Effects of upright posture on filtered QRS parameter of the signal-averaged electrocardiogram in healthy volunteers

Abstract

The effects of upright posture on signal-averaged electrocardiography were studied together with short-term spectral analysis of heart rate variability (HRV) in 20 healthy volunteers (mean age 34 ± 10 years). Filtered QRS duration from 5-minute epochs and corresponding spectral measures of HRV were computed from digital ambulatory electrocardiographic recordings both at supine rest and during upright tilt (60 degrees). Upright tilt was associated with a significant shortening of filtered QRS (88.2 ± 8 vs 82.6 ± 8 msec, p < 0.0001) and with an increase in its maximum amplitude (78.8 ± 39 vs 87 ± 48 mV, p < 0.004), whereas the average noise level remained unchanged. In the same time the ratio between low- and high-frequency components of HRV shifted in favor of sympathetic predominance (1.4 ± 1.2 vs 3.4 ± 2.9, p < 0.003). No correlation was found between changes in signal-averaged electrocardiography and HRV parameters associated with upright tilt. These data suggest that upright tilt-induced shortening in filtered QRS duration is rather vector-related and does not reflect changes in cardiac autonomic tone. This result must be considered when dynamic analysis of signal-averaged electrocardiography is attempted. (Am Heart J 1997;134:1002-4.)