Park's Pediatric Cardiology for Practitioners, 6th Ed.

Pathophysiology of Cyanotic Congenital Heart Defects

Clinical Cyanosis

Detection of Cyanosis

Cyanosis is a bluish discoloration of the skin and mucous membranes resulting from an increased concentration of reduced hemoglobin to about 5 g/100 mL in the cutaneous veins. This level of reduced hemoglobin in the cutaneous vein may result from either desaturation of arterial blood or increased extraction of oxygen by peripheral tissue in the presence of normal arterial saturation (e.g., circulatory shock, hypovolemia, vasoconstriction from cold). Cyanosis associated with desaturation of arterial blood is called central cyanosis; cyanosis with normal arterial oxygen saturation is called peripheral cyanosis.

Cyanosis is more difficult to detect in children with dark pigmentation. Although cyanosis may be detected on many parts of the body, including the lips, fingernails, oral mucous membranes, and conjunctivae, the tip of the tongue is a good place to look for cyanosis; the color of the tongue is not affected by race or ethnic background, and the circulation is not sluggish in the tongue. In a newborn, acrocyanosis may cause confusion. In addition, some newborns are polycythemic, which may contribute to the appearance of cyanosis without arterial desaturation (see later discussion). In older infants and children, chronic subclinical cyanosis produces clubbing.

When in doubt, arterial oxygen saturation should be obtained by a pulse oximeter or arterial Po2 by blood gas determination. Normal Po2 in a 1-day-old infant may be as low as 60 mm Hg. An arterial oxygen saturation of 90% or above does not completely rule out a cyanotic heart defect in a newborn infant. An arterial oxygen saturation of 90% can be seen with a Po2 of 45 to 50 mm Hg in newborns because of the normally leftward oxygen hemoglobin dissociation curve (see later section). In older children and adults, a Po2 of 60 to 65 mm Hg is needed to have 90% oxygen saturation.

Influence of Hemoglobin Level on Cyanosis

The level of hemoglobin greatly influences the occurrence of cyanosis. This effect is graphically illustrated in Figure 11-1. As stated earlier, about 5 g/100 mL of reduced hemoglobin in cutaneous veins is required for the appearance of cyanosis. Normally, about 2 g/100 mL of reduced hemoglobin is present in the venules so that an additional 3 g/100 mL of reduced hemoglobin in arterial blood produces clinical cyanosis. For a normal person with hemoglobin of 15 g/100 mL, 3 g of reduced hemoglobin results from 20% desaturation (because 3 is 20% of 15). Thus, cyanosis appears when the oxygen saturation is reduced to about 80%. Cyanosis is recognized at a higher level of oxygen saturation in patients with polycythemia and at a lower level of oxygen saturation in patients with anemia (see Fig. 11-1). For example, in a person with polycythemia with hemoglobin of 20 g/100 mL, 3 g of reduced hemoglobin results from only 15% desaturation (or at 85% arterial saturation). On the other hand, in a patient with a marked anemia (e.g., hemoglobin of 6 g/100 mL), cyanosis does not appear until arterial oxygen saturation is reduced to 50% (3 g of reduced hemoglobin results from 50% desaturation).


FIGURE 11-1 Influence of hemoglobin (Hgb) levels on clinical recognition of cyanosis. Cyanosis is recognizable at a higher arterial oxygen saturation in patients with polycythemia and at a lower arterial oxygen saturation in patients with anemia. See text for explanation.

BOX 11-1 Causes of Cyanosis

Reduced arterial oxygen saturation (i.e., central cyanosis)

Inadequate Alveolar Ventilation

Central nervous system depression

Inadequate ventilatory drive (e.g., obesity, Pickwickian syndrome)

Obstruction of the airway, congenital or acquired

Structural changes in the lungs or ventilation–perfusion mismatch (e.g., pneumonia, cystic fibrosis, hyaline membrane disease, pulmonary edema, congestive heart failure)

Weakness of the respiratory muscles

Desaturated Blood Bypassing Effective Alveolar Units

Intracardiac right-to-left shunt (i.e., cyanotic congenital heart defect)

Intrapulmonary shunt (e.g., pulmonary arterioventricular fistula, chronic hepatic disease resulting in multiple microvascular fistulas in the lungs)

Pulmonary hypertension with the resulting right-to-left shunt at the atrial, ventricular, or ductal levels (e.g., Eisenmenger’s syndrome, persistent pulmonary hypertension of the newborn)

Increased deoxygenation in the capillaries (i.e., peripheral cyanosis)

Circulatory shock

Congestive heart failure

Acrocyanosis of newborns


Ingestion of toxic substances (well water, aniline dye)

Congenital methemoglobinemia

Causes of Cyanosis

Cyanosis may result from a number of causes (Box 11-1). Central cyanosis (with reduced arterial oxygen saturation) may be caused by cyanotic congenital heart defects, lung disease, or central nervous system (CNS) depression. Cyanosis of cardiac origin must be diagnosed early for proper management, but the detection of mild cyanosis is not always easy.

Rarely, cyanosis is caused by methemoglobinemia. Methemoglobinemia may occur as a hereditary disorder or be caused by toxic substances. Toxic methemoglobinemia (such as that seen with ingestion of water high in nitrate or exposure to aniline teething gels) is more common than hereditary methemoglobinemia (absence of reductive pathways, or NADH-cytochrome b5 reductase deficiency). When methemoglobin (MHg) levels are greater than 15% of normal hemoglobin, cyanosis is visible, and a level of 70% MHg is lethal. Compensatory polycythemia may occur in this condition. In methemoglobinemia, color of the blood may remain brown even after a full oxygenation or a long exposure to room air.

Acrocyanosis, a bluish color of the fingers seen in neonates and infants, is a form of peripheral cyanosis and reflects sluggish blood flow in the fingers. It has no clinical significance unless associated with circulatory shock. Circumoral cyanosis refers to a bluish skin color around the mouth. This is a form of peripheral cyanosis seen in a healthy child with fair skin because of a sluggish capillary blood flow in association with vasoconstriction. Isolated circumoral cyanosis is of no concern unless it occurs as a result of a low cardiac output.

Cyanosis of Cardiac Versus Pulmonary Origin

Differentiation of cardiac cyanosis from cyanosis caused by pulmonary diseases is crucially important for proper management of cyanotic infants. The hyperoxitest helps differentiate cyanosis caused by cardiac disease from that caused by pulmonary disease. In the hyperoxitest, one tests the response of arterial Po2 to 100% oxygen inhalation. With pulmonary disease, arterial Po2 usually rises to a level greater than 100 mm Hg. When there is a significant intracardiac right-to-left shunt, the arterial Po2 does not exceed 100 mm Hg, and the rise is usually not more than 10 to 30 mm Hg, although some exceptions exist. See Chapter 14 for exceptions and further discussion.


