These areas of the heart are served by the posterior interventricular branch of the right coronary artery, which is therefore the most likely site of the thrombosis.
With a ventricular septal defect, blood in the left ventricle (which has just come from the lungs) flows through the defect into the right ventricle, and is circulated once again to the lungs. This results in pulmonary hypertension, and the extra workload on the heart causes myocardial hypertrophy, or thickening of the ventricular walls.
Systemic blood from the superior and inferior venae cavae would flow into the right atrium, then the right ventricle, then the aorta, and return to the systemic route without being oxygenated. Pulmonary blood returning from the lungs would enter the left atrium, then continue to the left ventricle, then pulmonary trunk, and return to the lungs without ever entering the systemic circulation; thus its oxygenated blood would not serve any organs on the systemic circuit. The infant would require immediate surgery.
The hiker’s heart would shift to the right, because with the right lung collapsed and the left lung inflating on each inspiration, pressure in the left half of the thoracic cavity would be greater than in the right. This displaces the mediastinum, including the heart, toward the low-pressure side.
As the left ventricle enlarges, the AV (mitral) valve orifice widens and the valve cusps no longer meet in the middle. Thus the closing valve cannot completely seal the orifice, and blood regurgitates into the left atrium.
