The most likely diagnosis in this patient is a patent foramen ovale with right-to-left shunt. He presents with features of platypnea-orthodeoxia syndrome, characterized by positional symptoms of cyanosis and dyspnea that generally occur when the patient is sitting and resolve in the supine position. Right-to-left shunting across an atrial septal defect or patent foramen ovale may rarely cause cyanosis and dyspnea owing to deformation of the atrial septum and redirection of shunt flow that result from increased right atrial pressure in the upright position. This patient had an inferior and right ventricular myocardial infarction with associated right heart enlargement and dysfunction and clinical features of hypotension. The right heart enlargement causes annular dilatation and tricuspid regurgitation. The foramen ovale stretches and becomes patent. The preferential cyanosis is caused by the hemodynamic alterations and preferential transfer of right atrial blood across the patent foramen in the upright position.

Mitral regurgitation due to papillary muscle injury or rupture is a recognized complication after myocardial infarction; however, the presentation generally is characterized by acute dyspnea and pulmonary edema rather than platypnea-orthodeoxia. Physical examination findings include a systolic murmur at the apex that increases during expiration rather than inspiration.

Severe left ventricular systolic dysfunction generally does not cause arterial oxygen desaturation. In addition, the initial assessment of left ventricular function was normal. The clinical picture in this patient is more compatible with right ventricular dysfunction.

Ventricular septal defect is a recognized complication after transmural myocardial infarction; however, the presentation generally includes acute dyspnea and pulmonary edema rather than oxygen desaturation. The left-to-right shunt associated with the ventricular septal defect causes left heart volume overload, rather than the right heart volume overload caused by right-to-left shunting seen in this patient. Physical examination findings in patients with ventricular septal defect following myocardial infarction include a holosystolic murmur at the left sternal border that does not change with respiration.