OBJECTIVE: QT interval prolongation and increased spatial QT dispersion are important factors increasing the risk in coronary heart disease. The authors studied the spontaneous beat-to-beat variability of ventricular repolarization (RT intervals) in normal subjects and in patients after myocardial infarction (MI) in order to define the determinants of abnormal temporal dispersion. METHODS: Seventy-six patients with a history of MI (17 female, 59 male, aged 52 +/- 10 years) comprised the study group. Forty-seven patients had preserved left ventricular ejection fraction (EF > or = 40%, MI-A) and 29 patients had left ventricular dysfunction (EF < 40%, MI-B). Twenty healthy volunteers (6 female, 14 male, aged 25 +/- 5 years) were included as the control group. An ECG signal of 512 heartbeats was recorded in the supine position. After analogue-to-digital conversion (16 bit, 2 kHz), the fiducial points of the R wave and T wave were determined. The RR and RT variability (V) assessed in the time domain as the standard deviations of RR and RT (ms), as well as the coefficients of scatterplots of RR and RT intervals. RESULTS: As expected, the standard deviation of RR was significantly reduced in MI patients. The magnitude of RTV in the time domain was similar in the controls and in both subgroups of MI patients. The complexity of heart rate variability (HRV) was slightly, but significantly, reduced in the MI-B group, but not significantly in the MI-A heart group. The complexity of RTV behaved in the opposite manner, being increased in both MI subgroups with the lower mean in the MI-B patients. The different behaviour of HRV and RTV was indicated by the increased ratio of RR/RT coefficients, which reached a significantly greater value in the MI-B group. CONCLUSION: The authors have described different patterns of scatterplot of short-term HRV and RTV in normal subjects, which confirmed that RTV is a less complex phenomenon than HRV. In patients after MI, the complexity of HRV diminishes, while the complexity of RTV increases. These opposing changes are more pronounced in patients with left ventricular dysfunction. A possible prognostic value of this feature is unknown and remains to be elucidated in future prospective studies.