What are the common right-to-left shunting lesions with reduced pulmonary blood flow?

What are the common right-to-left shunting lesions with reduced pulmonary blood flow?

Tetralogy of Fallot (TOF)

The combination of VSD, pulmonary valvular and/or right ventricular infundibular stenosis, right ventricular hypertrophy, and a large overriding aorta is known as TOF. It is the most common cyanotic defect seen after the first year of life, contributing to 10% of all congenital cardiac lesions. The degree of right ventricular outflow and/or pul-monic obstruction determines the onset and severity of cyanosis. With severe obstruction, cyanosis appears with closure of the ductus arteriosus in the neonatal period. Prostaglandin E₁ may be used to clinically stabilize the patient prior to surgical intervention. Many infants do not develop symptoms until 3–6 months of age and even then may not appear cyanotic at rest. However, episodes of severe cyanosis with hyperventilation and acidosis, known as hypercyanotic spells (or “tet” spells), may occur. These are caused by severe infundibular spasm, probably induced by changes in venous return and SVR. Reduction in SVR leads to decreased pulmonary blood flow, since blood tends to be shunted to the systemic circulation. Decreased venous return further decreases pulmonary blood flow. In older children, the squatting posture may improve symptoms through an increase in venous return from the lower extremities and by increasing SVR. The treatment of hypercyanotic spells is based on the goals of decreasing infundibular spasm by decreasing contractility and heart rate, and by increasing preload. Another goal (especially in fixed right ventricular outflow obstruction) is to increase SVR to decrease right-to-left shunting across the ventricular septal defect.

In TOF, both ventricles work at systemic pressure but volume overload does not occur and congestive heart fail-ure is rare.

These patients should arrive in the operating room well sedated. Preoperative fluid restriction should be mini-mized and/or maintenance fluid given intravenously to prevent hemoconcentration and hypovolemia. A smooth induction is important to prevent increases in oxygen demand or hypercyanotic spells. The agents used should have minimal peripheral vasodilating effects. Thus, halothane is theoretically preferable to isoflurane or sevoflurane for this purpose. Mild myocardial depression may relieve infundibular obstruction and is, therefore, desirable. If intravenous agents are used, they should be carefully titrated to prevent relative overdose. Intravenous barbiturate requirements may be halved. Ketamine can be safely used, particularly in very sick patients, since it main-tains SVR and does not cause “tet” spells.

Arterial oxygen saturation generally increases upon induction of anesthesia in cyanotic patients. The reasons for this are probably related to the reduction in oxygen consumption during anesthesia and the subsequent increase in venous saturation. Monitoring blood pressure may become problematic in patients with previous shunting procedures using the subclavian arteries. The contralateral arm should be used for invasive or non-invasive monitoring. For major surgery, intra-arterial and/or central venous pressures should be measured directly. This will also allow blood sampling for blood gas and acid–base measurements. Because the major myocar-dial stress rests on the right ventricle in these patients, central venous pressures can be used to assess cardiac performance.

Hypercyanotic spells may develop perioperatively because of the dynamic nature of the muscular infundibu-lar obstruction present in TOF. Strategies to prevent and treat these complications are outlined in Table 69.3.

These patients require endocarditis prophylaxis for life.

Transposition of the Great Arteries (TGA) and Complex Lesions

These patients will have undergone repair or palliation prior to any elective noncardiac surgery and will be dis-cussed below.