EFFECT OF RESPIRATION AND POSTURE ON HEART-RATE-VARIABILITY

The physiological control system of the heart produces a highly comple x pattern of cardiac rhythmicity which is reflected in the variability of heart rate. The aim of this study was to analyse the effects of po sture and breathing frequency on the cardiac control system by various noninvasive techniques. Seven healthy subjects (24+/-5 years, mean ag e +/- S.D.) were studied in the supine and sitting positions while bre athing spontaneously or at a fixed rate (3, 6, 12, 24, 48, 60 breaths. min(-1)). Time series of instantaneous beat-to-beat heart rates were e valuated by spectral analysis and by the dimensionless approximate ent ropy parameter. The total spectral power as well as the low (<0.05 Hz) and mid frequency (0.05-0.12 Hz) spectral components were higher in t he sitting position. Mean approximate entropy (+/- S.D.) (0.85+/-0.15 in sitting and 0.87+/-0.16 in lying subjects) was unaffected by postur al changes or breathing frequencies higher than 6 breaths.min(-1). Ana lysis in the frequency domain revealed that the activity of the autono mic components controlling heart rate was modified by ventilation and postural changes, whereas approximate entropy, a unique measure of the complexity and integrity of the cardiac control system, was almost un affected by respiration and posture.