Quantitative electrocardiography and vectorcardiography in postnatally developing rats. 
The aim of this study was to provide quantitative descriptions of developmental changes of the cardiac electrical activity of rats as background information useful for assessing abnormal development, for example, in spontaneously hypertensive rats.  The data are based on the findings of serial examinations of 11 animals, slightly anesthetized by ether, on whom Frank-ECGs were recorded on days 6, 14, 21, 30, 38, and 53 of life.  Typical features of the electrocardiograms during the physiologic development are P waves consisting of one positive deflection in all three leads (x, y, and z); and PQ intervals decreasing between days 6 and 14, whereas the heart rate increases during this period.  QRS vector loops and the maximum spatial vector (R) shift to the left, whereas the amplitudes of R first increase but decrease with later age.  The shape of the QRS magnitude curves is described, the peaks of which occur later during QRS in relation to increasing age.  QRS intervals are considerably shorter than in the adult even at day 53.  At this age rats have attained the adult configuration of the QRS loop.  From day 30 onward, QRS frequency merges with the T wave.  It is concluded that the ontogenetic development of the major electrical forces of the rat heart shows peculiarities compared to those seen with the development of other mammalian species.  The most probable morphologic and functional postnatal adjustments are discussed with reference to the causes of the developmental electrocardiographic changes.