Full bypass is instituted with high ascending aortic cannulation and two caval cannulae. In unstable patients, bypass is begun with one caval cannula, with the other added after bypass is initiated. These cannulae are inserted into the superior and inferior vena cava sufficiently to later open the right atrium if necessary. A temporary pulmonary artery vent is placed in the midline of the main pulmonary artery one-centimeter distal to the pulmonary valve.

The blood is cooled with the pump oxygenator , maintaining a 10°C gradient between arterial blood and bladder or rectal temperature. Surface cooling with both a head jacket and a cooling blanket is begun. With cooling the tympanic membrane measurements fall fastest, but circulatory arrest is not initiated until temperatures of the rectum or bladder are within a degree or two of the head temperatures.

During perfusion the venous saturations increase; saturations of 80% at 25°C and 90% at 20°C are typical. Hemodilution is used to decrease the blood viscosity during hypothermia and to optimize capillary blood flow; the hematocrit is maintained in the range of 18–25 during profound hypothermia .

Cooling generally takes 45 min to an hour, varying according to the body mass of the patient. When ventricular fibrillation occurs, a further vent is placed in the left ventricle through the right upper pulmonary vein. It is most convenient for the surgeon to be placed initially on the patient’s left side. During the cooling period, some preliminary dissection can be carried out. The aorta is freed from the right pulmonary artery. The superior vena cava is mobilized all the way to the innominate vein and also dissected free of the right pulmonary artery. All dissection of the pulmonary arteries takes place intrapericardially, and neither pleural cavity is entered. The distal right pulmonary artery is exposed (between the aorta and superior vena cava) by reflecting the pericardial covering upward, so that the takeoff of upper and middle lobes can be seen (Fig. 43.2).

An incision is then made in the right pulmonary artery from the lateral border of the ascending aorta, out toward the reflected superior vena cava , and entering the lower lobe branch of the pulmonary artery just after the takeoff of the middle lobe (Fig. 43.3). It is important that the incision stays in the center of the vessel and continues into the lower, rather than the middle lobe. The distal limit of the incision is dictated by the accessibility required to repair this subsequently. Any loose thrombus, if present, is now removed. In most cases no thrombus is present, and initially the pulmonary vascular bed may appear normal even in severe embolic pulmonary hypertension.

When the patient’s temperature reaches 20°C, the aorta is cross clamped and a single dose of cold cardioplegic solution (1 l) administered. Additional myocardial protection is obtained by the use of a cooling jacket. The entire procedure can now carried out with a single aortic cross clamp period with no further administration of cardioplegic solution .

A modified cerebellar retractor is placed between the aorta and superior vena cava, and this lifts the superior vena cava off the pulmonary artery and affords excellent exposure for the incision in this vessel. The approach medial to the superior vena cava, together with tilting the patient to the right, allows visualization of all distal vessels of the right pulmonary vascular bed.

Circulatory arrest is initiated, and the patient exsanguinated. Then, a microtome knife is used to develop the endarterectomy plane posteriorly, since any inadvertent egress at this site could be readily repaired or simply left alone (Fig. 43.4). The plane is not initiated at the initial incision site, unlike the situation in a carotid or femoral endarterectomy , since the residual thin pulmonary artery will not hold hemostatic sutures for repair. Dissection in the correct plane is critical because if the plane is too deep, the pulmonary artery may perforate, with fatal results, and if the dissection plane is not deep enough, inadequate amounts of the chronically thromboembolic material will be removed. The plane is in the media of the vessel.

When the proper plane is entered, the layer will strip easily. The ideal layer is marked with a pearly white layer, which strips easily. There should be no residual yellow plaque. If the dissection is too deep, a reddish or pinkish color indicates the adventitia has been reached. A more superficial plane should immediately be sought.

A full-thickness layer is left in the region of the initial incision, to ease subsequent repair (Fig. 43.5). The vessel is progressively endarterectomized with an eversion technique. As the fibrotic occluding layer becomes free, it is progressively grasped more distally until each subsegmental branch becomes free, and the entire cast is liberated. Although many of these vessels cannot be seen initially, progressive dissection and traction allow a complete endarterectomy of the entire pulmonary vascular bed. Absolute visualization in a completely bloodless field provided by circulatory arrest is essential. A perforation at subsegmental level will become inaccessible later. It is important that each subsegmental branch is followed and freed individually until it ends in a “tail,” beyond which there is no further obstruction. It is possible to remove occluding material as far distally as the diaphragmatic level.

The dissection is carried out with a dissector with a rounded tip, the body of which is attached to the cell-saver suction. The use of this dissector (“Jamieson dissector ,” Fehling Corporation) is essential for a complete endarterectomy. At the completion of the dissection, the lumen of the pulmonary artery is carefully inspected, and any residual debris removed.