If the right ventricle is defective and the left ventricle is relatively healthy, Stage I of the procedure turns the left ventricle into the main pumping chamber of the heart. Once again, this is accomplished by creating a pathway between the aorta and the pulmonary artery with a synthetic graft (Dacron). The new pathway is called a modified Blalock-Taussig shunt. It allows blood that is pumped by a healthy left ventricle to travel through the aorta and to both the lungs and the rest of the body (via the pulmonary artery). In this case, the right ventricle is completely bypassed.

During the procedure, the surgeon may cut away the septum (wall) between the two upper chambers atria. Therefore, oxygen-poor blood returning from the body into the right atrium can freely enter the left atrium, where it passes into the left ventricle and is then pumped out to the lungs, as well as the rest of the body. In this situation, the left ventricle becomes the main or only chamber responsible for pumping blood to the body. If the surgeon removes the septum, a heart-lung machine will be necessary to maintain blood supply to the body during this portion of the surgery.

Whether used for left or right ventricle defects, this first stage of the three-stage procedure accomplishes a few important goals. First, it allows for oxygen-rich blood to be pumped out to the tissues and organs of the body. Second, it allows for oxygen-poor blood to be pumped to the lungs, where it receives new oxygen.

However, it is only a partial solution. Because the oxygen-poor blood and the oxygen-rich blood are allowed to mix freely, some oxygen-poor blood is invariably pumped back out the tissues and organs of the body. Because of this, the infant’s oxygen saturation levels improve, often to the 70th or low 80th percentile, but not all the way to normal (98 to 100 percent oxygen saturation). Additionally, the shunt does not grow as the infant grows and so the amount of blood flowing through it is proportionally less over time. The opening of the shunt may also narrow as debris builds up inside it.

The next two stages of the procedure are designed to remedy this.