POSTOPERATIVE PHYSIOLOGY

Most patients with complex cyanotic lesions undergo repair or palliation in infancy, many within the first few days of life. Therefore, the practitioner is more likely to encounter patients with postoperative findings than those with the classic unoperated findings that have pop-ulated textbooks for many years. A wide variety of lesions that are unsuitable for complete correction are now palli-ated to the Fontan circulation, often in three stages (see Chapter 14). The following discussion reviews the phys-iology and typical physical findings of patients at each stage of this palliation.

Lesions with Functionally Single Ventricle After a Systemic-to-Pulmonary Artery Shunt In this physiologic state (also referred to as “Stage I” physi-ology in pediatric cardiphysi-ology circles), blood is pumped from a functionally single ventricle to the aorta,which sup-plies both the systemic circulation and the pulmonary cir-culation. The pulmonary circulation is fed through a tube graft, typically from the base of the innominate artery to the central pulmonary arteries (i.e., a modified Blalock-Taussig shunt) and is the physiologic equivalent of a some-what restrictive PDA. An alternative means of supplying pulmonary blood flow, which has been gaining popularity recently, is the placement of a 4- to 5-mm ventricular-to-pulmonary artery conduit (the Sano modification). In either approach, the ventricle is by definition volume-loaded because it supplies both the systemic and pul-monary circulation;the degree of volume overload depends on the amount of pulmonary blood flow (see Table 3-2).

The more cyanotic patients have less pulmonary blood flow and less volume load, so they have fewer symptoms and signs of CHF and better growth. The opposite is true of less-cyanotic patients with this physiology.

On physical examination, pulse oximetry is typically between 70% and 85% in room air and the baseline value should be noted for future reference. Those with higher saturations are more likely to have tachypnea. In those whose initial palliation involved an arch repair, it is important to determine that the lower extremity blood pressures are not lower than those in the upper extremity.

Recurrent arch obstruction occurs in about 10% of neonates who required an arch repair initially. If a classic Blalock-Taussig shunt, utilizing the subclavian artery instead of a tube graft, was performed, the blood pressure in the affected arm will be unreliable. The classic proce-dure is rarely performed today. Increased respiratory effort will be observed in those with larger volume loads, but the chest is usually clear. The precordial impulse is hyperdynamic. The most typical feature of the exam, if a Blalock-Taussig shunt is used, is a prominent continuous murmur arising from the shunt (commonly loudest under the right clavicle), which may obscure S₂. If heard, S₂is single, because there is not ejection by two ventricles into separate systemic and pulmonary circulations. If a ven-tricular to pulmonary artery conduit is used, a systolic ejection murmur, with or without a diastolic regurgitant murmur, can be heard arising from the conduit. A gallop may be audible in the setting of a large volume load. If atri-oventricular valve regurgitation is present, it may add to the systolic component of the murmur. The liver is enlarged in proportion to the degree of CHF. The femoral pulses should be carefully sought in patients with a history of arch repair. Frequently, the femoral pulses are bound-ing in patients with shunt physiology because of diastolic runoff from the aorta into the pulmonary arteries.

Specific ECG findings depend on the underlying lesion, but it is common to see atrial enlargement and ventricular hypertrophy. Chest x-ray typically shows mild to moderate cardiomegaly, with pulmonary vascular markings in proportion to the amount of blood flow.

Lesions with Functionally Single Ventricle after a Superior Cavopulmonary

Anastomosis

A superior cavopulmonary anastomosis connects the superior vena cava directly to the pulmonary artery (also referred to as “Stage II” physiology in pediatric cardiology circles). Specific operative techniques include the Glenn, the bidirectional Glenn, and the Hemi-Fontan (see Chapter 14). The systemic to pulmonary artery shunt is ligated at the same operation. Blood is no longer pumped to the pulmonary vascular bed, but rather it flows pas-sively and silently from the SVC into the pulmonary arter-ies. Inferior vena cava blood returns to the right atrium, the ventricle, and the aorta without being oxygenated in the lungs. The important physiologic consequence is that the ventricle now pumps only the systemic cardiac

output, and therefore symptoms and signs of heart failure usually improve or resolve. All the techniques involve surgery in the vicinity of the sinus node. Sinus node dys-function is a known long-term complication but is not usually symptomatic in early childhood.

On physical examination, pulse oximetry is typically in the 80s in room air and respiratory rate is normal. The baseline pulse oximetry value should be noted for future reference. Heart rate may be lower than expected for age if there is sinus node dysfunction. In the early post-operative period, frequently there is mild facial edema caused by the acute change in venous pressure, but this resolves within days to weeks. Inspection of jugular veins will demonstrate absence of the usual pulsations and may indicate elevated pressure. The precordium is quiet, and the second heart sound is single. In the absence of atrioventricular or semilunar valve pathology, there should be no murmurs or gallops. The liver is not enlarged, and peripheral pulses are no longer bounding.

ECG findings will vary with the underlying lesion, but signs of atrial enlargement may resolve. In a single right ventricle, criteria for right ventricular hypertrophy or biventricular hypertrophy will persist since the right ventricle is operating at high pressure. Cardiomegaly should resolve or improve after a superior cavopul-monary anastomosis.

