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Effect of generalised sympathetic activation by cold pressor test on cerebral haemodynamics in healthy humans
S Roatta1, G Micieli, D Bosone
1Department of Neuroscience, Centro Interuniversitario per la Neurofisiologia del Dolore, University of Torino Medical School, Italy.
Insights
Sympathetic nervous system activation, triggered by the cold pressor test (CPT), increases blood pressure but minimally affects cerebral blood flow (CBF) in healthy individuals. This suggests the sympathetic system regulates CBF by altering cerebrovascular resistance and compliance.
Area of Science:
- Neuroscience
- Physiology
- Cardiovascular Science
Background:
- The precise role of the sympathetic nervous system in regulating cerebral hemodynamics remains debated, particularly its effectiveness, operational range, and mechanisms of action.
- Understanding these effects is crucial before secondary autoregulatory mechanisms mask the direct sympathetic influence.
Purpose of the Study:
- To investigate the impact of generalized sympathetic activation on cerebral blood flow (CBF) in healthy humans.
- To elucidate the immediate effects on cerebral hemodynamics before metabolic or myogenic compensations occur.
Main Methods:
- A cold pressor test (CPT) was administered to 35 healthy volunteers (aged 20-35) for 5 minutes.
- Cerebral blood flow velocity in middle cerebral arteries was measured using transcranial Doppler sonography.
- Arterial blood pressure was monitored non-invasively; the ratio of arterial blood pressure to mean blood velocity (ABP/Vm) and Pulsatility Index (PI) were calculated.
Main Results:
- CPT significantly increased mean arterial blood pressure (ABP) but only slightly increased middle cerebral artery blood velocity (+2.4% ipsilateral, +4.4% contralateral).
- The ABP/Vm ratio increased, and Pulsatility Index (PI) decreased bilaterally during CPT, coinciding with the rise in ABP.
- These changes indicate increased cerebrovascular resistance and reduced middle cerebral artery compliance.
Conclusions:
- Generalized sympathetic activation, even with moderate increases in ABP, can limit concomitant increases in CBF.
- The sympathetic system appears to influence both small resistance and large compliant cerebral vessels.
- These regulatory effects occur even at blood pressure levels well below the autoregulation threshold.
Abstract:
There is no general agreement regarding several aspects of the role of the sympathetic system on cerebral haemodynamics such as extent of effectiveness, operational range and site of action. This study was planned to identify the effect of a generalised sympathetic activation on the cerebral haemodynamics in healthy humans before it is masked by secondary corrections, metabolic or myogenic in nature. A total of 35 healthy volunteers aged 20-35 underwent a 5 min lasting cold pressor test (CPT) performed on their left hand. The cerebral blood flow (CBF) velocity in the middle cerebral arteries and arterial blood pressure were recorded with transcranial Doppler sonography and with a non-invasive finger-cuff method, respectively. The ratio of arterial blood pressure to mean blood velocity (ABP/Vm) and Pulsatility Index (PI) were calculated throughout each trial. CPT induced an increase in mean ABP (range 2-54 mmHg depending on the subject) and only a slight, though significant, increase in blood velocity in the middle cerebral artery (+2.4 and +4.4% on ipsi- and contralateral side, respectively). During CPT, the ratio ABP/Vm increased and PI decreased in all subjects on both sides. These changes began simultaneously with the increase in blood pressure. The increase in ABP/Vm ratio is attributed to an increase in the cerebrovascular resistance, while the concomitant reduction in PI is interpreted as due to the reduction in the compliance of the middle cerebral artery. The results suggest that generalised increases in the sympathetic discharge, causing increases in ABP, can prevent concomitant increases in CBF by acting on both small resistance and large compliant vessels. This effect is also present when a slight increase in blood pressure occurs, which suggests a moderate increase in the sympathetic discharge, i.e. when ABP remains far below the upper limit of CBF autoregulation.