Effects of modified respiratory rhythm on heart rate variability during active orthostatic load.

The experiments were carried out to study the influence (a) of 3 different respiratory frequencies (0.12, 0.20 and 0.35 Hz) on heart rate power spectra during orthostatic load with special reference to the peak frequency of mid-frequency heart rate fluctuations, and (b) of variation in respiratory depth on this peak frequency in supine position. The peak frequency of mid-frequency heart rate fluctuations was shifted to lower frequency values in response to orthostasis despite the constant respiratory frequency for either posture. No differences in peak frequency were observed between respiratory frequencies of 0.20 and 0.35 Hz in both supine and standing position and between the two extreme patterns of respiratory depth. We suggest that the shift of the peak frequency of mid-frequency heart rate fluctuations to the lower end of the heart rate power spectrum during orthostasis represents an original effect independent of depth and frequency of respiration.