Electrocardiographic estimates of regional action potential durations and repolarization time subintervals reveal ischemia-induced abnormalities in acute coronary syndrome not evident from global QT.

Abstract: We evaluated electrocardiogram estimates of repolarization times (RTs) and action potential durations (APD) separately for initial and terminal repolarization periods in a reference group of 5376 healthy men and women and in 125 acute coronary syndrome patients with and 657 without diagnostic ST elevation (ST-elevation myocardial infarction [STEMI] and non-STEMI [NSTEMI], respectively). Two key covariates in the model are the rate-adjusted QT peak interval (QT_pa), assigned to earliest epicardial RT (RT_epi), and (T_p-T_xd), the rate-invariant interval from T_p to the inflection point (T_xd) at T wave downstroke. (T_p-T_xd) defines the crossmural RT gradient (XMRT_grad). Transmural RT_grad (TMRT_grad) is obtained as CosΘ(R_max|T_max)*XMRT_grad, where Θ is the spatial angle between the maximal QRS and T vectors. Derived endocardial variables are the XMRT_endo, equal to QT_pa + XMRT_grad and TMRT_endo, equal to QT_pa + TMRT_grad. Noting that excitation time (ET) and RT define APD, APD_epi = RT_epi - QR_p in V6 and TMAPD_endo = TMRT_endo - 10 milliseconds. Compared to the reference group, the estimates for APD_epi and TMAPD_endo were shortened in STEMI by 20 and 31 milliseconds, respectively, (p <0.001 for both) signifying transmural ischemia. In contrast, in NSTEMI, TMAPD_endo was shortened by 28 milliseconds (P <0.001) with a lesser, 5 millisecond shortening of APD_epi, signifying subendocardial ischemia. QT was prolonged by 6 milliseconds in STEMI (P <0.05) and by 8 milliseconds in NSTEMI (P <0.001). Prolonged QT with shortened APD_epi suggests that prolonged repolarization in terminal possibly non-ischemic regions accounts for QT prolongation in both myocardial infarction groups. These substantial differences in ischemia-induced regional manifestations of repolarization abnormalities revealed by the repolarization model were not evident from evaluation of the global QT. [Copyright &y& Elsevier]