Parallel recording of physical activity on commercial Holter recorders.

To accurately interpret heart rate variability (HRV) including circadian rhythm from Holter ECG, the simultaneous assessment of physical activity, which significantly affects HRV, is essential. In this study, to obtain this simultaneous assessment, the fundamental problems in implementing an accelerometer in a commercial Holter recorder were studied. In a treadmill exercise, three axial outputs of an accelerometer showed highly linear correlations with the running speed (correlation coefficient; vertical 0.94, forward-backward 0.97, sideways 0.97, three-dimensional amplitude 0.96, n = 8). The vertical acceleration showed a slightly sigmoidal increase with speed. Against a slope change, no significant increase in acceleration was observed except in the forward-backward direction. Individual calibration was found to be needed for the accurate estimation of physical load from body acceleration. A simplified calculation with the sum of the three axial absolute values correlated highly with the three-dimensional (3D) acceleration which shows the most reliable response to motions in any direction (r = 0.98, the slope of the regression line = 0.97) and, with this relation, the estimated 3D amplitude showed a sufficient degree of agreement. This substituted calculation was used in the Holter recordings. To know the posture of subjects, a piezoresistive accelerometer with the function of clinometer was used in another study. From played-back Holter recordings, the R-R interval and body acceleration were simultaneously sampled. The serial changes of both the power spectra of HRV and the body acceleration were observed over a 24 h period. Some cases with an abnormally reduced high-frequency component (HF) of HRV at night or an unusually high HF in the daytime were explained by physical conditions estimated with the accelerometer. The simultaneous assessment of patients' physical state by the present method provides the accurate interpretation of circadian rhythm in HRV.