ARDS: the role of PEEP
Dr John Griffiths DICM MRCP FRCA MA
The theory behind the application of PEEP
Amato et al got the "what level of PEEP" debate underway in 1998 when they demonstrated that setting PEEP using pressure volume curves was associated with an improved clinical outcome. From this starting point the theory behind the optimal setting and application of PEEP has evolved and it has been proposed clinicians should:
- set PEEP above the lower inflection point on the lung compliance curve, but below the upper inflection point
- ventilate with low tidal volumes (6mls/kg)
- maintain airway pressures below 30 cmH2O
By applying this approach one hopes to minimise Ventilator Associated Lung Injury ('VALI'). PEEP is thought to prevent VALI by preventing the cyclical opening and closing of the alveoli -often referred to as 'shear-stress injury'. These effects are related to lung 'recruitability' and were highlighted by the early CT studies of Gattinnoi et al. Gattinnoni demonstrated that the degree of lung that could be recruited varied widely between patients with ARDS - but was often as much as 40%. Patients with a a greater degree of potentially recruitable lung had heavier lungs, more dead space, worse oxygenation and ultimately increased mortality.
The clinical trials of PEEP in ARDS
The question that arose was that by targeting recruitable ARDS lung with the application of PEEP and recruitment manoeuvres, could survival be improved?
The study by Villar et al suggested that outcome from ARDS could indeed be improved by adopting a low tidal volume approach coupled with PEEP set above the lower inflection point on the compliance curve. The stage was set for the ARDSnet 'high' versus 'low' PEEP trial. The results from this trial were disappointing. Although higher levels of PEEP improved oxygenation, there was no survival benefit. Again there has been a failure to translate a physiological theory into improved clinical outcome. Whether this was a fault of the trial design or whether it is related to possible detrimental effects of high levels of PEEP remain unclear. One factor may be that different parts of the lung operate with different pressure-volume dynamics and what is good for one part of the lung can be deleterious to another.
Should a High or low PEEP strategy be adopted?
At the current time evidence suggests that 'high PEEP' appears to offer no clinical advantage over 'low PEEP'. However, there are proponents of both a 'low PEEP' and a 'high PEEP' strategy. Improved lung recruitment may not necessarily mean improved clinical outcome. Higher levels of PEEP and other recruitment maneouvres may improve oxygenation but may be associated with additional 'VALI'. What is needed are definitive trials using low-tidal volumes and different levels of PEEP to maintain airway pressures below 30cmH2O. Only then will the "what levcel of PEEP" debate be laid to rest.
Key PEEP References
Amato MB, Barbas CS, Medeiros DM, Magaldi RB, Schettino GP et al.
Effect of a protective-ventilation strategy on mortality in the acute respiratory distress syndrome.
New England Journal of Medicine 1998; 338: 347-354.
Gattinoni L, Mascheroni D, Torresin A, Marcolin R, Fumagalli R et al.
Morphological response to positive end expiratory pressure in acute respiratory failure. Computerized tomography study.
Intensive Care Medicine 1986; 12: 137-142.
Villar J, Kacmarek RM, Perez-Mendez L, Aguirre-Jaime A.
A high positive end-expiratory pressure, low tidal volume ventilatory strategy improves outcome in persistent acute respiratory distress syndrome: a randomized, controlled trial.
Crit Care Med. 2006;34(5):1311-8.
The National Heart, Lung, and Blood Institute ARDS Clinical Trials Network.
Higher versus Lower Positive End-Expiratory Pressures in Patients with Acute respiratory Distress Syndrome.
N Eng J Med 2004;351;327-36.