Congenital heart disease (CHD) consists of both shunting and/or obstructive lesions. In the presence of these defects, blood flow follows the path of least resistance. The amount of shunting between two circulatory beds is dependent on the relative gradient between these two systems as well as the size of the shunt. When there is an unrestrictive shunt, the amount and direction of blood flow are dependent solely on the difference in vascular resistances between the two sides. Any restriction (e.g., stenosis) typically serves as the point of maximal resistance, which limits the relative blood flow between two areas.
With normal physiology and no shunting present, the total pulmonary blood flow (Qp) is equal to the total systemic blood flow (Qs). Therefore, the ratio of Qp:Qs is equal to 1. When comparing the resistances of the two circulatory beds, the pulmonary vascular resistance (PVR) is normally lower than that on the systemic side. Consequently, in the presence of a shunt, blood will flow from the higher resistance systemic circulation to the lower resistance pulmonary circulation, making the relative Qp:Qs greater than 1. Conditions with this physiology are referred to as acyanotic lesions.
Acyanotic CHD is characterized by the relative increase in the pulmonary blood flow secondary to recirculation coming from the systemic circulation. As such, these lesions are also referred to as left-to-right shunts (through either intracardiac or extracardiac connections). Depending on variations in anatomy and whether or not there is an obstruction to systemic blood flow, patients with other lesions such as transposition of the great arteries, double outlet right ventricle (RV), and truncus arteriosus can also have high saturations in the neonatal period once PVR begins to drop.
ATRIAL SEPTAL DEFECT (ASD)
An ASD is a communication between the left and right atria secondary to a defect in the septum that separates these two chambers of the heart. There are four morphologic types of ASDs that are classified by their location:
Ostium Primum—Also known as a partial atrioventricular canal (AVC) defect.
Ostium Secundum—Due to deficiency in the fossa ovalis membrane. Accounts for 80% of ASDs.
Inferior and Superior Sinus Venosus Defects—Located at the junctions with the superior vena cava (SVC) and inferior vena cava (IVC) and most often associated with anomalous pulmonary drainage.
Shunts that are less than 0.5 cm will usually have no hemodynamic consequences and often go undetected. However, once they get larger (greater than 2 cm), there will be an increase in pulmonary blood flow that results in a systolic ejection murmur heard at the second left intercostal space. If there is uncorrected significant shunting, patients will develop dyspnea, supraventricular dysrhythmias (secondary to dilation of the right atrium), and ultimately right heart failure with pulmonary hypertension.
Many secundum ASDs can be closed with a device in the catheterization lab. ...