Tim Cripps

Risk stratification for arrhythmic events in postinfarction patients based on heart rate variability, ambulatory electrocardiographic variables and the signal-averaged electrocardiogram

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.

Baroreflex sensitivity and electrophysiological correlates in patients after acute myocardial infarction.

BACKGROUND Several studies have identified transient disturbances of autonomic function during the acute and recovery phases of myocardial infarction, and it has recently been suggested that survivors of acute myocardial infarction with depressed vagal tone may be at increased risk of sudden or arrhythmic death. METHODS AND RESULTS To investigate this hypothesis, parasympathetic function was assessed by arterial baroreflex sensitivity (BRS) testing (using the phenylephrine method) and by heart rate variability (HRV) analysis from 24-hour Holter recording in 68 patients at day 7-10 after infarction. The relation between autonomic tone and markers of arrhythmic propensity, including programmed ventricular stimulation (PVS) and late potentials in addition to other clinical variables, was examined. BRS for the whole group was 7.0 +/- 4.7 msec/mm Hg and was inversely correlated with age (r = 0.53, p less than 0.001) but not with left ventricular ejection fraction (r = 0.035, p = NS). In those patients in whom sustained monomorphic ventricular tachycardia (SMVT) was induced, BRS was significantly reduced (p = 0.001) as was HRV (p = 0.007) and left ventricular ejection fraction (p = 0.022). The strongest association between any variable (including HRV, BRS, late potentials, left ventricular ejection fraction, exercise testing, Q waves, and infarct site) and the induction of sustained monomorphic ventricular tachycardia was depressed BRS with a relative risk of 36.28 (95% confidence interval, 5-266). CONCLUSIONS This study confirms that depressed BRS identifies a subgroup at high risk for arrhythmic events after myocardial infarction and that programmed ventricular stimulation may be safely limited to this group without any loss of predictive accuracy.

Signal-averaged electrocardiography in hypertrophic cardiomyopathy

A major goal in the management of patients with hypertrophic cardiomyopathy is the prediction of sudden death. To evaluate the high gain signal-averaged electrocardiogram (ECG) in this setting, 64 patients with hypertrophic cardiomyopathy and 50 age- and gender-matched control subjects were studied. An abnormal signal-averaged ECG was more common in patients than in control subjects: 13 (20%) of 64 patients with hypertrophic cardiomyopathy had abnormalities compared with 2 (4%) of the 50 control subjects (p less than 0.001). There was a significant association between the presence of nonsustained ventricular tachycardia on 48 h ECG Holter monitoring and the presence of an abnormal signal-averaged ECG: 8 (47%) of the 17 patients with nonsustained ventricular tachycardia and 6 (86%) of 7 patients with more than three episodes of nonsustained ventricular tachycardia per 24 h had signal-averaged ECG abnormalities. There was no association between an abnormal signal-averaged ECG and a family history of premature sudden cardiac death, a history of syncope, symptomatic status, maximal left ventricular wall thickness, the presence of systolic anterior motion of the mitral valve or maximal rate of oxygen uptake on exercise. However, of four patients with a history of cardiac arrest, three had an abnormal signal-averaged ECG. Sensitivity was 50%; specificity was 93% and positive predictive accuracy was 77% for the signal-averaged ECG in detecting patients with electrical instability (defined as a history of cardiac arrest or the presence of nonsustained ventricular tachycardia, or both).(ABSTRACT TRUNCATED AT 250 WORDS)

Prognostic value of heart rate variability after myocardial infarction. A comparison of different data-processing methods

Reduced heart rate variability (HRV) has been reported as a predictor of mortality in recent myocardial infarction patients. However, its automated assessment in long-term ECG recordings is complicated by recording noise and beat-recognition errors which necessitate filtering of the computer-established sequence of beat-to-beat intervals, and visual checking and manual editing of the long-term recordings, making the whole method operator-dependent. To develop a fully automated method for analysis of HRV from 24 h ECG recordings, five filtering algorithms were combined with three methods of expressing HRV numerically and used to compare two groups of patients undergoing 24 h tape recordings of the ECG within the first two weeks after myocardial infarction. One group comprised 15 patients who later suffered death or ventricular tachycardia, the other group comprised 15 randomly selected uncomplicated cases. Using the same two groups of patients, three different methods of expressing HRV on a beat-to-beat basis were also compared empirically. The results show that alternative, operator-independent methods for establishing HRV from continuous long-term ECG recordings of postmyocardial infarction patients seem to be as effective as previously reported methods which rely on operator-dependent data post-processing techniques.

Prospective evaluation of clinical assessment, exercise testing and signal-averaged electrocardiogram in predicting outcome after acute myocardial infarction

The relative value of exercise testing, late potentials and simple clinical assessment in predicting ischemic and arrhythmic events during follow-up after acute myocardial infarction (AMI) was investigated prospectively in a population of 176 consecutive patients surviving to 7 days after AMI. During 15 +/- 9 (range 3 to 24) months of follow-up, there were 23 ischemic events (2 fatal reinfarctions, 6 nonfatal reinfarctions and 16 patients who underwent coronary artery bypass grafting, 1 after reinfarction) and 11 arrhythmic events (7 symptomatic ventricular tachycardias and 4 sudden cardiac deaths). Stepwise multiple regression analysis showed that out of 11 variables, including exercise testing, late potentials and clinical data, exercise testing was the only independent variable predicting the occurrence of ischemic events (p less than 0.05 not including coronary artery bypass grafting and p less than 0.002 including it). Arrhythmic events were predicted, in order of importance, by Killip class (p less than 0.05), late potentials (p less than 0.005), previous AMI (p less than 0.009), occurrence of in-hospital complications (p less than 0.005) and non-Q-wave AMI (p less than 0.02). The presence of late potentials provided independent prognostic information from the Killip class and the result of exercise testing in predicting both arrhythmic and ischemic events. Exercise testing, late potentials and clinical assessment provide complementary prognostic information in postinfarction patients.

