Nearly half of patients who suffer an out-of-hospital cardiac arrest (OHCA) develop acute respiratory distress syndrome (ARDS) within 48 hours. Those patients who develop ARDS after an OHCA spend more days in the ICU and on a ventilator, die more often, and are less likely to recover with good functional status.
“Cardiac arrest is not traditionally thought to be one of the main risk factors for ARDS,” said lead author Nicholas Johnson, MD, an attending physician in the emergency department, medical ICU and neurocritical care service at Harborview Medical Center, in Seattle. “We think, based on our findings, that it should be, and that patients with cardiac arrest should be included in future studies of ARDS treatments.” Dr. Johnson also is the associate program director for the Critical Care Medicine Fellowship and an assistant professor of emergency medicine at the University of Washington, in Seattle.
Dr. Johnson said he and his fellow researchers hypothesized that OHCA patients treated with lower tidal volumes would be less likely to develop ARDS. “After adjusting for patient demographics, cardiac arrest characteristics and other care factors, we actually did not find an association between initial tidal volume and ARDS development or neurologic outcome,” he said.
Patients who were on mechanical ventilation for at least 24 hours, had two consecutive arterial blood gases with a ratio of partial pressure of oxygen to the fraction of inspired oxygen of 300 mm Hg or less, and who had bilateral radiographic opacities were defined as having ARDS, according to the researchers, who examined the linear association between initial tidal volume (categorized into 0.5-mL/kg groups with collapsed tails) and ARDS and neurologic outcome.
“Of 978 OHCA patients transported to the study hospitals, 600 were mechanically ventilated and survived at least 48 hours,” the study stated. The mean age was 56 years, 74% were men, 69% were white, and overall survival was 59%. A total of 287 of the 600 patients (48%) in the study developed ARDS.
“Patients with ARDS had higher acute physiology scores and more comorbidities,” the researchers said. “Patients with ARDS had higher hospital mortality, longer ICU and hospital length of stay, more ventilator days, and were less likely to survive with full neurologic recovery to hospital discharge (35% vs. 55%; P<0.0001).” Ventilation with lower initial tidal volumes was linearly associated with lower ARDS incidence (P=0.02) but not neurologically intact survival, the researchers found, but this association was not present after adjusting for demographic and other care factors.
The researchers concluded that ARDS was associated with poor outcome, including reduced neurologic function, and ARDS patients required more resources. They said further research should clarify the effect of early low tidal volume ventilation on the outcome of OHCA patients.
Joseph Shiber, MD, FACEP, FACP, FCCM, an associate professor in the Department of Emergency Medicine, Neurology and Critical Care, and the director of the advanced lung/extracorporeal membrane oxygenation service at the University of Florida College of Medicine–Jacksonville, who was not associated with the study, said an occurrence of ARDS in patients who suffer an OHCA is “always bad in any situation,” and leads to worse outcomes.
Not all OHCA is due to primary cardiac arrest causes, he reminded the audience. If it is due to a respiratory cause—known as secondary cardiac arrest—then “I would expect the incidence of ARDS would be higher, as it is known that their expected mortality is higher.” Using an appropriate tidal volume—6 to 8 cc/kg of ideal body weight—“is always a good idea,” Dr. Shiber said.
While calling the study well done, Dr. Shiber said, “This study only looked at the initial tidal volume, so it may be too short of an exposure to cause any effect.”
Dr. Johnson said the effect of early low tidal volume ventilation on these patients was not known at this point. “There is one other recent study showing that early ventilation with lower tidal volumes is associated with improved outcome, and two others, including ours, that did not find an association.”
To know the answer, “we would need a prospective trial in which we randomize patients to low tidal volume ventilation or ventilation with traditional tidal volumes. While we await such a trial, I think it’s reasonable to use low tidal volume ventilation in these patients, provided it does not contribute to life-threatening acidemia, which can occur in this population,” he explained.