Mechanisms of sudden cardiac death in myocardial infarction survivors: insights from the randomized trials of implantable cardioverter-defibrillators

Pathogenesis of Sudden Unexpected Death in a Clinical Trial of Patients With Myocardial Infarction and Left Ventricular Dysfunction, Heart Failure, or Both

Background— The frequency of sudden unexpected death is highest in the early post–myocardial infarction (MI) period; nevertheless, 2 recent trials showed no improvement in mortality with early placement of an implantable cardioverter-defibrillator after MI.

Methods and Results— To better understand the pathophysiological events that lead to sudden death after MI, we assessed autopsy records in a series of cases classified as sudden death events in patients from the VALsartan In Acute myocardial infarctioN Trial (VALIANT). Autopsy records were available in 398 cases (14% of deaths). We determined that 105 patients had clinical circumstances consistent with sudden death. On the basis of the autopsy findings, we assessed the probable cause of sudden death and evaluated how these causes varied with time after MI. Of 105 deaths considered sudden on clinical grounds, autopsy suggested the following causes: 3 index MIs in the first 7 days (2.9%); 28 recurrent MIs (26.6%); 13 cardiac ruptures (12.4%); 4 pump failures (3.8%); 2 other cardiovascular causes (stroke or pulmonary embolism; 1.9%); and 1 noncardiovascular cause (1%). Fifty-four cases (51.4%) had no acute specific autopsy evidence other than the index MI and were thus presumed arrhythmic. The percentage of sudden death due to recurrent MI or rupture was highest in the first month after the index MI. By contrast, after 3 months, the percentage of presumed arrhythmic death was higher than recurrent MI or rupture (χ 2 =23.3, P <0.0001).

Conclusions— Recurrent MI or cardiac rupture accounts for a high proportion of sudden death in the early period after acute MI, whereas arrhythmic death may be more likely subsequently. These findings may help explain the lack of benefit of early implantable cardioverter-defibrillator therapy.

Optimal timing of implantable cardioverter-defibrillator implantation after myocardial infarction: a decision analysis

BACKGROUND

The optimal timing of implantable cardioverter defibrillator (ICD) placement for the primary prevention of sudden cardiac death after myocardial infarction (MI) remains unknown.

METHODS AND RESULTS

We developed a Markov model to investigate the optimal timing of ICD implantation after MI (no ICD, ICD at 60 days, 6 months, and 1 year) in patients who meet current guidelines. Estimates of arrhythmic death (baseline risk 6%, range 1-20% per year), nonarrhythmic death, and ICD efficacy were based upon MADIT-II and other contemporary post-MI clinical trials. We used both deterministic and stochastic modeling processes in our analysis. After 10 years follow-up, the baseline probability of survival was higher in those treated with ICD implantation versus not (42% vs 30%, P < 0.001). Survival was highest with ICD implantation at 60 days versus 6 months versus 1 year: 42.4%, 42.3%, and 42.0% (P = 0.0028). ICD implantation at 60 days provided a mean incremental survival of 0.28 months and 0.84 months per patient (compared with implantation at 6 months and 1 year). In sensitivity analyses, patients' competing risk for nonarrhythmic death was the primary determinant of benefit from ICD implantation. Overall, ICD implantation at 60 days resulted in the greatest life expectancy over a wide range of plausible nonarrhythmic and arrhythmic death rates.

CONCLUSIONS

The benefits of early ICD implantation are modest when compared with delayed implantation at 6 months/1 year. Our results suggest that making sure a patient receives an ICD, when appropriate, may be more important than the timing of the implantation procedure.

Redox modification of ryanodine receptors underlies calcium alternans in a canine model of sudden cardiac death

AIMS

Although cardiac alternans is a known predictor of lethal arrhythmias, its underlying causes remain largely undefined in disease settings. The potential role of, and mechanisms responsible for, beat-to-beat alternations in the amplitude of systolic Ca(2+) transients (Ca(2+) alternans) was investigated in a canine post-myocardial infarction (MI) model of sudden cardiac death (SCD).

