Atrial septal defect: Simple and complex

Anatomic considerations

The normal atrial septum viewed from its right side is a blade-shaped structure with a superoanterior margin that reflects the curvature of the ascending aorta, an inferior margin that borders the mitral annulus, and a posterior margin that is convex. The left side of the septum has a network of trabeculations that are remnants of the septum primum.

The fossa ovalis occupies about 28% of the septal area irrespective of age, is bordered by a limbus and guarded by a valve ( Fig. 12.1 ). After birth, the patent fetal foramen ovale closes by fusion of its valve with the limbus of the fossa ovalis as left atrial pressure exceeds right atrial pressure. The incidence of persistent patency of the foramen ovale declines from about one third during the first three decades of life to about one quarter during the fourth through eighth decades. The morphogenetic sequence of normal intrauterine formation and closure of the atrial septum is illustrated in Fig. 12.2 . Redundancy of the valve of the foramen is responsible for an atrial septal aneurysm. ^,

The commonest type of atrial septal defect is in the ostium secundum ( Figs. 12.3 , 12.4 , and 12.5 ). These defects lie in a folded area rather than on a flat plane. Their anatomy is more complex on the right side than on the left side of the septum. Ostium secundum defects result from shortening of the valve of the foramen ovale, excessive resorption of the septum primum, or deficient growth of the septum secundum (see Figs. 12.2 and 12.3 ). Occasionally, the atrial septal perforations resemble Swiss cheese, or the interatrial communication is represented by multiple openings. Next in frequency are ostium primum defects, also called atrioventricular septal defects because the atrioventricular septum is defective (absent) (see Figs. 12.2 , 12.3 , 12.6 , and 12.7 ).

Sinus venosus atrial septal defects are less common and constitute 2% to 3% of interatrial communications. During normal embryogenesis, the inferior vena cava and the right superior vena cava are incorporated into the right horn of the sinus venosus. Faulty resorption results in a communication near the orifice of the superior or the inferior vena cava. An understanding of normal cardiac development reveals the wide anatomical substrate that underscore the spectrum of sinus venosus defects. The right valve of the sinus venosus is a broad membrane that almost partitions the developing right atrium. Both vena cavas are located on the left side of the membrane. Superior vena caval sinus venosus defects are located immediately below the junction of the superior vena cava and the right atrium (see Figs. 12.3 and 12.8 ) and vary from small to non-restrictive. The orifice of the superior vena cava may override the defect which is therefore biatrial . Inferior vena caval sinus venosus defects are located below the foramen ovale and merge with the floor of the inferior cava (see Figs. 12.3 and 12.8 ). ^, As the valve of the inferior vena cava resorbs, its rudiment becomes the fetal eustachian valve that directs inferior caval blood across the foramen ovale. Persistence of a large eustachian valve channels inferior vena caval blood across an ostium secundum atrial septal defect ( Fig. 12.9 ) or across an inferior vena caval sinus venosus defect (see Fig. 12.3 ). ^, Atrial septal defects are usually located in only one of the foregoing locations, but separate ostium secundum, sinus venosus, and ostium primum defects occasionally coexist.

In coronary sinus atrial septal defects the defect is located at the site normally occupied by the right atrial ostium of the coronary sinus (see Fig. 12.3 ), and is characterized by an opening in the wall of the distal end of the sinus, or by unroofing caused by absence of the partition between the coronary sinus and left atrium. A left superior vena cava inserts into the upper left corner of the left atrium. A relatively rare combination consists of absence of the coronary sinus, a defect in the atrial septum in the location of the ostium of the coronary sinus, and a left superior vena cava connected to the left atrium, a combination that is necessarily cyanotic because blood from the left superior vena cava enters the left atrium directly.

Spontaneous closure of an ostium secundum atrial septal defect refers to sealing of a true tissue defect, and not to cessation of a left-to-right shunt through a valve-incompetent foramen ovale. ^, The mechanism(s) responsible for spontaneous closure remain to be established, but multiple small interatrial septal openings (diameters less than 5 mm) in newborns have a strong tendency to close during the first year of life.

Anatomic connections and physiologic drainage of pulmonary veins are important distinctions in atrial septal defects. Connection refers to a pulmonary vein that is anatomically contiguous—connected—with a morphologic left atrium or a morphologic right atrium. ^, Drainage refers to the physiologic pathway of blood from pulmonary veins into the left or right atrium. Pulmonary veins that connect normally can drain anomalously, but pulmonary veins that connect anomalously drain anomalously. Normal right pulmonary veins connect to the left atrium close to the rim of ostium secundum atrial septal defects, ^, so a substantial portion of right pulmonary venous blood preferentially drains into the right atrium even though the veins connect anatomically to the left atrium (see Fig. 12.4 ). Partial anomalous pulmonary venous connection refers to one or more but not all pulmonary veins that connect anomalously to the right atrium. ^, Total anomalous pulmonary venous connection exists when all four pulmonary veins connect anomalously to the right atrium directly or indirectly. Ten to 15% of ostium secundum atrial septal defects are associated with partial anomalous pulmonary venous connections. Eighty to 90% of superior vena caval sinus venosus defects are associated with anomalous connection of the right superior pulmonary vein to the right atrium or superior vena cava (see Figs. 12.3 and 12.8 ). About 90% of partial anomalous pulmonary venous connections join the right upper or middle lobe pulmonary veins into the right atrium or superior vena cava. ^, Partial anomalous connection of right pulmonary veins is usually associated with ostium secundum atrial septal defects, exceptionally is associated with an intact atrial septum, and may go unrecognized when associated with a restrictive sinus venosus defect ( Fig. 12.10 ). Anomalous connection of left pulmonary veins is far less prevalent (incidence about 10%) than anomalous connection of right pulmonary veins and is represented by anomalous connection to the innominate vein or to a persistent left superior vena cava that attaches to the innominate vein. Bilateral partial anomalous pulmonary venous connections are rare.

