6 How does PAH affect perioperative risk?

PAH is an independent predictor of perioperative morbidity and mortality in both adults and children, and the severity of baseline PAH correlates with the incidence of major complications, including cardiac arrest and pulmonary hypertensive crisis. Children with suprasystemic pulmonary artery pressures were eight times more likely to experience a major perioperative complication than were those with subsystemic pulmonary hypertension. Choosing a balanced anesthetic technique that minimizes hemodynamic changes and using perianesthetic pulmonary vasodilators are important in the care of patients with PAH.

7 How are shunts calculated?

Using cardiac catheterization data, relative flows in the pulmonary and systemic circulations can be calculated using the Fick principle (flow is inversely related to oxygen extraction):

where Qp = pulmonary blood flow, Qs = systemic blood flow, SaO₂ = systemic arterial oxygen saturation, SvO₂ = systemic mixed venous oxygen saturation, SpvO₂ = pulmonary venous oxygen saturation, and SpaO₂ = pulmonary arterial oxygen saturation.

8 How are pulmonary vascular resistance and systemic vascular resistance calculated?

Resistance is related to pressure and flow:

where PAP = pulmonary artery pressure, LAP = left atrial pressure, Qp = pulmonary blood flow, MAP = mean arterial pressure, CVP = central venous pressure, and Qs = systemic blood flow. The results of this equation are expressed in Wood units. Multiply by 80 to express in dyne • s • cm⁻⁵.

9 Do anesthetic drugs affect pulmonary vascular resistance?

Most anesthetics do not have undesired effects on the pulmonary vasculature. Inhalational anesthetics are vasodilators and probably have similar effects on both SVR and PVR. Fentanyl does not have direct effects on the pulmonary vasculature, but it can prevent the stress response to noxious stimuli. Ketamine can increase PVR significantly in some patients who have pulmonary hypertension, especially if accompanied by hypoventilation, but this increase can sometimes be attenuated by administering oxygen, ventilatory support, or a pulmonary vasodilator. Propofol may decrease SVR more than PVR. Almost any sedative drug can depress ventilation and cause changes in oxygen saturation or carbon dioxide tension that could increase PVR.

10 What causes cyanosis in congenital heart disease?

When at least 5 g/dl of desaturated hemoglobin is present in arterial blood, the lips, nailbeds, and mucous membranes appear blue, or cyanotic. Cyanosis occurs in patients with congenital heart lesions involving a right-to-left shunt and decreased pulmonary blood flow (including tetralogy of Fallot, pulmonary stenosis or atresia with septal defect, and tricuspid atresia), in lesions involving mixing of right- and left-sided blood without decreased pulmonary blood flow (including truncus arteriosus, anomalous pulmonary venous return, single ventricle, and double-outlet right ventricle), and in parallel right and left circulations (transposition of the great arteries).

11 Describe the clinical problems associated with cyanotic congenital heart disease

In response to chronic hypoxemia, these patients develop polycythemia. When hematocrit exceeds about 65%, increased blood viscosity is associated with a greater risk of intravascular thrombosis, stroke, coagulopathy, and poor flow in the microcirculation. The combination of hypoxemia and impaired blood flow can lead to tissue ischemia and organ dysfunction. In the heart ventricular dysfunction occurs as the myocardium is subjected to chronic ischemia and is exacerbated by the hypertrophy associated with ventricular outflow obstruction as in pulmonary stenosis.

In the presence of right-to-left shunting, air bubbles inadvertently injected into a vein can cross to the left side of the heart and enter the systemic arterial circulation, where they may cause stroke or myocardial ischemia.

12 What is tetralogy of Fallot? What are tet spells?

The tetrad of anatomic findings described by Fallot for this congenital heart lesion are pulmonary stenosis, overriding aorta, ventricular septal defect, and right ventricular hypertrophy. The pulmonary stenosis has a dynamic component: the subvalvular right ventricular outflow tract is muscular and contracts in response to inotropic stimuli (catecholamines). When such contraction occurs—or if SVR decreases significantly—less blood can flow into the pulmonary artery; thus more desaturated blood is shunted right to left across the ventricular septal defect into the left ventricle. An acute hypercyanotic episode, or tet spell, is the result (Figure 58-1).

Figure 58-1 In tetralogy of Fallot an acute increase in pulmonary outflow resistance causes an increase in right-to-left shunting of blood across the ventricular septal defect, resulting in a hypercyanotic spell. Treatment goals are to reduce pulmonary vascular resistance (PVR) and/or increase systemic vascular resistance (SVR).

(Reprinted from Hickey PR, Wessel DL: Anesthesia for treatment of congenital heart disease. In Kaplan JA [ed]: Cardiac Anesthesia, 2nd ed. New York, Grune & Stratton, 1987, p 648, with permission from Elsevier.)

13 How are tet spells treated?

Hypercyanotic spells and their treatment illustrate the importance of the balance between SVR and PVR (see Figure 58-1). In the presence of a shunt such as a ventricular septal defect, blood flow will follow the path of least resistance. If SVR is lower than PVR or right ventricular outflow tract resistance, as is the case in a tet spell, blood will shunt right to left. Treatment is aimed toward alteration of the resistance relationships (Table 58-2).

TABLE 58-2 Treatment of Hypercyanotic Spells

FiO₂, Fractional concentration of oxygen in inspired gas; SVR, systemic vascular resistance.

