Ventricular fibrillation complicating myocardial infarction

Effects of reperfusion on arrhythmias and death after coronary artery occlusion in the rat: increased electrical stability independent of myocardial salvage

OBJECTIVES

This study sought to delineate salvage-dependent from salvage-independent coronary reperfusion in acute myocardial infarction and the effects on spontaneously occurring arrhythmias and arrhythmic death in rats.

BACKGROUND

Reperfusion of the infarct-related artery might increase electrical stability independently of salvage of ischemic myocardium.

METHODS

In 98 conscious rats the electrocardiogram was monitored by telemetry for 48 h after MI, and all episodes of ventricular tachycardia (VT) and ventricular fibrillation (VF) were analyzed. Reperfusion at 45 min (RP45) (n = 15), 90 min (RP90) (n = 18) and 180 min (RP180) (n = 30) min was compared with permanent coronary artery occlusion (CAO) (n = 35) with respect to the post-reperfusion periods.

RESULTS

RP45, RP90 and RP180 reduced the incidence of VT by 93%, 98% and 88% and VF by 89%, 97% and 92%, respectively (all p < 0.01 vs. CAO). The all-cause mortality rate was reduced from 47% (CAO) to 8% (RP45, p < 0.05) and 0% (RP90, p < 0.01); after RP180 it was 17% (CAO 42%, p = 0.08). All reperfusion regimens reduced arrhythmic deaths: 47% to 8% (RP45, p < 0.05), 47% to 0% (RP90, p < 0.01) and 42% to 8% (RP180, p < 0.05). Infarct size was identical to that during CAO (49 +/- 10% [mean +/- SD]) and RP180 (49 +/- 10%), whereas preferentially epicardial salvage occurred at RP45 (36 +/- 8%, p < 0.001) and RP90 (38 < 10%, p < 0.001).

CONCLUSIONS

Early and late reperfusion reduce the incidence and duration of VT and VF in conscious rats with acute MI. Thereby, arrhythmia-related mortality is improved through the prevention of fatal VF episodes. Thus, reperfusion increases the electrical stability of the heart independently of myocyte salvage, as proposed by the open artery hypothesis.

Lidocaine prophylaxis for fatal ventricular arrhythmias after acute myocardial infarction

OBJECTIVE

To compare the efficacy and safety of a 40-hour lidocaine infusion after completion of a 8-hour open-label infusion for prophylaxis of primary ventricular fibrillation in patients with uncomplicated acute myocardial infarction.

METHODS

This was a double-blind, randomized placebo-controlled trial held in the coronary care unit of a large nonprofit hospital. We studied 200 patients with uncomplicated acute myocardial infarction in Killip class I or II who came to the hospital within 6 hours of onset of symptoms and 22 patients who had ventricular fibrillation before the start of the study. Intervention consisted of an 8-hour lidocaine infusion followed by placebo or lidocaine for an additional 40 hours. The infusion rate was adjusted in patients > or = 70 years old and in those < 50 kg or > or = 90 kg. Measurements recorded were baseline demographic characteristics, incidence of ventricular arrhythmias, adverse reactions, and death.

RESULTS

New congestive heart failure developed during the randomized phase in 9% of patients receiving lidocaine and in 2% of patients receiving placebo (p = 0.03). Ventricular fibrillation did not occur during the treatment period, and sustained ventricular tachycardia developed in one patient receiving placebo. The in-hospital mortality rate was comparable in both groups (4% versus 2%; p = 0.68) but was much higher (13.6%) in patients with initial ventricular fibrillation not included in the randomized study.

CONCLUSIONS

A 40-hour age- and weight-adjusted lidocaine infusion administered after an initial 8-hour infusion provoked more congestive heart failure than placebo. In view of the absence of ventricular fibrillation episodes with both infusions, caution should be used when lidocaine is administered for longer than 8 hours in patients with uncomplicated myocardial infarction.

Effects of thrombolytic therapy administered 6 to 24 hours after myocardial infarction on the signal-averaged ECG. Results of a multicenter randomized trial. LATE Ancillary Study Investigators. Late Assessment of Thrombolytic Efficacy

BACKGROUND

Thrombolytic therapy reduces mortality after acute myocardial infarction, even when treatment is initiated relatively late after onset of symptoms. The mechanism underlying this survival benefit is incompletely understood.

