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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

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Objective-To examine the influence of the duration of follow up on the values of heart rate variability (HRV) and the left ventricular ejection fraction (LVEF) for predicting mortality after infarction. Background-HRV is an index of autonomic balance that identifies patients at a high risk of arrhythmic events. The index is most depressed during the first few weeks after myocardial infarction whereas left ventricular function tends to deteriorate with time.Hypothesis-The value of depressed HRV measured before discharge from hospital for predicting mortality after infarction should decline with time. Methods-The HRV and the LVEF were assessed in 433 survivors of a first acute myocardial infarction: HRV < 20 units and LVEF < 40% were taken as cut off points. Kaplan Within the whole follow up period, HRV < 20 units and LVEF < 40% were both strongly associated with total cardiac mortality (p < 0*0001), but HRV was an independent predictor of total cardiac mortality only during the first six months of follow up. There were no deaths predicted by HRV < 20 units after the first year of follow up whereas LVEF < 40% had a sensitivity of 43% and a positive predictive accuracy of 90/o for predicting death during this period. HRV < 20 units was better than LVEF < 40%/o in predicting sudden deaths during the first year of follow up but was an independent predictor only of those sudden deaths occurring within six months of infarction.Conclusions-The duration of follow up affects the prediction of sudden death and total cardiac mortality from HRV.Reduced HRV as measured before discharge from hospital does not seem to retain independent prognostic value after six months of follow up. These findings have potential implications for the serial evaluation of HRV and for the prevention of sudden death after myocardial infarction. (Br Heart3J 1994;71:521-527) The balance between cardiac sympathetic and vagal efferent activity is reflected in beat by beat oscillations of the cardiac cycle.' 2 The magnitude of these oscillations can be assessed by measuring heart rate variability (HRV) from ambulatory electrocardiographic recordings. As sympathetic predominance predisposes to ventricular arrhythmias, whereas vagal stimulation is protective,3 the assessment of HRV provides a means of predicting mortality after myocardial infarction,45 particularly from arrhythmic events.67The HRV is most obviously depressed during the first few weeks after myocardial infarction,8'0 whereas left ventricular function tends to deteriorate with time, particularly after extensive infarction."1 Therefore, assuming causality between sympathovagal imbalance and mortality, the predictive value of a low HRV measured early after infarction should decline with time; whereas a low LVEF before discharge from hospital should be better for predicting late mortality. The implications of this hypothesis were examined by comparing the HRV and the LVEF as predictors of mortality during different periods of follow up after a first myocardial infarction.