Hyperthermia modulates regional differences in cerebral blood flow to changes in CO2

Area of Science:

• Physiology
• Cardiovascular Physiology
• Neurophysiology

Background:

• Cerebral blood flow regulation is crucial for brain function.
• Carbon dioxide (CO2) is a potent vasodilator influencing cerebral perfusion.
• The effects of hyperthermia on CO2 reactivity in specific cerebral and extracerebral arteries remain incompletely understood.

Purpose of the Study:

• To investigate blood flow responses to CO2 changes in the internal carotid artery (ICA), external carotid artery (ECA), and vertebral artery (VA).
• To compare these responses under normothermic and hyperthermic conditions.
• To determine if heat stress alters CO2 reactivity in these key arteries and the middle cerebral artery (MCA).

Main Methods:

• Eleven healthy subjects underwent passive whole-body heating to induce hyperthermia.
• Subjects were exposed to hypocapnic and hypercapnic challenges during both normothermic and hyperthermic states.
• Blood flow in the ICA, ECA, and VA, and mean blood velocity (Vmean) in the MCA were measured.

Main Results:

• Whole-body heating significantly increased ECA blood flow and cardiac output but decreased ICA, MCA, and VA blood flow/velocity.
• Hypocapnia reduced ICA, VA blood flows, and MCA Vmean, while hypercapnia increased them, irrespective of thermal conditions.
• Heat stress did not alter CO2 reactivity in the MCA and VA, but decreased it in the ICA and enhanced it in the ECA.

Conclusions:

• Hyperthermia significantly impacts blood flow distribution and regulation in major arteries supplying the brain.
• CO2 reactivity is differentially affected in the ICA and ECA during heat stress, suggesting altered dynamic cerebral blood flow control.
• These findings highlight the potential for impaired cerebral perfusion regulation under conditions of elevated body temperature.