Lesions with Functionally Single Ventricle After a Fontan Completion (Total

Cavopulmonary Anastomosis)

A Fontan completion involves incorporating the IVC blood into the pulmonary circulation. Flow is passive and frequently a fenestration is created at the operation between the Fontan pathway and the atrium to allow a small amount of deoxygenated blood to shunt from the venous circulation to the heart (see Chapter 14). The purpose of the fenestration is to decrease venous pres-sure and to augment preload to the ventricle in the immediate postoperative period. The fenestration is designed to be closed later by a transcatheter device if it does not close spontaneously to eliminate the risk of par-adoxical embolism. In Fontan physiology, the ventricle pumps a normal systemic cardiac output and the amount of pulmonary blood flow, which is entirely passive, equals the systemic blood flow minus any small amount that shunts across the fenestration.

On physical examination, pulse oximetry is typically in the high 80s to low 90s if the fenestration is patent. If the fenestration has closed, pulse oximetry will usually be 93% or above. Even with no right-to-left shunt, some patients with a Fontan circulation have sufficient V:Q mismatch to have saturations in the lower 90s. For future reference, it is important to know what saturation is normal for a patient with a Fontan when he or she is

well. Also, significant desaturation implies a baffle leak, decompressing vessels, or development of pulmonary arteriovenous malformations. The heart rate may be low if sinus node dysfunction is present. The respiratory rate is normal. Jugular veins lack the normal pulsations, and the height of the blood column may be elevated. The precordium is quiet and S₂is single. In the absence of atrioventricular or semilunar valve pathology, there should be no murmurs or gallops. Pulses are normal. In a well-functioning Fontan circulation, the liver is not enlarged significantly. (Progressive hepatomegaly is wor-risome for rising Fontan circuit pressures and failure of the Fontan.) Ascites or lower-extremity edema are partic-ularly ominous signs and are often associated with pro-tein-losing enteropathy, a known, although infrequent, complication of the Fontan circulation.

ECG and chest x-ray findings parallel those described for the superior cavopulmonary anastomosis above.

Cross-sectional studies have shown that although serum albumin is usually normal, ALT and AST are mildly ele-vated (at less than two times above the upper limit of normal) in the majority of patients.⁴⁷

Predicting longterm outcome in patients with a Fontan circulation is difficult. A multitude of factors, including surgical techniques, myocardial preservation techniques, intensive care unit management strategies, and age at operation have changed constantly over the past several decades.

Some consistent patterns have emerged and should give the primary practitioner a good idea of what to expect for their patient with a Fontan circulation.⁴⁷ When formal exercise testing is performed, patients with a Fontan circulation have diminished exercise capacity. Cardiac catheterization usually reveals a mildly depressed cardiac index. Subjectively, the majority of patients and their physicians identify either no functional limitations or only slight functional limitations. Sinus node dysfunction and atrial arrhythmias are common. In a cross-sectional study of Fontan survivors⁴⁷at a median of 5.4 years of follow-up, 9.4% of patients had a pace-maker and the prevalence of atrial flutter increased with duration of follow-up, reaching 33% at 10 years. Protein-losing enteropathy is reported in 2.6–13.4% of long-term Fontan survivors,^48–50and late stroke in 4%.⁴⁸ Institu-tional management style varies, but many cardiol-ogists maintain patients with Fontan circulations on antiplatelet or anticoagulant therapy because of the chronic venous stasis and the risk of stroke associated with this physiology. Afterload reduction therapy is also advocated by some, but no randomized controlled trials have been completed that examine this question.

Overall, the majority of patients with a Fontan circulation have good functional status, a remarkable achievement given the severity of the underlying heart disease in this group. Through research efforts and innovation,

practitioners are optimistic that outcomes will continue to improve for current and future generations of patients with single ventricle physiology.

MAJOR POINTS

● Cyanotic congenital heart disease, broadly defined, includes any malformations of the heart causing venous blood to be returned to the arterial side of the circulation without being oxygenated in the lungs.

● Cyanosis is a visible bluish discoloration of the skin and mucous membranes caused by arterial oxygen desaturation of any cause.

● The intensity of cyanosis is related to the concentration of arterial deoxyhemoglobin and depends on both the percent saturation of hemoglobin and the hemoglobin concentration.

● Cyanosis is not perceived consistently until desaturation is profound.

● Complications of chronic right-to-left shunting include stroke, brain abscess, scoliosis, and hyperviscosity syndrome.

● Right-to-left shunting occurs either because hemodynamic forces favor right-to-left flow across a communication between the two sides of the circulation or because the anatomy of the heart dictates return of venous blood to the arterial circulation.

● In most cyanotic lesions, the degree of cyanosis is primarily determined by the ratio of pulmonary to systemic blood flow.

● In cyanotic lesions with transposition physiology, the degree of cyanosis is determined primarily by the extent of mixing of systemic venous and pulmonary venous blood.

● A wide variety of cardiac malformations result in right-to-left shunting.

● Many forms of cyanotic congenital heart disease with only one functional ventricle are managed using the Fontan palliation, wherein all systemic venous blood flows passively to the lungs.

● Eisenmenger’s syndrome results in a reversal of the direction of shunting in acyanotic cardiac

malformations and, consequently, cyanosis.

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Left-to-Right Shunt Lesions