Cerebral blood flow regulation in intermittent hypoxia

Obstructive sleep apnea (OSA) patients are at increased risk of stroke independent of other risk factors; however, this mechanism remains unclear. This increased risk may be related to the OSA patient's impaired ability to regulate brain blood flow. The purpose of this study was to examine the effects of acute and chronic exposure to intermittent hypoxic apneas (IHA) on dynamic cerebral blood flow regulation in healthy subjects. Each subject performed one 30-s hypoxic apnea every one minute for 20 min on one day (acute protocol) or for 10 days (chronic protocol). During each hypoxic apnea event arterial oxygen saturation reached 80-85%. The rate of regulation (RoR) was calculated as an index of dynamic cerebral autoregulation (CA) from the response of arterial blood pressure and cerebral blood flow velocity to acute hypotension produced by the cuff release technique. The RoR during hypotension was significantly attenuated following the IHA protocol (0.78 +/- 0.09 to 0.47 +/- 0.07 /sec; P = 0.003), indicating a loss of dynamic CA as a result of IHA. In addition, RoR was significantly attenuated on days 1 and 10 of the chronic protocol from pre- to post- IHA values (0.78+/- 0.11 to 0.64 +/- 0.10 sec-1; P = 0.016 and 0.87 +/- 0.15 to 0.46+/- 0.05 sec-1; P = 0.026). The fluctuations of mean arterial pressure (MAP) and middle cerebral artery blood velocity (MCAV) in response to a single hypoxic apnea were not different before and after intermittent apneas indicating that there was not a loss of cerebral blood flow regulation under a hypertensive stimulus. These findings indicate the increased risk of stroke in the early morning waking hours when the OSA patient faces an orthostatic challenge, such as rising from the supine to the upright position.