Hyman1 has secured phonocardiograms of three types of fetal arrhythmia, namely, sinus arrhythmia, ventricular extrasystole and auricular fibrillation. Sinus arrhythmia is characterized by frequent slowing and speeding of the heart rate without the superimposition of additional beats or the elimination of expected beats. It should be easily recognized. The question of the cause of this phenomenon is of interest. In children one type of sinus arrhythmia is referred to as respiratory arrhythmia because the periods of rapidity and slowing of the heart beat usually conform to expiration and inspiration. While fetuses do exhibit on occasion what have been described as respiratory movements, such movements are minimal except in critical oxygen deprivation (Barcroft2) and would hardly, at their rate of 45 to 60 per minute, be responsible for the much less frequent alterations in heart rate shown in Hyman's tracings. In short, such arrhythmia is not the usual respiratory
SONTAG LW, NEWBERY H. INCIDENCE AND NATURE OF FETAL ARRHYTHMIAS. Am J Dis Child. 1941;62(5):991–999. doi:10.1001/archpedi.1941.02000170085007
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