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

IntroductionProtective 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.MethodsPEEP 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.ResultsProgressive 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.ConclusionThe 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.

Visualisation of Time-Variant Respiratory System Elastance in ARDS Models

“…It was found that E drs during increasing PEEP is significantly different to E drs during decreasing PEEP (p<0.005). Thus, this case shows the typical finding where PEEP should be titrated after the lung has been recruited [3][4][5]7]. However, a local minimum E drs was also found at 10 mbar during increasing PEEP titration, suggesting similar optimal PEEP settings.…”

“…Therefore, true minimal E drs can only be determined after a stabilisation period is provided at each PEEP level. Setting PEEP at minimum elastance theoretically benefits ventilation by maximising recruitment, reducing work of breathing and avoiding overdistension [3][4][5]9]. …”

“…E rs and R rs can be determined using multivariate regression or the integralbased method [7]. If R rs is assumed constant throughout a breath [3,4,7], E drs can be re-estimated using Equation (2) after R rs is estimated using Equation (1). (2) Patient-specific E drs is only analysed during the inspiration portion of the breathing cycle.…”

Paediatric lung recruitment: a review of the clinical evidence

“…21,22 In an experimental study, hyperinflated regions of the lung shifted to normally aerated areas at PEEP levels of 8cmH 2 O (mean of 30% decrease) and yet hyperinflated areas still appeared at zero end expiratory pressure. 23 Hyperinflation of easily distensible regions of the lung is an acknowledged limitation of recruitment manoeuvres. 18,20,[22][23][24] How to recruit lungs?…”

“…23 Hyperinflation of easily distensible regions of the lung is an acknowledged limitation of recruitment manoeuvres. 18,20,[22][23][24] How to recruit lungs?…”