Change in Phase Ratios Oscillations of Blood Pressure and Heart Rate in Orthostasis: Impact of Gravity Unloading

The study of the amplitude characteristics of variability in blood pressure (BP) and heart rate (HR) in the low-frequency range (LF—baroreflex waves, about 0.1 Hz) and high-frequency range (HF—corresponds to the respiratory rate) is widely used to assess the state of the cardiovascular system. In this work, there was studied the effect of 7-day dry immersion (DI) (a stage of pronounced hemodynamic shifts) on the phase characteristics of BP and HR variability in LF and HF ranges under orthostatic exposure. Eight young healthy men performed a passive head-up tilt test: 15 min in a lying position, then 15 min with orthostasis (60°). During the test, ECG, blood pressure, and respiratory rate were continuously recorded, and breathing rhythm was controlled by voice commands from a computer. Breathing control and experimental data analysis were performed using specially developed software. It has been shown that in healthy people with orthostasis, the degree of synchronization of LF oscillations of BP and HR increases, at the same time, there is a decrease in the phase difference (time shift) of the HF oscillations of BP and HR. After staying in the DI, the effects described above disappear, which reflects a disruption in the nervous regulation of the heart rhythm. A comprehensive analysis of the phase ratios of BP and HR oscillations in the LF and HF ranges can be used to assess changes in the nervous regulation of the heart under conditions of gravitational unloading, in the diagnosis of orthostatic instability in cardiovascular diseases, as well as in developing approaches to prevent the development of this condition.