Time-varying analysis of autonomic cardiovascular control during arousal from sleep

Obstructive Sleep Apnea Syndrome (OSAS) patients are subject to periodic episodes of upper airway collapse for over 10 seconds. Each apnea episode causes a drop in blood oxygen saturation (hypoxia), accumulation of carbon dioxide (hypercapnia) and a drop in Heart Rate and Blood Pressure. Usually, the resumption of normal respiration is either preceded or followed by a transient arousal from sleep. These episodes can occur at a considerable rate per night (easily reaching the higher hundreds). We hypothesize that long-term accumulation of sympathoexcitation following arousal may be a factor in the development of systemic hypertension observed in OSAS. In this work we examine the effects of arousals alone on the autonomic control of the cardiovascular function with the aim to help better describe the chain of events that take place at and after arousal, and to establish how the long-term effects of OSAS could arise from the repetitive occurrence apneas. Through recursive autoregressive spectral analysis of cardiovascular and respiratory signals, we aim to track the changes that acoustically-induced sleep interruption cause on the autonomic influence to the cardiovascular control of healthy subjects. The results point towards an increase in sympathetic activity, measured by a rise in Low Frequency oscillations of cardiovascular variability signals. This alteration seems to be lasting longer than the immediate changes in Blood Pressure, Heart Rate and EEG-induced arousal. Comparisons between healthy controls and OSAS show that, in this group, the pressor response to NREM arousal remains unchanged whereas the heart rate response is severely attenuated suggesting that long-term exposure to OSAS could impair the normal cardiac sympathoexcitatory reaction associated with NREM arousal. Moreover, in REM sleep, a stage with a generalized increase in sympathetic tone, healthy controls and OSAS patients show a similar impairment of the arousal response. The study of the mechanisms involved in the cardiovascular control with the help of a model aims to show how these autonomic influences may be translated into actual changes of the cardiovascular function. The Arterial Baroreflex (ABR) seems to be one of the mechanisms involved directly in the changes observed in both Blood Pressure and peripheral sympathoexcitation.