Right atrial ultrastructure in congenital heart disease. II. Atrial septal defect: effects of volume overload.

Portions of operatively resected right atrium from 15 patients with atrial septal defect were studied ultrastructurally to determine whether the cell hypertrophy in the right atrium of patients with increased right atrial blood flow and increased right atrial pressure is caused by the increased blood flow. In 12 patients with normal right atrial mean pressure but increased right atrial blood flow the atrium was dilated but no atrial arrhythmias were noted clinically. Ultrastructurally, the atrial myocardial cells in these patients were normal, measuring 6 to 10 mu in diameter, and there was no evidence of cell hypertrophy or degeneration. The remaining three patients had elevated right atrial mean pressure and increased right atrial blood flow. Ultrastructurally, the atrial myocardial cells in all three patients were hypertrophied, and two patients had evidence of focal cell degeneration; the atrium was markedly dilated, but atrial arrhythmias were not noted. The lack of cell hypertrophy in the right atrium of the 12 patients with increased blood flow but normal mean pressure suggests that in congenital heart disease volume overload alone does not lead to cell hypertrophy of the right atrial myocardium.