Traditional model of reexpansion pulmonary edema (RPE)
This model led to the recommendation that any volume of fluid could be removed as long as the pleural pressure was continuously measured with manometry and not allowed to fall under -20 cm. In the absence of manometry, it is recommended that we should avoid removing large volumes of fluid (variably defined as >1000 ml or >1500 ml), which could potentially lead to dangerously low pleural pressures (Daniels 2011).
Evidence: Risk of reexpansion pulmonary edema (RPE) after large-volume thoracentesis
Most notably, patients with RPE had slightly higher intrapleural pressure at the end of the procedure compared to patients without RPE (figure below, “closing pressure”). This argues strongly against the traditional model of RPE being due to negative intrapleural pressure “pulling” fluid into the lungs via Starling's law. These authors concluded that in the absence of manometry, pleural effusions should be drained dry unless the patient experiences vague chest discomfort.
New models of RPE: What implications do they have on thoracentesis volume?
How severe is RPE following thoracentesis if it does occur?
RPE risk in clinical context
Large-volume thoracentesis and ex vacuo pneumothorax
Another argument to avoid large-volume thoracentesis is to reduce the risk of ex vacuo pneumothorax. This occurs if the underlying lung is unable to expand, for example due to bronchial obstruction. Thoracentesis generates a low intrapleural pressure which transiently opens a tiny hole in the lung to allow air into the pleural space (alleviating the “vacuum”). Pneumothorax ex vacuo is a benign phenomenon which rarely enlarges or leads to tension pneumothorax, and generally should not be treated with a chest tube (Heidecker 2006).
Pneumothorax ex vacuo is rare, benign, and potentially a useful diagnostic finding. It probably doesn't make sense to limit the volume of thoracentesis to avoid this occurrence. This was explored in more detail in the last post.
Should we stop thoracentesis if the patient starts coughing or develops chest pain?
Guidelines and conventional wisdom
This is an arbitrary cutoff supported by little data. The British Thoracic Society 2010 guidelines made this as a Grade C recommendation. These guidelines admitted that it is “less clear how cases at risk of RPE can be predicted at higher volumes” of fluid removed. They also admitted that “advice has generally been conservative because of the morbidity associated with RPE and a mortality rate quoted as high as 20% (Mahfood 1988).” As discussed above, evidence supporting this mortality rate is flawed. No actual evidence is provided to explain the selection of 1500 ml as the maximal safe volume:
Although the safe aspiration of much larger volumes has been documented, it is also clear that complications are uncommon when aspirating <1.5 liters. This is therefore the recommended volume to be aspirated in one attempt.
- Re-expansion pulmonary edema is a rare and usually mild complication of thoracentesis.
- There is no evidence that removal of a larger volume causes re-expansion pulmonary edema, although a weak correlation likely exists. It is possible that the risk of re-expansion pulmonary edema relates to the volume of the initial effusion rather than the volume removed.
- There is little evidence to support any specific cutoff for the amount of fluid which may be safely removed.
- It is reasonable to drain large effusions completely, with premature termination of the procedure if the patient develops vague chest discomfort (Feller-Kopman 2007, Soberman 2007).
- Coughing or pleuritic chest pain during fluid removal are physiologic responses to lung re-expansion and catheter irritation of the pleura. Neither requires termination of the procedure.