletion, the increase in sympathetic nerve activity shifts the cerebral autoregulation curve to the right 17 and the vasoconstrictor sympathetic nerve activity overrides vasodilatation 18 .…”