Strategies to optimize alveolar recruitment and prevent lung overinflation are central to ventilatory management of patients with acute lung injury (ALI). The recent description of overinflation using multilinear regression analysis of airway pressure (Paw) and flow (V') data allows a functional assessment of lung mechanics. However, this technique has not been studied in ALI patients. During 15 positive end-expiratory pressure (PEEP) trials in 10 ALI patients, respiratory elastance was partitioned into volume-independent (E1) and volume dependent (E2VT) components, where Paw=(E1+E2VT)V+RrsV'+Po; where V is volume, VT is tidal volume, Rrs is respiratory resistance and Po is static recoil pressure at end expiration (equivalent to total PEEP). Then, %E2 was calculated as (100%E2VT)/(E1+E2VT); a measure of lung overinflation when %E2>30%. Alveolar recruitment, assessed as a PEEP-induced increase in V>50 mL at a constant P(aw) occurred in 14 of 15 trials (299±34 mL, mean+SEM), but was independent of the degree of lung inflation. Lung overinflation was common (six of 15 clinically set PEEP levels) and occurred despite a dynamic elastic distending pressure (Pel,dyn) <30 cmH2O during 18 of 36 PEEP titrations. During a PEEP titration the resultant %E2 was directly related to Δ(peak airway pressure-Po) (r(s)=0.86, p<0.001) and Δ(Pel,dyn-Po) (r(s)=0.89, p<0.001). The 95% predictive intervals for a 2 cmH2O change in either driving pressure were %E2 values of 30.4-68.1% and 32.8-69.2%, respectively. Single or continuous measurement of %E2 (a measure of lung inflation) is a readily available method for titrating ventilatorg parameters. Further, during a positive end-expiratory pressure titration a change in ventilatory driving pressure ≤2 cmH2O is indicative of overinflation.
- Acute lung injury
- Acute respiratory distress syndrome
- Lung overinflation
- Mechanical ventilation
- Multilinear regression analysis
- Volume-pressure curve