Changes of QT intervals associated with postural change in patients with chronic atrial fibrillation

Although different computerized systems have been developed to localize specific patterns in electrocardiographic (ECG) signals, it is still difficult to detect T waves and measure QT intervals during atrial fibrillation. This article demonstrates the use of an auto-correlation (ECG) based system that was used to investigate the dynamicity of QT intervals related to active postural change in patients with chronic atrial fibrillation. Twenty patients (9 male, mean age 63 years) with chronic atrial fibrillation (8 idiopathic, 12 organic heart disease) were examined. Seventeen of these patients were on digoxin, but patients with other conditions potentially affecting the autonomic nervous system were not included. A 3-channel ECG was recorded digitally during active postural change from supine to standing. Data were first analyzed by the Burdick Altair system and subsequently processed using an in-house software package evaluating auto-correlations of ECG signals. An ECG channel with suitable repolarization patterns was found in 15 patients. The mean QT interval of 409.8 +/- 11.1 ms (mean +/- SE) recorded during supine position shortened to 401.9 +/- 9.89 ms during the first minute of active standing (P < 0.05) and to 394.8 +/- 10.0 ms during the second minute of active standing (P < 0.005). It did not further change during the subsequent minutes of active standing. The study shows that automatic detection of QT intervals during atrial fibrillation is possible. Although the effect of position change of the heart cannot be completely excluded, the study suggests that QT interval is changed directly by autonomic nervous mechanisms rather than indirectly via the mean heart rate.