Positive end expiratory pressure in patients with acute respiratory distress syndrome - The past, present and future

Though well studied, acute respiratory distress syndrome (ARDS) is still not fully understood. Mechanical ventilation (MV) has been a key treatment for ARDS. However, the optimal ventilation settings of basic MV parameters are still significantly debated. Only recently, were low tidal volumes shown to have lower mortality rates among ARDS patients. Despite over two decades of study, no standardisation of therapy or approach to MV appears on the horizon. This problem is likely due to the heterogeneity of the ARDS patient and ARDS affected lung. Currently, external MV parameters are set to try and treat an internal condition. There is no way to determine if more harm than good is being done. Hence, there is a tradeoff in between the risk of and benefit. What is required a method to assess that tradeoff and thus the potential risk. The use of positive end expiratory pressure (PEEP) and tidal volume has been identified as key ventilation parameters when treating ARDS patients. Although the impact of both parameters has been studied extensively, only the use of low tidal volumes has been conclusively determined. In contrast, the application of PEEP is still widely disputed. This review discusses two different approaches to ventilation management and the impact on optimal PEEP. The first approach examines the use of imaging techniques to determine regional lung mechanics. In the past, computed tomography (CT) was seen as a way to optimise PEEP, but the risks associated with it have limited it to a research tool. Newer methods such as lung ultrasound and electrical impedance tomography (EIT) seem to offer a less riskier approach to assessing regional mechanics. The second approach examines model-based approaches to ventilation management. Models that take ventilation data and depict a physical picture offer the potential to assess the risks on a patient-specific basis. Models offer the benefit of creating an approach to a highly heterogeneously and patient-specific problem in a non-invasive manner. Given the added dynamic of a patient's evolution over time, a highly patient-specific approach is typical and also what is required. Although both approaches can potentially be used to help with clinical decision making with regard to PEEP, they both pose advantages and disadvantages. The use of a given approach will depend on the individual needs of each clinic. Although not currently deployed in the clinic, model-based methods represent a novel methodology in treating ARDS patients. Thus, model-based approaches represent a state of possible rather than currently practiced methods, and require further clinical validation before justifying their use in the clinic. (C) 2011 Elsevier Ltd. All rights reserved.