Cardiac output and pulmonary gas exchange at maximal exercise after atrial redirection for complete transposition

To assess the determinants of exercise capacity and exercise oxygenation after atrial redirection for complete transposition.

At graded bicycle ergometry, including respiratory and arterial blood gas analyses, intra-arterial blood pressure recording, and cardiac output determination (dye-dilution technique), we tested 17 post-Mustard/Senning patients, 8.9-22.0 years old (mean 14.5, SD 4.0). Reference data were obtained by similar methods. At maximal exercise, oxygen uptake (29.6 ml x kg(-1) x min(-1)) and heart rate (167 beats x min(-1)) were low (P<0.001). Right-to-left shunts were detected in five patients. Arterial oxygen partial pressure and saturation fell in all subjects (P<0.0001). In 15/16 (94%) the alveolar-arterial oxygen partial pressure difference was > +2 SD. In 13/15 (87%) stroke volumes fell during exercise. Cardiac output per oxygen uptake was low (P<0.0001), which implies a high arteriovenous oxygen difference and a low mixed venous oxygen content at peak exercise.

The low exercise capacity was caused by a combination of low maximally attained heart rate and falling stroke volumes. The impaired arterial oxygenation may be caused by a combination of pulmonary ventilation/perfusion mismatch, a low mixed venous content and atrial shunting in some patients.