An experimental model to study resistance index and systolic/diastolic ratio of uterine arteries in adverse canine pregnancy outcome

BLANCO P, , ; ARIAS D; RUBE A,; BARRENA JP; CORRADA Y; GOBELLO C

Viena

Congreso; 6th International Symposium on Canine and Feline Reproduction; 2008

International Symposium on Canine and Feline Reproduction

Doppler ultrasound has been a satisfactory and valuable method for evaluation of fetal condition during pregnancy in many species [2,6,8]. Uterine blood flow can be assessed by the resistance index (RI) and systolic/diastolic ratio (S/D) of the uterine arteries using Doppler ultrasound. Resistance index, also known as Pourcelot?s ratio, is defined as (S-D)/S where S is the peak systolic velocity and D the end diastolic velocity while S/D ratio is S/D [3]. Both RI and S/D progressively decrease throughout normal gestation [2,3,6,8]. In pregnant women, increasing RI and S/D indicate a high risk of adverse obstetric outcome [3,9]. Similar data is missing in veterinary medicine where no Doppler ultrasound indicators of potential abortion have been yet identified in dogs [4,5]. The aim of this study was to describe the RI and S/D of the uterine arteries in an experimental model of abnormal canine gestation pharmacologically induced. For this purpose, 16 cross and pure-bred pregnant (30-35 days from first mating) bitches, were randomly assigned to one of the following treatment groups: medicated group (MG; n = 8) which was administered a cabergoline and cloprostenol protocol to interrupt gestation [1] and a control group which did not receive any drug (CG; n = 8). Color and pulsed-wave Doppler examinations (Toshiba Core Vision Pro, Shimoishigami, Otawara-Shi Tochigi-Ken-Japan, 8-MHz linear-array transducer) of uterine arteries were carried out before the initiation of the study and then every other day up to abortion or parturition. In all the cases pregnancy and pregnancy termination were ultrasonagraphically diagnosed [4]. Resistance indexes and S/D of the left and right uterine arteries, before and after treatment, were compared between groups using T test. To further characterize results, a correlation analysis was carried out between days to abortion or parturition and both RI and S/D in the MG and CG, respectively (SPSS®, Chicago, USA, 10.01). P < 0.05 was considered significant. There were no RI nor S/D differences between left and right uterine arteries neither in the groups (CG vs. MG; P > 0.6 and P > 0.4) nor the periods (before vs. after treatment; P > 0.2 and P > 0.3) of the study, therefore results were calculated including both arteries. Pre treatment RI did not differ between CG and MG (0.57 ± 0.0 vs. 0.53 ± 0.0; P > 0.2). Neither did S/D (2.54 ± 0.3 vs. 2.22 ± 0.1; P > 0.1). All MG (8/8) but none (0/8) of the CG bitches terminated their pregnancy by abortion 6 ± 1.2 days after medication with a range of 2 to12 days. Post treatment RI of the MG and CG were 0.62 ± 0.1 and 0.53 ± 0.1 (P < 0.01), respectively. Post treatment S/D of the same groups were 2.96 ± 0.9 and 2.23 ± 0.3 (P = 0.01). Correlation between days to abortion or parturition and RI were 0.75 (P < 0.01) and - 0.78 (P < 0.01) for the MG and CG, respectively. The same correlation for the same groups were 0.79 (P < 0.01) and - 0.73 (P < 0.01) for S/D. As expected, this pharmacological protocol was useful to develop an experimental model of impending abortion as all medicated animals terminated their pregnancy. The similarity of RI and S/D between left and right arteries was also expected and probably due the even distribution of fetuses in the uterine horns [4]. In the present study, RI and S/D of uterine arteries progressively decreased to parturition and increased to abortion. Abnormal trophoblastic invasion of the hemochorial placenta has been described as the cause of increasing RI and S/D in women [3,9]. In spite of placental differences, increasing RI and S/D may also reflect histological abnormalities of the endotheliochorial carnivore placenta. In women, it has also been suggested that uterine blood flow increases with rising concentrations of serum progesterone [7]. Moreover, administration of oral progesterone was followed by a significant decrease in spiral arteries RI in one study [3]. Therefore, in the present canine model increasing RI and S/D could have been provoked by the pharmacologically induced progesterone deprivation [2]. In line with human obstetrics [3,9], in dogs, increasing RI and S/D seem to be predictors of adverse obstetrics outcome. If this is probed many cases of abnormal gestation could be diagnosed and prevented before other ultrasound sings appear. Further work on spontaneous abnormal gestation cases is still needed to confirm these initial experimental findings.