A chemoreflex model of relation between blood pressure and heart rate in sleep apnea syndrome

In obstructive sleep apnea syndrome (OSAS), pronounced low-frequency (LF) oscillations of blood pressure and interbeat interval (I) occur during recurrent apneas. We investigated the time relations between LF oscillations of diastolic pressure (D) and I in 12 patients with OSAS by means of spectral analysis. A high coherency between I and D was found, allowing a description of the relation by gain and phase. Oscillations in I and D were almost in counterphase in the LF range. Simple physiological models were implemented to interpret the observed features of LF oscillations. Model 1 describes the vagal and sympathetic influence by the carotid body chemoreflex on the circulation. From derivation of the frequency response of this model, gain and phase relations were obtained as would be expected from the action of the chemoreflex. We found that a range of phase relations can be induced by this reflex, depending on the relative vagal and sympathetic efferent influence on the circulation. This range of phase relations was indeed observed in 10 patients. Extended models that also included the orienting reflex (model 1a) or the baroreflex and a mechanical influence of breathing on the circulation (model 2) could not fit the data without a major contribution of the chemoreflex. We conclude that the relation between LF oscillations in I and D in OSAS can be explained by assuming that stimulation of the carotid body chemoreflex is the main source of these oscillations.