Caudal vena cava-to-aorta ratio in hemodynamically stable and unstable client-owned rabbits

Objective: To determine the feasibility of measuring caudal vena cava (CVC) and aorta size in non-contrast-enhanced CT scans of rabbits, variations in size and circularity depending on anatomical location, factors and conditions affecting the CVC-to-aorta (CVC:Ao) ratios, and changes of CVC:Ao ratios in hemodynamically unstable rabbits. Methods: This retrospective cohort study included consecutive client-owned rabbits that presented to the Cornell University Hospital for Animals between June 2022 and October 2024 and received an abdominal CT. Forty-nine CT scans from 46 individual rabbits were included. The CVC:Ao ratio was determined similarly to dogs. Hemodynamically instability was assessed by 2 independent blinded investigators. Results: 100% (95% CI, 92.8% to 100%) and 87.8% (95% CI, 75.2% to 95.4%) of the thoracic and abdominal CVC:Ao ratios, respectively, were measurable. Several factors were associated with abdominal and thoracic CVC:Ao ratios and their difference, including a higher difference between abdominal and thoracic ratio in rabbits with intestinal obstruction. The abdominal CVC:Ao ratio and the difference between abdominal and thoracic CVC:Ao ratio were significant predictors of hemodynamic instability, with a 6-fold and 8-fold increase in the odds of hemodynamic instability for every additional unit of these ratios and an area under the curve of 0.73 (95% CI, 0.56 to 0.91) and of 0.79 (95% CI, 0.64 to 0.93). Conclusions: In diseased rabbits, the aorta and CVC are larger and less circular in the thorax than in the abdomen. In hemodynamic unstable rabbits, the thoracic CVC:Ao ratio decreased while the abdominal CVC:Ao ratio increased, likely owing to gastric compression on the vena cava. Clinical Relevance: CVC:Ao ratios can be used as significant predictors of hemodynamic instability in diseased rabbits.