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, …