Electrocardiogram and Phonogram of Adult and Newborn Mice in Normal Conditions and Under the Effect of Cooling, Hypoxia and Potassium

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Abstract
The ecg. of adult mice does not show a distinct T wave in leads from any part of the body surface, but the terminal segment of the QRS complex is notched. Slowing of the heart from about 600 to 200 beats/min., by cooling or by hypoxia, separates the notch into a distinct wave, although no definite S-T segment is produced. K admn. increases the amplitude of the notch. It is assumed that the notch represents the T wave, and that depolarization and repolarization overlap due to the short duration of excitation. The QRS duration measured from the beginning of the QRS complex to the notch (0.01 sec.) is long enough to be accounted for by muscular conduction alone, but when measured to the end of the notch it is far too long (0.08 sec.) to be accounted for on this basis. There is an approx. linear relationship between the Q-T''intervals, measured from beginning of the QRS deflection to the end of the notch, and the heart rates of newborn and adult mice, rats and guinea pigs. The values of KQ-T, calculated according to Bazett''s formula, fall far outside the range for man. Comparison of the ecg. of mice, rats, and guinea pigs shows a transition from a notched QRS to a separate T wave. The ecg. of newborn mice, taken at room temp., shows a slower heart rate and a more distinct T wave than that of adult mice. The slower heart rate is due to the lower body temp.; warming the animal increases the heart rate, and the ecg. approaches the pattern of adult mice. The first heart sound in adult mice occurs about 0.015 sec. after the beginning of the QRS complex, and the sec. just before the onset of the P wave. Decrease of the heart rate by various agents lengthens all intervals (P-R, QRS, latent period, Q-T and mechanical systole duration, measured as interval between the 1st and 2d heart sound). However, the lengthening of the mechanical systole far exceeds that of the Q-T interval.

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