The pulmonary artery carries oxygen-depleted blood from the heart to the lungs. Because the anomalous left coronary artery receives its blood from the pulmonary artery rather than the aorta, it supplies the heart muscle with blood that is low in oxygen.

Secondly, as the blood pressure is three times lower in the pulmonary artery than in the aorta, blood will tend to flow backwards into the pulmonary artery from the coronary artery. The combination of lower oxygen and stealing of blood away from the heart muscle can have dire consequences resulting in heart attacks in children as early as the first year of life. This defect results in decreased heart function, poor feeding, and irritability.