Correlation between ventilation and brain blood flow during sleep

The relationships between brain blood flow (BBF) and ventilation (VI) were studied during sleep in 13 goats. Unilateral BBF was continuously measured with an electromagnetic flow probe; total and regional BBF were assessed by the radioactive microsphere technique in four animals. Interacting changes in VI and BBF occurred during both slow wave (SWS) and rapid eye movement (REM) sleep. During SWS, significant decreases in VI and increases in arterial P(CO₂) occurred compared to wakefulness. BBF during SWS correlated linearly with arterial CO₂ tension (PaCO₂); and the relationship was similar to that for awake goats breathing CO₂. During REM sleep, VI was significantly less than both the awake (W) and SWS states due principally to a decrease in tidal volume. BBF during REM sleep was significantly and substantially increased compared with both the W and SWS states; this increase was shared by all brain areas. The increase in BBF during REM sleep was greater than that predicted from changes in PaCO₂. In five goats provided with chronic sagittal sinus fistulae, arteriovenous oxygen difference was measured in separate studies and found to be significantly lower during REM sleep compared with W; brain O₂ consumption was similar in magnitude in the REM and W states. Thus, the high BBF of REM sleep was also unexplained by an increase of brain metabolic activity. We conclude that, during SWS, increases in BBF are explained by hypoventilation and hypercapnia. In contrast, during REM sleep, BBF is substantially in excess of that expected from PaCO₂ or brain metabolism. It is postulated that this excess of BBF during REM sleep could reduce the central chemoreceptor pH relative to that present in SWS. The combination of reduction of sensitivity to CO₂ and lower tissue P(CO₂) during REM sleep makes it likely that the output of the central chemoreceptors during this state is less than that during SWS and wakefulness. This may contribute to the low tidal volume and respiratory irregularities of this sleep period.