Ventricular epicardial activation sequence in the lamb

The ventricular epicardial activation sequence (VEAS) in 15 anesthetized lambs (near term fetus to 3.5 months of age) was determined using 40 simultaneously recorded bipolar electrograms for each animal. Isochrone maps were drawn by hand using relative activation times determined from the maximum first derivative of multiplexed signals recorded and analyzed by computer from single cardiac cycles. VEAS was similar for all ages studied. Earliest left ventricular (LV) activity appeared on the caudal dorsal and/or ventral free wall. Initial right ventricular (RV) activation appeared on the ventral anterior and/or lateral surface either simultaneously with or slightly later (2.5-3.5 msec) than initial LV activation. Excitation then proceeded circumferentially and in an apicobasilar direction and terminated on the right ventricular outflow tract.(RVO). All LV and RV epicardium except RVO activated within 7.5-12.5 msec. RVO required more than 1n.5 msec to activate in 12 of 15 animals. Total duration of RVO activation never exceeded 22.5 msec in any animal. Neither the duration nor the pattern of activation of LV or RV epicardium, including RVO, changed in a consistent fashion with age. These findings are similar to the known VEAS for adult ruminants. The data indicate that the VEAS assumes the adult pattern as of late gestation and suggest that changing right and left ventricular electromechanical events do not contribute to and probably are not affected by the process of ventricular epicardial activation. Speculation Although maturational changes in right and left ventricular electromechanical events do not appear to be related to the sequence of ventricular epicardial activation, such changes may be related to the distrubution of corresponding epicardial and intramural isopotentials.