Validation of a noninvasive measure of local myocardial repolarization in a conscious human model: Adaptation of repolarization to changes in rate

Rate Adaptation of Myocardial Repolarization. Introduction: A commercial pa cemaker sensor measure of the unipolar endocardial stimulus to T wave inter val may accurately reflect changes in the monophasic action potential durat ion at 90% repolarization (APD(90)). This sensor system was used to study t he kinetics of adaptation of repolarization duration to changes in heart ra te in humans. Methods and Results: Patients were studied using an external pacemaker capa ble of displaying all stimulus to T wave intervals for each paced beat. Rig ht ventricular stimulation was delivered via the pacemaker and compared sim ultaneously to APD(90). Steady-state pacing was simulated by 60 seconds of pacing at cycle lengths (CLs) 350 to 700 msec, Adaptation to a new ventricu lar rate was analyzed with a sudden %00-msec decrease in CL, The relation b etween repolarization measure and steady-state CL (n = 16) was linear with a slope of 0.16 and 0.19 for APD(90) and stimulus to T wave interval, respe ctively (P = NS), The adaptation of both repolarization measures to a sudde n change in rate were best modeled by a biexponential function. Stimulus to T wave interval exhibited a parallel course to APD(90), and an analysis of normalized differences between APD(90) and stimulus to T wave interval fol lowed an approximately normal distribution, with 93.5% of the paired differ ences within 2 SID of the mean. Conclusion: A pacemaker sensor measure of stimulus to T wave interval accur ately parallels APD(90) during both steady-state and sudden changes in rate . Repolarization in human endocardium follows a linear relation to steady-s tate CL and adapts to a new rate with a biexponential function. This model represents a novel method for studying human cardiac repolarization.