Acute Lung Injury The open lung approach

Current ventilation strategies involve using low tidal volumes with or without high levels of PEEP. The open lung approach attempts to optimize lung mechanics and minimize phasic damage by strategically placing PEEP above Pflex.

Two modern approaches to ventilating patients with acute lung injury are the open lung approach and the low tidal volume approach. These are not mutually exclusive. The premise of both is that phasic opening and closing of injured lung units causes further injury to lung tissue and can worsen the lung injury. The low tidal volume approach involves minimizing the amount of phasic stretch of lung units in inspiration, to prevent ventilator induced lung injury. This technique has been proven to be effective: in a landmark NIH coordinated multicenter trial, patients ventilated with tidal volumes of 2-6ml/kg had a 22% reduction in mortality than patients ventilated with tidal volumes of 10-12ml/kg (1).
The open lung approach takes a slightly different tack: it is believed that reinflating collapsed lung units also causes lung injury and cytokine release. By stenting the airways open at end expiration, using PEEP, it may be possible to reduce these shearing injuries. There has been a preliminary trial by Amato and colleagues (2), demonstrating the efficacy of this technique.
This group painstakingly constructed pressure volume curves on each patient to determine “Pflex” (the lower inflection point on the pressure volume curve), and applied PEEP just above this level. The patients invariably receive a higher than conventional PEEP level, with lower tidal volumes.
Critics of this technique have suggested that plotting pressure volume curves is difficult, that Pflex often is impossible to find, and that overdistension of less diseased tissues may occur. As a consequence of this controversy, the NIH is currently performing the “Alveoli” trial, which randomizes patients into two groups, low and high PEEP in patients on low tidal volume strategies (the trial is now complete an the early indications is that there was no difference in outcomes with this approach). It is important to note that while Amato and most other "open lung" practitioners performed recruitment maneuvers (moderate to high pressures are applied to the airway intermittently to re-open collapsed alveoli), this was not a part of the Alveoli study.

Above: Quasi Static volume pressure curve of an injured lung: the lungs are said to be most compliant between the lower inflection point of the curve and the upper inflection point, beyond which overdistension takes place.

References

(1) Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. The Acute Respiratory Distress Syndrome Network. N Engl J Med 2000; 342(18):1301-1308.
(2) Amato MB, Barbas CS, Medeiros DM, Magaldi RB, Schettino GP, Lorenzi-Filho G et al. Effect of a protective-ventilation strategy on mortality in the acute respiratory distress syndrome. N Engl J Med 1998; 338(6):347-354.

Copyright Patrick Neligan 2001-2002

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