Recruitment and Derecruitment during Acute Respiratory Failure

Abstract

In a model of acute lung injury, we showed that positive end-expiratory pressure (PEEP) and tidal volume (VT) are interactive variables that determine the extent of lung recruitment, that recruitment occurs across the entire range of total lung capacity, and that superimposed pressure is a key determinant of lung collapse. Aiming to verify if the same rules apply in a clinical setting, we randomly ventilated five ALI/ARDS patients with 10, 15, 20, 30, 35, and 45 cm H2O plateau pressure and 5, 10, 15, and 20 cm H2O of PEEP. For each PEEP-VT condition, we obtained computed tomography at end inspiration and end expiration. We found that recruitment occurred along the entire volume-pressure curve, independent of lower and upper inflection points, and that estimated threshold opening pressures were normally distributed (mode = 20 cm H2O). Recruitment occurred progressively from nondependent to dependent lung regions. Overstretching was not associated with hyperinflation. Derecruitment did not parallel deflation, and estimated threshold closing pressures were normally distributed (mode = 5 cm H2O). End-inspiratory and end-expiratory collapse were correlated, suggesting a plateau-PEEP interaction. When superimposed gravitational pressure exceeded PEEP, end-expiratory collapse increased. We concluded that the rules governing recruitment and derecruitment equally apply in an oleic acid model and in human ALI/ARDS.

A minimal model of lung mechanics and model-based markers for optimizing ventilator treatment in ARDS patients

“…This result suggests that recruitment and derecruitment is the dominant cause of volume change, rather than isotropic "balloon like", expansion of alveoli as had been traditionally thought, which is discussed in [16]. This elastic expansion hypothesis is also utilized by a variety of other clinical studies [18,19,20,8,13,14,21,22,9,23,24,25,26], and is in contrast to the in-vivo and clincally observed, more discrete open-closed behaviour employed here.…”

“…Threshold pressures are assumed to be normally distributed along pressure, based on work by Crotti et al [20] and Pelosi et al [24]. TOP and TCP distributions can therefore be modelled by a normal density (Gaussian) function.…”

Visual anatomical lung CT scan assessment of lung recruitability

“…The consolidated/collapsed areas, which radiologically appear as areas of homogeneous decrease of lung attenuation, correspond to not aerated lung regions in the quantitative analysis. We used an upper limit of airway pressure of 45 cmH 2 O because at this pressure almost 100 % of opening pressure is reached [39]. On the contrary at lower airway pressures a lower estimation of lung recruitment would be obtained.…”

Recruitment by sustained inflation: time for a change

“…Eliminating these interfaces is the primary objective of an ''open lung'' approach to safe ventilation [1]. It has been known for more than two decades that some collapsed but potentially aeratable units within the acutely injured lung may require opening pressures that exceed safe limits for ongoing tidal ventilation and that the process of tissue recruitment continues (albeit at a progressively declining rate) until further expansion is constrained by total lung capacity [2]. Once opened, most (but not all) lung units may continue to remain patent at combinations of positive end-expiratory pressure (PEEP) and tidal volume that do not pose undue risk for iatrogenic injury.…”

“…The first to be introduced-and until quite recently the most widely employed-was the application of 35-40 cmH 2 O airway pressure for 30-40 s. On rare occasion, even higher pressures may be needed to pry open large numbers of the most stubborn lung units, with some authors describing success with pressures that considerably exceed 50 cmH 2 O [6,7]. Because optimal duration of high pressure application varies inversely with pressure amplitude, the effective time of application may not be nearly as long as traditionally used at the bedside, as the data of this study indicate [3].…”