Heart rate and heart rate variability as indexes of sympathovagal balance.

According to the Rosenblueth-Simeone model, the heart rate (HR) is proportional to the sympathovagal balance. The individual proportionality constant is the intrinsic heart rate, which can only be determined invasively. The normalized low-frequency heart rate variability power (LF) has been raised as a calibrated noninvasive alternative. To concrete this assumption, we studied the individual LF-HR relation during incremental head-up tilt (0, 10, 20, 30, 40, 45, 50, 55, 60, 65, 70, 75, and 80 degrees) in 21 young, healthy males. HR (means +/- SD) increased from 61.0 +/- 9.1 beats/min at 0 degree to 85.9 +/- 18.3 beats/min at 80 degrees. LF increased from 45.8 +/- 16.7 nu at 0 degrees to 79.8 +/- 13.8 nu at 80 degrees (nu meaning normalized units). Individual regressions of LF on HR yielded correlation coefficients of 0.80 +/- 0.13 (means +/- SD). The demonstrated linear relation between LF and HR confirms the potential significance of heart rate variability as a noninvasive means of assessing the sympathovagal balance.