Thoracic ultrasound has proven effective for the diagnosis of left-sided congestive heart failure (L-CHF) dogs and cats with respiratory distress. The objective of this study was to determine whether ultrasonographic indices of the caudal vena cava (CVC) could be used to diagnose right-sided CHF (R-CHF) in dogs with cavitary effusions.

Dogs were prospectively enrolled in four groups: R-CHF (n = 34), L-CHF (39), cavitary effusions of noncardiac etiology (NC, 41), and pericardial effusion with tamponade (PCEFF, 17). Ultrasonographic indices included right ventricular to left ventricular ratio (RV:LV) and 2D and M-mode subxiphoid measures of CVC maximal and minimal size (CVCmax and CVCmin), CVCmax indexed to aortic dimension (CVC:Ao), and CVC collapsibility index (CVC-CI). Variables were compared between study groups using Kruskal-Wallis and Dunn's-Bonferroni testing.

All indices (RV:LV, CVCmax, CVCmin, CVC:Ao, and CVC-CI) were significantly different between R-CHF and NC dogs (p < 0.005). A CVC-CI less than 30% (in either M-mode or 2D) was 97% sensitive and 93% specific for diagnosis of R-CHF versus NC effusion. All CVC indices, but not RV:LV ratio, also differed between PCEFF and NC dogs (p < 0.005). Compared to NC, L-CHF dogs had higher CVC:Ao in both 2D (p = 0.017) and M-mode (p = 0.014); compared to R-CHF, L-CHF dogs had higher CVC-CI in both 2D and M-mode (p < 0.005).

Ultrasonographic indices of CVC size and collapsibility are useful to differentiate R-CHF versus NC disease as causes of cavitary effusions. Dogs with L-CHF demonstrate CVC measurements intermediate between R-CHF and NC dogs.