By James E. Barone, MD

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According to a new study, post-traumatic acute respiratory distress syndrome (ARDS) may be disappearing. Researchers have found the incidence of ARDS in trauma patients significantly decreased between 2001 and 2010.

According to lead author Kelly N. Vogt, MD, “The epidemiology of ARDS is changing. What was once an extremely common complication of severe trauma has now, for a multitude of reasons, almost completely disappeared in this population.” Dr. Vogt is a surgical critical fellow from the Division of Trauma and Surgical Critical Care at the University of Southern California (USC), Los Angeles.

The work was presented at the 2013 Critical Care Congress of the Society of Critical Care Medicine in San Juan, Puerto Rico.

Two different models of post-traumatic ARDS have been described. The early form, due to the degree of injury severity and resuscitation, develops within the first 48 hours. Late-onset ARDS is thought to be related to systemic inflammation and other factors associated with multiple organ failure.

Previously noted in as many as 25% of trauma patients, ARDS has been reported less frequently by others. The authors’ aim was to review the USC experience for ARDS incidence, morbidity and mortality over the period from 2001 to 2010.

AT A GLANCE

Of the 3,954 patients who met the inclusion criteria, 319 developed ARDS with rates of 14% in 2001 declining to 1.9% in 2010.

Regression analysis of risk factors showed that ARDS was associated with an injury severity score of 25 or higher, chest abbreviated injury scores of no less than 3, mean peak inspiratory pressures of no less than 35, red blood cell and plasma transfusions, and a positive fluid balance of at least 1 L in the initial 48 hours of treatment.

Tidal volume index in mL/kg and mean peak inspiratory pressures declined significantly over the years studied.

The researchers employed a standard definition of ARDS and included all trauma patients at least 16 years of age who survived more than 48 hours. Most of them were male, young (average age 40) and almost 75% had suffered blunt trauma.

Of the 3,954 patients who met the inclusion criteria, 319 (8%) developed ARDS with rates of 14% in 2001 declining to 1.9% in 2010 (R2=0.76).

Those who suffered ARDS were significantly more often hypotensive on admission and had higher chest abbreviated injury scores (AIS). Although the ARDS group had significantly lower mean injury severity scores (ISS), 56% had an ISS of 25 or higher compared with 32% in the non-ARDS cohort (P<0.01).

Regression analysis of risk factors showed that ARDS was associated with an ISS of 25 or higher, chest AIS of no less than 3, mean peak inspiratory pressures (PIP) of no less than 35, red blood cell (RBC) and plasma transfusions, and a positive fluid balance of at least 1 L in the initial 48 hours of treatment.

Tidal volume index in mL/kg and mean PIP declined significantly over the years studied. During those years, RBC transfusion rates declined, and the use of plasma was variable. Fluid balance in the first 48 hours was fairly stable, ranging from an average of about 600 mL to nearly 1,500 mL.

In-hospital mortality for patients who developed ARDS did not change significantly over the study period. It was 30% for those with ARDS versus 9% for those without ARDS (P<0.01) and was associated with significantly longer durations of mechanical ventilation and longer ICU and hospital lengths of stay.

After adjusting for markers of poor outcome, the development of post-traumatic ARDS was significantly associated with mortality (odds ratio, 2.7; 95% confidence interval, 1.8-4.2).

“Although this study was not designed to detect the reason why ARDS is declining, several possibilities exist,” said Dr. Vogt. She mentioned the use of lung-protective ventilation strategies and minimizing unnecessary blood transfusion and fluid administration as possible reasons for its decline.

Regarding the role of some of the newer treatments for ARDS, she said, “We do use airway pressure release ventilation, percussive ventilation and occasionally prone positioning in these patients.” However, given the extremely low incidence of ARDS in their trauma patients now, it would be “difficult to tease out the impact of these strategies on mortality.”

Dr. Vogt said that whether ARDS is a cause of death or merely a surrogate marker of serious illness is an important question, and one that remains unanswered.