Effect of Heart Rate and Body Position on the Complexity of the QRS and T Wave in Healthy Subjects VN Batchvarov 1 , G Bortolan 2 , II Christov 3 1 St Georges University of London, London, United Kingdom 2 Institute of Biomedical Engineering ISIB - CNR, Padova, Italy 3 Centre of Biomedical Engineering, Bulgarian Academy of Sciences, Sofia, Bulgaria Abstract 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 V resolution) acquired on 2 occasions in supine and standing position in 15 healthy subjects (8 men, age 28.6–7.5 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. 1. Introduction Principal component analysis (PCA) quantifies the complexity of the ECG waves by defining a set of independent forms (components) with decreasing relative value, which can fully describe their shape. Visibly more complex QRS or T waves with irregularities, notches, etc., which are frequently observed in diseased hearts are described by a greater number of independent components, with increasing relative value of the smaller components. Whilst the diagnostic [1 -3 ] and prognostic [4 ,5 ] value of PCA of the T wave has been demonstrated, the effect of physiological factors on the QRS and T wave complexity is unknown. The T wave shape or polarity can be influenced by age, sex, heart rate, body position, autonomic activity, respiration, temperature, electrolyte concentration, food and mental activity [6 -10 ]. It is possible that these factors can also affect T wave complexity. We investigated the effect of heart rate and body position on QRS and T wave complexity using an ECG database recorded in healthy subjects. 2. Methods A digital ECG database was previously recoded for another project in 15 subjects (8 men, 7 women, age 28.6–7.5 years, range 21.5  49.1 years) with negative medical history, normal physical examination and normal resting 12-lead ECG, who were not taking any medications during the project. Data were acquired after 5 minutes of rest in the supine position. In each subject, a continuous digital 12- lead ECG was recorded for 25 minutes (5 minutes supine → 10 minutes standing → 10 minutes supine) with Mason-Likar electrode positions [11 ] using a PC-based 12-lead ECG recorder (CardioSoft™, GE Medical Systems, Milwaukee, USA, 500 Hz, 4.88 V resolution). In each individual, the same protocol was repeated after approximately 1 week at the same time of the day (within±1 hour). The local Ethics Committee approved the study protocol and each subject provided a written consent. 2.1. Data analysis All ECG recordings were visually assessed on screen using the CardioSoft™ software programme in order to exclude segments with noise or premature beats. The first 2 minutes and the last 30 seconds of recording in each position were excluded from the study, and 2.5 minutes supine → 7.5 minutes standing → 7.5 minutes supine were analysed. The ECG were preprocessed suppressing power-line interference, electromyogram noise and baseline drift according to our previously published material on Q-onset and T-end delineation [12 ]. QRS detection was performed following the work of Christov [13 ], then one complex lead was synthesised from the initials 8 leads [14 ]. The QRS and the T wave boundaries as well as the RR interval were delineated and ISSN 0276-6574 225 Computers in Cardiology 2008;35:225-208.