Editor,—Yamagishi et al, studying 37 patients within seven weeks of Q wave myocardial infarction (MI), found that exercise induced ST segment elevation was closely associated with the presence of viable myocardium in the infarct territory.1 We also studied this in patients with previous MI and agree with the results2 ,3; however, viable myocardium may persist for a long time after an MI,4 and in these cases ST segment shift is not considered a specific indicator of transmural ischaemia and viability.

To increase the specificity of this sign in patients with an old (> 6 months) MI, we introduced an unconventional, but experimentally validated,5 ECG marker of transmural ischaemia—the stress induced shortening of QTc interval (QT interval corrected for heart rate using Bazett's formula) in Q wave leads—to identify hibernating myocardium in the infarct zone. Experimental studies demonstrated an increase in cellular K⁺ efflux at the onset of myocardial ischaemia accompanied by a progressive shortening of the action potential duration.5

We evaluated 15 …