The value of heart rate variability, ambulatory electrocardiographic (ECG) variables and the signal-averaged ECG in the prediction of arrhythmic events (sudden death or life-threatening ventricular arrhythmias) was assessed before hospital discharge in 416 consecutive survivors of acute myocardial infarction. During the follow-up period (range 1 to 1,112 days), there were 24 arrhythmic events and 47 deaths. The initial relation between several prognostic factors and arrhythmic events was explored with use of the Kaplan-Meier product limit estimates of survival function. Impaired heart rate variability less than 20 ms (p less than 0.0000), late potentials (p less than 0.0000), ventricular ectopic beat frequency (p less than 0.0000), repetitive ventricular forms (p less than 0.0000), left ventricular ejection fraction less than 40% (p less than 0.02) and Killip class (p less than 0.02) were identified as significant univariate predictors of arrhythmic events. When these variables were analyzed by using a stepwise Cox regression model, only impaired heart rate variability, followed by late potentials and repetitive ventricular forms remained independent predictors of arrhythmic events. The combination of impaired heart rate variability and late potentials had a sensitivity of 58%, a positive predictive accuracy of 33% and a relative risk of 18.5 for arrhythmic events and was superior to other combinations including those incorporating left ventricular function, exercise ECG, ventricular ectopic beat frequency and repetitive ventricular forms. These results suggest that a simple method of assessment based on heart rate variability and the signal-averaged ECG can select a small subgroup of survivors of myocardial infarction at high risk of future life-threatening arrhythmias and sudden death.

QT Dispersion. QT dispersion is defined as the difference in QT interval between (he different leads of the surface 12-lead ECG. This may provide an indirect measure of the underlying inhomogeneity of myocardial repolarization, which is believed to be important in arrhythmogenesis. Methodology for determining QT dispersion varie.s significantly between studies, and the results of these studies need to be interpreted in light of the methodology used. Although QT dispersion is developing into an important research tool, as yet it has no established role in clinical practice. Once standardization of methodology is achieved a clinical role may emerge, particularly in the assessment of patients hefore and after intervention aimed at reduction of arrhythmia risk. (J Cardiovasc Electrophysiol, Voi 5, pp. 672-685, August 1994) QT interval, ventricular arrhythmia, torsades de pointes

The rate of QT adaptation to abrupt changes in pacing rate was studied in seven patients with newly diagnosed complete heart block with a ventricular escape rate of less than 40 beats.min-1. Their median age was 70 (range 36-84) years, and none was taking any cardioactive medication known to affect the QT interval. From a baseline pacing rate of 50 or 110 beats.min-1 the ventricular rate was increased or decreased to a new level. The time taken for the ventricular paced QT interval to complete 90% of the change secondary to the change in rate was found to be 136(16) s (mean(SEM] when the rate was increasing and 189(25) s when the rate was decreasing (p less than 0.01). This time interval was independent of the magnitude of the rate change and the baseline heart rate from which the change occurred. Furthermore, the time course of QT adaptation was found to be exponential and was characterised by a time constant of 49.1(2.2) s when the rate was increasing and 60.4(2.0) s when the rate was decreasing (p less than 0.01). It is concluded that QT measurements in response to a change in pacing rate should take into account the time dependent nature of QT changes.

Background-Disturbances of autonomic function are recognised in both the acute and convalescent phases of myocardial infarction. Recent studies have suggested that disordered autonomic function, particularly the loss of protective vagal reflexes, is associated with an increased incidence of arrhythmic deaths. The purpose of this study was to compare the value of differing prognostic indicators with measures of autonomic function and to assess the safety of arterial baroreflex testing early after infarction.Methods-As part of a prospective trial of risk stratification in postinfarction patients arterial baroreflex sensitivity, heart rate variability, long term electrocardiographic recordings, exercise stress testing, and ejection fraction were recorded between days 7 and 10 in 122 patients with acute myocardial infarction.Results-During a one year follow up period there were 10 arrhythmic events. Baroreflex sensitivity was appreciably reduced in these patients suffering arrhythmic events (1-73 SD (1-49) v 7'83 (4-5) ms/mm hg, 95% confidence interval (CI) 4-8 to 7 3, p = 0-0001). Significant correlations were noted with age (r = -0-68, p < 0-001) but not left ventricular function. When baroreflex sensitivity was adjusted for the effects of age and ventricular function baroreflex sensitivity was still considerably reduced in the arrhythmic group (2-1 v 7-57 ms/mm Hg, p < 0-0001). Depressed baroreflex sensitivity carried the highest relative risk for arrhythmic events (23-1, 95% CI 7-7 to 69 2) and was superior to other prognostic variables including left ventricular function (10-4, 95% CI 3-3 to 32-6) and heart rate variability (10-1, 95% CI 5-6 to 18-1). No major complications were noted with baroreflex testing and in particular no patients developed ischaemic or arrhythmic symptoms during the procedure.Conclusions-Disordered autonomic function as measured by depressed baroreflex sensitivity or reduced heart rate variability was associated with an increase incidence of arrhythmic events in post-infarction patients. Baroreflex testing can be safely performed in the immediate post-infarction period.Established methods of risk stratification in post-infarction patients are based on clinical features; exercise stress testing; and the identification of complex ventricular arrhythmias, impaired left ventricular function, and multivessel coronary artery disease.l" Despite such diverse approaches many problems associated with the identification and treatment of patients at high risk of malignant arrhythmias and sudden death remain unsolved. In an attempt to improve the prediction of arrhythmic events, novel methods of risk stratification including the signal averaged electrocardiogram5 and programmed ventricular stimulation6 have been evaluated. More recently, with growing awareness of the key role of neural mechanisms in arrhythmogenesis, attention has been focussed on the prognostic value of autonomic function tests such as heart rate variability analysis and baroreflex sensitivity.7-10

The delivery of biphasic R wave synchronous shocks between the high right atrium and coronary sinus can terminate atrial fibrillation with very low energies. General anaesthesia is not required, and a minority of fully conscious patients are able to tolerate this method of cardioversion.