Analysis of heart rate variability indices with slowly changing heart rate

Cardiac autonomic function is often assessed by a variety of cardiac ECG indices. The ECG is usually assessed from recordings during stable ECG conditions with subjects in comfortable lying or sitting positions. However, it is accepted that heart rate may change during the recording, usually by becoming slower as the subject relaxes. Such changes are usually small, and therefore normally neglected. We were interested in assessing the effect of very small changes. ECGs were recorded in sequential one-minute samples, during sitting as this induces smaller changes than would be the case if subjects were asked to lie down. Recordings were made with electrodes positioned to give a stable positive QRS complex, and the RR was measured between consecutive complexes. All RR intervals were converted to instantaneous beat-to-beat heart rate. Indices were calculated from these data, and averaged over 1 min periods. Heart rate over one min periods changed from 75.9 ± 1.1 to 68.8 ± 1.7 (mean ± SD) beats/min (p<;0.001), with an 1 min average change over the 30 min period of -0.25 ±1.11 beats/min. The figures respectively for successive beat differences were -0.024 ± 0.383 to -0.025 ± 0.519 (p=NS) and 0.000 ± 0.037 beats/min, and for successive RMS beat differences were 0.27 ± 0.27 to 0.37 ± 0.37 (p=NS) and 0.0034 ± 0.088 beats/min. In spite of very significant changes in rate, the indices calculated from successive differences were less prone to the rate changes.