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Electrophysiological and histological observations were made on the heart of the African lungfish. Impulse origin and propagation were studied using simultaneously recorded epicardial and pericardial electrograms. The primary pacemaker site in the lungfish was found to be at the sinus venosus at its junction with the left cardinal vein. Under a variety of circumstances, pacemaker function shifted to other sites. In response to stress, probably under vagal influence, the regular and rapid sinus venosus rate was generally superseded by an irregular and slower atrial pacemaker. Heart rate and sinoatrial and atrioventricular conduction times varied with changes in temperature [Q10=3.77, 2.55, and 5.46, respectively]. Although alterations in impulse formation and conduction did occur, the site of impulse formation and the patterns of conduction between heart chambers were usually fixed, implying the existence of an organized conduction system. Nonetheless, extensive histological study failed to disclose either organized nodal structures or specialized conduction pathways.

1. Electrocardiographic and detailed histological studies are presented of a case of ventricular pre-excitation with progressive A-V and intraventricular block terminating in complete A-V dissociation. 2. Whereas the demonstration of a tract which leads from the atrial septal musculature to the A-V bundle bypassing the A-V node explains the short P-R interval, the delta wave is accounted for by the demonstration of unusually copious Mahaim fibers from the A-V bundle to the posterior portion of the muscular ventricular septum. 3. Thus, this appears to be the first demonstration of Mahaim fibers responsible for A-V transmission in the pre-excitation syndrome. 4. The correlation of the data permitted reconstruction of anatomic changes preceding, and responsible for, the gradual progression of the atrioventricular and intraventricular conduction disorder. 5. On the basis of this correlation it would appear that the two characteristic criteria of ventricular pre-excitation, shortened P-R and the delta wave, may in some cases have a different anatomic background. A separation of these parameters permits an explanation of certain atypical and puzzling aspects encountered in the syndrome of ventricular pre-excitation and related conditions.