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From DaNang lung to combat trauma-associated acute lung injury--closing the loop.

Acute lung injury (ALI) is a syndrome diagnosed by a combination of acute hypoxemic respiratory failure in the presence of bilateral radiographic lung infiltrates, which cannot be fully explained by hydrostatic pulmonary edema. The acute respiratory distress syndrome (ARDS) is considered a more severe subtype of ALI. Severe sepsis, aspiration and trauma are the most common conditions predisposing to ALI and it is estimated to affect 190,000 patients each year in the United States with mortality rates of about 35–45%. Although several investigators reported no attributable mortality of trauma-associated ALI, it was found to be an independent predictor of death in a recent study. The only intervention shown to reduce mortality of patients with ALI is use of low (6 mL/kg predicted body weight [PBW]) tidal volume (Vt) and distending alveolar pressures (targeting plateau pressure 30 cm H2O as a surrogate measure) as demonstrated in a multicenter randomized trial by the ARDS network, using volume assist control mode. This study followed on the experimental data of the injurious mechanical and biological effects of positive pressure ventilation, which can result in lung damage similar to ALI and termed ventilator-induced lung injury. Although other ventilatory methods are used in patients with severe ALI to improve oxygenation, none were shown to improve clinical patient outcomes when compared with the volume assist control mode. In this issue of Southern Medical Journal, Guerrero et al describe the successful care of a patient with trauma-associated ARDS. On first reading, one may consider this to be another report of a common complication of severe injury with an expected stormy course. However, this report serves to bring forth several key challenges involving the contemporary care of these critically ill patients. First, during his initial phase of ARDS the patient had been ventilated using Vt 600 mL. If this setting represents Vt of 6 mL/kg PBW, it would imply a predicted body weight of 100 kg, corresponding to a height of 6 8 , obviously well above that of an average adult. Unfortunately, the authors do not provide data on the patient’s PBW or lung mechanics. Despite a widespread endorsement for use of protective lung ventilatory techniques emulating the ARDS network protocol, low Vt continues to be underutilized. Second, the mode of mechanical ventilation was changed from assist-control to airway pressure release ventilation (APRV) following worsening oxygenation. APRV is a ventilator mode utilizing high level continuous positive airway pressure (CPAP) with timed pressure deflation (pressure release) to augment tidal breathing. Unlike other ventilatory modes used in patients with ALI, APRV allows spontaneous breathing throughout the ventilatory cycle and can be advantageous in these patients, with a potential to improve ventilation of the dorsal lung regions, better matching of ventilation to perfusion, decrease of intrapulmonary shunt, and avoiding alveolar overdistension. Although APRV was first described more than 2 decades ago, only limited clinical practice experience has been reported. The authors do not provide sufficient data to assess the cause of the patient’s progressive hypoxic deterioration on APRV. Third, mechanical ventilation in the prone position had been initiated following the patient’s refractory hypoxemia. Since being first proposed in 1974, numerous investigations of prone ventilation were reported. The mechanisms of gas exchange improvement with prone ventilation are incompletely understood, but are thought to be related to alveolar recruitment and redistribution of ventilation toward dorsal lung regions, resulting in improved ventilation to perfusion matching. Prone ventilation leads to rapid and usually sustainable improvement of oxygenation in about 70% of ALI patients. However, response cannot be predicted in individual patients, and no survival benefit was found in a recent metaanalysis. Thus, prone ventilation can be used as a temporizing measure in patients with severe hypoxic respiratory failure. However, patient selection criteria have not been clearly defined. APRV combined with prone position appears to have no oxygenation advantage during the first prone session, when compared with other ventilatory modalities. Prone ventilation is a labor-intensive and potentially risky undertaking, as highlighted in the present report. The associated complications include dislodgement of tracheal tubes, vascular catheters, and other draining devices, as well as pressure injuries to the face, eyes and limbs. As noted by the authors, no guidance is available on the use of prone ventilation in the ALI patient with an open abdominal wound, and the authors’ favorable experience following additional precautions can serve as a starting point for future research. Perhaps the most important message of Guerrero et al’s case report lies in the context of their patient’s care. Being injured by a gunshot wound to the chest during a convoy mission in Iraq, the patient had undergone prompt initial surFrom the Division of Pulmonary and Critical Care Medicine, Department of Internal Medicine, Texas Tech University School of Medicine at the Permian Basin, Odessa, TX.