P-R interval of the rat electrocardiogram

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Abstract
The variations observed in the configuration, duration, and amplitude of the P and Tp waves in the frontal plane leads of the rat ecg are presented. The prominent appearance of the Tp wave within the P-R segment appears to be due, in part, to the relatively high cardiac rate, the brief total duration of the activation and recovery phases of the rat atria, and a length of the P-R segment that is approximately double that of the P wave. The P-QRS-T deflections and the intervening segments in the rat ecg usually do not share a common baseline. Any given complex or wave may terminate at a level different from that at which it originated. Reasons are given for the selection of the termination point of the P-R interval as the best isoelectric reference level. Atrio-ventricular block following induction of ether anesthesia, interatrial block due to hyperkalemia, and manifestations of diaphragmatic activity in the rat ecg, are illustrated and discussed.