FIGURE 11-2 Result of hyperoxitest in cyanotic heart defects. A, Effect of a right-to-left shunt on the arterial PO2 in room air. The mixing of 1 L blood coming from normal ventilated alveoli (PO2 of 100 mm Hg) with 1 L of venous blood flowing through the cardiac defect (PO2 of 30 mm Hg) results in a significant decrease in arterial PO2 (41 mm Hg). B, Effect of a right-to-left shunt on the arterial PO2 in 100% oxygen. The mixing of 1 L of blood coming from normal ventilated alveoli (PO2 of 600 mm Hg) with 1 L of venous blood flowing through the shunt (PO2 of 30 mm Hg) results in arterial PO2 of 46 mm Hg. Breathing 100% oxygen does not significantly influence the hypoxemia because the arterial PO2 increases only from 41 to 46 mm Hg. Note that the oxygen content was calculated using an old number of 1.34 mL (rather than 1.36 mL), which can be bound to 1 g of hemoglobin. See text for a detailed description. (From Duc G: Assessment of hypoxia in the newborn: Suggestions for a practical approach. Pediatrics 48:469-481, 1971.)

Figure 11-2 explains why breathing 100% oxygen does not significantly increase Po2 in the presence of a right-to-left intracardiac shunt. Figure 11-2A, is a schematic illustration of the effect of a right-to-left shunt on the Po2 while breathing in room air. Assuming a cardiac output of 2 L/min, 1 L of venous blood is distributed to ventilated alveoli, and 1 L is shunted right to left through a cardiac defect. Mixing 1 L of venous blood with an oxygen content of 19.4 mL/100 mL (Po2 of 30 mm Hg) with 1 L of pulmonary venous blood containing 26.3 mL/100 mL (Po2 of 100 mm Hg) results in an oxygen content of 22.8 mL/100 mL. The corresponding Po2 from the dissociation curve is 41 mm Hg. Therefore, mixing 1 L of blood with a Po2 of 100 mm Hg with 1 L of blood with a Po2 of 30 mm Hg results in a Po2 of 41 mm Hg (see Fig. 11-2A), not an arithmetic average of 65 mm Hg. With the patient breathing 100% oxygen (see Fig. 11-2B), the alveolar Po2 becomes 600 mm Hg (with a corresponding oxygen content of 28.6 mL/100 mL, assuming a hemoglobin level of 20 g/100 mL; this figure is derived from 26.8 mg/100 mL of oxygen bound to hemoglobin plus 1.8 mL of oxygen dissolved in plasma [0.003 × 600]). When 1 L of blood with a Po2 of 600 mm Hg (oxygen content 28.6 mg/100 mL) is mixed with l L of venous blood with a Po2 of 30 mm Hg (oxygen content of 19.4 mL/100 mL), the resulting oxygen content is 24 mg/100 mL ([28.6 + 19.4]/2), with a corresponding Po2 of 46 mm Hg (see Fig. 11-2B). Thus, breathing 100% oxygen does not significantly alter the Po2 (an increase from 41 to 46 mm Hg) even though the alveolar Po2 increases from 100 to 600 mm Hg.

Hemoglobin Dissociation Curve

A full understanding of the hyperoxitest and the unique behavior of fetal hemoglobin requires the knowledge of the hemoglobin dissociation curve. The relationship between the Po2 and the amount of oxygen bound to hemoglobin and the relationship between the Po2 and the oxygen dissolved in plasma are different. The relationship is S shaped (sigmoid) for hemoglobin; the relationship is linear for plasma. For dissolved oxygen in plasma, the solubility coefficient is 0.003 mL/100 mL at a Po2 of 1 mm Hg at 37°C (or 0.3 mL of oxygen/100 mL plasma at 100 mm Hg of Po2).


FIGURE 11-3 Factors that influence the position of the oxygen-hemoglobin dissociation curve. Curve B is from a normal adult at 38°C, pH 7.40, and PCO2 35.0 mm Hg. Curves A and C illustrate the effect on the affinity for oxygen (P50) of variations in temperature (°C), pH, PCO2, 2,3-diphosphoglycerate (DPG), adenosine triphosphate (ATP), methemoglobin (MHg), and carboxyhemoglobin (CO Hb). Curve A is of the newborn. (From Duc G: Assessment of hypoxia in the newborn: Suggestions for a practical approach. Pediatrics 48:469-481, 1971.)

The sigmoid relationship between the Po2 and the amount of oxygen bound to hemoglobin is expressed by the oxygen-hemoglobin dissociation curve (Fig. 11-3). The Po2 at which 50% of hemoglobin is saturated has been chosen as the reference point, called P50. The P50 averages 27 mm Hg in adults and 22 mm Hg in fetuses and newborns. The position of the dissociation curve is an expression of the affinity of hemoglobin for oxygen. A newborn’s curve (curve A), with high oxygen affinity, favors the extraction of oxygen from the maternal circulation and suits the conditions of the intrauterine environment, but fetal hemoglobin is “stingy” hemoglobin; it does not allow easy release of oxygen to tissues as in adults. The adult curve (curve B), with a decreased affinity for oxygen, allows the release of more oxygen to tissues. The adult curve is reached by 3 months of age.

The pH, Pco2, and erythrocyte concentrations of 2,3-diphosphoglycerate (2,3-DPG), adenosine triphosphate (ATP), MHg, and carboxyhemoglobin influence the position of the dissociation curve (see Fig. 11-3).

1. A decrease in hydrogen ion concentration (or increased pH), Pco2, temperature, 2,3-DPG, and ATP concentrations shifts the curve to the left (curve A).

2. An increase in the above parameters shifts the curve to the right (curve C).

3. Fetal hemoglobin has considerably less affinity for 2,3-DPS (40%) than does the adult hemoglobin. This makes fetal hemoglobin behave as if 2,3-DPG levels are low, thus shifting the curve to the left.

4. The curve shifts to the right in compensation for high altitude, cyanosis, or anemia as a result of an increase in the red cell concentration of 2,3-DPG.

