Introduction
Atrial septal defects (ASDs) defect in the atrial septum that lead to shunting of blood between left and right atria. They constitute 10 to 15 per cent of congenital cardiac defects and up to 40 per cent of congenital defects presenting in adulthood. Over 80 per cent of defects are secundum ASDs or stretched patent foramen ovale (PFO).
The atrial septum is made up mainly of the flap valve of the oval fossa surrounded by the thickened superior rim or ‘limbus’ superiorly and laterally. This is the ‘true’ atrial septum, as the remainder of the ‘septum secundum’ is actually infolded atrial wall rather than septal structures. The foramen ovale lies at the superior end of the oval fossa and usually closes as the pressures in the atria reverse after birth (see Chapter 1), closing the flap valve against the limbus. Persistence of this hole, in the form of a PFO represents the commonest form of interatrial communication.
Types of ASD
Patent foramen ovale (PFO).
The oval fossa may be probe patent in 25 to 30 per cent of humans, detectable in up to 15 to 20 per cent of adults with echocardiography. The PFO can become stretched such that it becomes a significant opening located superiorly in the fossa ovalis. Large defects become indistinguishable from a secundum ASD.
Secundum ASD.
This involves defects of the true atrial septum, the septum secundum. There is complete or partial absence of the floor of the oval fossa, and there may be remnant strands of septal tissue that create the appearance of fenestrations or of multiple neighbouring defects. Frequently these defects extend close to the inferior vena cava (IVC) opening.
Sinus Venosus ASD.
This is an interatrial communication (10 per cent) related to the point where the superior vena cava (SVC) or the IVC enters the atrium (these defects can also be described as ‘the caval vein having a biatrial connection’). The majority are superior sinus venosus ASDs, at the mouth of the SVC, which is associated in 90 per cent of cases with an anomalous right upper pulmonary vein from the upper and/or middle lobe draining into this junction.
Primum ASDs.
Also called ‘partial atrio-ventricular septal defects’, these defects constitute an absence of the primum atrial septum. The defect is crescentic in shape, and the medial margin is the valve leaflets of the atrio-ventricular (A-V) valves – thus there is no atrial tissue between the defect and the valve leaflets. These defects are part of the spectrum of atrio-ventricular septal defects and are dealt with in detail in Chapter 10.
Coronary Sinus ASD.
These are varying communications between the coronary sinus and the left atrium (<3 per cent), also known as ‘unroofed coronary sinus’. When the coronary sinus is completely unroofed, the orifice of the coronary sinus is essentially an opening into the left atrium – hence an ASD. This defect can be associated with persistent left SVC (see Figure 7.1).
Pathophysiology and Natural History
ASDs permit left-to-right shunting, causing volume overload of the right heart and increased pulmonary blood flow. The magnitude and direction of flow through any ASD depend on the size of the ASD and the relative diastolic filling properties/compliance of the left and right ventricles.
The shunt in isolated ASD is left to right. A significant shunt is defined as one that has a pulmonary-to-systemic blood flow ratio Qp:Qs greater than 1.5:1 or is any shunt causing significant right heart dilatation. This chronic volume overload of the right heart causes pulmonary congestion which can predispose to recurrent chest infection, but haemodynamic symptoms are due to gradual right ventricular failure causing effort intolerance (not usually until the fourth or fifth decade). This can be worsened by the development of functional tricuspid incompetence and/or atrial fibrillation/flutter secondary to the chronic volume overload of the right heart. The high pulmonary blood flow associated with significant ASD is at low pressure and generally does not tend to precipitate increased pulmonary vascular resistance. Nevertheless, there is a small group of patients with ASDs (5 per cent of adults with untreated large ASDs) who will develop pulmonary hypertension if the ASD is not treated, although debate continues as to whether this is a manifestation of primary pulmonary hypertension rather than the ASD being the cause.
In patients operated on before age 25, right ventricle (RV) size and function would be expected to return to normal and life expectancy return to that of the normal population, However, repair at >25 years, although still conveying both symptomatic improvement and improved life expectancy, does not restore to normality. When ASDs are closed in childhood, the long-term risk of arrhythmias is not much higher than that of the general population, but after age 11, this risk increases so that about 50 per cent of 60-year-olds with ASD have atrial flutter or fibrillation or sinus node dysfunction.
ASDs rarely cause symptoms in children and are usually found incidentally or through investigations for recurrent chest infections. Rarely, if the defect is very big, there may be heart failure and failure to thrive (1 per cent of patients). If an infant or small child presents with heart failure, other associated lesions must be excluded, e.g. patent arterial duct or lesions causing high left atrial pressure. In older children and adolescents, there may be reduced exercise tolerance in comparison to peers. In older adults, presentation may be at the onset of atrial arrhythmias. Congestive heart failure may occurs in up to a third of patients over age 40.
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
