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Summary: The responses of common carotid blood flow (CCF), pressure (BP), and resistance (R) to variations in respiratory gases were compared during waking periods in 10 sleep apnea patients (SA) and 10 healthy controls (N) of similar age, Respiratory gases were altered by 3-min CO 2 rebreathing (RB), 3-min hyperventilation (HV), and 4-min hypoxia (HYP) procedures, CCF was measured continuously by a 5-MHz pulsed Doppler duplex scanner and R was calculated using brachial BP. During RB, which increased end-tidal Pco 2 (P AC0 2 ) by 15 mm Hg, SA had a lower CCF and greater BP response and therefore a significantly different (positive) change in R compared with N, The ventilatory responses to CO 2 were not significantly different. With HV the P A C0 2 fell by l3 mm Hg in both groups and CCF fell more markedly in SA than N with the same change in BP; therefore, R was increased significantly more in SA. The HYP results did not demonstrate a difference between groups, These results suggest that abnormal cerebrovascular responses to P ACOZ, initiated either by unusual vasoactive properties of cerebral resistance vessels or peculiar venous outlow patterns, may initiate or potentiate periodic breathing in SA by prolonging lung-to-brain circulation time, Key Words: Sleep apnea-Cerebral blood flow-Rebreathing-Hyperventilation-Hypoxia-Circulatory time lags.The malfunctions of physiological, anatomical, or neurological mechanisms which result in the clinical manifestation of the sleep apnea syndrome are not well understood. Numerous factors such as obesity, unusual airway and oral anatomy and oropharyngeal muscle tone, central neurological lesions, and insensitive or unstable operation of chemoreceptor responses to respiratory stimuli have all been implicated (1-3). The separation of sleep apnea syndromes into "obstructive" and "central" types is also no longer categorical because of increased resolution provided by better measurement techniques in sleep laboratories. Recent reports have demonstrated the propensity of healthy individuals to develop CheyneStokes respiration ,or sleep apnea with transient ascents to higher altitudes (4), and some transient episodes of sleep apnea are often noted in healthy individuals when their sleep is monitored. Sleep studies of randomly selected elderly patients who were subjectively not afflicted with sleep disorders revealed that about one-third had clinically significant sleep apnea (5). These and other observations have lead to the general consensus that the abnormal

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