Stability over time of short-term heart rate variability.

Heart rate variability (HRV) is a widely used method to assess cardiac autonomic control. However, the reproducibility of especially short-term HRV has not been properly evaluated. Therefore, we assessed the stability of short-term HRV over a three to four month period. We had seven consecutive electrocardiographic (ECG) recordings from 89 subjects with stable coronary artery disease obtained during a large multicenter study. The HRV assessments were performed from these 40-minute ECG-recordings simulating normal daily activities, i. e., recordings consisting of 5 to 10 minute periods of rest, paced breathing, standing, submaximal exercise and recovery. Both time and frequency domain HRV analyses were conducted from the whole 40-minute recordings and from the 5-minute periods of rest and paced breathing. The coefficient of variation (CV) varied between 5.1-16.7% for the 40-minute and 6.0-37.1% for the 5-minute time domain and 4.4-11.0 % for the 40-minute and 7.2-16.5 % for the 5-minute frequency domain measurements. The mean of the RR intervals and the total power showed the highest stability over time. The most unstable measure was the standard deviation of all NN intervals (SDNN). In conclusion, most short-term HRV measures were highly stable over time indicating low physiological variation. However, the SDNN showed large variability in consecutive recordings.