Consequences and Complications

1. Polycythemia. Low arterial oxygen content stimulates bone marrow through erythropoietin release from the kidneys and produces increased number of red blood cells (RBCs). Polycythemia, with a resulting increase in oxygen-carrying capacity, benefits cyanotic children. However, when the hematocrit reaches 65% or higher, a sharp increase in the viscosity of blood occurs, and the polycythemic response becomes disadvantageous, particularly if the patient has congestive heart failure (CHF). Some cyanotic infants have a relative iron deficiency state, with normal or lower than normal hemoglobin and hypochromia on blood smear. A normal hemoglobin in a cyanotic patient represents a relative iron deficiency state. Although less cyanotic, these infants are usually more symptomatic and improve when iron therapy raises the hemoglobin.

2. Clubbing. Clubbing is caused by soft tissue growth under the nail bed as a consequence of central cyanosis. The mechanism for soft tissue growth is unclear. One hypothesis is that megakaryocytes present in the systemic venous blood may be responsible for the change. In normal persons, platelets are formed from the cytoplasm of the megakaryocytes by fragmentation during their passage through the pulmonary circulation. The cytoplasm of megakaryocytes contains growth factors (e.g., platelet-derived growth factor and transforming growth factor B). In patients with right-to-left shunts, megakaryocytes with their cytoplasm may enter the systemic circulation, become trapped in the capillaries of the digits, and release growth factors, which in turn cause clubbing. Clubbing usually does not occur until a child is 6 months or older, and it is seen first and is most pronounced in the thumb. In the early stage, it appears as shininess and redness of the fingertips. When fully developed, the fingers and toes become thick and wide and have convex nail beds (see Fig. 2-1). Clubbing is also seen in patients with liver disease or subacute bacterial endocarditis and on a hereditary basis without cyanosis.

3. CNS complications. Either very high hematocrit levels or iron-deficient RBCs place individuals with cyanotic congenital heart defects at risk for disorders of the CNS, such as brain abscess and vascular stroke. In the past, cyanotic congenital heart diseases accounted for 5% to 10% of all cases of brain abscesses. The predisposition for brain abscesses may partially result from the fact that right-to-left intracardiac shunts may bypass the normally effective phagocytic filtering actions of the pulmonary capillary bed. This predisposition may also result from the fact that polycythemia and the consequent high viscosity of blood lead to tissue hypoxia and microinfarction of the brain, which are later complicated by bacterial colonization. The triad of symptoms of brain abscesses are fever, headache, and focal neurologic deficit.

Vascular stroke caused by embolization arising from thrombus in the cardiac chamber or in the systemic veins may be associated with surgery or cardiac catheterization. Cerebral venous thrombosis may occur, often in infants younger than 2 years of age who have cyanosis and relative iron-deficiency anemia. A possible explanation for these findings is that microcytosis further aggravates hyperviscosity resulting from polycythemia.

4. Bleeding disorders. Disturbances of hemostasis are frequently present in children with severe cyanosis and polycythemia. Most frequently noted are thrombocytopenia and defective platelet aggregation. Other abnormalities include prolonged prothrombin time and partial thromboplastin time and lower levels of fibrinogen and factors V and VIII. Clinical manifestations may include easy bruising, petechiae of the skin and mucous membranes, epistaxis, and gingival bleeding. RBC withdrawal from polycythemic patients and replacement with an equal volume of plasma tend to correct the hemorrhagic tendency and lower blood viscosity.

5. Hypoxic spells and squatting. Although most frequently seen in infants with tetralogy of Fallot (TOF), hypoxic spells may occur in infants with other congenital heart defects (see a later section of TOF for further discussion).

6. Scoliosis. Children with chronic cyanosis, particularly girls and patients with TOF, often have scoliosis.

7. Hyperuricemia and gout. Hyperuricemia and gout tend to occur in older patients with uncorrected or inadequately repaired cyanotic heart defects.

The pathophysiology of individual cyanotic heart defects is discussed in the section to follow.


FIGURE 11-4 Circulation pathways of normal “in series” circulation (A) and the “in parallel” circulation of transposition of the great arteries (B). Open arrows indicate oxygenated blood; closed arrows indicate desaturated blood. AO, aorta; LA, left atrium; LV, left ventricle; PA, pulmonary artery; RA, right atrium; RV, right ventricle.


FIGURE 11-5 Diagrammatic presentation of the hemodynamics of transposition of the great arteries with inadequate mixing (A) and with good mixing at the atrial level (B) Numbers within the diagram denote oxygen saturation values, and those outside the diagram denote pressure values. AO, aorta; LA, left atrium; LV, left ventricle; PA, pulmonary artery; PV, pulmonary vein; RA, right atrium; RV, right ventricle; VC, vena cava.

Common Cyanotic Heart Defects

Complete Transposition of the Great Arteries

Complete transposition of the great arteries (D-TGA) is the most common cyanotic congenital heart defect in newborns, at least in Western countries. In this condition, the aorta arises from the right ventricle (RV), and the pulmonary artery (PA) arises from the left ventricle (LV). As the result, the normal anteroposterior relationship of the great arteries is reversed, so that the aorta is anterior to the PA (transposition) but the aorta remains to the right of the PA; thus, the prefix D is used for dextroposition. In levotransposition of the great arteries (L-TGA, or congenitally corrected TGA), the aorta is anterior to and to the left of the PA; therefore, the prefix L is used (see Chapter 14). The atria and ventricles are in normal relationship. The coronary arteries arise from the aorta, as in a normal heart. Desaturated blood returning from the body to the right atrium (RA) flows out of the aorta without being oxygenated in the lungs and then returns to the RA. Therefore, tissues, including vital organs such as the brain and heart, are perfused by blood with a low oxygen saturation. Conversely, well-oxygenated blood returning to the left atrium (LA) flows out of the PA and returns to the LA. This results in a complete separation of the two circuits. The two circuits are said to be in parallel rather than in series, as in normal circulation (Fig. 11-4). This defect is incompatible with life unless communication between the two circuits occurs to provide the necessary oxygen to the body. This communication can occur at the atrial, ventricular, or ductal level or at any combination of these levels.

