M-mode ultrasonic assessment of equine fetal heart rate

Embryonic heart rate was studied during 87 examinations in 56 mares on Days 25 to 174 post ovulation. Real-time B-mode using a stopwatch and M-mode ultrasonography were compared. For analyses of heart rates, data were grouped into weeks or months. For the stopwatch and M-mode techniques, respectively, heart rates averaged 123 and 133 beats per minute during Month 1, peaked during Month 3 at 172 or 196 beats, then gradually decreased over the duration of the study to 126 and 130 beats during Month 6. There was a significant increase in heart rate from Months 1 to 2 and Months 2 to 3 and a significant decrease from Months 3 to 4 and Months 5 to 6 for both techniques. There was a tendency for a difference (P<0.08) between techniques during Month 1 and a significant difference between techniques for each of Months 2 and 3. The lower heart rate values for the stopwatch technique during Months 1, 2 and 3 may have reflected the difficulties associated with maintaining the transducer field on the fetal heart continuously for an adequate period of time (Months 1 and 2) and difficulty counting fast enough to keep up with the fetal heart (Months 2 and 3). When data were grouped weekly, the largest increase in fetal heart rate occurred between Weeks 4 and 5.