PEEP selection

Mechanical ventilation may induce per se a lung injury when leading to unphysiological stress and strain, inflammatory response and mechanical lesions. It is widely accepted and proved effective, that low tidal volume ventilation (6 mL/kg ideal body weight) and airway plateau pressures limited to 30 cmH2O may prevent lung injury limiting global stress and strain. This settings are part of a more integrated ventilator strategy known as “lung protective strategy” which includes also the prevention of intra-tidal collapse of pulmonary units by providing a PEEP value sufficient to keep the lung open throughout the respiratory cycle. The adequate PEEP selection and its efficacy in lung injury prevention, however, has not been proved and is still subject of debate. The most recent and largest clinical studies on PEEP application (ALVEOLI, LOV and ExPress Study) were not able to find any difference in outcome between patients ventilated with high vs. low PEEP values. However, in the ExPress and LOV studies it has been clearly shown that patients randomized to higher PEEP had a significantly lower rate of application of rescue therapy, finally leading to a survival benefit in the most severe ARDS patients. It is conceivable, that the best way for setting high or low PEEP levels is on the severity of the pathology and on the potential for lung recruitment evaluated by CT scan. The debate on PEEP selection includes also the basis on which an adequate level may be applied at the bedside. Several methods have been proposed over the years: lung mechanics (setting PEEP 2 cmH2O greater than lower inflection point, analyzing the shape of the inspiratory/expiratory pressure-time curve, considering the changing of the compliance of the respiratory system, testing the gas exchange variations (oxygenation or CO2 decrease). Recently it has been proposed to evaluate the best PEEP according to the esophageal pressure measurement in order to keep the lung open. We found no differences between the various methods, equally inadequate to cope with lung recruitability.