The scimitar syndrome, described in 1836 by Chassinat, is a rare anomaly characterized by connection of all of the right pulmonary veins into the inferior vena cava.

The ipsilateral lung and pulmonary artery are usually hypoplastic ( Figs. 12.11 and 12.12 ). The syndrome rarely involves the left lung. The term scimitar refers to a radiologic shadow that resembles the shape of a Turkish sword (see Fig. 12.12 ). The lower portion of the right lung is perfused by systemic arteries from the abdominal aorta. ^,

In ostium secundum atrial septal defects, anatomical studies of the mitral, tricuspid, and pulmonary valves have disclosed morphologic and architectural modifications. The mitral valve abnormalities consist of thickening and fibrosis of leaflets and chordae tendineae attributed to traumatic cusp movements that result from deformity of the left ventricular cavity. The lesions are believed to be the basis for age-related mitral regurgitation. ^, Superior systolic displacement of the mitral leaflets (mitral valve prolapse) occurs because leaflets with normal area and chordal length are housed in a left ventricular cavity that is reduced in size and abnormal in shape owing to the leftward position of the ventricular septum ( Fig. 12.13 B).

In patients with non-restrictive atrial septal defects and pulmonary vascular disease, the hypertensive proximal pulmonary arteries dilate aneurysmally and contain mural calcification and intraluminal thrombi that can be massive and occlusive ( Figs. 12.14–12.16 ). Aneurysmal proximal pulmonary arteries may rupture. Abnormalities of medial smooth muscle, elastin, collagen, and ground substance reside in the walls of these pulmonary arteries, and are held responsible for dilation that is out of proportion to hemodynamic or morphogenetic expectation.

Physiologic consequences

The physiologic consequences of atrial septal defects depend on the magnitude and chronicity of the left-to-right shunt and on the behavior of the pulmonary vascular bed. When the defect is restrictive , size per se determines the magnitude of the shunt. When the defect is non-restrictive , there is no pressure difference between the right and left atrium, so shunt volume is determined by the relative compliance of the two ventricles. During diastole, all four cardiac chambers are in common communication, so blood can flow from the left atrium through the atrial septal defect into the right atrium and across the tricuspid valve into the right ventricle, or can flow directly into the left ventricle across the mitral valve (see Fig. 12.4 ). Alternatively, blood from the right atrium can flow across the atrial septal defect into the left atrium across the mitral valve into the left ventricle, or directly into the right ventricle across the tricuspid valve. In an ostium secundum atrial septal defect, the right ventricle is thinner and more compliant than the left ventricle, so blood flow is from left atrium through the atrial septal defect across the tricuspid valve into the relatively compliant right ventricle, thus establishing a left-to-right shunt ( Fig. 12.17 ). The shunt reaches its peak in late systole and early diastole, diminishes throughout diastole, and in late diastole is supplemented by atrial contraction (see Fig. 12.17 ). ^, A small transient right-to-left shunt coincides with the onset of ventricular systole (see Fig. 12.17 ). Clinical and experimental studies of instantaneous flow across atrial septal defects have confirmed these flow patterns.

The fetal circulation is not altered by an atrial septal defect because in utero interatrial flow is normally from right to left through a patent foramen ovale (see Fig. 12.1 ). At birth, there is little or no shunt in either direction across an atrial septal defect because the compliance of the right and left ventricles is virtually identical. ^{, ,} The right ventricle gradually becomes thinner and more compliant than the left ventricle in response to the fall in neonatal pulmonary vascular resistance, so left atrial blood then flows across the atrial septal defect into the more compliant right ventricle. Pulmonary blood flow that is received by the right pulmonary veins is channeled into the right atrium because of proximity of the right pulmonary veins to the rim of the atrial septal defect (see earlier and see Fig. 12.4 ). Pulmonary blood flow received by the left pulmonary veins is channeled directly into the left atrium and is then shunted across the atrial septal defect. Accordingly, the right ventricle is volume overloaded and the left ventricle is volume underloaded .

The mature right ventricle is a compliant chamber that readily adapts to volume overload and ejects its increased stroke volume into the low-resistance pulmonary vascular bed. Right ventricular function is usually maintained through the fourth decade. Ischemic heart disease and systemic hypertension conspire to reduce left ventricular compliance and thus to increase the left-to-right shunt (see Fig. 12.13 ). The additional volume overload of the right atrium provokes atrial fibrillation and atrial flutter which further increase the left-to-right shunt and result in heart failure.

Left ventricular end-diastolic volume, stroke volume, ejection fraction, and cardiac output are decreased in infants and adults with an atrial septal defect, and ejection fraction tends to fall with exercise. Diminished left ventricular functional reserve is related to the mechanical effects of right ventricular volume overload which displaces the ventricular septum into the left ventricular cavity, reducing its size and changing its shape from ovoid to crescentic (see earlier) (see Fig. 12.13 B). ^{, ,} In addition, coronary reserve is compromised in the volume overloaded right ventricle if the left main coronary artery is compressed by a dilated pulmonary trunk.

