Reference range of gestational uterine artery resistance index in small canine breeds

The objectives of this study were to evaluate the uterine artery blood flow during the second half of normal pregnancy in small breed dogs and to determine the RI reference values of this artery in these bitches. Fifty-two, 1.5-6.5 kg, purebred healthy pregnant bitches were included in this study. Doppler ultrasonographic examinations of the uterus were performed every 10 days from Day 30 of the estrous cycle to parturition. Peak systolic velocity (PSV) and end diastolic velocity (EDV) of the uterine artery were measured while RI [(PSV-EDV)/PSV] was automatically calculated. Values of PSV, EDV and RI were analyzed by repeated measures ANOVA followed by Tukey test using age, body weight, litter size and parity as covariates. Resistance index reference ranges were constructed by regressing this index on gestational age. Equation of the polynomial regression curves was used to calculate the mean, 95th and 5th centiles for each gestational age (GA). A gradual increase of PSV (P < 0.01) and EDV (P < 0.01) and a decrease in RI (P < 0.01) were found throughout the study period. PSV and EDV were influenced by litter size (P < 0.05), while RI was not. None of the Doppler parameters were influenced by age, body weight or parity. A second-degree polynomial described the relationship between RI of uterine artery and GA in the different time points: [RI = 0.993 + 8.4x10-5 x GA2- 0.0117 x GA]. It is concluded that uterine artery blood flow progressively increased throughout normal pregnancy in small breed bitches. In addition, reference ranges of uterine artery RI were 0.64-0.79, 0.57 to 0.75, 0.53 to 0.71 and 0.52 to 0.66 on Days 30, 40, 50 and 60 of gestation, respectively.