You said the patient has "known" R --> L shunt, but it wasn't clear if there was TTE evidence of a shunt or if you were assuming that based on the sats. Generally speaking, there are no intracardiac shunts remaining in a Fontan, unless a fenestration was intentionally left as a pop-off to preserve cardiac output at the expense of O2 sats. And oftentimes those clot/close off/are closed off once the Fontan has stabilized post-operatively. It's certainly possible that there's a fenestration still open, and there's R-->L flow across it, causing hypoxia, but there are also many other reasons a failing Fontan might be hypoxic, especially one of this size/with these comorbidities.
