• canine
• congestive heart failure
• right atrial size
• right ventricular enlargement
• tricuspid regurgitation

Right atrial (RA) dilation, right ventricular (RV) enlargement and RV systolic dysfunction are strongly associated with worse prognosis in humans with pulmonary hypertension (PH). Echocardiography is the most commonly-used tool to diagnose PH in dogs. Right heart size and function in dogs with PH have been mainly assessed using subjective or M-mode evaluation. In humans, quantitative assessment of the right heart is recommended, and two-dimensional echocardiography is considered more accurate than M-mode. Only recently, reference intervals for right heart size and function have been described in healthy dogs. The aim of the study was to describe objective indicators of RA size, RV size and RV systolic function in dogs with PH, and verify the association with disease severity and presence of right-sided congestive heart failure (R-CHF).
Healthy dogs (control group) and dogs with naturally occurring PH were prospectively included in the study. PH was classified according to tricuspid regurgitation pressure gradient as mild (36-50 mmHg), moderate (51-75 mmHg), and severe (>75 mmHg). RA size was assessed using the RA area indexed to the body surface area (RAA index), and RV size was assessed using the RV end-diastolic area indexed to the body surface area (RVEDA index). RV systolic function was evaluated with the tricuspid annular plane systolic excursion (TAPSE) and the RV fractional area change (FAC), normalized for body weight (TAPSEn and FACn, respectively).
The sample population included 166 client-owned dogs, 89 with PH and 74 control dogs. In healthy dogs, RA area and RV end-diastolic area showed a strong positive correlation with the body surface area. Normal reference range for RAA index (4.2-10.2 cm2/m2) and RVEDA index (4.9-10.9 cm2/m2) were described in healthy dogs. Both RAA index and RVEDA index were higher in dogs with moderate and severe PH than in those with mild PH and control dogs (P<0.001 for each contrast). The RAA index and the RVEDA index were significantly higher in dogs with R-CHF in comparison to dogs without R-CHF (P<0.0001). The RAA index had the most accurate cut-off value to predict R-CHF (>12.3 cm2/m2; sensitivity 100%, specificity 84%). The severity of tricuspid regurgitation was the only independent predictor of RAA index and RVEDA index based on the multivariate analysis. TAPSEn and FACn were not significantly different among degrees of PH severity, and no differences in TAPSEn and FACn were found between dogs with and without R-CHF.
In conclusion, RAA index and RVEDA index can be used to evaluate RA and RV size in dogs. Tricuspid regurgitation severity is the main determinant of right heart enlargement in dogs with PH, and it seems to play a key role in the occurrence of R-CHF. RV systolic function, assessed with TAPSEn and FACn, does not seem to be a predictor of R-CHF in dogs with PH. Further longitudinal studies are needed to verify whether the RAA index and RVEDA index represent prognostic factors in dogs with PH.