The effect of hypoxia on baroreflexes and pressor sensitivity in sleep apnea and hypertension.

Many persons with sleep apnea are hypertensive. Forty-two subjects of similar age and weight were divided into four groups of hypertensives and normotensives with and without sleep apnea. All subjects had heart rate, blood pressure (BP), baroreflex sensitivity and pressor sensitivity to phenylephrine measured while breathing room air or 15% oxygen. Hypoxia raised heart rate and lowered BP in all groups (p < 0.001), with the greatest hypotensive effect among hypertensives. Hypertensives had blunted baroreflex sensitivity, and breathing a hypoxic mixture lowered baroreflex sensitivity of all four groups (p = 0.008). The apneic subjects tended to lower their baroreflex sensitivity more in response to hypoxia and also had an enhanced pressor response to phenylephrine, whether breathing room air or 15% oxygen. Episodes of sleep apnea lead to hypoxia, an initial period of hypotension and a subsequent increase in sympathetic nervous activity. Our studies suggest that apneics could have an exaggerated pressor sensitivity to norepinephrine. They might also have difficulty returning BP to normal levels, because hypoxia impaired baroreflexes.