What maintains the metabolic cost at maximal exercise in heart transplant recipients and coronary artery disease patients?

Background: In this study we assess the influence of disease status on hemodynamic and cardiac output values, as measured by oxygen utilization at peak aerobic exercise, in heart transplant recipients (HTRs) and coronary artery disease patients (CAD).

Methods: Fifteen CAD patients and 13 HTRs (40.2 +/- 12.6 and 41.7 +/- 11.7 years, respectively) underwent a peak cardiopulmonary exercise test on bicycle ergometry. Arterial oxygen was defined on the basis of echocardiography and venous oxygen content.

Results: At rest, except for cardiac output, oxygen uptake and lactate levels, all variables were significantly (p < 0.01) different between groups. At peak exercise, compared with HTRs, CAD patients had significantly (p < 0.0001) higher values for cardiac output (12.4 +/- 0.8 and 20.2 +/- 1.7 liters/min, respectively), stroke volume (87.3 +/- 5.4 and 129.3 +/- 9.7 ml, respectively) and oxygen uptake (22.7 +/- 3.6 and 29.7 +/- 2.7 ml/kg/min, respectively) (p < 0.01), whereas (a - v)O2 was significantly lower (127.0 +/- 4.3 and 141.9 +/- 6.4 O2 ml/liter, respectively; p < 0.0001).

Conclusions: The differences in oxygen utilization at peak exercise may be attributed to differences in energy metabolism, namely higher oxygen extraction in HTRs, compensating for the dramatically reduced oxygen delivery. It is further suggested that CAD patients and HTRs respond to a greater extent to maximal aerobic testing by reducing their left ventricular systolic function despite increased after-load.