Echocardiographic study of heart rate in dog and cat fetuses in utero.

The heart rate of dog and cat fetuses in utero was studied using real-time B and M mode ultrasonography. Fifteen bitches and 15 queens were monitored from day 19 after the luteinizing hormone (LH) peak in the bitch, and from day 15 after the first mating in the cat, up to parturition, using a 7.5 MHz linear transducer. The first fetal cardiac movements and the cardiac frequency were detected from days 23 and 29 after the LH peak and from days 18 and 25 after mating, in the bitch and in the queen, respectively. In bitches, heart rates were higher [230.2 +/- 15.4 beats per minute (b.p.m.)] than values previously reported. Initial average values were 214 +/- 13.3 b.p.m. and these significantly increased thereafter, with an average of 238.2 +/- 16.1 b.p.m. at day 40. A reduction was noted near parturition. In queens, the average values recorded were 228.2 +/- 35.5 b.p.m. Contrary to what has been observed in the bitch, the cardiac frequencies remained stable during pregnancy.