Inversion of the normal interatrial septum convexity has been described in patients with right atrial pressure or volume overload, but there is no reference to this abnormality in acute myocardial infarction. A group of 576 consecutive patients with acute infarction and serial echocardiographic studies were prospectively evaluated during a mean follow-up period of 406 days. Inverted interatrial septum convexity was found in 30 patients (5.2%); 29 of the 30 presented with inferior infarction with right ventricular involvement (29 [24.4%] of 119) and the remaining presented with cardiac tamponade secondary to heart rupture. The incidence of inverted interatrial septum convexity rapidly decreased, and after 3 months it was present in only five patients. All patients with inverted interatrial septum convexity had a right atrial pressure greater than or equal to pulmonary capillary pressure, a relation found in only 2 of 43 patients with right ventricular involvement and normal septal convexity. In patients with right ventricular infarction, right atrial pressure was higher in the presence of inverted septal convexity (15.9 +/- 4.1 versus 10.5 +/- 4.1 mm Hg, p less than 0.0001) and the incidence of hypotension (10 [34.4%] of 29 versus 15 [17.4%] of 90, p = 0.04) and third degree atrioventricular block (10 [34.4%] of 29 versus 11 [12.2%] of 90, p = 0.006) as well as the mortality rate after 3 months (9 [31%] of 29 versus 11 [12.2%] of 90, p = 0.04) were higher in the presence of inverted convexity than in patients with normal septal convexity.(ABSTRACT TRUNCATED AT 250 WORDS)