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/lower limb (ACP) counterpressures on end-expiratory volume (EEV) and respiratory muscle activity in humans breathing at 40 cmH(2)O of continuous positive airway pressure (CPAP). Expiratory activity was evaluated on the basis of the inspiratory drop in gastric pressure (Delta Pga) from 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% increase in EEV because of a high level of expiratory activity (Delta Pga = 24 +/- 5 cmH(2)O) contrasting with a reduction in PTPdi from 17 +/- 2 to 9 +/- 7 cmH(2)O.s(-1).cycle(-1) during 0 and 40 cmH(2)O of CPAP, respectively. When ACP, TCP, or both were added, hyperventilation decreased 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 muscle activity and restore diaphragmatic activity.