Ultrasonographic Assessment of Caudal Vena Cava Collapsibility Index, Caudal Vena Cava-to-Aorta, and Femoral Vein-to-Artery Ratios in Healthy Sedated Adult Horses

(1) Background: Ultrasonography of major vessels helps evaluate fluid status and responsiveness in critical human and canine patients. Aims: transrectal and inguinal ultrasonography of caudal vena cava (CVC), aorta (Ao), and femoral artery (FA) and vein (FV) in horses; calculate CVC collapsibility index (CI CVC) and vein-to-artery ratios; compare B- and M-mode; and evaluate repeatability and reproducibility. (2) Methods: B-mode and M-mode video loops were recorded twice by transrectal (Ao, CVC) and inguinal ultrasonography (FV, FA) by two operators on 17 healthy, sedated adult horses. Diameters and areas were measured. CI CVC, CVC-to-Ao ratio (CVC/Ao), FV-to-FA ratio (FV/FA), reproducibility, and repeatability were calculated. (3) Results: Vessels were successfully visualized (mean time: rectal 4 min, inguinal 3 min). CVC displayed respiratory-related dimension changes. Ratios included CI CVC (mean diameter B-mode 30 ± 13%, M-mode 33 ± 12%; area 36 ± 15%), CVC/Ao (mean diameter B-mode 0.43 ± 0.15, M-mode 0.43 ± 0.11; median area 0.56, IQR 0.53-0.64), and FV/FA (mean diameter 3.80 ± 1.02; median area 26.74, IQR 25.23-32.40). M-mode was inadequate for inguinal analysis. B-mode and M-mode measurements did not differ significantly. Repeatability and reproducibility were excellent (transrectal ratios 93.9% and 94.4%, respectively); FV/FA 96.9%. (4) Conclusions: Assessment of CI CVC, CVC/Ao, and FV/FA is feasible, highly repeatable, and reproducible.