1. Why is this topic important?
In patients who receive mechanical ventilation, acute lung injury and the development of ARDS are frequent complications associated with significant morbidity and mortality. The ideal ventilatory strategy is unknown, but the ARDSnetwork has demonstrated that lung protective ventilation in patients with ARDS reduces mortality, ICU length of stay, and pulmonary infections. In the emergency department our ventilatory strategies may affect patient outcomes days down the road as lung injury has an insidious onset. Therefore the question remains whether in patients who are at risk for developing acute lung injury or acute respiratory distress syndrome benefit from the same lung protective ventilation early in the disease course i.e. beginning in the emergency department.
2. What does the study attempt to show?
Three reviewers from the department of CCM in Brazil reviewed the available literature regarding the use of lung-protective ventilation in patients without acute respiratory distress syndrome, or acute lung injury (berlin definition of ALI and ARDS). Articles were selected if they evaluated two types of ventilation in patients without ARDS or ALI: low tidal volumes (6-8cc/kg IBW) vs. traditional tidal volumes (10-12cc/kg IBW). Studies included: prospective observational studies (before/after, cohort, cross-sectional), and RCT in a variety of settings. The study attempts to identify if lung protective ventilation is beneficial in all patients prior to the onset of ALI or ARDS.
3. What were the findings?
Twenty articles which included, with 2,822 participants analyzed. Table 1 summarizes the characteristics of included studies, in which there was significant heterogeneity and none of the studies included began in the emergency department setting. Baseline characteristics were similar between the two groups except for several important ventilator characteristics: respiratory rate was higher in the low TV ventilation group (18 vs. 12) , while minute ventilation remained similar (8.6 vs. 8.4 L/min). The plateau pressure was lower in the low TV group although the mean Pplateau remained below 30 in both groups. PEEP was higher in the low TV group, and PaCO2 was also higher in the low TV group. See table 2 for all of the numbers.
There was a decrease in lung injury development in the low TV ventilation group (RR 0.33 95% CI, 0.23-0.47 with a NNT 11), and a reduction in mortality in the low TV groups (RR 0.64, 95% CI 0.46-0.89; NNT 23.) The results were similar when you evaluated both randomized and nonrandomized studies.
4. How is patient care impacted?
In patients who require endotracheal intubation lower tidal volume ventilation is suggested by this meta-anyalsis to reduce both morbidity and mortality. This study included a wide-variety of patients including post-surgical, and medical patients. The intervention in this group is free, effective, and poses minimal risks to patients other than slight worsening in acidosis and increased PaCO2 which can be counter-balanced by increase respiratory rate and maintaining minute ventilation, and optimizing PEEP in order to reduce atelectasis.
5. Is this an area of controversy?
Absolutely. Many physicians point to the fact that this is a summation of many non-randomized studies and studies in which potentially injurious lung volumes (800cc) were given to patients. In addition no prospective studies from the emergency department have EVER been completed. To complicate this issue in the emergency department the diagnosis of why a patient required endotracheal intubation is often obscured and the ability of an emergency physician to predict who may or may not develop acute lung injury is difficult if not impossible.
Other physicians argue that low tidal volume ventilation increases atelectasis and in patients without ARDS this atelectasis may increase the risk of pulmonary infection, lower PaO2, and increase PCO2 worsening underlying acidosis, if present.
6. Major limitations of the study?
This study is comprised of multiple prospective observational studies, non-RCT, in a heterogeneous population. Patients who are post-surgical are certainly at risk of developing ARDS, however the mechanism may be inherently different than that of a patient with septic shock. Since ARDS and ALI are clinical syndromes, and the pathophysiology for the causes is inherently different it is difficult to recommend one ventilatory strategies for all critically ill patients.
My take on the study is that low tidal volume ventilation is potentially beneficial in all patients requiring mechanical ventilation. As long as you can manage the patients respiratory parameters with PEEP and RR while monitoring PCO2, pH, PaO2 and always keeping the plateau pressure < 30, all patients should receive lung protective ventilation. I believe this for several reasons, one of which is patients lungs who require intubation in the ED are not post-surgical they rarely are being intubated to be extubated in a few hours and we anticipate that intubation is going to be for days. Therefore, we should expect all of our patients to be at risk for ALI, and ARDS. In addition, the TV is not spread evenly throughout the lungs as the lungs often have a patchy distribution of injury, for example a LLL pneumonia means that region of the lung is filled with pus and the rest of the TV (which cannot reasonable aerate that portion of the lung) will be dispersed to the "healthy" lung possibly causing over distention and lung injury.
Another important fact to remember is when assessing for lung protective ventilation it is based on IBW, so Jaba the hut gets ventilated based on height not weight! Rarely is a patient tall enough to require >600cc, but see the PDF in included for volume recommendations.
The evidence is weak so read for yourself and be flexible to change as the evidence changes in the coming years, but for now I think it is reasonable to use lung protective ventilation in all emergency department patients requiring endotracheal intubation.