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Variability of R-R, P wave-to-R wave, and R wave-to-T wave intervals.

We analyzed the effect of changing posture from supine to standing on the variability of R-R, P-R, and R-T intervals in 10 healthy volunteers using power spectral analysis. An electrocardiogram and respiratory trace were recorded before and after posture change. Variability in the P-R and R-T intervals was much less than in the R-R interval and demonstrated a lower low-frequency (LF)-to-high-frequency (HF) ratio. Changing from a supine to a standing position showed no change in indexes of vagal influence on the P-R and R-T variability, in contrast to the well-documented decrease in the indexes of vagal influence on the R-R variability (HF power decreased from 2.33 to 0.41 ms2, P = 0.003; amplitude of the respiration-to-heart rate impulse response decreased from 31.6 to 14.4 ms ⋅ ml-1 ⋅ s-1, P = 0.03; and LF/HF increased from 1.96 to 5.22, P = 0.005). We concluded from this study that the effects of standing were an observed reduction in vagal influence on the heart rate variability of the R-R interval and maintenance of lung volume-related vagal modulation of the P-R and R-T intervals.