increased left atrial pressure Mitral stenosis, a dysfunctional left ventricle, aortic stenosis, coarctation of the aorta, systemic hypertension, patency of the arterial duct, and ventricular septal defect are examples, and atrial shunting may improve with treatment of the underlying problem A small amount of left-toright shunting is usual in the newborn and young infant14 and defects less than 6 mm at this age are likely not to be important later in life; however, probe patency of the oval fossa is present in up to one-third of these individuals as adults Cardiac Response to the Interatrial Communication In childhood, the usual hemodynamic characteristics of an uncomplicated interatrial communication are a large net left-to-right shunt and normal pulmonary arterial pressure Left-to-right flow across the defect is phasic and occurs predominantly in late ventricular systole and early diastole.16 Numerous studies have documented that most patients with a typical defect within the oval fossa also have a small right-to-left shunt.15,17 This small shunt is not detectable by oximetry, so there is no systemic desaturation, but it can be demonstrated by indicator dilution techniques,18 contrast echocardiography,19 and Doppler studies.20 The contribution of the pulmonary venous return from each lung to the total left-to-right shunt is unequal In a typical large defect, 80% of the pulmonary venous return from the right lung shunts left to right This is in contrast to between 20% and 40% of the pulmonary venous return from the left lung.14,15,18 In a sinus venosus defect, the anomalously connected right pulmonary veins provide most of the shunted blood Interestingly, this preferential shunting from the right lung does not occur to any great extent with an ostium primum interatrial communication As discussed, a large left-to-right shunt at the atrial level leads to enlargement of both the right atrium and the right ventricle.21 The left atrial and left ventricular dimensions are usually normal in childhood, and systemic cardiac output is almost always normal in children In contrast, left ventricular enddiastolic volume may be less than normal,22 and systemic cardiac output has been found to be decreased in up to half the patients with an atrial septal defect who are older than 18 years.23 Numerous studies in adults have shown significant left ventricular dysfunction, which may persist even after surgical correction.22,24,25 Despite the greatly increased flow of blood to the lungs, pulmonary arterial pressure is rarely elevated in children and pulmonary vascular resistance is low, frequently less than 1 Wood unit.26 The incidence of pulmonary hypertension in patients younger than 20 years is no more than 5% in most studies but increases to 20% of those aged from 20 to 40 years and is found in half of the patients older than 40 years.3,15,27,28 However, severe elevation of resistance and the Eisenmenger reaction are unusual and are decreasing in frequency as surveillance techniques improve.29,30 The changes in the pulmonary vascular bed at this stage are similar no matter what the cause of pulmonary vascular disease may be, including a predominance of intimal fibrosis and endothelial proliferation, albeit with less medial muscular hypertrophy than is seen in patients with ventricular septal defects.31 The Eisenmenger reaction is not a uniform response in older age, and such a response is somewhat idiosyncratic.23,32 However, a progressive rise in pulmonary artery pressure with worsening cardiopulmonary function is the norm (presumably due to increased shunting as left ventricular compliance worsens with age and right ventricular function worsens as a result), thus providing the rationale for surgery in childhood Nonetheless, congestive heart failure rarely occurs before the fourth or fifth decade.23,33 Rarely an isolated defect may cause congestive heart failure in infancy; early surgery may be indicated in such cases, although a search for other precipitating factors is a crucial part of the evaluation of these infants.34–36 Another cardiac consequence of the long-standing left-to-right shunt is the occurrence of atrial arrhythmias, particularly atrial flutter and fibrillation They presumably result from chronic stretching of the atria and occur most commonly in patients older than 40 years of age.32,37,38 As with the other complications associated with interatrial communications, atrial arrhythmias rarely occur in childhood Nonetheless, electrophysiologic studies have demonstrated a high incidence of subclinical dysfunction of the sinus node, along with conduction disturbances, in children prior to operative intervention.39–42 Clinical Findings Presentation Mild dyspnea on exertion and/or easy fatigability are the most common early symptoms of an interatrial communication They are not usually present during infancy or early childhood However, not infrequently, parents report increased activity and stamina after repair, and “asymptomatic” adults frequently report improved exercise tolerance after repair A left-to-right shunt may exacerbate other conditions, such as asthma and other chronic pulmonary diseases; again, earlier than usual closure should be considered under these circumstances Symptoms become much more common in the fourth or fifth decade for reasons already discussed Physical Examination The general physical examination is usually normal Associated noncardiac abnormalities are uncommon in individuals with a defect within the oval fossa or a sinus venosus defect.9 Nonetheless, when there is a diagnosed syndrome known to be associated with a defect (e.g., Noonan syndrome), screening echocardiography is indicated unless the physical findings are unequivocally normal Skeletal anomalies of the forearm and hand do occur occasionally in association with Holt-Oram syndrome.11 Noncardiac anomalies much more commonly accompany ostium primum defects Notable examples include Down syndrome43 and the visceral anomalies typically present with isomerism of the atrial appendages.44 The jugular venous pulse is usually normal, as are blood pressure and peripheral arterial pulses A left parasternal lift may be present, but precordial motion is often normal, especially if the left-to-right shunt is not large Rarely, there is asymmetric development of the chest with a protuberance of the lower left aspect of the thorax, reflecting more severe enlargement of the right heart The heart sounds are almost always abnormal The first sound at the lower left sternal border may be accentuated because of prominent closure of the tricuspid valve,45 but this can be subtle The second sound is characteristically widely and fixedly split, with little or no variation in the width of the split during the respiratory cycle Several reasons for the lack of respiratory variation in splitting