An important and poorly understood deviation from the prevailing pattern of an asymptomatic onset of the left-to-right shunt across an atrial septal defect is the occasional infant who develops a large shunt and right ventricular failure. ^, A left-to-right shunt that begins before the increase in right ventricular compliance has been attributed to more complete emptying of the right ventricle (reduced resistance to discharge). Right ventricular failure ensues because the neonatal right ventricle is volume overloaded before involution of its free wall thickness. Although right ventricular failure is occasionally intractable, there is a propensity for clinical improvement because of spontaneous closure of the atrial septal defect. ^,

The paucity of pulmonary vascular disease in patients with non-restrictive atrial septal defects has been ascribed to the onset of the left-to-right shunt after pulmonary arterial pressure and pulmonary vascular resistance have normalized. A low-resistance low-pressure pulmonary vascular bed accommodates an appreciable increment in blood flow without a rise in pressure. ^, An exception is the propensity for pulmonary hypertension in patients born at high altitude with atrial septal defects. ^, Pulmonary vascular disease with a right-to-left shunt at sea level occurs in less than 10% of patients with an atrial septal defect, and is believed to represent the coincidence in young females of idiopathic pulmonary arterial hypertension and an ostium secundum atrial septal defect ( Fig. 12.18 , see also Fig. 12.16 ).

Increased resistance to right ventricular discharge can also result from massive occlusive thrombus in dilated hypertensive proximal pulmonary arteries (see earlier) (see Figs. 12.14 and 12.19 ). Older adults experience a moderate rise in pulmonary artery pressure with persistence of the left-to-right shunt. Thus, pulmonary hypertension with a non-restrictive atrial septal defect at sea level is bimodal, and is represented in young females with coexisting idiopathic pulmonary arterial hypertension , or in older adults—male or female—who have moderate pulmonary hypertension with a persistent left-to-right shunt.

The physiologic consequences of an atrial septal defect with partial anomalous pulmonary venous connection are similar if not identical to an isolated atrial septal defect with an equivalent net shunt because the hemodynamic fault remains the left-to-right shunt at atrial level. ^, However, flow through anomalous pulmonary veins into the right atrium is obligatory and is therefore established earlier than shunt flow across an atrial septal defect. When partial anomalous pulmonary venous connection occurs with an intact atrial septum, flow is increased in the segment of lung with the anomalous pulmonary veins because right atrial pressure is lower than left atrial pressure (intact atrial septum). The pressure gradient across the anomalously draining lung is therefore greater than across the normally draining lung. ^,

Anomalous systemic venous drainage from a normally aligned inferior vena cava results in cyanosis with increased pulmonary blood flow. In an ostium secundum atrial septal defect, small amounts of inferior vena caval blood transiently stream across the defect in early systole, a pattern appropriate for the direction of fetal blood from inferior vena cava across a foramen ovale. A large persistent eustachian valve sometimes extends from the orifice of the inferior vena cava to the margin of an ostium secundum atrial septal defect, selectively channeling inferior caval blood into the left atrium and causing cyanosis. ^, A large eustachian valve in the presence an inferior vena caval sinus venosus atrial septal defect channels inferior caval blood directly into the left atrium. .

The history

The female/male ratio is at least 2:1 in patients with an ostium secundum atrial septal defect, while the sex ratio in sinus venosus defects and in ostium primum defects is approximately equal. Ostium secundum atrial septal defects are sometimes familial, can recur in several generations, ^, and have been found in identical twins. Familial scimitar syndrome has been reported. ^, Autosomal dominant inheritance is a feature of atrial septal defect with the Holt-Oram syndrome (see Physical Appearance). Autosomal dominant inheritance tends to be the mode in inheritance in ostium secundum defects with prolonged atrioventricular conduction. In some members of a family, the atrial septal defect occurs with PR interval prolongation, other family members have PR prolongation with an intact atrial septum, and still others experience sudden death. Mutations in the NKX 2.5 gene have been associated with familial atrial septal defect and progressive prolongation of atrioventricular conduction. Concordant familial segregation of atrial septal defect has been reported with the Axenfeld-Rieger anomaly (see Physical Appearance).

Atrial septal defects may go unrecognized for decades because symptoms are mild or absent and physical signs are subtle. The soft pulmonary midsystolic flow murmur in children and young adults is often dismissed as innocent (see Chapter 2 ). Conversely, an atrial septal defect may first come to light in a routine chest x-ray. An important exception is the symptomatic infant with an ostium secundum atrial septal defect and congestive heart failure followed by spontaneous closure (see earlier). ^{, ,}

About half of patients with a scimitar syndrome are either asymptomatic or only mildly symptomatic when the diagnosis is made, despite varying degrees of hypoplasia of the right lung (see Fig. 12.12 ). ^, However, a small but important group of scimitar syndrome patients consists of symptomatic infants with cyanosis and pulmonary hypertension. Older children and young adults come to light because an x-ray discloses the scimitar sign and the hypoplastic right lung because of an atrial tachyarrhythmia, recurrent lower respiratory infections, or a murmur. ^{, ,}

Paul Wood remarked, “ In any series of geriatric necropsies atrial septal defect is always represented .” Ostium secundum atrial septal defects are among the commonest congenital cardiac malformations in adults, accounting for 30% to 40% of unoperated patients over 40 years of age. ^, Patients often survive to advanced age, but life expectancy is not normal. Three quarters are alive through the third decade, but three quarters are dead by age 50 years, and 90% are dead by age 60 years. Sporadic survivals have been recorded beyond age 70 years, and there are rare examples of patients in their 80s or 90s. An exceptional patient died 3 months before his 95th birthday. Death is often unrelated to the malformation, but when a relationship exists, cardiac failure is the commonest cause.

