Auscultatory and phonocardiographic studies on the cardiovascular system of the newborn thoroughbred foal.

In 20 newborn thoroughbred foals cardiovascular examinations were made on the heart rate and detailed auscultation of the cardiovascular murmurs, which were also recorded by phonocardiography, was conducted between birth and two weeks of age. A transient bradycardia, which was slower than the near-term fetal heart rate, was noted in neonates immediately after birth. Rapid and progressive increase in the heart rate between 5 and 40 minutes postpartum was observed together with a slower decrease after that time. Cardiovascular murmurs in newborn foals consisted of a continuous murmur arising from the ductus arteriosus and a systolic murmur auscultated over the aortic area. The continuous murmur appeared 10-15 minutes' after birth and disappeared 60-84 hours postpartum in most foals. Thus the physiologic closure of the ductus arteriosus appeared to occur around the postnatal 3rd day. The systolic murmur became auscultable at around the time of the disappearance of the continuous murmur and also could be heard for a short time soon after delivery, suggesting that this murmur may be related to a rise in systemic vascular resistance accompanied by closure of the ductus arteriosus or the umbilical vessels. Furthermore, we examined the effect of changes in the heart rate due to injection of isoproterenol, atropine and propranolol on the intensity of the systolic murmur and discussed the occurrence of the murmur in newborn foals.