Study ofcardiac rhythm inhealthy newborn infants

were recorded inthefirst 10daysoflife on 134 healthy full-term infants withbirthweights greaterthan2-5kg.Thehighest heart ratea minute, measured overninebeats, was 175±19(SD). Thelowest rates,measured overthree, five, andnine beats were 82± 12,87+12,and93±12,respectively. Attheir lowest rates109infants hadsinus bradycardia and25hadjunctional escaperhythms. Thirty-three infants showedchanges inP wave configuration with orwithout pronounced variation inPRinterval. Atrial prematurebeats were present in19infants butonly onehadmore than12perhour. Inarandomly selected subgroup of71infants sinus pauseswere found in51(72%). Five(7%) hadelectrocardiographic patterns andrhythm disturbances that could notbedifferentiated fromthose previously described as complete sinuatrial exitblock or sinus arrest, eight (11%)hadpatterns indistinguisable from2:1sinuatrial block, and23(32%)hadpatterns indistinguishable fromsinuatrial Wenckebach block. Thisstudy shows that normal infants havevariations inheart rateandrhythm hitherto considered tobeabnormal.