The clinical course of ostium secundum atrial septal defects spans the reproductive years, and the majority of patients are female. It is therefore reassuring that despite the gestational increase in cardiac output and stroke volume, young gravida with an atrial septal defect generally endure pregnancy—even multiple pregnancies—without tangible ill effects. However, brisk hemorrhage during delivery provokes a rise in systemic vascular resistance and a fall in systemic venous return, a combination that augments the left-to-right shunt, sometimes appreciably. There is also a peripartum risk of paradoxical embolization from leg veins or pelvic veins because emboli carried by the inferior vena cava traverse the atrial septal defect and enter the systemic circulation.

Dyspnea and fatigue are early symptoms of an ostium secundum atrial septal defect. The large left-to-right shunt is responsible for a decrease in pulmonary compliance and an increase in the work of breathing. Orthopnea may be experienced because the supine position increases the work of breathing in patients with reduced lung compliance. Platypnea-orthodeoxia is a rare syndrome characterized by orthostatic provocation of a right-to-left shunt across an atrial septal defect or a patent foramen ovale. ^, Platypnea (dyspnea induced by the upright position and relieved by recumbency) and orthodeoxia (arterial desaturation in the upright position with improvement during recumbency) are features of this rare disorder. Clinical suspicion may originate from the patient who reports that dyspnea is provoked by standing upright.

Recurrent lower respiratory infections are common, especially in children. Although the pulmonary valve is theoretically susceptible to infective endocarditis owing to the rapid rate of ejection, only a single case has been reported.

Older patients deteriorate chiefly on three counts. First , a decrease in left ventricular distensibility associated with aging, ischemic heart disease, systemic hypertension, or acquired calcific aortic stenosis augments the left-to-right shunt (see Fig. 12.13 ). ^, Second, an age-related increase in prevalence of paroxysmal atrial tachycardia, atrial fibrillation, and atrial flutter precipitates congestive heart failure. Third , mild to moderate pulmonary hypertension in older adults occurs with a persistent left-to-right shunt, so the aging right ventricle is doubly beset by both pressure and volume overload. Pulmonary vascular disease with reversed shunt is believed to represent the coincidence of idiopathic pulmonary arterial hypertension with an ostium secundum atrial septal defect, typically presenting in young females who are predisposed to both lesions (see Fig. 12.18 ) (see earlier). Importantly, the outlook is better when idiopathic pulmonary hypertension occurs with an atrial septal defect or a patent foramen ovale that permits the right heart to decompress.

A patent foramen ovale is the most common remnant of the fetal circulation (see Fig. 12.1 ), occurring in 10% to 15% of normal adults, and in 20% to 30% of normal hearts at postmortem. ^{, ,} The patent foramen varies in anatomical and functional size, and is implicated in paradoxical embolization, transient ischemic attacks, venoarterial gas embolism, decompression sickness, and platypnea-orthodeoxia (see earlier). ^,

Atrial septal aneurysm is characterized by protrusion beyond the plane of the atrial septum and by rapid, dramatic phasic cardiorespiratory oscillations. ^{, ,} An atrial septal defect may coexist. Cerebral emboli originate from fibrin-platelet aggregates on the left atrial side of the aneurysm and are dislodged by the phasic excursions. Atrial septal aneurysms have been incriminated in atrial arrhythmias in children and adults as well as in the fetus.

Physical appearance

Children with an atrial septal defect may have a delicate gracile habitus with weight more affected than height ( Fig. 12.20 A), and may have a left precordial bulge with Harrison’s grooves (see Fig. 12.20 B). Newborns subsequently found to have an atrial septal defect are on average smaller than their normal siblings. Symptomatic infants may be cyanotic because of the effects of congestive heart failure. Cyanosis also occurs when a large eustachian valve (see earlier) selectively channels inferior vena caval blood into the left atrium through an ostium secundum atrial septal defect or through an inferior vena caval sinus venosus defect. ^{, ,}

The distinctive physical appearance of the Holt-Oram syndrome heightens suspicion of a coexisting ostium secundum atrial septal defect ( Fig. 12.21 ), ^{, ,} less commonly of an ostium primum defect. The thumb is hypoplastic with an accessory phalanx that results in triphalangism, a crooked appearance, and difficulty in apposition of thumb to fingertips. The abnormality becomes more obvious when the palms are supinated (see Fig. 12.21 B). The thumb may be rudimentary or absent, and the metacarpal bone may be small or absent with hypoplasia extending to the radius ( Fig. 12.22 ). The bony anomaly ranges from minor changes identified on x-ray to absence of the arm (abrachia) or absent arms with persistent underdeveloped hands (phocomelia). Other cardiac malformations occur without prevailing patterns. Mutations in a gene on chromosome 12q2 play an important role in skeletal and cardiac development, and produce a wide range of partial phenotypes of the Holt-Oram syndrome.

