Influence of Head-Up Tilt on Respiration-Related Oscillations of Arterial Pressure and Heart Rate at Different Inspiration/Expiration Phase Ratios

Cardiovascular diseases are commonly associated with disturbances in parasympathetic heart rhythm control; therefore, the development of new methods for assessing vagal cardiotropic effects is an important biomedical task. The purpose of this research was to study the synchronization of respiration-related oscillations of mean arterial pressure (MAP) and heart rate (HR) depending on the duration of the expiration phase, during which cardiac vagal effects increase. The study involving nine young men included a passive head-up test performed at a fixed respiratory rate of 0.2 Hz (12 cycles/min) and different ratios of inspiration and expiration phase durations (30/70 and 70/30%). Blood pressure, HR, and pulmonary ventilation were continuously recorded during the experiment. In the supine position, the power of HR oscillations at the respiratory frequency was significantly higher with longer expiratory phase. The power of MAP oscillations, phase synchronization index, and phase difference between HR and MAP oscillations did not depend on the breathing pattern. The power of HR oscillations decreased during body verticalization: to a lower level during 30% expiratory phase than during 70% expiratory phase. The power of MAP oscillations increased and the phase difference between MAP and HR oscillations decreased during head-up tilt regardless of the duration of the inspiratory phase; however, the increased phase synchronization index of MAP and HR oscillations were observed only during a short inspiratory phase. Therefore, the phase structure of the respiratory cycle can significantly influence the power of respiration-related HR oscillations and their synchronization with MAP oscillations of the corresponding frequency.