METHODS AND RESULTS

Post-MI dogs had preserved left ventricular (LV) function and susceptibility to ventricular fibrillation (VF) during exercise. LV wedge preparations from VF dogs were more susceptible to action potential (AP) alternans and the frequency-dependence of Ca(2+) alternans was shifted towards slower rates in myocytes isolated from VF dogs relative to controls. In both groups of cells, cytosolic Ca(2+) transients ([Ca(2+)](c)) alternated in phase with changes in diastolic Ca(2+) in sarcoplasmic reticulum ([Ca(2+)](SR)), but the dependence of [Ca(2+)](c) amplitude on [Ca(2+)](SR) was steeper in VF cells. Abnormal ryanodine receptor (RyR) function in VF cells was indicated by increased fractional Ca(2+) release for a given amplitude of Ca(2+) current and elevated diastolic RyR-mediated SR Ca(2+) leak. SR Ca(2+) uptake activity did not differ between VF and control cells. VF myocytes had an increased rate of reactive oxygen species production and increased RyR oxidation. Treatment of VF myocytes with reducing agents normalized parameters of Ca(2+) handling and shifted the threshold of Ca(2+) alternans to higher frequencies.

CONCLUSION

Redox modulation of RyRs promotes generation of Ca(2+) alternans by enhancing the steepness of the Ca(2+) release-load relationship and thereby providing a substrate for post-MI arrhythmias.

Sudden cardiac death: The larger problem... The larger genome

This perspective considers progress in understanding how genetic influences modulate susceptibility to lethal ventricular arrhythmias in cardiac patients and the population at large, as opposed to those with rare inherited arrhythmic conditions, such as the Long-QT and Brugada syndromes. It addresses largely unresolved issues, such as how important these effects may be and what we know of underlying mediators and pathways. Attention is given to newly revealed mechanisms of genomic function and the problem of identifying new susceptibility genes and targets useful in developing improved strategies for sudden death prevention.

Sudden death after myocardial infarction

CONTEXT

Sudden cardiac death after myocardial infarction (MI) has not been assessed recently in the community. Risk stratification for sudden cardiac death after MI commonly relies on baseline characteristics and little is known about the relationship between recurrent ischemia or heart failure and sudden cardiac death.

OBJECTIVE

To evaluate the risk of sudden cardiac death after MI and the impact of recurrent ischemia and heart failure on sudden cardiac death.

DESIGN, SETTING, AND PARTICIPANTS

Population-based surveillance study of 2997 residents (mean [SD] age, 67 [14] years; 59% were men) experiencing an MI in Olmsted County, Minnesota, between 1979 and 2005, and followed up through February 29, 2008.

MAIN OUTCOME MEASURES

Sudden cardiac death defined as out-of-hospital death due to coronary disease; and observed survival free of sudden cardiac death compared with that expected in Olmsted County, Minnesota.

RESULTS

During a median follow-up of 4.7 years (25th-75th percentile, 1.6-7.1 years), 1160 deaths occurred, 282 from sudden cardiac death (24%). The 30-day cumulative incidence of sudden cardiac death was 1.2% (95% confidence interval [CI], 0.8%-1.6%). Thereafter, the rate of sudden cardiac death was constant at 1.2% per year yielding a 5-year cumulative incidence of 6.9% (95% CI, 5.9%-7.9%). The 30-day incidence of sudden cardiac death was 4-fold higher than expected (standardized mortality ratio, 4.2; 95% CI, 2.9-5.8). The risk of sudden cardiac death has declined significantly over time (hazard ratio [HR], 0.62 [95% CI, 0.44-0.88] for MIs that occurred between 1997 and 2005 compared with between 1979 and 1987; P = .03). The recurrent events of ischemia (n = 842), heart failure (n = 365), or both (n = 873) occurred in 2080 patients. After adjustment for baseline characteristics, recurrent ischemia was not associated with sudden cardiac death (HR, 1.26 [95% CI, 0.96-1.65]; P = .09), while heart failure markedly increased the risk of sudden cardiac death (HR, 4.20 [95% CI, 3.10-5.69]; P < .001).

CONCLUSIONS

The risk of sudden cardiac death following MI in community practice has declined significantly over the past 30 years. Sudden cardiac death is independently associated with heart failure but not with recurrent ischemia.

123 I-mIBG scintigraphy to predict inducibility of ventricular arrhythmias on cardiac electrophysiology testing: a prospective multicenter pilot study

BACKGROUND

Disturbances of autonomic function after infarction are associated with both total mortality and sudden death. Although many imaging techniques for assessing the cardiac autonomic nervous system have been studied, the clinical usefulness of these techniques remains uncertain. This exploratory pilot study examined the relationship between abnormalities of ventricular sympathetic innervation delineated by scintigraphic imaging with (123)I-mIBG and inducible ventricular tachyarrhythmias in patients with left ventricular dysfunction and previous myocardial infarction.