14 What effects do anesthetic agents have on shunting in patients with cyanotic congenital heart disease?

Anesthetics can affect myocardial contractility, SVR, and PVR, and shunting can be influenced by any of these. Although it has been postulated that halothane or isoflurane may cause hypercyanotic spells by decreasing SVR, studies have demonstrated that right-to-left shunting usually does not change significantly as long as ventilation and oxygenation are maintained. Nevertheless, as with any patient with cardiovascular disease, it is wise to avoid great changes in hemodynamic parameters by careful dosing, combining anesthetic drugs to achieve a balanced technique, and using muscle relaxants to facilitate lower anesthetic doses.

15 What is the main problem associated with ventricular obstructive lesions?

Obstruction to ventricular outflow such as is observed in aortic stenosis, interrupted aortic arch, coarctation of the aorta, or pulmonary stenosis is associated with progressive deterioration of ventricular performance because of ventricular hypertrophy and myocardial ischemia. The hypertrophy develops because the ventricle must work extra hard to overcome the obstruction and maintain stroke volume. Eventually oxygen demand exceeds supply, and myocardial ischemia occurs. Fibrosis and ventricular failure follow. Neonates with left-sided obstructive lesions commonly present with ventricular failure and acidosis and have a high mortality rate.

16 What is a ductal-dependent lesion?

In some congenital heart defects a patent ductus arteriosus is the only route by which blood can flow into either the pulmonary artery or the systemic aorta. For example, neither tricuspid atresia nor pulmonary atresia allows direct blood flow from the right ventricle into the pulmonary artery. Instead blood must be shunted into the left side of the heart across an atrial or ventricular septal defect, flow into the aorta, and then flow through the ductus arteriosus into the pulmonary artery for gas exchange in the lungs. Similarly, the hypoplastic left heart has no direct blood flow from the heart to the aorta for systemic circulation; blood must flow from the pulmonary artery across the ductus arteriosus into the descending aorta. In these cases maintenance of ductal patency by infusion of prostaglandin E₁ is essential to support life until a palliative or corrective procedure can be performed.

17 Why can oxygen be dangerous in patients with single ventricle physiology?

Maintenance of high PVR is of key importance in providing adequate systemic (and coronary) perfusion in infants with single ventricle physiology such as hypoplastic left heart syndrome. Ventilation with subambient oxygen concentrations (FiO₂ 0.16 to 0.18) or hypercarbic mixtures are frequently used for this purpose. If 100% oxygen is inadvertently administered, the acute decrease in PVR can rapidly lead to pulmonary edema from high Qp and to systemic hypotension and coronary ischemia from low Qs (Figure 58-2).

Figure 58-2 Hypoplastic left heart syndrome. In single ventricle physiology, blood flow from the heart follows the path of least resistance. A, Maintenance of high pulmonary vascular resistance promotes flow across the ductus arteriosus to the aorta, thus supporting systemic and coronary perfusion. B, If a high FiO₂ is inadvertently administered, most of the cardiac output may flow to the lungs at the expense of the systemic circulation.

18 What is the best anesthetic technique for patients with congenital heart disease?

There is no simple recipe for anesthetic management of the patient with congenital heart disease, and many rational approaches can be used. Choices depend on a thorough assessment of the patient’s condition, the type of surgical procedure to be performed, and postoperative goals (e.g., mechanical ventilation vs. extubation). Review of cardiac catheterization and echocardiographic data will provide information concerning shunts, valve function, pulmonary hypertension, and ventricular function. It is necessary to understand the anatomy and pathophysiology of the patient’s cardiac lesion so shunt balance, ventricular function, and rhythm can be better maintained; maintain good oxygenation and ventilation; and be familiar with the cardiovascular effects of anesthetic drugs and avoid cardiovascular depressants. A balanced technique using more than one anesthetic can often ensure adequate anesthesia while minimizing side effects. Be careful!

19 How soon does cardiac function return to normal after surgical repair?

Although restoration of improved cardiac function is a very rewarding aspect of pediatric cardiac surgery, some experts believe that normal function is rarely achieved. Long-term cardiac function following repair of congenital heart disease depends on the duration and severity of cardiac impairment before correction and on the surgical result. Myocardial ischemia and fibrosis associated with obstructive or cyanotic lesions are irreversible changes that can have permanent effects on cardiac function. Residual postoperative valvular stenosis or regurgitation may be associated with progression of cardiac disease. Inadequate myocardial preservation during cardiopulmonary bypass can injure the heart. Even years after straightforward repair of ventricular septal defects, cardiac function has been observed to be abnormal. Therefore, do not assume that cardiac function is normal in a child or adult who has undergone successful surgical repair of congenital heart disease in the past.

20 What is subacute bacterial endocarditis and how can it be prevented?

Turbulent or high-velocity blood flow in the heart associated with congenital heart defects can cause damage to the endocardium of the heart or valves. Damaged endocardium is rough and can be a nidus for infection in the presence of bacteremia or septicemia. Bacteremia can occur during dental or surgical procedures in the oral cavity, the alimentary tract, or the genitourinary system. Prophylactic administration of antibiotics during these procedures has been recommended for many years to prevent the development of endocarditis, but current recommendations of the American Heart Association have been revised to limit such prophylaxis to patients with complex cyanotic lesions.