In the most frequently encountered form of D-TGA, only a small communication exists between the atria, usually a patent foramen ovale (PFO) (Fig. 11-5A). The newborn is notably cyanotic from birth and has an arterial oxygen saturation of 30% to 50%. The low arterial Po2, which ranges from 20 to 30 mm Hg, causes an anaerobic glycolysis, with resulting metabolic acidosis. Hypoxia and acidosis are detrimental to myocardial function. The normal postnatal decrease in pulmonary vascular resistance (PVR) results in increased pulmonary blood flow (PBF) and volume overload to the LA and LV. Severe hypoxia and acidosis (with a resulting decrease in myocardial function) and volume overload to the left side of the heart cause CHF during the first week of life. Therefore, chest radiographs show cardiomegaly and increased pulmonary vascularity. Unless hypoxia and acidosis are corrected, the condition of these infants deteriorates rapidly. Hypoxia and acidosis stimulate the carotid and cerebral chemoreceptors, causing hyperventilation and a low Pco2 in the pulmonary circulation. Other metabolic problems encountered are hypoglycemia, which is probably secondary to pancreatic islet hypertrophy and hyperinsulinism, and a tendency toward hypothermia. The electrocardiogram (ECG) shows right ventricular hypertrophy (RVH), but RVH may be difficult to diagnose in the first days of life because of the normal dominance of the RV at this age. Usually no heart murmur is noted in a neonate with D-TGA, although a murmur is commonly found in other forms of cyanotic heart defects. The S2 is single, mainly because the pulmonary valve is farther from the chest wall, causing the P2 to be inaudible. A deeply cyanotic newborn with increased pulmonary vascular markings and cardiomegaly without heart murmur can be considered to have TGA until proved otherwise.


FIGURE 11-6 Diagrammatic presentation of the hemodynamic abnormalities in transposition of the great arteries with a large ventricular septal defect (VSD) (A) and with VSD and pulmonary stenosis. (B) Numbers within the diagram denote oxygen saturation values, and those outside the diagram denote pressure values. Abbreviations are the same as those in Figure 11-5.

The presence of a large atrial septal defect (ASD) is most desirable in infants with TGA. When a large ASD is present, infants have good arterial oxygen saturation (as high as 80% to 90%) because of good mixing (Fig. 11-5B). Therefore, hypoxia and metabolic acidosis are not the problems in these children. In fact, the idea of the balloon atrial septostomy (Rashkind procedure) was derived from the natural history of infants with TGA and large ASDs. However, the frequency of a large ASD occurring naturally in TGA is low. Infants who have had successful balloon atrial septostomies behave like those with naturally occurring ASDs. As the PVR falls after birth, PBF increases, with an increase in the size of the LA and LV. Although these infants are not hypoxic or acidotic, CHF develops because of volume overload to the left side of the heart. Because the RV is the systemic ventricle, RVH becomes evident on the ECG.

When associated with a large ventricular septal defect (VSD), only minimal arterial desaturation is present, and cyanosis may be missed (Fig. 11-6A). Therefore, metabolic acidosis does not develop, but left-sided heart failure results within the first few weeks of life as the PBF increases with decreasing PVR. Chest radiographs reflect this, showing cardiomegaly with increased pulmonary vascularity. The ECG may show biventricular hypertrophy (BVH) when the VSD is large: RVH because of the systemic RV and left ventricular hypertrophy (LVH) because of volume overload of the left side of the heart. A heart murmur of VSD is present, and the S2 is single because the P2 is inaudible or pulmonary hypertension is present.

When the VSD is associated with pulmonary stenosis (PS) in infants with TGA, although the VSD helps good mixing, the volume of fully saturated blood returning from the lungs is inadequate (see Fig. 11-6B). Likewise, even after a well-performed Rashkind procedure, the arterial oxygen saturation does not increase much because of the decreased PBF. These infants have severe hypoxia and acidosis and may succumb early in life. This is a good illustration of how the magnitude of PBF affects the arterial oxygen saturation in a given cyanotic congenital heart defect. Because PBF is not increased, the left cardiac chambers are not under increased volume work; therefore, cardiac enlargement and CHF do not develop. Chest radiographs, therefore, show normal heart size and normal or decreased pulmonary vascularity. The ECG shows evidence of BVH; LVH is present because of PS, and RVH is present because of the nature of TGA. Physical examination reveals a PS murmur and a single S2 in addition to cyanosis.


FIGURE 11-7 Diagrammatic presentation of persistent truncus arteriosus (A) and a common form of single ventricle (B). AO, aorta; LA, left atrium; LV, left ventricle; PA, pulmonary artery; PV, pulmonary vein; RA, right atrium; RV, right ventricle; SV, single ventricle; TA, truncus arteriosus; VC, vena cava.

Persistent Truncus Arteriosus and Single Ventricle

In persistent truncus arteriosus (Fig. 11-7A), a single arterial blood vessel (truncus arteriosus) arises from the heart. The PA or its branches arise from the truncus arteriosus, and the truncus continues as the aorta. A large VSD is always present in this condition. In single ventricle (also called “double-inlet ventricle”) (Fig. 11-7B), two atrioventricular (AV) valves empty into a single ventricular chamber from which a great artery (either the aorta or PA) arises. The other great artery arises from a rudimentary ventricular chamber attached to the main ventricle. The opening between the single ventricle and the rudimentary chamber is called the “bulboventricular foramen.” No ventricular septum of significance is present (see Fig. 14-61).

The following similarities exist between persistent truncus arteriosus and single ventricle from a hemodynamic point of view:

1. There is almost complete mixing of systemic and pulmonary venous blood in the ventricle, and the oxygen saturation of blood in the two great arteries is similar.

2. Pressures in both ventricles are identical.

3. The level of oxygen saturation in the systemic circulation is proportional to the magnitude of PBF.

In addition to the level of PVR, the magnitude of PBF is determined by the caliber of the PA in the case of persistent truncus arteriosus and by the presence or absence of PS and the size of the VSD (i.e., the bulboventricular foramen) in the case of single ventricle. When the PBF is large, the patient is minimally cyanotic but may develop CHF because of an excessive volume overload placed on the ventricle. In contrast, when the PBF is small, the patient is severely cyanotic and does not develop CHF because there is no volume overload. This latter group of patients and those with TOF share similar clinical pictures.

Physical examination reveals varying degrees of cyanosis, depending on the magnitude of PBF. A heart murmur of the VSD is rarely audible because of the presence of a huge defect. There may be an ejection systolic murmur caused by the stenosis of the pulmonary valve or of the PA branches. An early diastolic murmur of truncal valve regurgitation may be heard. The ECG usually shows BVH in both conditions. In single ventricle, the QRS complexes of all precordial leads (i.e., V1–V6) are recorded over one ventricle, and therefore, they are similar (with poor R/S progression), suggestive of BVH. Chest radiographic findings are determined by the magnitude of PBF—if the magnitude of the PBF is large, the heart size is large, and the pulmonary vascularity increases; if the magnitude is small, the heart size is small, and the pulmonary vascularity decreases. With increased PBF and resulting pulmonary hypertension, CHF and later pulmonary vascular obstructive disease (i.e., Eisenmenger’s syndrome) may develop.


