 Cardiovascular instability during liberation from mechanical ventilation was first described more than 30 years ago by Beach et al 3 in post-operative cardiac surgery patients. Since then, a few other studies have described the hemodynamic changes both in patients who underwent successful and unsuccessful weaning form mechanical ventilation 1,4,5,6,7. Lemaire et al. 4 emphasized the use of a pulmonary artery catheter as the gold standard to detect left ventricular dysfunction during spontaneous breathing trial SBT. They reported a markedly increased in pulmonary artery occlusion pressure PAOP in patients who were suffering from COPD or heart failure who failed to wean from mechanical wentilation. Jubran et al. 5 reported an increase in PAOP, decreased mixed venous oxygen saturation SvO2 and an increase in the oxygen extraction ratio in a group of patients who failed to wean from mechanical ventilation. In contrast, De Backer et al. 6 described an increase in cardiac index in a group of patients who were successfully weaned after cardiac surgery. A recently published study by Frazier et al. 7 corroborated previous data by comparing the hemodynamic changes between a group of patients who were successfully weaned and a group of patients who failed to wean from mechanical ventilation. The cardiac output and stroke volume were estimated by the differential Fick partial rebreathing technique, with carbon dioxide as the indicator substance. The group of patients who were successfully weaned had a statistical significant increase in both stroke volume and cardiac index, unlike the group that failed weaning. 