Doppler ultrasonography: an improvement in real-time recipient selection in equine ET programs

Abstract

In this study, we test the hypothesis that a CL with greater blood perfusion has a more significant impact than luteal size and P4 concentrations on pregnancy per embryo transfer (P/ET). The study was carried out from November/2021 to May/2022 on a stud farm of the Mangalarga Marchador breed in Itaperuna-RJ, Brazil. Recipient mares (n=110), aged between 3-16 years, weighing 370-460 kg, with adequate BCS and on days 3 to 9 after spontaneous ovulation, received a single fresh grade I or II blastocyst, retrieved by uterine flushing of donor mares. Immediately before the ET, the CL characteristics were evaluated by B-mode and Color-Doppler ultrasonography (US) and a blood sample was collected for plasma P4 assay. Recipients were retrospectively classified according to CL area [small (≤6 cm2) or large (>6 cm2)], CL blood perfusion [low ≤55 % or high >55%] and plasma P4 concentration [low ≤9.98 ng/mL or high >9.98 ng/mL]. Pregnancy diagnosis was performed 30 days after ET and P/ET analysis was performed using the GLIMMIX of SAS procedure. The overall P/ET was 73% (83/110). The P/ET was not influenced by the luteal area, but effects of CL blood perfusion class (P=0.007) and the P4 plasma concentration class (P=0.02) on P/ET were observed. The P/ET was greater in the high luteal perfusion group [89.1%, 41/46 vs.65.6%, 42/64] and in the high P4 group [85.4%, 41/48 vs. 67.7%, 42/62]. When area, luteal blood perfusion and P4 concentration were compared with the probability of pregnancy through ROC curve analysis, luteal blood perfusion (AUC=0.66) was the most significant (P=0.01) predictor of pregnancy compared to area (AUC=0.53) and plasma P4 (AUC=0.60). Finally, recipients were grouped into four different groups by analyzing CL blood perfusion classes and P4 (high CLBP and high P4 vs. High CLBP and low P4 vs. low CLBP and high P4 vs. low CLBP and low P4) and compared with the P/ET using the Fisher test. P/ET differed between groups (P=0002), as indicated by the higher P/ET in the high CLBP and high P4 95% (19/20) and high CLBP and low P4 84.6% (22/26) groups, compared to animals with low CLBP and low P4 55.5% (20/36). In conclusion, although the P4 concentration is a potential indicator of pregnancy success, it cannot be used in real-time when selecting the most appropriate recipient in a commercial ET program. Also, luteal blood perfusion determined by Doppler-US is an innovative and easily accessible tool that can be used in real-time to select the recipients with the greatest chances of maintaining pregnancy in ET programs in horses.