Exercise studies in congenital heart block

Abstract

Bicycle exercise studies were performed on 4 patients with congenital heart block. Cardiac output was measured using the indirect (CO2) Fick technique. Two patients were found to have a normal capacity for work but they both had low cardiac outputs at high work loads. The 2 patients who were unable to perform a normal amount of work had normal cardiac outputs on exercise at relatively low work loads. The degree to which a patient increased his ventricular rate or stroke volume was not related to the maximum work which he could perform. It is suggested that in these patients with very slow heart rates at work normal amounts of work can only be done if the patient can tolerate a low cardiac output, a large arteriovenous oxygen difference, and a low venous oxygen tension.