Circadian cerebrovascular reactivity to CO2.

Cerebrovascular reactivity (CVR) assesses the ability of the cerebral vasculature to adjust cerebral blood flow in response to changes in arterial carbon dioxide (CO2), and is used as an indicator of cerebrovascular health. A common method of estimating CVR is to measure the increase in blood flow velocity of the middle cerebral artery (MCAv), using trans-cranial Doppler ultrasound, in response to a CO2 stimulus. We used this method to measure the CVR of 10 subjects in the mornings and evenings of two consecutive days. Mean arterial pressure (MAP) was also measured, and CVR was determined solely from tests where MAP remained unchanged in response to CO2. CVR was measured as the slopes of MCAv responses to a ramp CO2 stimulus fitted with linear regression, and significantly increased from evening to morning each day, with no significant day-to-day differences. We concluded that these measurements of CVR exhibited a circadian rhythm, and were repeatable from one day to the next.