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

## TL;DR

This study explores using ultrasound to assess blood vessel measurements in sedated horses to help evaluate their fluid status, similar to techniques used in human and small animal medicine.

## Contribution

The study introduces the feasibility of using transrectal and inguinal ultrasonography to assess vascular parameters in horses for the first time.

## Key findings

- Transrectal and inguinal ultrasonography approaches are well-tolerated and feasible in horses.
- Caudal vena cava collapsibility index and vein-to-artery ratios can be reliably measured with high repeatability and reproducibility.
- B-mode and M-mode measurements did not differ significantly, but M-mode was inadequate for inguinal analysis.

## Abstract

This is a prospective, observational pilot study conducted at a private equine referral hospital that aims to establish baseline data in adult horses for ultrasonographic evaluation of the caudal vena cava respiratory collapsibility index and large vessel vein-to-artery ratios, which are currently used in human and small animal emergency medicine to guide fluid therapy. This study showed that both transrectal and inguinal ultrasonographic approaches are well-tolerated and feasible and offers valuable insights for the future development of non-invasive tools to assess intravascular volume status and fluid responsiveness in 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.