ACTIVATION AND REPOLARIZATION PATTERNS ARE GOVERNED BY DIFFERENT STRUCTURAL CHARACTERISTICS OF VENTRICULAR MYOCARDIUM - EXPERIMENTAL-STUDY WITH VOLTAGE-SENSITIVE DYES AND NUMERICAL SIMULATIONS

Activation and Repolarization Patterns. Introduction: Substantial prog ress has been made in our understanding of transmural activation acros s ventricular muscle through studies of excitation patterns and potent ial distributions. In contrast, repolarization sequences are poorly un derstood because of experimental difficulties in mapping action potent ial durations (APDs) using extracellular electrodes. Methods and Resul ts: Langendorff-perfused guinea pig hearts and isolated coronary-perfu sed left ventricular sheet preparations were stained with the voltage- sensitive dye RH-421 and optical APs were recorded with a photodiode a rray. Epicardial maps were constructed using a triangulation method ap plied to matrices of activation and repolarization times determined fr om (dF/dt)(max) and (d(2)F/dt(2))(max), respectively. Numerical simula tions were carried out based on: (1) a modified Luo-Rudy model; (2) th e three-dimensional architecture of ventricular fibers; and (3) the in trinsic spatial distribution of APDs. In ventricular sheets, epicardia l stimulation elicited elliptical activation patterns with the major a xis aligned with the longitudinal axis of epicardial fibers. When the pacing electrode was progressively inserted from epicardium to endocar dium, the major axes rotated gradually, clockwise by 45 degrees, and t he eccentricity decreased from 2 to 1.14. Repolarization showed a rela tively uniform pattern, independent of pacing site, beginning at the a pex and spreading to the base. Conclusion: In experiments and simulati ons, the helical rotation of epicardial excitation isochrones caused b y pacing at increasing depth in the myocardium correlated with the hel ical three-dimensional architecture of ventricular fibers. In contrast , repolarization was independent of the activation sequence and was ma inly guided by spatial differences in APDs between apex and base.