FIGURE 11-8 Hemodynamics of acyanotic (A) and cyanotic (B) tetralogy of Fallot. Numbers within the diagram denote oxygen saturation values, and those outside the diagram denote pressure values. In both conditions, the systolic pressure in the right ventricle (RV) is identical to that in the left ventricle (LV) and the aorta (AO) and there is a significant pressure gradient between the RV and the pulmonary artery (PA). Whereas in the acyanotic form (A), pulmonary blood flow is slightly to moderately increased, in the cyanotic form (B), pulmonary blood flow is decreased. Other abbreviations are the same as those in Figure 11-5.

Tetralogy of Fallot

The classic description of TOF includes the following four abnormalities: VSD, pulmonary stenosis (PS), right ventricular hypertrophy (RVH), and overriding of the aorta. From a physiologic point of view, TOF requires only two abnormalities—a VSD large enough to equalize systolic pressures in both ventricles and a stenosis of the right ventricular outflow tract (RVOT) in the form of infundibular stenosis, valvular stenosis, or both. RVH is secondary to PS, and the degree of overriding of the aorta varies widely and it is not always present. The severity of the RVOT obstruction determines the direction and the magnitude of the shunt through the VSD. With mild stenosis, the shunt is left to right, and the clinical picture resembles that of a VSD. This is called acyanotic or pink TOF (Fig. 11-8A). With a more severe stenosis, the shunt is right to left, resulting in “cyanotic” TOF (Fig. 11-8B). In the extreme form of TOF, the pulmonary valve is atretic, with right-to-left shunting of the entire systemic venous return through the VSD. In this case, the PBF is provided through a patent ductus arteriosus (PDA) or multiple collateral arteries arising from the aorta. In TOF, regardless of the direction of the ventricular shunt, the systolic pressure in the RV equals that of the LV and the aorta (see Fig. 11-8A and B). The mere combination of a small VSD and a PS is not TOF; the size of the VSD must be nearly as large as the annulus of the aortic valve to equalize the pressure in the RV and LV.

In acyanotic TOF, a small to moderate left-to-right ventricular shunt is present, and the systolic pressures are equal in the RV, LV, and aorta (see Fig. 11-8A). There is a mild to moderate pressure gradient between the RV and PA, and the PA pressure may be slightly elevated (because of a less severe stenosis of the right ventricular outflow tract). Because the presence of the PS minimizes the magnitude of the left-to-right shunt, the heart size and the pulmonary vascularity increase only slightly to moderately. These increases are indistinguishable from those of a small to moderate VSD. However, unlike VSDs, the ECG always shows RVH because the RV pressure is always high. Occasionally, LVH is also present. The heart murmurs are caused by the PS and the VSD. Therefore, the murmur is a superimposition of an ejection systolic murmur of PS and a regurgitant systolic murmur of a VSD. The murmur is best audible along the lower left and mid-left sternal borders, and it sometimes extends to the upper left sternal border. Therefore, in a child who has physical and radiographic findings similar to those of a small VSD, the presence of RVH or BVH on the ECG should raise the possibility of acyanotic TOF. (A small VSD is associated with LVH or a normal ECG rather than with RVH or BVH). Right aortic arch, if present, confirms the diagnosis. Infants with acyanotic TOF become cyanotic over time, usually by 1 or 2 years of age, and have clinical pictures of cyanotic TOF, including exertional dyspnea and squatting.


FIGURE 11-9 Comparison of ejection systolic murmurs in tetralogy of Fallot (A) and isolated pulmonary valve stenosis (B) (see text). EC, ejection click.

In infants with classic cyanotic TOF, the presence of severe PS produces a right-to-left shunt at the ventricular level (i.e., cyanosis) with decreased PBF (see Fig. 11-8B). The PAs are small, and the LA and LV may be slightly smaller than normal because of a reduction in the pulmonary venous return to the left side of the heart. Therefore, chest radiograph films show a normal heart size with decreased pulmonary vascularity. The systolic pressures are identical in the RV, LV, and aorta. The ECG demonstrates RVH because of the high pressure in the RV. The right-to-left ventricular shunt is silent, and that the heart murmur audible in this condition originates in the PS (ejection-type murmur). The ejection systolic murmur is best audible at the mid-left sternal border (over the infundibular stenosis) or occasionally at the upper left sternal border (in patients with pulmonary valve stenosis). The intensity and the duration of the heart murmur are proportional to the amount of blood flow through the stenotic valve. When the PS is mild, a relatively large amount of blood goes through the stenotic valve (with a relatively small right-to-left ventricular shunt), thereby producing a loud, long systolic murmur (Fig. 11-9A). However, with severe PS, there is a relatively large right-to-left ventricular shunt that is silent, and only a small amount of blood goes through the PS, thereby producing a short, faint systolic murmur (see Fig. 11-9A). In other words, the intensity and duration of the systolic murmur are inversely related to the severity of the PS. These findings are in contrast to those seen in isolated PS (Fig. 11-9A and B). Because of low pressure in the PA, the P2 is soft and often inaudible, resulting in a single S2. The heart size on chest radiograph films is normal in TOF because none of the heart chambers handle an increased amount of blood. If a cyanotic infant has a large heart on the chest radiograph films, especially with an increase in pulmonary vascularity, TOF is extremely unlikely unless the child has undergone a large systemic-to-PA shunt operation. Another important point is that an infant with TOF does not develop CHF. This is because no cardiac chamber is under volume overload, and the pressure overload placed on the RV (not higher than the aortic pressure, which is under baroreceptor control) is well tolerated.

The extreme form of TOF is that associated with pulmonary atresia, in which the only source of PBF is through a constricting PDA or through multiple aortic collateral arteries (feeding into pulmonary arteries). All systemic venous return is shunted right to left at the ventricular level, resulting in a marked systemic arterial desaturation. Probably the more important reason for such severe cyanosis is the markedly reduced PBF, with resulting reduction of pulmonary venous return to the left side of the heart. Unless the patency of the ductus is maintained, the infant may die. Infusion of prostaglandin E1 has been successful in keeping the ductus open in this and other forms of cyanotic congenital heart defects that rely on the patency of the ductus arteriosus for PBF. Heart murmur is absent, or a faint murmur of PDA is present. RVH is present on the ECG as in other forms of TOF. Chest radiographs show a small heart and a markedly reduced PBF.