METHODS AND RESULTS

Fifty patients underwent electrophysiological (EP) testing and 15-minute and 4-hour planar and single photon emission computed tomography (SPECT) imaging with (123)I-mIBG and SPECT imaging with (99m)Tc-tetrofosmin. The primary efficacy variables were the 4-hour heart:mediastinum ratio (H/M) and the (123)I-mIBG/(99m)Tc-tetrofosmin SPECT mismatch score. EP studies were categorized as positive (EP(+)) or negative (EP(-)) for inducibility of sustained (>30 seconds) ventricular tachyarrhythmias. Thirty patients were EP(+), and 20 were EP(-). There were no significant differences in the 4-hour H/M ratios or (123)I-mIBG/(99m)Tc-tetrofosmin SPECT mismatch scores between the two groups. In a multivariable analysis using all (123)I-mIBG and (99m)Tc-tetrofosmin SPECT measurements, the only variable that showed a significant difference between EP(+) and EP(-) patients was the 4-hour (123)I-mIBG SPECT defect score. A 4-hour (123)I-mIBG SPECT defect score of > or =37 yielded a sensitivity of 77% and specificity of 75% for predicting EP results.

CONCLUSIONS

The standard indices of (123)I-mIBG imaging (H/M and innervation-perfusion mismatch score) are not predictive of EP test results. The association of (123)I-mIBG SPECT defect severity with EP test inducibility in this exploratory study will require confirmation in a larger cohort of patients.

Sudden cardiac death in patients with stable coronary artery disease and preserved left ventricular systolic function

Although sudden cardiac death (SCD) has been extensively studied in patients with coronary artery disease (CAD) and low ejection fraction, prediction of SCD among individuals with preserved left ventricular systolic function is less well understood. We randomized 8,290 patients with stable CAD with preserved left ventricular systolic function to trandolapril or placebo in a secondary coronary prevention trial, and we used Cox proportional hazards models to identify independent baseline predictors of SCD during 4.8 year follow-up (median). Using a risk scoring algorithm based on simple clinical characteristics, we were able to distinguish individuals at higher risk for SCD. Independent determinants of SCD included age (p <0.001), current angina pectoris (p = 0.002), ejection fraction >40% to <50% (as opposed to >50%) (p <0.001), and diuretic (p <0.001) and digitalis use (p <0.001). Negative predictors included having prior coronary revascularization (p = 0.01) and being female (p = 0.02) or Caucasian (p = 0.006). Trandolapril neither increased nor decreased SCD. Thus, among patients with stable CAD with preserved left ventricular systolic function receiving current standard-of-care including coronary revascularization, clinical characteristics can identify individuals at higher risk for SCD.

Electrotonic remodeling following myocardial infarction in dogs susceptible and resistant to sudden cardiac death

Passive electrical remodeling following myocardial infarction (MI) is well established. These changes can alter electrotonic loading and trigger the remodeling of repolarization currents, a potential mechanism for ventricular fibrillation (VF). However, little is known about the role of passive electrical markers as tools to identify VF susceptibility post-MI. This study investigated electrotonic remodeling in the post-MI ventricle, as measured by myocardial electrical impedance (MEI), in animals prone to and resistant to VF. MI was induced in dogs by a two-stage left anterior descending (LAD) coronary artery ligation. Before infarction, MEI electrodes were placed in remote (left circumflex, LCX) and infarcted (LAD) myocardium. MEI was measured in awake animals 1, 2, 7, and 21 days post-MI. Subsequently, VF susceptibility was tested by a 2-min LCX occlusion during exercise; 12 animals developed VF (susceptible, S) and 12 did not (resistant, R). The healing infarct had lower MEI than the normal myocardium. This difference was stable by day 2 post-MI (287 +/- 32 Omega vs. 425 +/- 62 Omega, P < 0.05). Significant differences were observed between resistant and susceptible animals 7 days post-MI; susceptible dogs had a wider electrotonic gradient between remote and infarcted myocardium (R: 89 +/- 60 Omega vs. S: 180 +/- 37 Omega). This difference increased over time in susceptible animals (252 +/- 53 Omega at 21 days) due to post-MI impedance changes on the remote myocardium. These data suggest that early electrotonic changes post-MI could be used to assess later arrhythmia susceptibility. In addition, passive-electrical changes could be a mechanism driving active-electrical remodeling post-MI, thereby facilitating the induction of arrhythmias.