Grönkvist M, Bergsten E, Eiken O. Lung mechanics and transpulmonary pressures during unassisted pressure breathing at high G_z loads. Aviat Space Environ Med 2008; 79:1041–6.Background: Positive pressure breathing (PPB) is commonly used in modern fighter aircraft as part of the anti-G ensemble. PPB is combined with a chest counterpressure bladder which is pressurized to the same magnitude as the breathing mask (balanced PPB). The chest counterpressure is expected to reduce the expiratory work of breathing, reduce the risk for lung rupture, and increase G tolerance. In a previous study we did not find any effect from chest counterpressure on G tolerance or G endurance. The aim of this study was to investigate the effects of chest counterpressure on the work of breathing and the risk for lung rupture. Methods: Eight male test subjects were exposed to 20-s periods of PPB at +1.0, 5.0, 6.0, 7.0, and 8.0 G_z. Each G_z level was accomplished twice, with and without pressurization of the chest bladder. Inspiratory and expiratory flows were measured and esophageal pressures were measured in the lower and upper third of the thorax. Subsequently, work and power of breathing and apical transpulmonary pressure were estimated. Results: The apical transpulmonary pressure was slightly larger without than with chest counterpressure at 1.0 G_z, while chest counterpressure did not affect apical transpulmonary pressure at increased G_z load. Nor did the chest counterpressure affect work or power of breathing at any G_z load. Conclusion: Inflation of the chest bladder does not seem to have any effects on work or power of breathing or risk for lung rupture during PPB at high G_z loads.