Arterial chemoreceptor reflex and hypertension.

In this issue, Fletcher et al 1 report that repetitive brief hypoxia applied during a 7-hour period per day for up to 35 days causes diurnal elevation of blood pressure in rats. This significant finding examines the mechanism of chronic, reversible arterial hypertension correlated with sleep apnea syndrome.The hemodynamic pattern of obstructive sleep apnea (OSA) (blood pressure rise at the nadir of arterial blood oxygen saturation [Sac^] and bradycardia or bradyarrythmias with secondary ventricular tachyarrythmias-) mimics those of the circulatory chemoreceptor reflex response in the absence of breathing and lung inflation. To interpret diurnal elevation of blood pressure after periods of brief hypoxic episodes, recent progress in the elucidation of functional characteristics of arterial chemoreceptors should be considered. In contrast to rapidly adapting baroreceptors, chemoreceptors proved to be facilitated by prolonged hypoxic stimulation. Consequently, resetting of the arterial chemoreceptor reflex is opposite to that of the baroreceptor reflex. In this regard it is surprising that arterial chemoreceptors were long considered irrelevant to hypertension, quite opposite to baroreceptors, which have been and still are a classic field of study in hypertension research. A contribution of the arterial chemoreceptor reflex to mechanisms of primary hypertension was recognized late." Apparently, the reason for the low level of interest was a common observation that in healthy normoxic subjects, chemoreceptor reflex drive seemed not to influence resting blood pressure and sympathetic activity, unlike ventilation, which is significantly dependent on peripheral chemoreceptor drive. More recently, however, it has been shown in rats that abrupt hyperoxic inactivation of arterial chemoreceptors induced a transient fall of arterial blood pressure and of some sympathetic activity. Sympathoexcitatory chemoreceptor reflex drive may be of significance in aged rats, possibly contributing to an increase in sympathetic activity with aging.