A A PILLAI, A HANDA, R CHANDRAMOHAN The pathophysiology of isolated atrial septal defects (ASDs) depends upon the relationship of pulmonary and systemic resistances, the compliance of the right and left ventricles and the size of the defect. Perinatal physiology—In utero, pulmonary arterial blood flow in the foetus is limited by high pulmonary vascular resistance. As a result, the blood that flows into the right atrium is shunted across the isolated ASD into the left atrium, similar to the blood flow through the normal patent foramen ovale. At birth, left atrial pressure becomes greater than right atrial pressure resulting in left-to-right shunting across the defect. Initially, the volume of blood shunted from left to right is small because the right ventricle is still relatively thick-walled and noncompliant. As the right ventricle remodels in response to the decreased pulmonary vascular resistance, its compliance increases and the mean right atrial pressure decreases. As a result, the left-to-right shunting increases in volume. In some neonates, transient right-to-left shunting may also occur during the cardiac and respiratory cycles, resulting in mild cyanosis. In these patients, there is a drop in atrial pressure at the onset of ventricular contraction due to atrial relaxation that is more rapid in the left than the right atrium. During inspiration, the decrease in intrathoracic pressure results in an increase in systemic venous return and a decrease in pulmonary venous return, decreasing left atrial pressure and increasing right atrial pressure, which results in right-to-left shunting. Postnatal physiology—Although there are several forms of ASDs, the pathophysiology is similar. With a small ASD, left atrial pressure is slightly higher than right atrial pressure, resulting in continuous flow of oxygenated blood from the left to the right atrium across the defect. The pressure gradient between the two atria and the amount of shunt flow depends upon the size of the defect and the relative distensibility of the right and left sides of the heart. Left-to-right shunting occurs primarily in the late ventricular systole and early diastole and with augmentation during atrial systole. Even when the right and left atrial pressures are equal, as will be seen with a large defect, left-to-right shunting still occurs because of the greater compliance of the right ventricle compared with the left ventricle. The shunt flow constitutes a circuit through the right atrium, right ventricle, pulmonary circulation, left atrium and defects back to the right atrium. Thus, the volume of blood flow in the pulmonary circulation is greater than that in the systemic circulation. The increased flow leads to right-sided dilatation, evident on chest radiograph and echocardiographic imaging. Right ventricular function is also occasionally decreased. The main pulmonary arteries dilate and the pulmonary vascularity is increased. These pulmonary vascular changes may be evident on the chest radiograph, and large vessels in both the lower and upper lobes may be seen. Heart failure is unusual before the age of 30, but the prevalence increases substantially in older uncorrected patients over time; right-sided volume overload is usually well tolerated for years. Other complications in older patients include atrial arrhythmias such as flutter and fibrillation, thought to result from chronic stretching of the atrial muscle, and occasionally, pulmonary arteriopathy leading to progressive pulmonary hypertension resulting in right-to-left shunting of blood (i.e., Eisenmenger syndrome). The natural course of isolated ASDs, which are primarily secundum ASDs, varies from spontaneous closure to enlarging defects and increasing symptoms. Spontaneous closure—Spontaneous closure, or a decrease in size, is most likely to occur in younger patients and those with defects less than 7–8 mm in diameter.1 In a review of 101 infants diagnosed at a mean age of 26 days and with an average follow-up of 9 months, spontaneous closure occurred in all 32 patients with ASDs <3 mm in diameter, 87% of patients with 3–5 mm ASDs, 80% of patients with 5–8 mm ASDs and in none of the four infants with defects larger than 8 mm.2
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Pathophysiology
NATURAL HISTORY
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