OBJECTIVES

We report on a previously unrecognized form of the long QT syndrome (QT interval prolongation andpause-dependent polymorphic ventricular tachycardia [VT]) entirely related to myocardial infarction (MI).

BACKGROUND

Polymorphic VT in the setting of acute MI generally occurs during the hyperacute phase, is related to ischemia, and is not associated with QT prolongation. Although QT prolongation after MI is well described, typical pause-dependent polymorphic VT (torsade de pointes) secondary to uncomplicated MI was previously unknown.

METHODS

Of 434 consecutive admissions for acute MI, 8 patients had progressive QT prolongation that led to typical torsade de pointes. None of these patients had active ischemia or other known causes of QT prolongation. These patients were compared with 100 consecutive patients with uncomplicated MI who served as controls.

RESULTS

The incidence of torsade de pointes following MI was 1.8% (95% confidence interval 0.8% to 3.6%). The QTc intervals of patients and controls were similar on admission. The QTc lengthened by day 2 in both groups, but more so in patients with torsade de pointes (from 470 ± 46 to 492 ± 57 ms [p < 0.05] and from 445 ± 58 to 558 ± 84 ms, respectively [p < 0.01]). Maximal QT prolongation and torsade de pointes occurred 3 to 11 days after infarction. Therapy included defibrillation, magnesium, lidocaine and beta-blockers. Three patients required rapid cardiac pacing. The long-term course was uneventful.

CONCLUSIONS

Infarct-related torsade de pointes is uncommon but potentially lethal. An acquired long QT syndrome should be considered in patients recovering from MI who experience polymorphic VT as specific therapeutic measures are mandatory.