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Cerebral blood flow is regulated by changes in blood pressure and in blood viscosity alike
Insights
Blood viscosity significantly impacts cerebral blood flow (CBF) when autoregulation is impaired. Changes in blood viscosity, particularly in defective autoregulation zones, necessitate compensatory vessel adjustments, suggesting a shared mechanism with pressure autoregulation.
Area of Science:
- Neuroscience
- Physiology
- Cerebrovascular Research
Background:
- Cerebral blood flow (CBF) regulation is complex, with ongoing debate on blood viscosity's role.
- Autoregulation mechanisms normally maintain stable CBF despite pressure fluctuations.
- The caudate nucleus and ectosylvian cortex are key areas for studying cerebrovascular dynamics.
Purpose of the Study:
- To investigate the influence of blood viscosity on CBF in cats.
- To determine if blood viscosity changes affect CBF differently in areas with intact versus impaired autoregulation.
- To explore the potential link between blood viscosity autoregulation and pressure autoregulation.
Main Methods:
- CBF was measured using microspheres in 23 cats.
- Autoregulation was intentionally disrupted in the left caudate nucleus via microsurgical occlusion.
- Intravenous mannitol was administered to induce changes in blood viscosity, alongside induced hypertension or hypotension.
Main Results:
- Mannitol administration caused significant, albeit temporary, decreases in blood viscosity.
- CBF in the left caudate nucleus (impaired autoregulation) showed greater sensitivity to viscosity and pressure changes compared to the right caudate nucleus (intact autoregulation).
- Specific findings include: CBF decreased 18% with higher viscosity and 21% with hypotension in the left caudate; CBF increased 47% with lower viscosity and 56% with hypertension in the left caudate.
Conclusions:
- Changes in blood viscosity necessitate compensatory adjustments in cerebral vessel diameter.
- These viscosity-induced adjustments are impaired in brain regions with defective pressure autoregulation.
- The study proposes 'blood viscosity autoregulation' and hypothesizes a shared regulatory mechanism with pressure autoregulation.
Abstract:
There is still considerable controversy regarding the influence of blood viscosity upon CBF. We have measured CBF with microspheres in 23 cats. Autoregulation was disturbed in the left caudate nucleus by microsurgical occlusion of the left middle cerebral artery. Induced hypertension or hypotension was used and i.v. mannitol (1 g/kg) administered. In all cats blood viscosity decreased an average of 16% at 15 minutes and, in 16 cats, increased 10% at 75 minutes post-mannitol. CBF in the right caudate was 79 +/- 6 ml/100g/min, in the left 38 +/- 6 (p less than 0.001). Only minor changes of CBF occurred in areas with presumed normal autoregulation, including the right caudate, in conjunction with pressure or viscosity changes. In the left caudate CBF decreased 21% with hypotension and 18% with higher viscosity, more than on the right (p less than 0.01 and p less than 0.2, respectively). CBF increased in the left caudate 56% with hypertension and 47% with lower viscosity, again much more than on the right (p less than 0.001 and p less than 0.01, respectively). In the other area which is (nearly) exclusively supplied by the middle cerebral artery of the cat, i.e., the ectosylvian cortex, results were similar to those in the caudate nucleus. These results show that viscosity changes must result in compensatory readjustments of vessel diameter, but that these adjustments do not occur where autoregulation to pressure changes is known to be defective. The adjustments to viscosity changes might be called blood viscosity autoregulation of CBF. We hypothesize that pressure autoregulation and blood viscosity autoregulation share the same mechanism.