Fontan physiology

Physiology

•The essence of Fontan physiology is a near-complete dependence of ventricular preload on pulmonary venous return that is venovenous in nature (total cavopulmonary connection). An increase in PVR thus has greater potential to significantly impair cardiac filling, as well as causing significant venous hypertension. The presence of a fenestration or baffle leak provides a ‘bypass’ of this circulation, creating a margin of safety.

•Following the Fontan operation, the capacitance of the venous system increases significantly because all systemic veins now drain into the also compliant pulmonary circulation. This can further diminish ventricular preload.

•Acute increases in hepatic venous pressure may cause hepatic congestion, dysfunction, coagulopathy, or hypoglycemia. In these cases, ensure fenestration patency.

•In the early postoperative period, hepatic congestion, bleeding, and effusive drainage can also cause hypovolemia, further compromising ventricular preload.

•Postoperative assessment of the Fontan circulation includes pulse pressure and pulse strength (as markers of preload sufficiency), Fontan pressure and common atrial pressure. Transpulmonary gradient (Fontan-CAP) >8-10 mmHg may represent elevated PVR, though this can be exacerbated by arterial hypertension. A more specific (concerning) finding is the simultaneous presence of sinus tachycardia, hypotension, low CAP (<3-5 mmHg) and elevated Fontan pressure, which suggests preload insufficiency.

•Presence of PaO2>90 mmHg on 21% FiO2 suggests decreased flow through fenestration; however, hyperoxic ventilation can mask fenestration shunting due to dissolved oxygen fraction.

•Recall that pulmonary and systemic lymphatics drain into the thoracic duct, which in turn drains into the innominate vein. Venous hypertension may inhibit lymphatic resorption of fluid within the alveolus, thoracic or peritoneal cavity. Thus, prolonged pleural and peritoneal drainage are important considerations. Treatment considerations

•When possible (not precluded by arrythmias or significant bleeding or low cardiac output), early extubation allows for negative intrathoracic pressure and promotes PBF.

•The prevention of atelectasis (particularly accumulation of effusion or blood in the thorax) is paramount to maintain low PBF.

•If BP tolerates, early vasodilation may diminish venous pressure and diminish effusive losses. Similarly, early fluid restriction, diuresis and low fat feeding may diminish duration of effusive drainage. All of these approaches typically require low PVR and early extubation to be successful. •Consider ASA to maintain fenestration patency if present. Consider systemic anticoagulation (heparin±warfarin) for higher risk patients or those with extracardiac conduit.