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The study by Waller et al 1 did not attempt to compare the clinical benefit, cost effectiveness, or relative convenience of the two alternative chest drainage systems—a flutter valve drainage bag or the conventional underwater seal. Instead it claims that the flutter valve offers “a physiologically more effective” alternative to the underwater system. However, a pneumothorax is a pathological rather than a physiological condition in which it is imperative to remove air from the pleural cavity. In this situation the pleural pressures will be higher than in the normal physiological state and, indeed, air will only exit the pleural cavity via a chest drain when the pleural pressure exceeds atmospheric pressure. Any drainage system for a pneumothorax that renders the pleural pressure more negative (relative to the atmospheric pressure) will therefore reduce the exit of air, other than by more forcible expiration, and reduce its effectiveness.
A further theoretical but serious consideration is that of re-expansion pulmonary oedema (REPE). This has been associated with excessively negative pleural pressures after removal of air2 or fluid3 from the pleural space.
The study concludes that an ideal postoperative drainage system will “allow the maintenance of a negative intrapleural pressure, even in the presence of an air leak”. In our view a negative pleural pressure in this situation would only serve to encourage the egress of more air from the damaged lung surface into the pleural cavity, thus worsening the situation.
Whilst accepting that the postoperative situation may differ somewhat from that which pertains in a spontaneous pneumothorax because of the presence of fluid/blood in the pleural cavity, we would expect the same physiological principles to apply, especially in the presence of an air leak. There is a risk that the conclusions from this paper might be extrapolated into general respiratory practice and the scenario of spontaneous pneumothorax. For all of the above reasons we would urge caution before abandoning the standard and well tried technique of drainage of the pleural cavity (underwater seal) in favour of the suggested alternative, at least before data from larger, controlled clinical trials become available.
authors' reply We thank Dr Henry and colleagues for their comments regarding our study. We must first emphasise that we were not intending to make any conclusions regarding the relative clinical or financial benefits of either drainage system, and that we were specifically addressing the situation of postoperative air leaks. We have shown that, at resting tidal volume, flutter valve drainage bags maintain a more negative (but not excessively negative) intrapleural pressure than underwater seal systems and therefore are more likely to restore the normal physiology of the pleural space.
The aim of chest drainage in this situation is to restore and maintain the negative intrathoracic pressure necessary for lung re-expansion which will allow reactivation of the surface forces promoting visceral and parietal pleural apposition.1-1 This is dependent on removing excess gas or fluid from the pleural space. We cannot therefore agree with the assertion of Dr Henry and colleagues that achieving negative intrapleural pressure will reduce the exit of air (by which we assume they mean will reduce lung expansion). While it has been shown that excessive pleural suction in the presence of a high flow bronchopleural fistula may perpetuate air leakage,1-2 Henry et al are ill advised to extrapolate this principle to the more common clinical scenario of a smaller volume air leak.
Re-expansion pulmonary oedema is a recognised but uncommon complication of chest drainage which reflects the rapidity of lung re-expansion.1-3 This is rare in prolonged chest drainage for persistent air leakage and the concerns of Dr Henry and colleagues cannot be justified.
We welcome the opportunity to discuss this common but seldom debated area of respiratory practice. The standard and well tried method of underwater seal drainage is a source of many misunderstandings and complications.1-4 The Heimlich flutter valve, whose physics we have studied, has been shown to be clinically effective by Grahamet al 1-5 and others and further data from a randomised clinical trial in prolonged air leakage will soon be forthcoming.