Ikeda et al, AJC, 2002

This study of 850 post MI patients constitutes the largest MTWA study to date. The patients were studied an average of 2.7 months post MI. The mean LVEF was 51%. Average follow-up was 25 months. The primary end point was SCD or resuscitated VF. The secondary endpoint was sustained VT. In addition to MTWA - LVEF, NSVT, and SAECG were also measured.

TWA was the best predictor of primary and secondary endpoints. For the primary endpoint TWA has a relative risk of 11.4 a sensitivity of 92% and a negative predictive value of 99.5%. Multivariate analysis revealed that only TWA and LVEF were significant independent predictors of the primary endpoint.

The authors note the importance of not performing the MTWA test immediately post MI as was done in the Tapainen 2001 study. MTWA status is unstable during the first few weeks post MI and thus cannot be used for long term predictions.

Conclusion: This paper provides a powerful rationale for the testing of all patients with a recent MI. A positive MTWA test indicates a risk of approximately 20% for the occurrence of SCD, VF, or sustained VT over the next two years. Patients with a negative MTWA test have an extremely low event rate (~0.5% for the primary endpoint). Every post MI patient should have a MTWA test.

Kitamura et al, JACC 2002

This study is of 104 patients with nonischemic dilated cardiomyopathy. Mean LVEF 41%. Mean follow-up 21 months. The endpoint of the study was SCD, VF, or sustained VT. MTWA was a powerful predictor of events with a positive predictive value of 23.9%, negative predictive value of 97.3% and relative risk of 8.8. It was far better than LVEF, SAECG, or NSVT. The authors demonstrate that patients with an onset heart rate <= 100 bpm had a worse prognosis that those with an onset heart rate between 101 and 110. However, if you considered only patients with an onset heart rate <= 100 bpm to be positive, the negative predictive value would fall to 94.9% and the relative risk would fall to 7.4. Thus there is no reason to change the criteria for MTWA positivity for these patients.

Conclusion: MTWA is an extremely effective predictor of arrhythmic events in DCM patients. This is an important patient population because EPS is generally acknowledged as being of no value in these patients. DCM patients should undergo MTWA testing. Patients who test positive are at high risk; patients who test negative are at low risk.

Klingenheben et al, JACC 2002

This is a study of the effect of two beta blockers (metoprolol, sotalol) on pacing induced MTWA in 54 high risk patients. The authors found that at equivalent heart rates, beta blockers reduced MTWA voltage. Beta blockers converted 8/48 patients from a positive to a negative MTWA test.

The authors conclude that beta blockers tend to reduce MTWA. Beta blockers are also known to reduce SCD. The authors go on to argue that patients undergoing MTWA testing for risk stratification should be tested without changing their pharmacologic regimen (ie not witholding beta blockers or anti-arrhythmics*) and using B Rules for interpretation of exercise tests (in exercise tests B Rules allow one to classify a test as negative if the test was stopped because of patient fatigue or symptoms, a MaxHR of at least 80 bpm was achieved, and the MaxNegHR is within 5 bpm of the MaxHR). B Rules must be used if beta blockers are not with held – because many patients on beta blockers cannot achieve a heart rate of 105.

Conclusion: Beta blockers may reduce MTWA independent of their effect on heart rate.

*Sakabe et al, ANE 2001 shows that MTWA is predictive of arrhythmic events in patients tested while on anti-arrhythmic agents.