Patau syndrome (trisomy 13) is characterized by polydactyly, flexion deformities of the fingers, palmar crease, microcephaly, holoprosencephaly, cleft lip, cleft palate, and low-set malformed ears. Edward syndrome (trisomy 18) is characterized by clenched fists, rocker bottom feet, prominent occiput, low-set malformed ears, and micrognathia. The most common congenital cardiac malformations in trisomy 13 and in trisomy 18 are atrial septal defect, ventricular septal defect and patent ductus arteriosus. An atrial septal defect is associated with the Axenfeld-Rieger anomaly, a genetically heterogeneous autosomal dominant disorder characterized by ocular abnormalities with glaucoma, and non-ocular abnormalities that include maxillary hypoplasia, dental anomalies, umbilical hernia, and/or hypospadias.

The arterial pulse

The arterial pulse is normal even though left ventricular ejection fraction tends to be decreased. Left ventricular output is maintained during the Valsalva maneuver despite a fall in systemic venous return because of the large volume of blood pooled in the lungs. Tachycardia is less pronounced during the straining phase, and there is a smaller decrease in pulse pressure. Bradycardia is less pronounced after cessation of straining, and the systolic overshoot is smaller. These abnormal responses can be identified by palpating the brachial arterial pulse.

The jugular venous pulse

Most important is left atrialization of the jugular venous wave form . The crests of the A and V waves tend to be equal as they are in the left atrium ( Fig. 12.23 ) because the two atria are in common communication through a non-restrictive atrial septal defect. The A wave amplitude varies with heart rate as in normal subjects. When left ventricular compliance decreases, left atrial pressure rises and with it the right atrial pressure. ^, Pulmonary vascular disease results in an increased force of right atrial contraction and a dominant if not giant A wave ( Fig. 12.24 ).

Precordial movement and palpation

In 1934, Roesler called attention to the conspicuous thrust of the right ventricle in atrial septal defect. The impulse is hyperdynamic but not sustained because the volume-overloaded right ventricle contracts vigorously and empties rapidly into a low-resistance pulmonary vascular bed. ^, The impulse is especially prominent at the left sternal border during held exhalation , and in the subxiphoid area during held inspiration . Anterior movement at the left sternal border is accompanied by retraction at the apex because the enlarged right ventricle occupies the apex. ^, A dilated pulsatile pulmonary trunk is palpable in the second left intercostal space, but a systolic thrill is seldom present despite hyperkinetic right ventricular ejection into a dilated pulmonary trunk.

Auscultation

Auscultatory signs are the same in all varieties of isolated non-restrictive atrial septal defects. The first heart sound is split at the lower left sternal edge and apex, and the tricuspid component is loud ( Fig. 12.25 ). ^{, ,} Increased diastolic flow across the tricuspid valve depresses the bellies of the leaflets into the right ventricle, and vigorous right ventricular contraction causes abrupt cephalad excursion of the leaflets, generating a loud tricuspid component of the first heart sound. ^{, ,} A pulmonary ejection sound is uncommon despite dilatation of the pulmonary trunk ( Fig. 12.26 ). ^,

The pulmonary midsystolic systolic flow murmur begins immediately after the first heart sound because right ventricular isovolumetric contraction is short. The murmur is grade 2/6 of 3/6, is maximum in the second left intercostal space over the pulmonary trunk, is impure and superficial because of proximity of the dilated pulmonary trunk to the chest wall, and is crescendo-decrescendo, peaking in early or midsystole, and ending well before the second heart sound ( Figs. 12.27 and 12.28 ). ^{, , ,} Origin in the pulmonary trunk has been confirmed by intracardiac phonocardiography (see Fig. 12.28 ) and by phonocardiograms recorded from the surface of the pulmonary trunk during surgery. The murmur radiates to the apex because the right ventricle occupies the apex. A louder murmur is reserved for coexisting pulmonary valve stenosis. Systolic murmurs widely distributed in the right chest, axillae, and back are generated by rapid flow through peripheral pulmonary arteries (see Fig. 12.27 ). ^,

The pulmonary component of the second sound is prominent because of proximity of the dilated pulmonary trunk to the chest wall and because of brisk elastic recoil ( Fig. 12.29 ). Wide fixed splitting is an auscultatory hallmark of atrial septal defect. ^{, ,} The aortic and pulmonary components are widely split during expiration, and the degree of splitting does not change during inspiration (see Figs. 12.26 , 12.27 , and 12.30 ) or during the Valsalva maneuver ( Fig. 12.31 ). Wide splitting is caused by a delay in the pulmonary component associated with an increase in pulmonary vascular capacitance and an increase in “hangout interval” between the descending limbs of the pulmonary arterial and right ventricular pressure pulses. With a rise in pulmonary arterial pressure, the hangout interval decreases. The split then becomes a function of the relative duration of right and left ventricular electromechanical systole which is the same in both ventricles because a potential increase in duration of right ventricular systole due to volume overload is countered by accelerated ejection. A normal child examined in the supine position may exhibit relatively wide but not fixed splitting of the second heart sound, but in the sitting position, respiratory splitting is normal. ^, The duration of diastole affects the degree of splitting by influencing the relative volumes of the right and left ventricles. As diastole shortens the split narrows, and as diastole lengthens the split widens, patterns that are evident in the beat-to-beat variations in cycle length with atrial fibrillation in which splitting tends to vary inversely with the duration of the preceding diastole.