METHODS AND RESULTS

In a prospectively designed ancillary study of a randomized, placebo-controlled trial of late thrombolytic therapy (LATE), the signal-averaged (SA) ECG was recorded before hospital discharge in an effort to assess the effect of thrombolytic therapy on arrhythmia substrate. Three hundred ten patients were enrolled at 23 participating sites; 160 patients received placebo, and 150 patients received recombinant tissue-type plasminogen activator (rTPA) therapy 6 to 24 hours after onset of symptoms. Compared with placebo, rTPA tended to reduce the frequency of SAECG abnormality (filtered QRS duration > 120 milliseconds) by 37% (95% CI, -64%, +6%; P = .087) and the filtered QRS duration (105.7 +/- 13.8 versus 108.8 +/- 14.6 milliseconds, P = .05). In the prespecified subgroup of 185 patients with ST elevation on the qualifying ECG, rTPA resulted in a 52% reduction (95% CI, 4% to 77%, P = .011) of SAECG abnormality and a shorter filtered QRS duration (105.7 +/- 10.9 versus 110.7 +/- 15.9 milliseconds, P = .01). No benefit was seen in patients without ST elevation on ECG.

CONCLUSIONS

Late thrombolytic therapy produced a more stable electrical substrate, which probably represents an important mechanism of mortality benefit.

The role of thrombolytic therapy in patients with acute myocardial infarction presenting later than six hours after the onset of symptoms

Thrombolytic therapy clearly reduces mortality in patients with acute myocardial infarction, especially when initiated within 6 hours of onset of symptoms. Some studies have also suggested that thrombolytic therapy may improve survival even when initiated 6-24 hours after the onset of symptoms by mechanisms other than infarct size limitation, such as reduced expansion, reduced electrical instability, and improved healing of the infarct. However, in view of the possibility that late thrombolytic therapy may also be associated with an increased risk of cardiac rupture, the risk-benefit ratio needs to be more clearly defined. Ongoing randomized trials are expected to clarify the situation in the near future. In the meantime, efforts to initiate reperfusion as soon after the onset of myocardial infarction as possible should continue, since early treatment is still the best treatment.

Decreased incidence of ventricular late potentials after successful thrombolytic therapy for acute myocardial infarction

In some patients with acute myocardial infarction, low-amplitude potentials that prolong the QRS complex, termed "late potentials," can be recorded on a signal-averaged electrocardiogram. The presence of these late potentials is known to be associated with an increase in the risk of ventricular tachycardia and sudden death. Because patients with acute myocardial infarction who receive thrombolytic therapy have a reduced incidence of ventricular tachyarrhythmia and sudden death, we sought to determine whether such patients also have a decreased incidence of late potentials. We studied 106 patients less than 75 years of age who were admitted with a first myocardial infarction and in whom a signal-averaged electrocardiogram was recorded within 48 hours of admission. Within four hours of the onset of chest pain, tissue plasminogen activator (t-PA) was given to 44 patients, and 62 were treated conventionally. In the t-PA group, late potentials were recorded in 2 of 44 patients (5 percent), as compared with 14 of 62 (23 percent) in the conventionally treated group (P = 0.01). Furthermore, among the patients treated with t-PA, continued occlusion of the infarct-related artery was related to the presence of late potentials. In the t-PA group, late potentials were recorded within 24 hours of angiography in 2 of the 6 patients with an occluded infarct-related artery, as compared with none of the 38 patients with a patient infarct-related artery. Our data suggest that successful thrombolytic therapy is associated with a marked reduction in the incidence of late potentials on the signal-averaged electrocardiogram. Long-term follow-up will be required to determine whether this finding predicts a reduced incidence of subsequent ventricular tachyarrhythmia and sudden death.

One-year prognosis of primary ventricular fibrillation complicating acute myocardial infarction. The GISSI (Gruppo Italiano per lo Studio della Streptochinasi nell'Infarto miocardico) investigators

The 1-year prognosis of 293 patients discharged alive from the hospital after acute myocardial infarction (AMI), who experienced primary ventricular fibrillation (VF) in the acute phase, was compared with that of a reference group of 6,337 patients identified from the same population included in the Gruppo Italiano per lo Studio della Streptochinasi nell'Infarto miocardico (GISSI) trial. There was no difference in the 6- and 12-month mortality between the patients with primary VF and the reference group (3.7 vs 2.7% and 4.1 vs 4.2%, respectively). Survival of the 2 groups was also similar when patients were stratified according to infarct site (anterior and posterior), and whether or not they received treatment with streptokinase during AMI. Thus, long-term mortality of patients discharged alive after AMI complicated by primary VF is low and is not influenced by previous fibrinolytic therapy or by infarct site. The excess mortality of patients with primary VF is confined to the hospital phase, after which survivors represent a low-risk subgroup.

Thrombolytic reperfusion of acute myocardial infarction: resolved and unresolved issues

Intensive clinical investigation has led to the acceptance of many fundamental aspects of reperfusion therapy, especially its salutary effects on improved survival, left ventricular function and reduced infarct size. However, many unresolved issues are currently being addressed or will be the focus of future clinical trials. These include patient selection with respect to risk profile and time of presentation, the choice of a thrombolytic agent, the role of adjunctive therapies and the optimal use and timing of follow-up coronary revascularization procedures. The rapidly evolving status of myocardial reperfusion therapy is reviewed, with specific attention to these important yet unresolved aspects.