EFFECTS OF CHEST-WALL COUNTERPRESSURES ON LUNG-MECHANICS UNDER HIGH-LEVELS OF CPAP IN HUMANS

We assessed the respective effects of thoracic (TCP) and abdominal/low er limb (ACP) counterpressures on end-expiratory volume (EEV) and resp iratory muscle activity in humans breathing at 40 cmH(2)O of continuou s positive airway pressure (CPAP). Expiratory activity was evaluated o n the basis of the inspiratory drop in gastric pressure (Delta Pga) fr om its maximal end-expiratory level, whereas inspiratory activity was evaluated on the basis of the transdiaphragmatic pressure-time product (PTPdi). CPAP induced hyperventilation (+320%) and only a 28% increas e in EEV because of a high level of expiratory activity (Delta Pga = 2 4 +/- 5 cmH(2)O) contrasting with a reduction in PTPdi from 17 +/- 2 t o 9 +/- 7 cmH(2)O.s(-1).cycle(-1) during 0 and 40 cmH(2)O of CPAP, res pectively. When ACP, TCP, or both were added, hyperventilation decreas ed and PTPdi increased (19 +/- 5, 21 +/- 5, and 35 +/- 7 cmH(2)O.s(-1) .cycle(-1), respectively), whereas Delta Pga decreased (19 +/- 6, 9 +/ - 4, and 2 +/- 2 cmH(2)O, respectively). We concluded that during high -level CPAP, TCP and ACP limit lung hyperinflation and expiratory musc le activity and restore diaphragmatic activity.