QRS and T-wave alternans in acute coronary syndrome: Do they provide complementary insights?

We greatly appreciate Dr Selvaraj's remarks on our recent study demonstrating the usefulness of continuous T-wave alternans (TWA) monitoring in ultra-short-term prediction of impending life-threatening ventricular tachyarrhythmia (VTA) on emergent reperfusion in acute coronary syndrome (ACS) patients.1 As he pointed out, our sample size was small despite a relatively long enrolment period of 2 years and 5 months. Indeed, our hospital treated 95 ACS patients (68 with ST-segment elevation myocardial infarction, 14 with non-ST-segment elevation myocardial infarction, and 13 with unstable angina) undergoing emergent reperfusion therapy in this period. However, the monitoring device was not available in 22 patients because the device was already occupied or the person in charge of the study was absent during the patients’ emergency room visit. Ten patients declined to participate in the study. In addition, nine patients with atrial fibrillation, 15 with high-grade atrioventricular block or pacemaker rhythm, 15 with cardiogenic shock on arrival, and four with pre-procedural thrombolysis in myocardial infarction flow grade 3 were excluded. Ultimately, 20 consecutive patients were enrolled. Although our ethics committee did not permit the enrolment of critically ill patients with cardiogenic shock, it will be worthwhile to evaluate whether TWA monitoring is also useful in patients with haemodynamic instability. We agree with Dr Selvaraj that a component of TWA observed in the setting of acute myocardial ischaemia is secondary to depolarization alternans. It is intriguing that the patient with both visible QRS-wave alternans and TWA finally developed monomorphic ventricular tachycardia (VT), which may have resulted from depolarization (conduction) abnormality. On the other hand, two patients with TWA but without visible QRS-wave alternans developed ventricular fibrillation (VF), which resulted from repolarization abnormality. Furthermore, early afterdepolarizations (EADs) have been implicated in Torsades de Pointes or some episodes of monomorphic VT. And 2:1 conduction of EADs can lead to alternating changes in the amplitude of the T-wave in the form of TWA.2 These considerations along with our findings indicate that it may be feasible to detect risk not only for VF but also for some types of monomorphic VT by TWA monitoring in this setting. Patients with cardiomyopathy such as were enrolled in the study by Selvaraj et al.3 may have more widespread myocardial damage than ACS patients, who have localized myocardial ischaemia while the rest of the myocardium is normal. Therefore, TWA may be more narrowly distributed in patients with ACS than in those with cardiomyopathy. For this reason, we agree that there is value in optimizing detection of maximum TWA using multichannel unipolar precordial recordings in ACS patients. However, there is a practical trade-off in the number of leads that can be employed in the setting of emergent reperfusion therapy to avoid impeding the procedure. The use of radiopaque electrodes reduces concerns regarding visibility. We concur that it is important to obtain unipolar precordial recordings from leads that overlie the ischaemic area. Finally, this emerging evidence suggests that TWA monitoring deserves further evaluation as an independent risk marker of life-threatening VTA in ACS patients in future large-scale studies. Conflict of interest: R.L.V. is co-inventor of the Modified Moving Average method for T-wave alternans analysis with patent assigned to Beth Israel Deaconess Medical Center and licensed by GE Healthcare, Inc., and Medtronic, Inc.