Congenital Heart Disease 1-Atrial Septal Defect

Dr.AhmadDr.Ahmad مدير عام
تم تعديل 2009/03/25 في طب الأطفال Pediatrics
Atrial Septal Defect

An atrial septal defect (ASD) is defined as an opening in the interatrial septum that enables the mixing of blood from the systemic venous and pulmonary venous circulations.
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Embryology

The atrial and ventricular septa form between the third and sixth weeks of fetal development. After the paired heart tubes fuse into a single tube folded onto itself, the distal portion of the tube causes an indentation to form in the roof of the common atrium. Near this portion of the roof, the septum primum arises and extends into a crescentic formation toward the atrioventricular (AV) junction. The gap remaining between the septum primum and the developing tissues of the AV junction is called the ostium primum. Before the septum primum fuses completely with the endocardial cushions, a series of fenestrations appear in the septum primum that coalesce into the ostium secundum. During this coalescence, the septum secundum grows downward from the roof of the atrium, parallel to and to the right of the septum primum. The septum primum does not fuse, but creates an oblique pathway, called the foramen ovale, within the interatrial septum. After birth, the increase in left atrial pressure normally closes this pathway, obliterating the interatrial connection. 4

Anatomy

ASDs can be classified into three different types: (1) sinus venosus defects, comprising approximately 5 to 10% of all ASDs; (2) ostium primum defects, which are more correctly described as partial atrioventricular canal defects; and (3) ostium secundum defects, which are the most prevalent subtype, comprising 80% of all ASDs (Fig. 19-1). 5

Pathophysiology

ASDs result in an increase in pulmonary blood flow secondary to left-to-right shunting through the defect. The direction of the intracardiac shunt is predominantly determined by the compliance of the respective ventricles. In utero, the distensibility, or compliance, of the right and left ventricles is equal, but postnatally the left ventricle (LV) becomes less compliant than the right ventricle (RV). This shift occurs because the resistance of the downstream vascular beds changes after birth. The pulmonary vascular resistance falls with the infant's first breath, decreasing RV pressure, whereas the systemic vascular resistance rises dramatically, increasing LV pressure. The increased LV pressure creates a thicker muscle mass, which offers a greater resistance to diastolic filling than does the RV; thus, the majority of flow through the ASD occurs from left to right. The greater volume of blood returning to the right atrium causes volume overload in the RV, but because of its lower muscle mass and low-resistance output, it easily distends to accommodate this load. 5,6
The long-term consequences of RV volume overload include hypertrophy with elevated RV end-diastolic pressure and a relative pulmonary stenosis across the pulmonary valve, because it cannot accommodate the increased RV flow. The resistance at the level of the pulmonary valve then contributes a further pressure load on the RV, which accelerates RV hypertrophy. Compliance gradually decreases as the right ventricular pressure approaches systemic pressure, and the size of the left-to-right shunt decreases. Patients at this stage have a balanced circulation and may deceptively appear less symptomatic.
A minority of patients with ASDs develop progressive pulmonary vascular changes as a result of chronic overcirculation. The increased pulmonary vascular resistance in these patients leads to an equalization of left and right ventricular pressures, and their ratio of pulmonary (Qp) to systemic flow (Qs), Qp:Qs, will approach 1. 5,7 This does not mean, however, that there is no intracardiac shunting, only that the ratio between the left-to-right component and the right-to-left component is equal.
The ability of the right ventricle to recover normal function is related to the duration of chronic overload, because those undergoing ASD closure before age 10 years have a better likelihood of achieving normal RV function in the postoperative period. 3
The physiology of sinus venosus ASDs is similar to that discussed above except that these are frequently accompanied by anomalous pulmonary venous drainage. This often results in significant hemodynamic derangements that accelerate the clinical course of these infants.
The same increase in symptoms is true for those with ostium primum defects because the associated mitral insufficiency from the "cleft" mitral valve can lead to more atrial volume load and increased atrial level shunting.

