Influence of head-up tilt on respiratory-related oscillations of blood pressure and heart rate at different inspiration/expiration phase ratio

Сardiovascular diseases are commonly associated with disturbances in parasympathetic heart rhythm control, so the development of new methods for assessing vagal cardiotropic influences is an important biomedical task. This work aimed a studying 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 influences increase. In the study involving nine young men, a passive head-up test was performed at a fixed respiratory rate of 0.2 Hz (12 cycles/min) and different ratios of the 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; the phase synchronization index, and the phase difference between HR and MAP oscillations did not depend on the breathing pattern. During verticalization of the body, the power of HR oscillations decreased: with 30% expiratory phase – to a lower level than with 70% expiratory phase. The power of MAP oscillations increased and the phase difference between MAP and HR oscillations decreased during orthostasis regardless of the duration of the inspiratory phase, but increased synchronization of MAP and HR oscillations was observed only during a short inspiratory phase. Thus, the phase structure of the respiratory cycle can significantly affect the power of respiration-related HR oscillations and their synchronization with MAP oscillations of the corresponding frequency.