Effect of heart rate and body position on the complexity of the QRS and T wave in healthy subjects

We analysed the effect of heart rate and body position on the complexity of the QRS and T wave quantified by the ratio of 2^nd/1^st eigenvalue from principal component analysis (PCA) (QRS-PCA, T-PCA) using continuous 25 min 12-lead digital ECGs (500 Hz, 4.88 muV resolution) acquired on 2 occasions in supine and standing position in 15 healthy subjects (8 men, age 28.6plusmn7.5 years). In the group as a whole, QRS-PCA and T-PCA did not differ significantly between the two positions. However, the linear correlation coefficient between the PCA parameters and the RR interval varied widely between different subjects in the supine position (QRS-PCA: from 0.002 to 0.61; T-PCA: from 0.01 to 0.65) and even more in standing position (QRS-PCA: from -0.55 to 0.48; T-PCA: from -0.63 to 0.51). In both positions, the intra-subject variability of QRS-PCA and T-PCA was significantly smaller than the inter-subject variability.