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Notes: Effect of Sustained Load on EP Parameters. Introduction: It is well known that myocardial stretch can elicit ventricular arrhythmias in experimental models. However, previous reports have predominantly documented stretch-induced arrhythmias during short, pulsatile stretch. The arrhythmogenic mechanism of sustained static stretch is incompletely understood. Methods and Results: To examine the influence of sustained load on several electrophysiologic parameters, a latex balloon was placed into the left ventricle of ten isolated Langendorffperfused rabbit hearts and filled with a neutral volume of fluid. The heart was paced from a catheter inside the right ventricle (apicoseptal endocardial position), and the following parameters were studied during steady-state pacing with a cycle length of 500 msec (S1) and during extrastimulation (S2, base drive of 8 beats): monophasic action potential (MAP) durations at 90% repolarization (APD90) from 5 to 6 epicardial electrodes located on both ventricles and one right ventricular endocardial contact electrode; dispersion of APD90 (range of MAP durations from all electrodes); effective refractory period (ERP) and longest activation time (pacing stimulus to MAP upstroke). After baseline recordings, the balloon inside the left ventricle was filled with a volume of 1.0 mL of fluid by means of a servo-controlled pump. The ERP was significantly shortened from 198 ± 9 msec at baseline to 183 ± 8 msec during sustained load (P 〈 0.03). Similarly, the average APD90 was shortened from 180 ± 5 msec at baseline to 175 ± 6 msec during sustained load (P 〈 0.006) with steady-state pacing and from 178 ± 6 msec to 170 ± 8 msec during premature extrastimulation (P 〈 0.03). At the same time, dispersion of APD90 was increased from 27 ± 5 msec to 38 ± 6 msec (P 〈 0.002) during steady-state pacing and from 28 ± 4 msec to 38 ± 6 msec (P = 0.013) during premature extrastimulation. The longest activation time among all MAP recordings was increased from 39 ± 2 msec to 43 ± 3 msec (P = 0.003) during steady-state pacing and from 56 ± 6 msec to 69 ± 6 msec during premature extrastimulation (P 〈 0.003). Conclusions: Sustained load shortens the ERP and the mean APD90 and at the same time increases dispersion of APD90 and prolongs activation times. These findings provide additional insight into the arrhythmogenic mechanisms of sustained mechanical load.