Positive end-expiratory pressure at minimal respiratory elastance represents the best compromise between mechanical stress and lung aeration in oleic acid induced lung injury

Abstract

Introduction: Protective ventilatory strategies have been applied to prevent ventilator-induced lung injury in patients with acute lung injury (ALI). However, adjustment of positive end-expiratory pressure (PEEP) to avoid alveolar de-recruitment and hyperinflation remains difficult. An alternative is to set the PEEP based on minimizing respiratory system elastance (Ers) by titrating PEEP. In the present study we evaluate the distribution of lung aeration (assessed using computed tomography scanning) and the behaviour of Ers in a porcine model of ALI, during a descending PEEP titration manoeuvre with a protective low tidal volume.

Methods: PEEP titration (from 26 to 0 cmH2O, with a tidal volume of 6 to 7 ml/kg) was performed, following a recruitment manoeuvre. At each PEEP, helical computed tomography scans of juxta-diaphragmatic parts of the lower lobes were obtained during end-expiratory and end-inspiratory pauses in six piglets with ALI induced by oleic acid. The distribution of the lung compartments (hyperinflated, normally aerated, poorly aerated and non-aerated areas) was determined and the Ers was estimated on a breath-by-breath basis from the equation of motion of the respiratory system using the least-squares method.

Results: Progressive reduction in PEEP from 26 cmH2O to the PEEP at which the minimum Ers was observed improved poorly aerated areas, with a proportional reduction in hyperinflated areas. Also, the distribution of normally aerated areas remained steady over this interval, with no changes in non-aerated areas. The PEEP at which minimal Ers occurred corresponded to the greatest amount of normally aerated areas, with lesser hyperinflated, and poorly and non-aerated areas. Levels of PEEP below that at which minimal Ers was observed increased poorly and non-aerated areas, with concomitant reductions in normally inflated and hyperinflated areas.

Conclusion: The PEEP at which minimal Ers occurred, obtained by descending PEEP titration with a protective low tidal volume, corresponded to the greatest amount of normally aerated areas, with lesser collapsed and hyperinflated areas. The institution of high levels of PEEP reduced poorly aerated areas but enlarged hyperinflated ones. Reduction in PEEP consistently enhanced poorly or non-aerated areas as well as tidal re-aeration. Hence, monitoring respiratory mechanics during a PEEP titration procedure may be a useful adjunct to optimize lung aeration.

Figure 1

Time plot of Paw during the PEEP titration procedure. The baseline ventilation, with a PEEP of 5 cmH₂O, and the recruitment maneuver followed by the descending PEEP titration are shown. At the end of each PEEP step, a CT scan was performed at end-expiratory (left) and end-inspiratory (right) pauses. (CT scan images from a representative animal are shown.) CT, computed tomography; Paw, airway opening pressure; PEEP, positive end-expiratory pressure.

Figure 2

Ers, Rrs and morphological analysis of the CT scans during PEEP titration for animals I, II, III and VI. The median and range of Ers and Rrs, and the distribution of lung aeration are plotted as a function of PEEP. Red diamonds indicate hyperinflated areas, blue circles indicate normally aerated areas, light grey squares indicate poorly aerated areas, and black triangles indicate non-aerated areas. The filled and open symbols indicate lung aeration changes at end-inspiration and end-expiration, respectively. Regions of interest on the CT scan images obtained during the PEEP titration in a representative case (animal I) are also presented in the upper panel. Aeration titration in a representative case (animal I) are also presented in the upper panel. Aeration status is colour coded in the images. Red indicates hyperinflated areas, and blue, light grey and black indicate normally aerated, poorly aerated and non-aerated areas, respectively. CT, computed tomography; Ers, respiratory system elastance; PEEP, positive end-expiratory pressure; Rrs, respiratory system resistance.

Figure 3

Ers, Rrs and morphological analysis of the CT scans during PEEP titration for animal IV. The regions of interest of the CT scan images obtained during the PEEP titration are also shown in the upper panel. For details, see legend to Figure 2. CT, computed tomography; Ers, respiratory system elastance; PEEP, positive end-expiratory pressure; Rrs, respiratory system resistance.

Figure 4

Ers, Rrs and morphological analysis of the CT scans during PEEP titration for animal V. The regions of interest of the CT scan images obtained during the PEEP titration are also shown in the upper panel. For details, see legend to Figure 2. CT, computed tomography; Ers, respiratory system elastance; PEEP, positive end-expiratory pressure; Rrs, respiratory system resistance.

Figure 5

Comparative changes in lung aeration at different anatomic levels. Images from the apex to diaphragm level during an end-expiratory pause and an end-inspiratory pause for two studied animals (left and right columns). The computed tomography (CT) scans were acquired near the lung hilus (upper), near the carina (middle) and at juxta-diaphragmatic (lower) levels; the respective histograms of density are also shown (bottom).