FIGURE 11-10 Simplified concept of tetralogy of Fallot that demonstrates how a change in the systemic vascular resistance (SVR) or right ventricular outflow tract obstruction (pulmonary resistance [PR]) affects the direction and the magnitude of the ventricular shunt. AO, aorta; LV, left ventricle; PA, pulmonary artery; RV, right ventricle.

It is important to understand what determines the amount of PBF, which in turn determines the degree of cyanosis, in patients with TOF because this concept relates to the mechanism of the “hypoxic” spell of TOF. Because the VSD of TOF is large enough to equalize systolic pressures in both ventricles, the RV and LV may be viewed as a single chamber that ejects blood to the systemic and pulmonary circuits (Fig. 11-10). The ratio of flows to the pulmonary and systemic circuits (Qp/Qs) is related to the ratio of resistance offered by the right ventricular outflow obstruction (shown as pulmonary resistance [PR] in Fig. 11-10) and the systemic vascular resistance (SVR). Either an increase in the pulmonary resistance or a decrease in the SVR will increase the degree of the right-to-left shunt, producing a more severe arterial desaturation. On the contrary, more blood passes through the right ventricular outflow obstruction when the SVR increases or when the pulmonary resistance decreases. Although controversies exist over the role of the spasm of the RVOT as an initiating event for the hypoxic spell, there is no evidence that the spasm actually occurs as a primary event. Pulmonary valve stenosis has a fixed resistance and does not produce spasm. The infundibular stenosis, which consists of disorganized muscle fibers intermingled with fibrous tissue, is almost nonreactive to sympathetic stimulation or catecholamines. Hypoxic spell also occurs in patients with TOF with pulmonary atresia in which the presence or absence of spasm would have no role in the spell. Therefore, it is more likely that changes in the SVR plays a primary role in controlling the degree of the right-to-left shunt and the amount of PBF. A decrease in the SVR increases the right-to-left shunt and decreases the PBF with a resulting increase in cyanosis. In this case, the RVOT dimension may decrease, but it is likely secondary to the decreased amount of blood flowing through it rather than primary spasm. Conversely, an increase in SVR decreases the right-to-left shunt and forces more blood through the stenotic RVOT. This results in an improvement in the arterial oxygen saturation. Therefore, the likelihood of the RVOT spasm initiating the right-to-left shunt is remote. Also, excessive tachycardia or hypovolemia can increase the right-to-left shunt through the VSD, resulting in a fall in the systemic arterial oxygen saturation. The resulting hypoxia can initiate the hypoxic spell. Tachycardia or hypovolemia may narrow down the RVOT, and hypovolemia with reduction of blood pressure can initiate a hypoxic spell by increasing right-to-left ventricular shunt. Slowing of the heart rate by β-adrenergic blockers, volume expansion, or interventions that increase the SVR have all been used to terminate the hypoxic spell.

The hypoxic spell, also called the cyanotic spell, tet spell, or hypercyanotic spell, occurs in young infants with TOF. It consists of hyperpnea (i.e., rapid and deep respiration), worsening cyanosis, and disappearance of the heart murmur. This occasionally results in complications of the CNS and even death. Any event such as crying, defecation, or increased physical activity that suddenly lowers the SVR or produces a large right-to-left ventricular shunt may initiate the spell and, if not corrected, establishes a vicious circle of hypoxic spells (Fig. 11-11). The sudden onset of tachycardia or hypovolemia can also cause the spell as discussed earlier. The resulting fall in arterial Po2, in addition to an increase in Pco2 and a fall in pH, stimulates the respiratory center and produces hyperpnea. The hyperpnea, in turn, makes the negative thoracic pump more efficient and results in an increase in the systemic venous return to the RV. In the presence of fixed resistance at the RVOT (i.e., pulmonary resistance) or decreased SVR, the increased systemic venous return to the RV must go out the aorta. This leads to a further decrease in the arterial oxygen saturation, which establishes a vicious circle of hypoxic spells (see Fig. 11-11).


FIGURE 11-11 Mechanism of hypoxic spell. A decrease in the arterial PO2 stimulates the respiratory center, and hyperventilation results. Hyperpnea increases systemic venous return. In the presence of a fixed right ventricular outflow tract (RVOT), the increased systemic venous return results in increased right-to-left (R-L) shunt, worsening cyanosis. A vicious circle is established. SVR, systemic vascular resistance.

Treatment of hypoxic spells is aimed at breaking this circle by using one or more of the following maneuvers:

1. Picking up the infant in such a way that the infant assumes the knee–chest position and traps systemic venous blood in the legs, thereby temporarily decreasing systemic venous return and helping to calm the baby. The knee–chest position may also increase SVR by reducing arterial blood flow to the lower extremities.

2. Morphine sulfate suppresses the respiratory center and abolishes hyperpnea.

3. Sodium bicarbonate (NaHCO3) corrects acidosis and eliminates the respiratory center–stimulating effect of acidosis.

4. Administration of oxygen may slightly improve arterial oxygen saturation.

5. Vasoconstrictors such as phenylephrine raise SVR and improve arterial oxygen saturation.

6. Ketamine is a good drug to use because it simultaneously increases SVR and sedates the patient. Both effects are known to help terminate the spell.

7. Propranolol has been used successfully in some cases of hypoxic spell, both acute and chronic. Its mechanism of action is not entirely clear. When administered for acute cases, propranolol may slow the heart rate and perhaps reduce the spasm of the RVOT (although not likely as discussed earlier). More important, propranolol may also increase SVR by antagonizing the vasodilating effects of β-adrenergic stimulation. The successful use of propranolol in the prevention of hypoxic spells is more likely the result of the drug’s peripheral action. The drug may stabilize vascular reactivity of the systemic arteries, thereby preventing a sudden decrease in SVR (see Chapter 14).

Infants and toddlers with untreated TOF often assume a squatting position after playing hard. During playing, these infants become tachypneic and dusky. When they assume a squatting position and rest a little while, these symptoms disappear, and then they resume playing. What is the mechanism of recovery from these symptoms during squatting? The squatting position is the same as the knee–chest position (which is used to treat hypoxic spells). Squatting or the knee–chest position increases systemic arterial oxygen saturation as shown in an experimental study (Fig. 11-12). Three mechanisms may be involved. First, reduction of the systemic venous return by trapping venous blood in the lower extremities reduces right-to-left shunt at the ventricular level (evidenced by a reduced arterial lactate levels in Fig. 11-12). Second, a reduced arterial blood flow to the legs reduces venous washout from the leg muscles. Third, squatting might also increase SVR, a known mechanism to reduce right-to-left ventricular shunt.

