Which position is used to decrease atelectasis and improve refractory hypoxemia

Mechanical ventilation is the cornerstone of treatment and comprises different ventilatory modes and/or maneuvers that aim to improve the effective gas exchange surface: PEEP level, recruitment maneuvers, pressure regulated ventilation modalities, inverse inspiration-expiration ratio, airway pressure release ventilation [APRV], and high-frequency oscillation ventilation [HFOV].

Lung disease is characterized by heterogeneous distribution between healthy alveolar units and units with different degrees of alveolar collapse, which globally reduce the surface available for gas exchange. The application of pressure to the respiratory system can decollapse the damaged alveoli. The main difficulty, however, is to reach a sufficient level of pressure to recruit [decollapse] the diseased alveolar units while simultaneously avoiding cyclic opening and closing [atelectrauma], overdistension of the healthy alveoli, and adverse hemodynamic effects [alteration of the ventilation–perfusion ratio and of cardiac output].

There are two main maneuvers for securing the greatest possible surface for gas exchange, distinguished by their intensity and duration of application. Positive end-expiratory pressure [PEEP] applies continuous pressure during ventilation, with slow and progressive recruitment, while recruitment maneuvers [RMs] involve high pressures maintained for short periods of time, with recruitment of as many collapsed alveoli as possible.

In recent years, the debate regarding PEEP has focused on the application of moderate PEEP versus high PEEP. The results show an improvement of PaO2/FiO2, with no influence upon survival. The ALVEOLI study9 compared low volume-controlled ventilation in two PEEP regimens, with the observation of improved oxygenation and respiratory mechanics in the high PEEP group. However, there were no differences in mortality, days without mechanical ventilation or in the development of barotrauma–though this possibly could be explained by the existence of a certain imbalance between the groups. In this same line, Meade et al.10 compared volume-controlled ventilation [moderate PEEP] versus pressure-controlled ventilation [PEEP high], and observed improvements in refractory hypoxemia and death due to hypoxemia in the high PEEP group, though without differences in either global mortality or the incidence of barotrauma. The study published by Mercat et al.11 in turn compared low volume-controlled ventilation in a standard group and in a group with PEEP elevation to a plateau pressure of 28–30cmH2O. Not only were no benefits observed, but adverse effects were even recorded in the high PEEP group with mild respiratory failure.

Gordo-Vidal et al. reviewed the effect of different PEEP levels in four studies of high methodological quality among the 12 studies regarded as relevant.12 The authors selected the studies of Amato et al.,13 Ranieri et al.,14 Brower et al.9 and Villar et al.,15 and concluded that PEEP level did not affect either mortality or the incidence of barotrauma. The same analysis, without considering the ALVEOLI study published by Brower et al.,9 yielded significant reductions in mortality and barotrauma [RR=0.6; 95%CI 0.4–0.8, and RR=0.2; 95%CI 0.1–0.7, respectively]. The difference was based on the fact that in these three studies PEEP was adjusted according to identification of the lower inflexion point of the ascending arm of the pressure/volume loop.

In sum, on the basis of the described studies, it can be concluded that the use of high PEEP to increase oxygenation affords better results. However, PEEP elevations that do not result in an increase in alveolar surface imply increases in transpulmonary pressure, which in turn is related to VILI.16 PEEP monitoring according to P-V loops or the “stress index” is advised in order to avoid deleterious hemodynamic or pulmonary effects.

Recruitment maneuvers expand the effective gas exchange surface through an intense and transient increase in transpulmonary pressure, resulting in the recruitment of collapsed alveoli. Studies such as that published by Oczenski et al.17 report temporary improvements in blood gas parameters. Pressures of 50cmH2O were generated during 30s, with the improvement of oxygenation for 30min. The same results were obtained by Meade et al.,18 with adverse effects, such as hypotension or barotrauma. On the other hand, the PEEP required after RM can be adjusted by identifying the inflexion point of the ascending arm of the pressure–volume curve, as in the study of Amato et al.,13 or by desaturation in the context of progressive PEEP reduction, as in the study published by Girgis et al.19 The review carried out by Fan et al.20, involving 40 studies in patients with ALI, confirmed the temporary improvement of patient oxygenation, with fewer adverse effects than in other articles. The authors concluded that the procedure should not be regarded as a routine measure but as an individualized option in patients in the context of life-threatening situations.

Recruitment maneuvering is the subject of debate, offering transient effects, and with no established optimum methodology, timing or frequency of application. We hope that the OLA multicenter study will offer clarifying results in the near future.