Fetal and maternal heart rate ratio in pregnant bitches: effect of gestational age and maternal bodyweight

Introduction and aim. The most widely used method to evaluate fetal health is the monitoring of fetal heart rate (FHR). In case of fetal distress a bradycardia is commonly detectable, but the relationship between FHR and maternal heart rate (MHR) has been poorly investigated (1). Pregnancy induces some hemodynamic changes and MHR tends to be higher to ensure adequate blood supply to the fetus (2). However, several factors such as maternal age, breed, bodyweight, temperament could also affect the MHR (3). The availability of reference values of the ratio FHR/MHR, could better contribute to the evaluation of the fetal health at different gestational ages than the single FHR values. For this purpose, the trend of FHR, and FHR/MHR ratio in bitches of different pre-gestational bodyweight was evaluated during uncomplicated pregnancies. Materials and methods. Pregnant bitches (n=17) of different breeds, pre-gestational bodyweights (5.8-68 kg) and ages (2-7 years) were examined by ecocolordoppler (SonoAce 8800, Medison Co. Ltd., Seoul, Korea) twice a week (5 bitches) from day 21 after ovulation (progesterone 4-10 ng ⁄ml), and at week 4, 7, and 9 of pregnancy (12 bitches). To reduce and control the stress-effect induced by the restraint, MHR was evaluated at the aortic valve three times: at the beginning of the examination, after 10 minutes and at the end of the examination. The FHR of at least 3 different puppies (in litter size >3) was recorded. Only data from uncomplicated pregnancies (n=11) were included in the statistical analysis. The relationship between FHR, and FHR/MHR ratio, and independent variables (pre-gestational maternal bodyweight, and gestational age, in terms of days from parturition) was analyzed by a quadratic regression (p≤0.05). Results. Fetal heart rates and FHR/MHR ratio significantly fitted a multiple quadratic regression for all independent variables (p<0.02 and p<0.05, respectively); FHR increased from 35 days to 20 days before parturition, and then a decreasing pattern until parturition followed. Higher values of FHR were observed in bitches of lowest and highest bodyweight. As seen for FHR, the FHR/MHR ratio resulted higher in low and high bodyweight, and it reached the maximum values at about 20 days from parturition. The FHR/MHR ratio was well expressed by a quadratic law, in terms of pre-gestational bodyweight and gestational age. The equation derived was as follows: z= 1.8284-0.0137x + 0.00014x2+0.05071y-0.00099y2,where z = FHR/MHR ratio, x = pre-gestational maternal bodyweight (kg), y= days from parturition. Conclusion. This study suggests that the maternal pre-gestational bodyweight and the gestational age influence FHR and MHR. The patterns of FHR and FHR/MHR ratio were similar, but the ratio better describes the effect of the independent variables on the data. The highest significance of FHR/MHR compared to FHR alone, encourages the application of this ratio to evaluate fetal wellbeing. The derived equation could be applied in clinical practice to obtain the expected values in healthy pregnancies. References. 1) Johnston et al., Canine and Feline Theriogenology 2001; WB Saunders, Philadelphia. 2) Lucio et al., Reprod Dom Anim 2009;44(suppl 2):133-6. 3) Hezzell et al., JSAP 2013;54:318-24. 4) Assali et al., Am J Obstet Gynecol 1977;129:748-59.