The human heart has 4 chambers: 2 collecting chambers (atria) and 2 pumping chambers (ventricles) to allow blood flow within two distinct circuits: "pulmonary" and "systemic". The pulmonary circuit directs the blood to the lungs to receive oxygen and the systemic circuit delivers oxygen-rich blood throughout the body. In children born with a single ventricle, blood from these two circuits mixes within the heart resulting in lower oxygen levels in the blood delivered to the body (cyanosis).

The Fontan procedure is a palliative surgery which bypasses the need for a ventricle to deliver blood to the lungs, as blood from the body flows passively to the lungs by a man-made connection (whereby two large body veins [cavae] are sewn to the pulmonary arteries), thereby preventing mixing of blood through restoration of two distinct circuits without mixing of blood. Although the Fontan operation effectively eliminates cyanosis and enables survival into adulthood, increased systemic venous pressure is an unavoidable systemic complication and low cardiac output (CO) is pervasive finding.

Despite excellent pediatric surgical results, the risk of late complications and death dramatically increases in the decades following Fontan surgery. A chronically low CO state secondary to decreased forward flow of blood to the lungs can result in end-organ dysfunction and shortened life expectancy. Short of heart transplantation, deemed suitable only for a minority of patients, effective therapies for low CO are largely absent. The investigators aim to investigate a novel, non-invasive, ambulatory therapy which can augment CO. Specifically, external suction is applied intermittently to the chest wall, much like a vacuum, to increase CO, called negative pressure ventilation (NPV) using a Cuirass® ventilator (Hayek Medical). Although used in patients with lung disease, the investigators' proposal is to evaluate this novel, portable, ventilation system would be the first study of its kind in adults with congenital heart disease, specifically those with a Fontan palliation.

Inclusion Criteria:

• Fontan patients age > 13 years.

Exclusion Criteria:

• unable to complete MRI (pacemaker/defibrillator/retained wires, pregnancy, claustrophobia);
• evidence of Fontan failure (requiring ongoing medical / device therapy) protein-losing enteropathy, intracardiac thrombus, anatomical obstruction to the Fontan circuit;
• patent Fontan fenestration;
• oxygen saturation < 90%;
• ongoing arrhythmia;
• ejection fraction < 50% on echocardiography or CMR;
• moderate or severe valve insufficiency on echocardiography or CMR;
• Obesity (BMI >35);
• severe obstructive sleep apnea (AHI>20);
• chronic obstructive lung disease (FEV1/FVC<60%);
• severe chest wall deformities (scoliosis, kyphosis, kyphoscoliosis);
• acute or chronic kidney disease (eGFR<60)
• unable to provide informed consent.