PURPOSE OF REVIEW: Pleural effusions (PLEF) occur frequently in mechanically ventilated patients (MVP). There have been improvements in bedside diagnosis, quantification, and drainage techniques. Nevertheless, information on the impact of PLEF on heart-lung performance and pathophysiologic considerations for drainage in MVP is limited. RECENT FINDINGS: Only four studies on physiological effects of PLEF drainage in MVP were found. Variable oxygenation response is reported. Older work suggests that the space occupying effect of PLEF is shared between chest wall expansion and lung collapse. Normally, the compliant chest wall accepts most of it, buffering its effects on lung volume and gas exchange. In MVP chest wall compliance and airway pressure probably play major roles on the response to PLEF drainage. PLEF reduces inspiratory muscle efficiency; hence drainage can help in weaning. PLEF can increase heart filling and pericardial pressures, dissociating intrathoracic vascular pressures from preload. Tamponade-like effects with diastolic chamber collapse and circulatory impairment that reverses with PLEF drainage have been reported. SUMMARY: PLEF drainage does not always improve oxygenation in MVP with acute respiratory failure. An assessment of chest wall compliance and a lung recruitment maneuver may help predict the response. Procedural complication risks should be considered when choosing the best approach.