Fixed splitting means that the width of the split remains constant throughout active respiration and during the Valsalva maneuver. Persistent splitting means that the split widens during inspiration and narrows during expiration. Atrial septal defect is characterized by splitting of the second heart sound that is wide and fixed (see Fig. 12.26 ). During inspiration, the aortic and pulmonary components are equally delayed or do not move at all. In the normal heart, inspiratory splitting is due chiefly to a delay in the pulmonary component of the second sound because the increase in pulmonary capacitance during inspiration is accompanied by an increase in the hangout interval ( Fig. 12.32 ). The high pulmonary capacitance in atrial septal defect precludes an additional increase during inspiration, so there is no inspiratory delay in the pulmonary component of the second sound. Normal phasic changes in systemic venous return during respiration are associated with reciprocal changes in volume of the left-to-right shunt, minimizing the respiratory variations in right and left ventricular filling. ^{, ,} Inspiration is accompanied by an increase in systemic venous return, so right ventricular filling is maintained or increased while the left-to-right shunt decreases reciprocally, so left ventricular filling is maintained or is increased by the same amount. An inspiratory decrease in left-to-right shunt has been demonstrated in experimental animals and in human subjects. Wide fixed splitting is unlikely in neonates because there is little or no shunt in either direction.

These patterns of splitting do not apply when partial anomalous pulmonary venous connection occurs with an intact atrial septum ( Fig. 12.33 ). ^, Increased venous return during inspiration is not accompanied by a reciprocal fall in left-to-right shunt because the atrial septum is intact. Accordingly, the aortic component of the second heart sound moves toward the first heart sound and the pulmonary component moves away , so the split widens with inspiration and narrows with expiration (see Fig. 12.33 ).

Rarely, an opening sound of the tricuspid valve follows the pulmonary component of the second heart sound. ^, Echocardiographic timing confirms that the sound coincides with abrupt arrest of the opening movement of the tricuspid valve in early diastole. Mid-diastolic murmurs are due to augmented tricuspid flow ( Fig. 12.34 ). ^{, , ,} Origin of the flow murmur at the tricuspid orifice has been demonstrated experimentally in animals and by intracardiac phonocardiography in human subjects. ^, Tricuspid flow murmurs are medium frequency, impure, soft, short, presystolic or mid-diastolic, localized at the lower left sternal border, and do not increase with inspiration despite their right-sided origin. Intracardiac phonocardiograms identify low intensity inaudible diastolic murmurs within the atrial septal defect itself. The combination of superficial impure presystolic and mid-diastolic murmurs together with a superficial impure midsystolic murmur occasionally creates the impression of a pericardial rub. Occasionally, a rub in fact does occur, and attention has been called to roughened pericardium in patients with an atrial septal defect (see Fig. 12.17 ). ^, A diastolic murmur of low-pressure pulmonary regurgitation is uncommon (see Fig. 12.28 ) and is reserved for aneurysmal dilatation of the pulmonary trunk. Continuous murmurs through restrictive atrial septal defects are rare. Atrial septal defects with pulmonary vascular disease and reversed shunts are accompanied by auscultatory signs of pulmonary hypertension ( Figs. 12.35 and 12.36 ). ^{, ,}

The electrocardiogram

Sinus node dysfunction has been identified as early as age 2 to 3 years, and accelerated atrial rhythms have been recorded on 24-hour ambulatory electrocardiograms in 35% of children with an atrial septal defect. The incidence of atrial fibrillation, atrial flutter, and supraventricular tachycardia increases in the fourth decade ( Figs. 12.37 and 12.38 ). ^, Interestingly, sinus arrhythmia does not occur in adults with an atrial septal defect, and is minimal or absent in children (see Fig. 12.38 ). Sinus arrhythmia requires separation of the systemic and pulmonary venous returns. With an atrial septal defect, the two venous returns are by definition not separated.

Atrioventricular conduction defects are intrinsic components of atrial septal defects, and are usually age related. ^{, ,} The PR interval tends to be prolonged. Atrioventricular node dysfunction begins in older children, and is less frequent than sinus node dysfunction. ^, Advanced first-degree atrioventricular nodal block occurs with familial or less commonly nonfamilial atrial septal defect, and occasionally culminates in complete heart block. Some family members have an ostium secundum atrial septal defect and first-degree heart block, while others have PR prolongation with an intact atrial septum. In the Holt-Oram syndrome (see Figs. 12.21 and 12.22 ), PR interval prolongation, sinus bradycardia, and ectopic atrial rhythms are relatively common.

Abnormal right atrial P waves are peaked rather than tall ( Fig. 12.39 ), but P wave configurations are usually normal ( Fig. 12.40 ). Prolonged P wave duration occurs when the terminal force is written by an enlarged right atrium. The P wave axis with an ostium secundum atrial septal defect is inferior and to the left with upright P waves in leads 2, 3, and aVF (see Figs 12.39 and 12.40 ). With a superior vena caval sinus venosus atrial septal defect, the atrial pacemaker is ectopic because the defect occupies the site of the sinoatrial node. The P wave axis is then leftward, and the P waves are inverted in leads 2, 3, and aVF and upright in lead aVL ( Fig. 12.41 ). Superior vena caval sinus venosus defects are occasionally accompanied by shifts from sinus rhythm with a normal P axis to an ectopic atrial rhythm with a leftward P axis.

