Observer variations in short period spectral analysis of heart rate variability

In spectral analysis of heart rate variability (HRV), the ratio between power in the low-frequency (LF) and power in the high-frequency (HF) regions is widely used to assess the balance between cardiovagal and sympathetic activity (LF/HF ratio). We studied the variation between three observers in the analysis of seven healthy volunteers and eight patients with renovascular hypertension under three conditions (rest, nitroprusside and phenylephrine infusion), in order to determine whether manual selection of the most representative and steady periods increases the reliability of the method. For each condition a period of 6 min of assumed steady-state ECG activity was marked for further analysis. LF/HF ratios were calculated for 3-min periods, using three analysis methods: ‘all manual’, ‘fixed frequency’ and ‘automatic’. In ‘all manual’, the observer selected the optimal 3-min period as well as the LF and HF regions. In ‘fixed frequency’, the manually selected optimal 3-min periods were used with fixed frequency regions for LF (0.04–0.15 Hz) and HF (0.15–0.40 Hz). In ‘automatic’, the fixed frequency regions (0.04–0.15 and 0.15–0.40 Hz) were used and the periods were systematically selected regardless of HR data. The variance in this ‘automatic’ analysis is considered to reflect intrasubject variability. For all analysis methods, the mean LF/HF ratio was highest in the nitroprusside condition, in which the largest interobserver as well as intrasubject variations were found. Calculated per method and per patient condition, the standard deviation between observers was always >25% of the corresponding mean LF/HF ratio. The smallest interobserver variations were found for the ‘fixed frequency’ method. The data showed that it is advantageous to select the 3-min ECG periods but not to select the frequency regions. Selection of the latter led to an increase in interobserver variation. The results of this study give a realistic impression of the intrasubject and interobserver variation to be expected when measuring the LF/HF ratio. This variation is considerable.