Lung Entrapment & Trapped Lung- The Role of Pleural Fluid Manometry

Malignant pleural effusions are common in clinical practice. In my specialty, I often see Rheumatoid related pleural effusions. These are often self limited, and may response to NSAIDs or steroids. However, not infrequently, they can be large and cause dyspnoea or chest pain, requiring aspiration.

One of the principal concerns while aspirating large effusions, whether they be malignant or inflammatory to dryness, is the risk of reperfusion related pulmonary oedema or RPO. This is often heralded by chest pain. The risk of RPO is increased in subjects where the lung fails to re-expand fully, as the pleural pressure in these subjects is more negative than they would otherwise be.

There are two situations where the lung fails to re-expand- the first is acute, because of an inflammatory visceral pleural peel around a segment of the lung, or because of increased elastic recoil of the lung due to lymphangitis carcinomatosa, or due to associated endobronchial obstruction stopping the lungs from re-expanding. The accepted term for this acute phenomenon is "lung entrapment". The pleural fluid in such cases is directly related to the underlying malignancy or serositis, and is an exudate. Such cases, if symptomatic, may be dealt with via a tunnelled pleural catheter.

The second situation is when the inability of the affected portion of the lung to re-expand is because of a fibrotic peel around the visceral pleura due to remote inflammation. Here, there is chronic negative pressure in the pleural cavity, and the effusion is the consequence of this negative pressure. This condition is called "trapped lung". The effusion is a transudate, and requires no treatment.

Pleural fluid manometry measures the relationship between intrapleural pressure and the volume of pleural fluid. Normal intrapleural pressure is slightly negative~3-5 cm H2O at functional residual capacity (FRC), allowing the lung to re-expand after expiration. When pleural fluid accumulates, intrapleural pressure rises. In the case of transudates such as with heart failure or hepatic hydrothorax, or exudates where the lung is able to re-expand, the pleural pressure falls steadily during aspiration until the normal slightly negative pressure of -3-5 cm H2O is reached at FRC.

The relationship between change in pleural pressure and pleural fluid volume is described as pulmonary elastance or Pel. The unit for Pel is cm H2O/litre.

In the case of lung entrapment, the pressure falls gradually during the first part of aspiration, but when more than 2000 ml is taken out, the pressure drops rapidly, giving rise to a steep curve in its later half. Pel is usually higher than 19 cm H2O/l.

With a trapped lung, the ambient pleural pressure is negative to start with, unlike the other two situations. In addition, as pleural fluid is withdrawn, the intrapleural pressure drops steeply, without an intervening flat trajectory. Thus, Pel tends to be very high, >25 cm H2O per litre.

These three situations are represented in the figure.

Thus, by observing the relationship between intrapleural pressure and pleural fluid volume during manometry, one can a)predict the chances of successful pleurodesis following aspiration of malignant effusions. One study found that Pel>19 cm H2O/litre during the first 500 ml of aspiration predicted unsuccessful pleurodesis (by failing to appose the visceral and parietal pleura) b) predict whether there is lung entrapment or trapped lung. and c) guard against RPO, as these typically tend to occur at pleural pressures more negative than -40 cm H2O. Some authors stop aspirating at intrapleural pressures below -20 cm H2O, or at the first sign of chest pain.

Reference:
Feller-Kopman D. Should Pleural Manometry Be Performed Routinely During Thoracentesis? Yes. Chest.2012;141:844-845.