Pulmonary and Ventilatory Effects of End-expiratory Positive Pressure Optimization in Intubated ICU Patients With Healthy Lungs or Acute Respiratory Distress Syndrome. A Randomized Controlled Trial

Management of ICU patients may require the use of ventilatory support requiring tracheal intubation and invasive mechanical ventilation.

Any mechanically ventilated patient is exposed to the formation of atelectasis (collapsed pulmonary alveoli), which occurs systematically after endotracheal tube insertion, after any de-recruiting action (tracheal suction, disconnection) or simply if protective ventilation is used, combining small tidal volumes (6 to 8 mL/kg of theoretical ideal body weight - IBW) and an end-expiratory positive pressure (PEEP) that is sometimes insufficient. It is thus proposed to perform alveolar recruitment maneuvers (ARMs), which remove atelectasis by temporarily increasing intrathoracic pressure. To avoid alveolar re-collapse, it is necessary to apply a sufficient level of PEEP.

The opening pressure (P) necessary for the re-expansion of a collapsed alveolus is inversely proportional to its radius (r), following Laplace law P = 2.γ/r where γ is the surface tension. The pressure necessary to the re-expansion of a collapsed cell depends on its radius. Amato's team has showed in 2006 that within the same lung, several levels of alveolar aeration and thus several opening pressures coexist. The distribution of pressures was bimodal, with a peak around 30 cmH2O and a second around 40 cmH2O. Consequently, to allow complete re-expansion of atelectasis within a lung, it is necessary to apply a pressure at least equal to 30 cmH2O. The application of insufficient pressures cannot be expected to result in complete re-expansion of the lung, but rather in an increase in aeration of already aerated alveoli (whose radius is larger and whose opening pressure is much lower), what, in turn, can lead to over-distension. This is probably what can happen if the PEEP is increased without any previous ARM.

The application of an ARM can also lead to an overdistension phenomenon during a reduced period of time (20 to 30 seconds), contrary to the direct application of a high PEEP which could led to an overdistension lasting a much longer period of time (possibly several hours) and aggravated with each administration of a tidal volume (and thus several times per minute). Chronic lung exposure to overdistension phenomena can induce a disintegration of alveolar collagen fibers (volotrauma), leading to local inflammation (biotrauma) and systemic inflammation by releasing pro-inflammatory molecules (cytokines...) into the bloodstream and led to apoptosis in distant organs (kidney, digestive tract for example).

The optimization of mechanical ventilation requires the search for the optimal PEEP: insufficient, it cannot prevent atelectasis formation; too high, it would lead to alveolar overdistension. In current practice, the PEEP is determined arbitrary or following a stepwise titration, either by incremental or decremental steps. To date, scientific literature is not unequivocal concerning the use of ARMs and their safety. Thus, some teams prefer not to use ARMs and usually apply an upward PEEP level.