 The physiologic changes that explain the above mentioned findings in hemodynamic parameters during weaning are complex and not fully understood 1. It is well known that weaning can place an excessive burden on the heart and it can be considered a form of exercise 2. As studied by exercise physiologists, the increase in oxygen consumption during exercise in healthy individuals is matched both by an increase in cardiac output and oxygen extraction. In patients with overt heart failure, the increase in cardiac output is inadequate, and the predominant response constitutes and increase in oxygen extraction. Return to spontaneous breathing induces alteration in intrathoracic pressures that influence cardiovascular function, producing, among other changes, an increase in venous return. Other postulated physiologic changes include increase sympathetic tone, weaning-induced myocardial ischemia and increased ventricular interdependence 2. 