Tricuspid Atresia

In tricuspid atresia, the tricuspid valve and a portion of the RV do not exist (Fig. 11-13). Because no direct communication exists between the RA and RV, systemic venous return to the RA must be shunted first to the LA through an ASD or PFO. There is usually a VSD (or PDA) for the pulmonary arteries to receive some blood for survival. For the right-to-left shunt to occur, the RA pressure is elevated in excess of the LA pressure, and enlargement of the RA results (i.e., right atrial hypertrophy [RAH] on the ECG and right atrial enlargement on chest radiographs). The LA and LV receive both systemic and pulmonary venous returns and thereby dilate (i.e., enlargement of the LA and LV on chest radiographs). The volume overload placed on the LV is unopposed by the hypoplastic RV, with resulting LVH on the ECG. Therefore, the ECG shows RAH and LVH, and the chest radiographs show an enlargement of the RA, LA, and LV. In addition, a “superior” QRS axis is a characteristic ECG finding in tricuspid atresia, as in endocardial cushion defect. Embryologically there is a similarity between these two defects; tricuspid atresia results from an abnormal development of the endocardial cushion tissue, that is, an incomplete shift of the common atrioventricular (AV) canal to the right. Developmental abnormalities in the endocardial cushion tissue may explain the similar QRS axis in both conditions.


FIGURE 11-12 Hemodynamic changes with squatting. An adult patient with tetralogy of Fallot was studied during cardiac catheterization with determinations of arterial oxygen saturation and arterial lactate levels. The latter was used as an indicator of the change in the systemic venous return. With exercise, there is an immediate drop in the arterial saturation and an increase in systemic venous return. With squatting (a knee–chest position), there is an immediate rise in arterial oxygen saturation and a drop in systemic venous return.(From Guntheroth WG, et al. Venous return with knee-chest position and squatting in tetralogy of Fallot. Am Heart J 75:313-318, 1968.)


FIGURE 11-13 Hemodynamics of tricuspid atresia with normally related (A) and transposed (B) great arteries. Numbers within the diagram denote oxygen saturations, and those outside the diagram denote pressure values. Abbreviations are the same as those in Figure 11-5.

Oxygen saturation values are equal in the aorta and the PA because there is a complete mixing of systemic and pulmonary venous blood in the LV, from which both the systemic and the pulmonary circulations receive blood. The level of arterial saturation directly relates to the magnitude of PBF. Anatomically, the great arteries are normally related in about 70% of cases and transposed in about 30% of cases (Fig. 11-13Aand B). In patients with normally related great arteries (see Fig. 11-13A), the PBF is generally reduced because it comes through a small VSD, hypoplastic RV, or small PAs. Therefore, arterial oxygen saturation is low, and the infant is notably cyanotic. In infants with transposed great arteries (see Fig. 11-13B) the PBF is usually increased. Therefore, these infants are only mildly cyanotic; their heart size is large, and their pulmonary vascular markings are increased. However, because of an interplay of other factors such as the size of the VSD, the presence or absence of PS or pulmonary atresia, as well as the patency of the ductus arteriosus, some infants with normally related great arteries may have increased PBF, and some infants with TGA may have decreased PBF. The magnitude of PBF determines not only the level of arterial oxygen saturation but also the degree of enlargement of the cardiac chambers.

No physical findings are characteristic of tricuspid atresia. These infants have varying degrees of cyanosis, and most have a heart murmur of VSD. A PS murmur, if present, is characteristic. In patients with increased PBF, an increased amount of blood passing through the mitral valve may produce an apical diastolic rumble. The liver may be enlarged because of increased pressure in the RA, which may result from an inadequate interatrial communication or heart failure.

In summary, tricuspid atresia is the most likely diagnosis if a cyanotic infant has an ECG that shows a “superior” QRS axis, RAH, and LVH and chest radiographic films that show enlargement of the RA (with or without left atrial enlargement), a concave PA segment, and decreased pulmonary vascularity.

Pulmonary Atresia

In pulmonary atresia, direct communication between the RV cavity and the PA does not exist; the PDA (or collateral arteries) is the major source of blood flow to the lungs. The systemic venous return to the RA must go to the LA through an ASD or a PFO. The RA enlarges and hypertrophies to maintain a right-to-left atrial shunt (resulting in right atrial enlargement on radiographic films and RAH on the ECG). The RV is usually hypoplastic with a thick ventricular wall, but occasionally the RV is normal in size; tricuspid regurgitation (TR) is usually present in the latter situation. Systemic and pulmonary venous returns mix in the LA and go to the LV to supply the body and the lungs (Fig. 11-14). The volume load placed on the left side of the heart (i.e., LA and LV) is proportionally related to the magnitude of PBF. Because the PDA is the major source of PBF and it may close after birth, the PBF is usually decreased. When multiple collateral arteries are the only source of PBF, they are usually not adequate and PBF is reduced. Therefore, the infant is severely cyanotic, and the overall heart size is normal or only slightly increased. The hypoplasia of the RV and possible volume overload to the LV produce LVH on the ECG.

The infant is usually notably cyanotic, and the S2 is single because there is only one semilunar valve to close. A faint, continuous murmur of PDA may be present. Closure of the ductus results in a rapid deterioration of the infant’s condition unless there are enough collateral arteries supplying PBF. Reopening or maintaining the patency of the ductus arteriosus with infusion of prostaglandin E1 increases the PBF, improves cyanosis, and stabilizes the infant’s condition.


FIGURE 11-14 Hemodynamics of pulmonary atresia. The chambers that enlarge are similar to those in tricuspid atresia; therefore, radiographic findings are similar in tricuspid atresia and pulmonary atresia. The electrocardiogram also shows left ventricular hypertrophy but without the characteristic “superior” QRS axis of tricuspid atresia. Because of the decreased pulmonary blood flow, the aortic saturation is low, and the infant is notably cyanotic. Abbreviations are the same as those in Figure 11-5.

In summary, a severely cyanotic newborn with decreased pulmonary vascularity and normal or slightly enlarged heart size on chest radiographic films and RAH or biatrial hypertrophy (BAH) and LVH on the ECG may have pulmonary atresia. The QRS axis is usually normal, in contrast to the “superior” QRS axis seen in tricuspid atresia. Either a faint, continuous murmur of PDA or a soft regurgitant systolic murmur of TR may be present.

