Reducing atelectasis attenuates bacterial growth and translocation in experimental pneumonia

How to ventilate patients with acute lung injury and acute respiratory distress syndrome

PURPOSE OF REVIEW

The purpose of this paper is to review the mechanisms of ventilator-induced lung injury as a basis for providing the less damaging mechanical ventilation in patients with acute respiratory failure.

RECENT FINDINGS

In normal lungs, high tidal volume causes an immediate gene upregulation and downregulation. Although the importance of alveolar inflammatory reaction is well known, recent findings suggest the potential role of airway distension in causing ventilator-induced lung injury. The initial activation has been shown to occur in the airways, accounting for the damages induced by high peak flow. The healthier lung regions are more exposed to the injury, since they may be subjected to strain. Challenge with endotoxin enhances in a synergistic manner the pulmonary inflammation induced by mechanical ventilation. However, mechanical strain and endotoxin seem to trigger lung inflammation through two different pathways. Despite convincing experimental and clinical evidences of lung injury, the clinical implementation of low tidal volume ventilation is still limited and has not yet become part of standard clinical practice. Setting positive end-expiratory pressure remains an open problem because the ALVEOLI study did not provide any exhaustive answers, likely because of methodologic problems and, unphysiologic design.

SUMMARY

Gentle lung ventilation must be standard practice. Because stress and strain are the triggers of ventilator-induced lung injury, their clinical equivalents should be measured (transpulmonary pressure and the ratio between tidal volume and end-expiratory lung volume). For a rational application of positive end-expiratory pressure, the potential for recruitment in any single patient should be estimated.

Acute lung injury and bacterial infection

The relationships between acute lung injury and bacterial infection are complex. Indeed, sepsis and in particular pneumonia are leading causes of acute lung injury. Bacterial superinfection of the lung is a frequent complication of acute lung injury. Because of impaired host defenses and prolonged mechanical ventilation, more than one third of patients with the acute respiratory distress syndrome acquire ventilator-associated pneumonia, with resistant pathogens in most instances. This complication is responsible for more than a doubling of the time on mechanical ventilation but does not seem to increase mortality.