Reproducibility of methods used for the assessment of autonomous nervous system's function

The objective of this study is to investigate the influence of the day-to-day variability of the measures of heart rate variability (HRV) on the sample size calculation for the study of cardiac autonomic neuropathy.

We analyzed HRV in the frequency domain [very low (VLF), low (LF), and high frequency (HF) bands] and in the time domain [the root mean squared of successive RR intervals differences (RMSSD); the mean RR intervals (RRNN); the standard deviation of RR intervals (SDNN) and the coefficient of variation (CV)] during a 5-min electrocardiogram record. We also analyzed the heart rate response to deep breathing [expiration:inspiration ratio], to the Valsalva maneuver and to standing [maximum:minimum ratio] and the blood pressure response to standing. The day-to-day variability was assessed by calculating the within-subject standard deviations (WSSD), limits of agreement, typical errors and the ratio of the WSSD to the mean values obtained on days 1 and 2 (WSSD/GM).

Sixty-seven healthy subjects (45 females), aged 27 (19-39) years, were recruited. The RMSSD, CV, VLF, LF, HF and blood pressure response to standing showed marked variability (WSDD/GM (%)=237.7, 455.1, 69.9, 126.5, 81.3 and 380.5, respectively), while the RRNN, SDNN, Valsalva, expiration:inspiration and maximum:minimum ratio showed less variability (WSSD/GM (%)=6.4, 24.5, 18.6, 11.0 and 14.1, respectively). The minimum differences expected to be statistically significant for the autonomic measurements were calculated.

Some tests that assess HRV showed adequate reproducibility. This study allows the determination of a sample size calculation for longitudinal or drug-testing studies.