Total Anomalous Pulmonary Venous Return

In total anomalous pulmonary venous return (TAPVR), the pulmonary veins drain abnormally to the RA, either directly or indirectly through its venous tributaries. An ASD is usually present to send blood from the RA to the LA and LV. Depending on the drainage site, TAPVR may be divided into the following three types (see Fig. 14-30):

1. Supracardiac: The common pulmonary vein drains to the superior vena cava through the vertical vein and the left innominate vein.

2. Cardiac: The pulmonary veins empty into the RA directly or indirectly through the coronary sinus.

3. Infracardiac (or subdiaphragmatic): The common pulmonary vein traverses the diaphragm and drains into the portal or hepatic vein or the inferior vena cava.

Physiologically, however, TAPVR may be divided into two types, obstructive and nonobstructive, depending on the presence or absence of an obstruction to the pulmonary venous return. The infracardiac type is usually obstructive, and the majority of the cardiac and supracardiac types are nonobstructive.

The hemodynamics of the nonobstructive types of TAPVR are similar to those of a large ASD. The amount of blood that goes to the LA through the ASD, rather than to the RV, is determined by the size of the interatrial communication and the relative compliance of the ventricles. Because right ventricular compliance normally increases after birth, with a rapid fall in PVR, and the ASD may be inadequate in size, more blood enters the RV than the LA. Thus, volume overload of the right side of the heart and the pulmonary circulation results, with enlargement of the RA, RV, PA, and pulmonary veins (Fig. 11-15A). Chest radiographs show this as an enlargement of the RA and RV, a prominent PA segment, and increased pulmonary vascular markings. The pressures in the RA, RV, and PA are slightly elevated. The ECG shows right bundle branch block or RVH, as in secundum ASD, and occasional RAH. Because there is complete mixing of systemic and pulmonary venous blood in the RA, oxygen saturation values are almost identical in the aorta and the PA. Cardiac examination reveals an ejection systolic murmur of PS (at the upper left sternal border) and a diastolic murmur of tricuspid stenosis, because the pulmonary and tricuspid valves handle three arrows (see Fig. 11-15A). The S2 splits widely for the same reasons as it does for ASD. This contributes to the characteristic “quadruple” rhythm of TAPVR, which consists of an S1, a widely split S2, and an S3 or S4. Children with large PBF are only minimally desaturated, and cyanosis is often missed because the arterial oxygen saturation ranges from 85% to 90% (see Fig. 11-15A).


FIGURE 11-15 Hemodynamics of total anomalous pulmonary venous return without (A) and with (B) obstruction to the pulmonary venous return. In the nonobstructive type (A), the hemodynamics are similar to those of a large atrial septal defect, with the exception of a mild systemic arterial desaturation. In the obstructive type (B), the hemodynamics are characterized by pulmonary venous hypertension, pulmonary edema, pulmonary arterial hypertension, and marked arterial desaturation. The heart size is not enlarged on chest radiographs. Severe right ventricular hypertrophy is present on the electrocardiogram. Abbreviations are the same as those in Figure 11-5.

If there is an obstruction to the pulmonary venous return, the hemodynamic consequences are notably different from those without pulmonary venous obstruction. The obstruction to the pulmonary venous return causes pulmonary venous hypertension and secondary PA and RV hypertension (see Fig. 11-15B), a situation similar to that seen in mitral stenosis (see Fig. 10-4). Pulmonary edema results when the hydrostatic pressure in the capillaries exceeds the osmotic pressure of the blood. As long as a large ASD permits a right-to-left shunt, the RV cavity remains relatively small (i.e., smaller than one arrow). This is because the RV hypertension prevents the RV compliance from increasing, and the PVR remains elevated. Therefore, chest radiographs show a relatively small heart and characteristic patterns of pulmonary venous congestion or pulmonary edema (i.e., “ground-glass” appearance). The ECG reflects the high pressure in the RV (i.e., RVH). The oxygen saturation values are equal in the aorta and the PA because of the complete mixing of systemic and pulmonary venous return at the RA level, and the arterial saturation is much lower than that found in patients without obstruction. The degree of arterial desaturation or cyanosis inversely relates to the amount of PBF. Infants with obstruction have severe cyanosis and respiratory distress. The latter results from pulmonary edema and may cause pulmonary crackles on auscultation. The pulmonary valve closure sound (P2) is loud because of pulmonary hypertension, which results in a single, loud S2. The heart murmur may be absent or soft because of normal or decreased flow through the pulmonary or tricuspid valve (i.e., smaller than one arrow) (see Fig. 11-15B).

Full comprehension of the relationship between the magnitude of PBF and the systemic arterial oxygen saturation helps in understanding and managing most cyanotic congenital heart defects. The two extreme examples of TAPVR shown in Figure 11-15 discuss this relationship.

If the PBF is three times as great as the systemic blood flow (i.e., Qp/Qs = 3:1), as in most nonobstructive cases (see Fig. 11-15A), the arterial oxygen saturation is close to 90%, and cyanosis does not become obvious. This figure is derived as follows. An assumed pulmonary vein saturation of 96% and a vena caval saturation of 60% result in an average mixed venous saturation of 87%. Of course, the aortic saturation will be 87% in this case.


The difference in the arterial and venous oxygen saturation is kept at 27% to indicate the absence of heart failure (see Fig. 11-15A).

If an obstruction to the pulmonary venous return exists and the PBF is small, a marked arterial desaturation results, based on the following calculation. It assumed that the PBF is 70% of the systemic flow (i.e., Qp/Qs = 0.7:1) and the pulmonary vein saturation is 96%. The RA saturation (and thus the aortic saturation) will be 56%.


It is also assumed that the infant is not experiencing heart failure (i.e., the systemic AV difference is 28%) (see Fig. 11-15B).

This relationship holds true for other forms of cyanotic congenital heart defects. For a given defect, an increase in the magnitude of PBF results in a rise in the systemic arterial oxygen saturation; a decrease in PBF results in a decrease in the arterial oxygen saturation. An improvement in cyanosis after a systemic-to-PA shunt operation in a cyanotic infant with decreased PBF is an example of this relationship. (Of course, in infants with obstructive pulmonary venous return, a systemic-to-PA shunt operation is not the right operation; it make the situation worse.) Conversely, infants with a single ventricle may be in CHF from a large PBF but not be cyanotic. CHF improves after a PA banding operation (which decreases PBF and lowers PA pressure), but the arterial oxygen saturation usually decreases, and cyanosis may appear.