The QRS duration is slightly prolonged in atrial septal defects because of slurring of the terminal force (see Fig. 12.40 ). The duration increases with age and may culminate in a pattern resembling complete right bundle branch block (see Fig. 12.37 ). The QRS axis is vertical with clockwise depolarization that writes q waves in leads 2, 3, and aVF (see Figs. 12.40 and 12.39 ). Right-axis deviation is reserved for symptomatic pulmonary hypertensive infants, or for young females with pulmonary vascular disease. Left-axis deviation is exceptional, and represents acquired left anterior fascicular block in older adults (see Fig. 12.37 ).

An electrocardiographic hallmark of atrial septal defect is an rSr prime or an rsR prime in right precordial leads (see Figs. 12.40 and 12.39 ). This finding, although widely described, has limitations in terms of diagnostic accuracy. The r prime in lead V1 and aVR is slurred in contrast to thin terminal r waves in 5% of normal electrocardiograms. Q waves are small or absent in left precordial leads because the shunt does not traverse the left ventricle (see Figs. 12.39–12.41 ). The outflow tract of the right ventricle is the last portion of the heart to depolarize. Enlargement and increased thickness caused by right ventricular volume overload are responsible for the rightward, superior, and anterior direction of the terminal force of the QRS, as well as for increased QRS duration. The term incomplete right bundle branch block is a misnomer.

A notch near the apex of the R waves in inferior leads of ostium secundum and sinus venosus atrial septal defects has been called crochetage because the notch resembles the work of a crochet needle ( Fig. 12.42 ). Crochetage is independent of the terminal force direction of the QRS, but when the rSr prime pattern occurs with crochetage in each of the inferior limb leads, the specificity of the electrocardiographic diagnosis of atrial septal defect is remarkably high. Although crochetage has been correlated with shunt severity, the pattern has also been reported with a patent foramen ovale, and has been suggested as an electrocardiographic marker of a patent foramen.

The x-ray

Increased pulmonary arterial vascularity extends to the periphery of the lung fields ( Figs. 12.43 and 12.44 ). The pulmonary trunk and its proximal branches are dilated (see Fig. 12.44 ). The left branch is usually obscured by an enlarged pulmonary trunk (see Fig. 12.44 ), but the lateral view discloses dilation of both branches (see Figs. 12.44 and 12.45 ). The ascending aorta is seldom border forming because the intracardiac shunt does not traverse the aortic root (see Figs. 12.43 and 12.44 ). ^, However, angiographic and echocardiographic assessments indicate that the intrinsic caliber of the ascending aorta is not significantly reduced. A sinus venosus atrial septal defect may be accompanied by localized ampullary dilation of the superior vena cava proximal to its attachment to the right atrium ( Fig. 12.46 ). Infants with large left-to-right shunts exhibit both pulmonary arterial and pulmonary venous vascularity with enlargement of all four cardiac chambers. In older adults with moderate pulmonary hypertension and persistent left-to-right shunt, the pulmonary trunk and proximal branches are occasionally aneurysmal ( Fig. 12.47 ). In young adults with pulmonary vascular disease and a balanced or reversed shunt, the pulmonary trunk and its branches are strikingly enlarged and calcified (see Figs. 12.15 and 12.18 ).

Right atrial enlargement is characteristic (see Figs. 12.44 A and 12.16 ), but the left atrium seldom enlarges despite a left-to-right shunt (see Fig. 12.44 B) because a major portion of pulmonary venous return enters the right atrium directly owing to the proximity of the right pulmonary veins to the rim of the ostium secundum atrial septal defect (see earlier). ^, Left atrial enlargement is reserved for older adults with atrial fibrillation. ^, Volume elastic properties of the right and left atrium also play a role in determining relative enlargement. For equal increments in volume, the right atrium is more distensible than the left.

An enlarged right ventricle occupies the apex, and forms an acute angle with the left hemidiaphragm (see Fig. 12.43 ). Dilation of the outflow tract causes smooth continuity with the enlarged pulmonary trunk above (see Fig. 12.43 ). In the lateral projection, the dilated right ventricle encroaches on the retrosternal space (see Figs. 12.44 B and 12.45 ), and displaces the left ventricle posteriorly. The size of the left ventricle is normal because its stroke volume is normal or reduced. The relationship between the inferior vena caval shadow and the left ventricular shadow in the lateral projection distinguishes posterior displacement of the left ventricle from intrinsic dilation ( Fig. 12.48 ). The lateral projection in adults often shows disproportionate anterior bowing of the upper third of the sternum (see Fig. 12.44 B). A progressive and often dramatic increase in heart size is initiated by atrial tachyarrhythmias, especially atrial fibrillation with congestive heart failure (see Figs. 12.19 and 12.49 ).

In the scimitar syndrome , the confluence of right pulmonary veins forms a distinctive shadow parallel to or behind the right cardiac silhouette as the anomalous venous channels course downward to join the inferior vena cava (see Fig. 12.12 ). ^{, ,} The heart is displaced into the right hemithorax because of hypoplasia of the right lung, but the anomalous venous channels usually remain visible (see Fig. 12.12 ).

