QRS Amplitude of ECG in Normal Humans: Effects of Orthostatic Challenge on Linear and Nonlinear Measures of Beat-to-Beat Variability

When respiratory signal is not available, it can be derived from the surface electrocardiogram (ECG) with some limitations. This is particularly useful to understand the contribution of respiratory variability in several conditions where there is an increased risk of cardiovascular mortality. ECG-derived respiratory signal is also more valuable in situations of 24-h ECG records, where the continuous respiratory signal is not usually available. We have previously shown that respiratory variability in tidal volume significantly increases during standing posture compared to supine posture. In this study, we obtained respiratory signal derived from the ECG in 17 normal adult controls without a history of heart disease and quantified the time of occurrence of peaks and amplitudes or the QRS complex and performed cross-spectral analysis between R-R (interbeat) interval and the QRS-amplitude time series sampled at 4 Hz. Our findings show that the supine QRS amplitude HF power (0.15–0.5 Hz) correlates significantly with the R-R HF power (r (0.62; n (17; p ((0.004). However, this was negatively correlated in standing posture (r (−0.5; n (17; p (0.04). While there was a significant decrease of R-R HF power upon standing (p(0.01), there was a significant increase in QRS amplitude HF power (p (0.004). These findings indicate that the variability of QRS amplitude behaves differently in standing posture compared to R-R time series and thus the supine QRS amplitudinal changes may reflect more closely, the respiratory variability. These findings are discussed in relation to the increased QRS amplitude variability in conditions such as coronary artery disease and other populations at risk for increased cardiac mortality.