Invasive Mechanical Ventilation in ARDS Patients: Role of End-Positive Expiratory Pressure (PEEP) Levels

Recently, three large clinical trials [21-23], including acute lung injury/ARDS patients ventilated with low tidal volume, have compared different PEEP strategies (high vs. low), but none of them could show a significant difference in mortality. Moreover, a recent meta-analysis has pooled those trials [24], revealing some combined benefit of the high PEEP strategy; still, the survival benefit was modest and limited to the subgroup of ARDS patients with PaO₂/FiO₂ < 200. Conceptually, one could argue that none of the “high PEEP” strategies was designed to test the “open-lung hypothesis” postulated by Lachmann [25-28], that is, the hypothesis that most of the collapsed lung tissue observed in early ARDS can be reversed at an acceptable clinical cost, potentially resulting in better lung protection. According to a recent study by Borges and colleagues [29], a straight test of the “open-lung hypothesis” would certainly require more aggressive recruiting maneuvers in association with individualized, decremental PEEP titration. Thus, one can speculate that the limited results reported above were related to suboptimal ventilatory strategy. Recently, de Matos and colleagues [30] reported the experience with maximal recruitment rtrategy (MRS) in 51 patients with ARDS. MRS consisted of 2-min steps of tidal ventilation with pressure- controlled ventilation, fixed driving pressure of 15 cmH₂O, respiratory rate of 10 breaths/minute, inspiratory/expiratory ratio of 1:1, and stepwise increments in PEEP levels from 10 to 45 cmH2O (recruitment phase). After that, PEEP was decreased to 25 cmH2O and, then, from 25 to 10 cmH2O (PEEP titration phase) in steps of 5 cmH2O, each one lasting 4 min. At each of the steps, computer tomography image sequences from the carina to the diaphragm were acquired during an expiratory pause of 6-10 s. Lung collapse was assessed online by visual inspection, for immediate clinical decision, and offline for quantitative measurements.

Fig. 3.2 Detailed thoracic Tomographic analysis of nonaerated (HU from -100 to +100U), poorly aerated (HU form -100 to -500 U), normally aerated (HU from -500 to -900U), and hyperinflated ( HU more than -900U) in 12 patients with moderate/severe ARDS during maximal recruitment manuevers and PEEP titration (Reprinted with permission from Medical Evidence Percorso Formativo 2015, yr. 8, n. 104, www.ati14.it)

MRS showed a statistically significant decrease in nonaerated areas of the ARDS lungs that was accompanied by a significant increment in oxygenation. The opening plateau pressure observed during the recruitment protocol was 59.6 (±5.9 cmH₂O), and the mean PEEP titrated after MRS was 24.6 (±2.9 cmH₂O). Mean PaO₂/FiO₂ ratio increased from 125 (±43) to 300 (±103; p < 0.0001) after MRS and was sustained above 300 throughout 7 days. Nonaerated parenchyma decreased significantly from 53.6% (interquartile range (IQR): 42.5-62.4) to 12.7% (IQR: 4.9-24.2) (p < 0.0001) after MRS. The potentially recruitable lung was estimated at 45% (IQR: 25-53). ICU mortality was 28% and hospital mortality was 32%. The independent risk factors associated with mortality were older age and higher driving pressures (or higher delta pressure control). There were no significant clinical complications with MRS or barotrauma. A better evolution of these ARDS patients with less necessity of oxygen supplementation in the recovery phase of the disease and a better quality of life must be tested in prospective, controlled clinical trials. A recent meta-analysis [23] showing beneficial effects on mortality using higher PEEP levels compared with lower PEEP in ARDS patients corroborates the results of our clinical case series of ARDS patients submitted to MRS. A detailed thoracic tomographic analysis performed in 12 of these ARDS patient thoracic computed tomographies during recruitment phase and PEEP titration phase showed a significant increment in normal aerated lungs and decrement of nonaerated during maximal recruitment maneuvers and PEEP titration (Fig. 3.2). These results demonstrated that keep sufficient PEEP levels after recruitment is crucial in ARDS patients (Fig. 3.3).

Fig. 3.3 Maximal recruitment manuevers can open up the collapsed lung in ARDS, and high PEEP levels are crucial to keep the lungs open (Reprinted with permission from Medical Evidence Percorso Formativo 2015, yr. 8, n. 104, www.ati14.it)