What's new in critical illness and injury science? Managing acute respiratory distress syndrome is still a challenge

Abstract

In this issue, the authors of “A comparison between two different recruitment maneuvers in patients with ARDS” describe in detail the effects of two maneuvers on gas exchange, pulmonary and systemic hemodynamics, and lung compliance. From the 1970s to 1990s, various investigations were done to discover means to improve oxygenations by different ventilation strategies. For a number of years, critical care text books recommended using tidal volumes in the range of 10 to 15 ml/kg of actual body weight.[1] These tidal volumes were likely appropriate at the time for postabdominal surgery patients; however, for 10 to 15 years after recognition of acute respiratory distress syndrome (ARDS), the same tidal volumes were generally accepted and used. Hickling et al. in 1990s proposed that using lower tidal volumes improved mortality in patients with ARDS; however, the studies were small and limitations were there.[2 3] In 1994, the NIH National Heart, Lung, and Blood Institute (NHLBI) ARDS Clinical Trials Network (ARDSNet) was launched. Initially, a number of pharmacologic interventions in ARDS were being considered for the inaugural study by this group; however, based on lack of data on best strategy to ventilate this patient population, the group proceeded with study to determine if 6 ml/kg predicted (not actual) body weight and plateau airway pressure limitation to 30 cm/H2O would provide better outcomes in patients with acute lung injury (ALI) than 12 ml/kg predicted body weight. The trial was stopped earlier than anticipated due to a significant improvement in mortality in the lower tidal volume group. Based on this historic trial by the ARDSNet group, using lung protective ventilation in patients with ARDS has become standard of care.[4] It has been well established that maintaining positive pressure throughout the respiratory cycle improves oxygenation by recruiting alveoli and at the same time prevents damage that occurs by repetitive opening and closing of alveoli. Transient increase in transpulmonary pressure further promotes recruitment of collapsed alveoli.[5] As a result of this recruitment, ventilation perfusion matching improves resulting in better gas exchange.[6–8] A number of methods have described over the years to accomplish such recruitment. To our knowledge, to date, there are neither any data to suggest mortality benefit nor a comparison of different recruitment maneuvers. Despite being a widely accepted practice in the arena of critical care, there still is no clarity in regards to optimum pressure, duration and frequency of such maneuvers. A recent systematic review by Fan et al. showed that the most common complications of such recruitment maneuvers included hypotension (12%) and desaturation (9%); however, oxygenation generally improved after such maneuvers. Serious complications such as barotraumas and arrhythmias are infrequent (about 1%). Improved oxygenation was more commonly noticed in patients with smaller difference in pre- and post-recruitment PEEP (≤5 cm H2O), lower baseline PaO2/FiO2 ratio (<150 mm Hg, and in patient with lower pre-recruitment lung compliance (<30 ml/cm).[5] There has also been reported decline in the improvement in oxygenation noticed immediately after a recruitment maneuver.[5] The application of higher positive end expiratory pressure (PEEP) post-recruitment maneuver likely helps in maintaining a sustained response, although there is likely considerable variability among different patients.[9 10] There are several non-ventilator strategies (inhaled pulmonary vasodilator therapy, conservative fluid management, avoidance of systemic vasodilators, prone positioning, corticosteroid therapy, and nutritional supplementation) which can also be implemented and result in improvement in gas exchange.[11] These strategies were nicely outlined by Raoof et al. in a recent review article. One can make an argument that these non-ventilatory strategies may be easier and safer to implement when compared with recruitment maneuvers when one is faced with severe hypoxemic respiratory failure. Among these non-ventilator strategies, the ones that can be routinely practiced are avoidance of factors that may contribute to deterioration in oxygenation (such as systemic vasodilators which include commonly used antihypertensive medications such as nifedipine and nitroglycerine and vasopressors such as dopamine and dobutamine) and conservative fluid management.[11] In addition to management, early identification of a number of risk factors may lead to interventions to prevent development of ALI or ARDS.[12] The authors have done a great job in describing impact of two recruitment maneuvers on various parameters. Both of the maneuvers resulted in improvement of PaO2/FiO2 ratio compared with baseline; however, more improvement was noticed in the extended sigh method. Both group of patients tolerated the maneuvers well and no significant hemodynamic effects were seen. The gas exchange parameters were monitored for only up to 20 minutes after the recruitment maneuver and no outcome data were provided, leaving the question of whether a sustained response or long-term benefit is seen with these maneuvers unanswered. Based on current evidence, it is difficult to recommend routine use of recruitment maneuvers due to the lack of sustained effect and lack of mortality benefit, even though they generally improve oxygenation and are safe when performed in a controlled and well-monitored setting. In addition to lung protective ventilation strategies, conservative fluid management and avoidance of factors that may contribute to worsening gas exchange should be routine practice by physicians involved in managing the critically ill population.