Observations on the electrical activity of the snake heart

Summary o 1. Body surface electrocardiograms have been recorded in the anesthetized and unanesthetized snake from leads approximating the conventions of limb leads. 2. They shown evidences of a sequence of activity in this form: from a pacemaker in the sinus venous to the atria and to the ventricles, by an electrocardiogram including an SV complex preceding the P wave. 3. The ventricular T wave in the snake is generally in the same direction as the major deflection R; Q and S are poorly developed. The T wave varies greatly in amplitude and may become completely inverted without apparent change in the position of the leads relative to the heart. 4. Direct leads and myograms from the individual chambers of the snake heart confirm the designations applied to the body surface electrocardiogram. 5. T waves have been detected following the P wave as well as the SV wave in direct leads, confirming the potentially biphasic nature of all the complexes of the electrocardiogram. 6. Studies of the duration of electrical systole related to the cardiac cycle yield ratios of 0.20, 0.40, and 0.60 for the sinus venosus, atria, and ventricle in the same heart and in comparable circumstances. They suggest a growing preponderance of the contractile element of ventricular systole. 7. As the heart deteriorates and dies the relative and absolute amplitude of the SV complex recorded from direct leads increases remarkably. 8. Spontaneous premature sinus waves and sino-atrial blocks appear as clear-cut electrographic entities.