A nonlinear model of cardiac autonomic control in obstructive sleep apnea syndrome

Using the Volterra-Wiener approach, we employed a minimal model to quantitatively characterize the linear and nonlinear effects of respiration (RCC) and arterial blood pressure (ABR) on heart rate variability (HRV) in normal controls and subjects with moderate-to-severe obstructive sleep apnea syndrome (OSAS). Respiration, R-R interval (RRI), blood pressure (BP) and other polysomnographic variables were recorded in eight normal controls and nine OSAS subjects in wakefulness, Stage 2 and rapid eye-movement sleep. To increase respiratory and cardiovascular variability, a preprogrammed ventilator delivered randomly timed inspiratory pressures that were superimposed on a baseline continuous positive airway pressure. Except for lower resting RRI in OSAS subjects, summary statistical measures of RRI and BP and their variabilities were similar in controls and OSAS. In contrast, RCC and ABR gains were significantly lower in OSAS. Nonlinear ABR gain and the interaction between respiration and blood pressure in modulating RRI were substantially reduced in OSAS. ABR gain increased during sleep in controls but remained unchanged in OSAS. These findings suggest that normotensive OSAS subjects have impaired daytime parasympathetic and sympathetic function. Nonlinear minimal modeling of HRV provides a useful, insightful, and comprehensive approach for the detection and assessment of abnormal autonomic function in OSAS.