Disclosed is a pulmonary artery catheter ("PAC") that is used in determining myocardial oxygen consumption. Myocardial oxygen consumption is of critical importance because decreased myocardial energy utilization during acute illness may lead to tissue hypoperfusion, multiple organ failure, and eventually death. The inventor has discovered that myocardial oxygen consumption is a function of the difference in oxygen levels in atrial and mixed venous blood. The invention has further discovered that differences in lactate, glucose or any other measurable blood concentration metabolite in atrial or superior vena cava and mixed venous blood can also be used in determining myocardial oxygen consumption.