Incomplete atrioventricular canal complicated by cardiac tamponade and bidirectional shunting in an adult dog.

An incomplete atrioventricular (AV) canal with bidirectional shunting and cardiac tamponade in a 6-year-old dog was initially diagnosed echocardiographically as a common atrium. The dog failed to respond to medical therapy and was euthanized. Upon necropsy, the defect was confirmed as an incomplete AV canal. A mechanism for the potential sequence of clinical events demonstrated in this dog is proposed.