Heart beat dynamics during sleep and wake phases: a feedback control approach

In this paper, we study some aspects of the heart rate variability (HRV) of subjects with normal sinus rhythm (NSR) during wake and sleep phases. To this end, a structure function is employed to quantify the fluctuations of the heart rhythm, which is subsequently used to interpret the HRV from a feedback control framework. As made in classical control theory, a frequency-domain methodology is used to gain some insights on the main mechanisms controlling the dynamics of the cardio-respiratory system. In this way, it is shown that the HRV of young NSR subjects reflects a cardio-respiratory system with the same robust frequency response during sleep and wake conditions. On the contrary, for a significant percentage of old NSR subjects one finds a different response performance during the wake phase, which, according to a feedback control framework, could indicate a reduced capacity of the cardio-respiratory system to respond to daily activity. The HRV of subjects with congestive heart failure (CHF) was used to compare the results. It is found that the HRV of CHF subjects reflects a cardio-respiratory system with a decreased sensitivity in a wide frequency range during both sleep and wake phases. These results seem to indicate that discrimination between NSR and anomalous states can be made on the basis of HRV measurements of both sleep and wake phases.