The echocardiogram

Echocardiography with color flow imaging and Doppler interrogation establishes the location and size of an atrial septal defect and its physiologic consequences ( Video 12.1 ). Transesophageal echocardiography has refined the anatomic assessment of the interatrial septum, ^, and identifies partial anomalous pulmonary venous connections. ^{, ,} Pulsed Doppler imaging can identify anomalous pulmonary venous connections in the fetus. Real-time three-dimensional transesophageal echocardiography has optimized assessment of atrial septal defects. An ostium secundum atrial septal defect is represented by an echo-free space in the mid portion of the atrial septum (see Fig. 12.17 ). Color flow imaging confirms that the echo-free space is a true tissue defect, and pulsed Doppler imaging characterizes instantaneous flow patterns across the defect (see Fig. 12.17 ). A foramen ovale can be identified (see Fig. 12.1 ), and color flow can determine its patency, however, agitated saline injection is often necessary to demonstrate a right-to-left shunt (see Fig. 12.1 ; Video 12.2 ). ^, An in utero patent foramen can be distinguished from an ostium secundum atrial septal defect. Transthoracic and transesophageal echocardiography establish the diagnosis of atrial septal aneurysm and color flow imaging or echocontrast determines whether an atrial septal defect coexists. ^,

Transesophageal echocardiography identifies a divided right atrium that consists of a shelf separating an anterior supra-tricuspid component from a posterior systemic venous sinus, to which the cava are connected. A rare double atrial septum is imaged as a midline chamber between the left and right atria.

Real-time imaging discloses a dilated hyperkinetic right ventricle with paradoxical motion of the ventricular septum, and vigorous pulsations of the pulmonary trunk and its branches. Right ventricular volume overload impacts the right ventricular outflow tract. The consequences of significant left to right shunting are right ventricular volume overload which results in right ventricular enlargement and diastolic septal flattening which is best seen on short-axis echocardiography at the ventricular level ( Video 12.3 ).

A superior vena caval sinus venosus defect lies near the junction of the superior cava and the right atrium, the imaging of which can be optimized with specific echocardiographic windows. ^, An inferior vena caval sinus venosus defect lies just beyond the rim of the inferior vena cava. An unroofed coronary sinus is identified by dilation of the sinus and by a defect between the sinus and left atrium and is easily visualized with transesophageal echocardiography. Developments in advanced forms of cardiac imaging including 3-dimensional echocardiography have impacted early diagnosis and improved timely management of atrial shunts. ^,

Summary

Ostium secundum atrial septal defects predominate in females and often come to light in asymptomatic children or young adults. Patients usually reach their fourth decade with little or no handicap. Pre-adolescents sometimes appear delicate and gracile. Dyspnea, fatigue, and recurrent lower respiratory infections are common. The left-to-right shunt in older adults is augmented by ischemic heart disease, systemic hypertension, and acquired calcific aortic stenosis that decrease left ventricular compliance. Atrial tachyarrhythmias, especially atrial fibrillation, precipitate congestive heart failure. The crests of the jugular venous A and V waves are equal (left atrialization). The right ventricular impulse is hyperdynamic, and a systolic pulsation is palpated over the dilated pulmonary trunk. The first heart sound is split with a loud tricuspid component. A grade 2/6 or 3/6 pulmonary midsystolic murmur is followed by fixed splitting of the second heart sound and a tricuspid mid-diastolic flow murmur.

P waves are peaked but seldom tall. The QRS axis is vertical with clockwise depolarization and q waves in leads 2, 3, and aVF. Terminal forces of the QRS are prolonged and point to the right and anterior, producing an rSr prime in lead V1. Notching of the R waves in inferior leads results in a distinctive crochetage pattern. The x-ray shows increased pulmonary arterial vascularity, an inconspicuous ascending aorta, dilation of the pulmonary trunk and its proximal branches, and enlargement of the right atrium and right ventricle. The scimitar syndrome is characterized by partial anomalous right pulmonary venous connection with infradiaphragmatic drainage and hypoplasia of the right lung. Echocardiography with color flow imaging and Doppler interrogation establishes the location and size of the atrial septal defect, its physiologic consequences, and the presence of partial anomalous pulmonary venous connections. A non-restrictive ostium secundum atrial septal defect with pulmonary vascular disease is believed to represent the coexistence of primary pulmonary hypertension in young females.

A superior vena caval sinus venosus atrial septal defect is clinically similar to an equivalent ostium secundum defect with two exceptions: first, the atrial pacemaker is ectopic so the P wave axis is leftward and inverted P waves appear in leads 2, 3, and aVF; second, the right hilus may show localized ampullary dilation of the distal superior vena cava as it joins the right atrium. Echocardiography confirms the location of the defect.

Ostium secundum atrial septal defects in the young are among the most readily diagnosed congenital malformations of the heart, but these same defects sometimes defy clinical recognition because of diagnostic ambiguities. The diagnosis of mitral stenosis is entertained ( Box 12.1 ) because of dyspnea, orthopnea, atrial fibrillation, an increased jugular venous V wave, a right ventricular impulse, a loud first heart sound, a delayed pulmonary component of the second heart sound mistaken for an opening snap, a tricuspid flow murmur mistaken for a mitral diastolic murmur, shunt vascularity mistaken for pulmonary venous congestion, and dilation of the pulmonary trunk, the right atrium, and occasionally the left atrium. Mitral regurgitation may be misdiagnosed as acquired ( Box 12.2 ) because the holosystolic murmur of tricuspid regurgitation is well heard at the apex, a delayed pulmonary component of the second sound followed by a tricuspid flow murmur is mistaken for an opening snap and the mid-diastolic murmur of mitral stenosis, and a tricuspid flow murmur is mistakenly attributed to flow across the mitral valve. Diagnostic ambiguity in older adults results from atrial arrhythmias, ischemic heart disease, systemic hypertension, and inverted left precordial T waves ( Box 12.3 ).