THE SYMPATHETIC NERVOUS-SYSTEM AND OBSTRUCTIVE SLEEP-APNEA - IMPLICATIONS FOR HYPERTENSION

Patients with obstructive sleep apnea experience repetitive apneic eve nts during sleep, with consequent hypoxia and hypercapnia. Hypoxia and hypercapnia, acting via the chemoreflexes, elicit increases in sympat hetic nerve activity. The sympathetic responses to hypoxia and hyperca pnia are potentiated during apnea, when the sympathetic inhibitory inf luence of the thoracic afferent nerves is eliminated. As a consequence of the sympathetic vasoconstrictor response to apneic events, patient s with obstructive sleep apnea manifest marked increases in blood pres sure during sleep, especially evident at the end of the apnea. The inc reases in sympathetic activity and blood pressure during sleep in thes e patients appear to carry over into the daytime such that patients wi th sleep apnea have an increased prevalence of hypertension and high l evels of sympathetic nerve activity. Although the mechanism underlying the persistent elevation in sympathetic activity during the daytime i s not known, it is likely that the increased sympathetic drive is impl icated in the higher daytime blood pressures in these patients. Wherea s patients with sleep apnea have an increased prevalence of hypertensi on, in those patients with sleep apnea who do have hypertension, the s ympathetic response to apneic events may be potentiated. This may be s econdary to impaired baroreflex sensitivity, since the baroreflexes ex ert an inhibitory influence on the chemoreflex responses to hypoxia. T reatment with continuous positive airway pressure results in an acute reduction in blood pressure and sympathetic activity during sleep. Pro longed effective treatment of sleep apnea may also reduce daytime bloo d pressure levels. This review examines the physiology of the chemoref lex responses to hypoxia, hypercapnia and apnea, as well as the physio logic responses to sleep in normal humans. These physiologic responses are compared with the pathophysiologic sympathetic and hemodynamic re sponses that characterize obstructive sleep apnea. Increases in sympat hetic activity and blood pressure in patients with obstructive sleep a pnea may play a role in linking sleep apnea to hypertension and cardia c and vascular events. (C) Rapid Science Publishers ISSN 0263-6352.