Fontan Circulation: Ventilation
Advanced, Clinical Subspecialties
The Fontan operation is the final stage palliation surgery for single ventricle disease states. The Fontan circulation dissociates the single systemic ventricle and blood from the pulmonary blood flow. Venous return to the lungs is passive (since there is no pulmonary ventricle), coming from the superior vena cava to pulmonary artery anastomosis created during the Glenn procedure and the baffle of the inferior vena cava to the pulmonary arteries created during the Fontan operation. Cardiac output is dependent on this passive pulmonary blood flow and is very sensitive to changes in intrathoracic pressure and pulmonary vascular resistance (PVR).
Intrathoracic Pressure: increased intrathoracic pressure affects the pressure gradient for venous return, decreased pulmonary blood flow, and therefore decreased cardiac output.
- Positive pressure ventilation decreases venous return.
- Spontaneous respiration creates negative intrathoracic pressure with inspiration, increasing systemic venous return, optimizing hemodynamics. If at all possible, spontaneous ventilation should be maintained in a patient with a Fontan so long as hypercarbia is avoided.
- Maintaining FRC by adding PEEP optimizes PaO2, minimizes atelectasis, and avoids hypoxic vasoconstriction.
Pulmonary vascular resistance: increases in PVR decrease the passive blood flow to the lungs and thus decreases cardiac output. The most common causes of perioperative increases in PVR are pain, hypercarbia, acidosis, vasoactives, and increased intrathoracic pressure.
- Vasodilators like inhaled nitric oxide reduce pulmonary vascular resistance without reducing systemic blood pressure and counter hypoxemia induced pulmonary vasoconstriction
- Low respiratory rates, short inspiratory times, low PEEP, and tidal volumes of 5-6 mL/kg allow adequate pulmonary blood flow, normocarbia, and low PVR.
- Hyperventilation tends to impair pulmonary blood flow despite the induced respiratory alkalosis because of the increased mean intrathoracic pressure.