Inducibility of sustained monomorphic ventricular tachycardia as a prognostic indicator in survivors of recent myocardial infarction: A prospective evaluation in relation to other prognostic variables

The prognostic significance of sustained monomorphic ventricular tachycardia inducible with up to three extrastimuli was assessed in relation to other prognostic markers, including clinical assessment, signal-average electrocardiogram (ECG), Holter monitoring, ejection fraction measurement and exercise testing, in 75 patients after recent myocardial infarction. Among eight patients with inducible sustained monomorphic ventricular tachycardia, six suffered arrhythmic events during a median follow-up period of 16 months. No patient without inducible sustained monomorphic ventricular tachycardia suffered an arrhythmic event. Multivariate analysis showed that of all the variables examined, inducible sustained monomorphic ventricular tachycardia was the only independent predictor of arrhythmic events during the follow-up period. The sensitivity for predicting arrhythmic events by this response was 100%, the specificity 97% and the positive predictive accuracy 75%. Individually, the other prognostic variables were less sensitive and much less accurate predictors of arrhythmic events, but the combination of the occurrence of acute phase complications or frequent ectopic activity with an abnormal signal-averaged ECG approached the sensitivity and accuracy of inducible sustained monomorphic ventricular tachycardia. The prognostic utility of programmed ventricular stimulation in patients with recent myocardial infarction is limited because comparable information can be obtained less invasively. However, the test may have a role in selecting therapy in patients judged to be at risk from arrhythmias on the basis of noninvasive assessment.

The management of electromechanical dissociation

Electromechanical dissociation occurs when there is no effective cardiac output in spite of the presence of a continuing normal or nearly normal electrocardiographic rhythm. A patient with electromechanical dissociation is usually unconscious and apnoeic; no heart sounds are audible, yet electrocardiograpic monitoring shows a continuing rhythm. This paradox has been observed since the earliest days of electrocardiographic monitoring [l] but its precise aetiology and correct treatment remains in many cases obscure. Electromechanical dissociation is a less frequent finding at cardiac arrest than ventricular fibrillation or ventricular tachycardia. In one study of a paramedic-based out of hospital resuscitation service, electromechanical dissociation was present in 13% of 667 cardiac arrests, the remainder being accounted for by asystole (11%) and ventricular tachycardia or fibrillation (76”/u) [2,3]. Greene reports a 5% incidence for EMD in 5199 episodes of out of hospital cardiac arrest in Seattle, 1971-1987 [ 41. Ambulatory monitoring performed during sudden death [5,6] suggests that 25-30X of sudden deaths may be due to electromechanical dissociation or asystole. When cardiac arrest occurs in hospital, electromechanical dissociation appears to be more common, perhaps because the patients have more advanced cardiac disease, and may receive a wider range of drugs during prolonged resuscitation attempts. Raizes [7] found that 15 out of 23 (68%) of in-hospital monitored sudden deaths in the context of acute myocardial infarction were due to electromechanical dissociation. Vincent [8] reported that 36 of 54 (67%) monitored in-hospital cardiac arrests occurred because of electromechanical dissociation, and all were fatal. The prognosis of electromechanical dissociation is very poor, only l/90 patients presenting in one series with electromechanical dissociation surviving to hospital discharge [2]. In the Seattle series [4] survival during long term follow-up in patients where EMD was the arrest rhythm was only 6%. This should be contrasted with the observation that patients resuscitated from ventricular fibrillation during the acute phase of myocardial infarction have a prognosis comparable to those admitted to hospital with uncomplicated myocardial infarction [9].

Long-term spectral analysis of heart rate variability ― an algorithm based on segmental frequency distributions of beat-to-beat intervals

Reduced heart rate variability has been reported as a predictor of long-term mortality in recent myocardial infarction patients. However, it has not been systematically investigated whether the reduction in heart rate variability in those post myocardial infarction patients who later suffer death or severe arrhythmias is caused by a reduction of short-term variability of heart rate (such as respiratory arrhythmia) or whether the differences in long term variability (such as diurnal rhythm) are involved. In order to perform such an evaluation, a new algorithm has been developed which permits different wavelength components (including the long-term components due to diurnal rhythm) of heart rate variability to be approximated. In general, the method uses segmental frequency distributions of durations of intervals between successive normal cardiac beats. To assess the spectral components of heart rate variability, a scale of wavelength limits is used and for each limit of this scale, the algorithm excludes the rate changes of wavelength longer than the given bound. The method was applied to the analysis of electrocardiograms recorded in 14 post myocardial infarction patients who later suffered death or ventricular tachycardia, and in 14 other randomly selected patients with an uncomplicated course following acute myocardial infarction. The rate variability spectra obtained for both groups of patients were compared statistically and the results showed that the groups of positive and negative cases were most significantly distinguished when including both short- and long-term components of heart rate variability. Separate evaluation of different wavelength components showed that the very long-term components of heart rate variability were more powerful in distinguishing between positive and negative cases than the short term components.

PREDICTION OF SERIOUS POST-INFARCTION CARDIAC ARRHYTHMIAS

Simple clinical findings suggestive of a large myocardial infarction identify most patients at risk from serious post-infarction arrhythmias (defined as sustained ventricular tachycardia and sudden death not associated with reinfarction). However, similar findings are seen in many other patients who remain free of complications. The presence of late potentials and frequent ventricular ectopics on long-term electrocardiographic monitoring add to prognostic accuracy in patients with clinical complications, with a predictive accuracy of nearly 90% when all three features are present.