Our observations regarding platypnea and orthodeoxia were based on the clinical appearance and arterial saturation changes observed at the bedside; the patient's precipitous deterioration precluded more extensive evaluation by tilt-table testing using echocardiography or subsequent catheterization [1]. However, we presumed that the increased right-to-left shunting across the atrial septal defect was due to preferential streaming of blood from the inferior vena cava associated with upright posture as described in the case by Sorrentino and Resnekov [2] and previously from our institution [3] in seven cases documented by catheterization, right atrial angiocardiography, and tilt-table contrast echocardiography.

Why positional change causes variation in shunting in this condition remains obscure. As Dr. Sorrentino correctly points out, most patients have had normal right heart pressures, and upright posture has been presumed to alter the anatomy of the fossa ovalis or to result in subtle changes in venous return or right ventricular compliance, the net effect of which enhances a small preexisting right shunt. Indeed, although most reported patients have been clinically acyanotic while supine, all such cases, including the one reported by Dr. Sorrentino, have shown supine right-to-left shunting (ranging in our previous study from 6% to 24% supine and increasing with the upright position to 18% to 41%). Thus, a spectrum of severity appears to accompany positional desaturation; our patient represents an extreme example, no doubt due to the marked impairment of right ventricular filling.