Left To Right Shunt Lesions: Atrial Septal Defect (ASD)
The Different Shunts
This is the commonest congenital heart disease found in adults. This is characterized by the presence of one or more defects in the inter-atrial septum. Based on the developmental defect there are mainly three types- Ostium secondum, Ostium Primum and the sinus venosus type of defects. Ostium secondum defect occurs in the region of the fossa ovalis and this is the commonest. This results from the maldevelopment of the septum second. The only abnormality is the presence of atrial septal defect.
Ostium primum defect is the result of abnormal development of the endocardial cushion. The defect is in the lower portion of the inter-artrial septum. This may be associated with cleft mitral or tricuspid valve resulting in their incompetence. Sinus venosus type of defect occurs in the upper portion of the inter-atrial septum. It is due to the abnormality of development of the upper part of the inter-atrial septum and it is associated with anomalous connection of one or more pulmonary veins to the superior vena cava or right atrium.
In ASD, the blood is shunted from left atrium to right atrium and then to the right ventricule. The right ventricular output and the pulmonary blood flow are considerably increased. This leads to progressive enlargement of the right atrium, right ventricle and pulmonary arteries.
Artria septal defect is found more in females. This may occur as an isolated anomaly or in association with other intracardiac anomalies like pulmonic stenosis or ventricular septal defect. Rheumatic mitral stenosis may coexist with ASD (Lutembacher’s syndrome).
Palpitation and recurrent respiratory infection are the usual symptoms. Effort intolerance usually develops in the third and fourth decades and thereafter features of congestive heart failure develop. Clinically, there is cardiac enlargement with hyperactive precordium as a result of right ventricular enlargement. Pulmonary artery pulsation is felt in the left second intercostals space. In some cases, a systolic thrill may be felt over the upper part of the sternum on the left. A pulmonic ystolic murmur may be heard. The murmur and the thrill are produced by increased blood flow through the pulmonary valve. The second sound in the pulmonary area is widely split since the pulmonic valve closure is delayed by the larger volume of blood to be ejected by the right ventricle. During inspiration more venacaval blood flows into the right side of the heart and during expiration more of left atrial blood flows through the shunt. These phenomena account for the ejection of the same quantity of blood across the pulmonary valve during inspiration and expiration. Therefore, the split of the pulmonary second sound remains fixed. This is the single most characteristic feature of ASD. Increased flow across the tricuspid valve during diastole may produce a tricuspid mid-diastolic murmur, best heard along the left lower sterna border. This is usually preceded by a right ventricular third heart sound.
Course And Prognosis
Small atrial septal defect may remain asymptomatic. When the shunt is large, the right-sided chambers progressively enlarge. Pulmonary hypertension develops in course of time. The pulmonary pressure may be elevated to high levels and the same is reflected in the right ventricule and right atrium. When right atrial pressure exceeds that of the left atroum, the blood flow is reversed and this leads to the development of central cyanosis. This phenomenon is called Eisenmenger Syndrome. Thereafter, the course is rapidly downhill and death occurs due to congestive heart failure. Primum type of ASD carries a poorer prognosis than the secondum type. Other complications include paradoxical embolism, arrhythmias like atrial fibrillation and very rarely infective endocarditis.
Atrial septal defect can be repaired surgically. Surgery is preferably done before marked elevation of pulmonary artery pressure develops. Cases with severe irreversible pulmonary hypertension are not suitable for surgery.
© 2014 Funom Theophilus Makama