Diagnosis

Patients with ASDs may present with few physical findings. Auscultation may reveal prominence of the first heart sound with fixed splitting of the second heart sound. This results from the relatively fixed left-to-right shunt throughout all phases of the cardiac cycle. A diastolic flow murmur indicating increased flow across the tricuspid valve may be discerned, and, frequently, an ejection flow murmur can be heard across the pulmonary valve. A right ventricular heave and increased intensity of the pulmonary component of the second heart sound indicates pulmonary hypertension and possible unrepairability.
Chest radiographs in the patient with an ASD may show evidence of increased pulmonary vascularity, with prominent hilar markings and cardiomegaly. The electrocardiogram shows right axis deviation with an incomplete bundle-branch block. When right bundle-branch block is associated with a leftward or superior axis, an AV canal defect should be strongly suspected. 8
Diagnosis is clarified by two-dimensional echocardiography, and use of color-flow mapping facilitates an understanding of the physiologic derangements created by the defects. 9 Echocardiography also enables the clinician to estimate the amount of intracardiac shunting, can demonstrate the degree of mitral regurgitation in patients with ostium primum defects, and with the addition of microcavitation, can assist in the detection of sinus venosus defects. 5
The advent of two-dimensional echocardiography with color-flow Doppler has largely obviated the need for cardiac catheterization because the exact nature of the ASD can be precisely defined by echo alone. However, in cases where the patient is older than age 40 years, catheterization can quantify the degree of pulmonary hypertension present, because those with a pulmonary vascular resistance (PVR) greater than 12 U/mL are considered inoperable. 10 Cardiac catheterization also can be useful in that it provides data that enable the calculation of Qp and Qs so that the magnitude of the intracardiac shunt can be determined. The ratio (Qp:Qs) can then be used to determine whether closure is indicated in equivocal cases, because a Qp:Qs greater than 1.5:1 is generally accepted as the threshold for surgical intervention. Finally, in patients older than age 40 years, cardiac catheterization can be important to disclose the presence of coronary artery disease.
In general, ASDs are closed when patients are between 4 and 5 years of age. Children of this size can usually be operated on without the use of blood transfusion and generally have excellent outcomes. Patients who are symptomatic may require repair earlier, even in infancy. Some surgeons, however, advocate routine repair in infants and children, as even smaller defects are associated with the risk of paradoxical embolism, particularly during pregnancy. In a recent review by Reddy and colleagues, 116 neonates weighing less than 2500 g who underwent repair of simple and complex cardiac defects with the use of cardiopulmonary bypass were found to have no intracerebral hemorrhages, no long-term neurologic sequelae, and a low operative-mortality rate (10%). These results correlated with the length of cardiopulmonary bypass and the complexity of repair. 11 These investigators also found an 80% actuarial survival at 1 year and, more importantly, that growth following complete repair was equivalent to weight-matched neonates free from cardiac defects. 11

Treatment

ASDs can be repaired in a facile manner using standard cardiopulmonary bypass (CPB) techniques through a midline sternotomy approach. 7 The details of the repair itself are generally straightforward. An oblique atriotomy is made, the position of the coronary sinus and all systemic and pulmonary veins are determined, and the rim of the defect is completely visualized. Closure of ostium secundum defects is accomplished either by direct suture or by insertion of a patch. The decision of whether patch closure is necessary can be determined by the size and shape of the defect as well as by the quality of the edges.
Sinus venosus ASDs associated with partial anomalous pulmonary venous connection are repaired by inserting a patch, with redirection of the pulmonary veins behind the patch to the left atrium. Care must be taken with this approach to avoid obstruction of the pulmonary veins or the superior vena cava, although usually the superior vena cava is dilated and provides ample room for patch insertion.
These operative strategies have been well established, with a low complication rate and a mortality rate approaching zero. As such, attention has shifted to improving the cosmetic result and minimizing hospital stay and convalescence. Multiple new strategies have been described to achieve these aims, including the right submammary incision with anterior thoracotomy, limited bilateral submammary incision with partial sternal split, transxiphoid window, and limited midline incision with partial sternal split. 4,12–14 Some centers use video-assisted thoracic surgery (VATS) in the submammary and transxiphoid approaches to facilitate closure within a constricted operative field. The morbidity and mortality of all of these approaches are comparable to those of the traditional median sternotomy; however, each has technical drawbacks. The main concern is that operative precision be maintained with limited exposure. Luo and associates recently described a prospective randomized study comparing ministernotomy (division of the upper sternum for aortic and pulmonary lesions, and the lower sternum for septal lesions) to full sternotomy in 100 consecutive patients undergoing repair of septal lesions. 14 The patients in the ministernotomy group had longer procedure times (by 15 to 20 minutes), less bleeding, and shorter hospital stays. These results have been echoed by other investigators from Boston who maintain that ministernotomy provides a cosmetically acceptable scar without compromising aortic cannulation or limiting the exposure of crucial mediastinal structures. 12 This approach also can be easily extended to a full sternotomy should difficulty or unexpected anomalies be encountered. 13
First performed in 1976, transcatheter closure of ASDs with the use of various occlusion devices is gaining widespread acceptance. 15 Certain types of ASDs, including patent foramen ovale, secundum defects, and some fenestrated secundum defects, are amenable to device closure. Complications reported to occur with transcatheter closure include air embolism (1 to 3%); thromboembolism from the device (1 to 2%); disturbed atrioventricular valve function (1 to 2%); systemic/pulmonary venous obstruction (1%); perforation of the atrium or aorta with hemopericardium (1 to 2%); atrial arrhythmias (1 to 3%); and malpositioning/ embolization of the device requiring intervention (2 to 15%). 4,16 Thus, although percutaneous approaches are cosmetic and often translate into shorter periods of convalescence, their attendant risks are considerable, especially because their use may not result in complete closure of the septal defect.

Results

Surgical repair of ASDs should be associated with a mortality rate near zero. 4,5,7,8,11 Early repairs in neonates weighing less than 1000 g have been increasingly reported with excellent results. 11 Uncommonly, atrial arrhythmias or significant left atrial hypertension may occur soon after repair. The latter is caused by the noncompliant small, left atrial chamber and generally resolves rapidly.

التعليقات

  • تم تعديل 2009/03/25
    السلام عليكم ورحمة الله وبركاته ECG may provide astrong diagnostic clue: secundumASD-right axis deviation due to right ventricular enlargment . partial AVSD-left axis deviation a so-called (superior QRS axis). this occurs because there is adefect of middle part of heart where the atrioventricular node is. the displaced node then conducts to the ventricles superiorly giving the abnormal axis
  • Dr.AhmadDr.Ahmad مدير عام
    تم تعديل 2009/03/25
    thanks dr.APLY for your add