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Non-uniform recovery of myocardial excitability may be essential in triggering malignant ventricular tachycardia in patients after cardiac surgery. Beat-to-beat heart rate and QT interval variability was investigated in 27 patients scheduled for elective coronary artery bypass graft surgery (CABG) and 20 control subjects. 35 channel ECG was recorded for 6 minutes, followed by off line computer analysis. Heart rate interval duration, RR SD, QT SD and power spectra of RR variability were computed from 256-second stable heart rate and QT interval series using MECG software. Study showed that CABG caused increased QT interval variability in face of nearly constant or slowly changing heart rate, suggesting a loss of autonomic coupling between heart rate and ventricular repolarization for sympathovagal modulation.