Heart rate and heart rate variability in pregnant warmblood and Shetland mares as well as their fetuses

Heart rate (HR) is an important parameter of fetal well-being. In horses, HR and heart rate variability (HRV) can be determined by fetomaternal electrocardiography (ECG) from mid-pregnancy to foaling. Normal values for physiological parameters in larger breeds are often used as reference values in ponies. However, HR increases with decreasing size of the animal and in ponies is higher than in warmblood horses. It is not known if fetal HR is affected by breed and if values obtained in larger breeds can be used to assess Shetland fetuses. We have determined fetomaternal beat-to-beat (RR) interval (inversely correlated to HR) and HRV in warmblood (n = 6) and Shetland pregnancies (n = 7) at days 280 and 300 of gestation by ECG. Maternal RR interval was lower in pony than in warmblood mares (day 280: Shetland: 958 ± 110, warmblood: 1489 ± 126 ms, p < 0.01) The SDRR (standard deviation of RR interval) and the RMSSD (root mean square of successive RR differences) did not differ between breeds at any time. Also RR interval as well as HRV did not differ between warmblood and pony fetuses (RR interval day 280: Shetland: 606 ± 39, warmblood: 589 ± 38 ms). In conclusion, although maternal RR interval is clearly higher in Shetland than in warmblood mares, fetal RR interval in the two breeds is on the same level.