Recruitment and derecruitment during acute respiratory failure - An experimental study

We aimed to elucidate the relationships between pleural (Ppl), esophageal ( Pes), and superimposed gravitational pressures in acute lung injury, and to understand the mechanisms of recruitment and derecruitment. In six dogs wi th oleic acid respiratory failure, we measured Pes and Ppl in the uppermost , middle, and most dependent lung regions. Each dog was studied at positive end-expiratory pressure (PEEP) of 5 and 15 cm H2O and three levels of tida l volume (VT; low, medium, and high). For each PEEP-VT combination, we obta ined a computed tomographic (CT) scan at end-inspiration and end-expiration . The variations of Ppl and Pes pressures were correlated (r = 0.86 +/- 0.0 7, p < 0.0001), as was the vertical gradient of transpulmonary (PL) and sup erimposed pressure (r = 0.92, p < 0.0001). Recruitment proceeded continuous ly along the entire volume-pressure curve. Estimated threshold opening pres sures were normally distributed (mode = 20 to 25 cm H2O). The amount of end -expiratory collapse at the same PEEP and PL was significantly lower when v entilation was performed at high VT. End-inspiratory and end-expiratory col lapse were highly correlated (r = 0.86, p < 0.0001), suggesting that as mor e tissue is recruited at end-inspiration, more remains recruited at end-exp iration. When superimposed pressure exceeded applied airway pressure (Paw), collapse significantly increased.