When a large defect occurs between the left and right heart or great vessel, the pressures are equal in both sides of the lesion. The direction of blood flow is determined by the pathway of least resistance. In an atrial septal defect, blood flows from left to right, as the resistance offered by the right heart and lungs is less than that of the left heart and periphery.
ASD blood flow
Blood flow across an atrial septal defect can be modified by a change in resistance. Pulmonary vascular obstructive disease may develop in an adult. The result is an increase in right-sided resistance of the pulmonary vascular bed and right ventricle. If severe, this may result in reversal of the shunt with right to left flow. In some older patients, increased left-sided resistance may occur as the left ventricle becomes less compliant with the development of coronary artery disease or systemic hypertension. This may result in increased left to right shunting.
Shunt magnitude
Quantitation of the magnitude of the left to right shunt can be achieved by several methods. The magnitude is usually expressed as a ratio of pulmonary to systemic blood flow. Physical examination can provide a rough estimate. In our patient, for example, the presence of a diastolic flow murmur indicates the pulmonary to systemic flow ratio of at least 2:1. A smaller shunt would not produce this finding. Echocardiographic measurement of pulmonary trunk and aortic areas and Doppler flow velocity across the respective valves permits the calculation of the volume of left- and right-sided flow. A more accurate assessment of shunt magnitude can be obtained during cardiac catheterization by measuring pulmonic and aortic blood flow.
Hemodynamic data
Typical hemodynamic data obtained at catheterization from a patient with findings similar to our are demonstrated. Pulmonary blood flow measured 10 L/min and systemic blood flow measured 4 L/min. The pulmonary to systemic, or Qp/Qs was 2.5, reflecting a large shunt. The patient's pulmonary vascular resistance was normal.
Cleft mitral valve
This is a systolic still-frame of the left ventricular angiogram from another patient with an ostium primum defect associated with a cleft mitral valve and mitral regurgitation. The balloon tip catheter is in the left ventricle where contrast material has been injected. The left ventricle, the aortic valve leaflets and the aorta are visualized. The arrows identify the area of the mitral valve. It has a cleft in the anterior leaflet causing mitral regurgitation. In the real-time study that follows, note the cleft in the mitral valve best seen in the presystolic phase and the systolic jet of mitral regurgitation. The atrial septal defect is not clearly demonstrated.