Come One, Come All – Low tidal volumes improve outcomes

Low Tidal Volumes Improve Outcomes in Non-ARDS Patients

Since the landmark ARDSnet trial of low tidal volume ventilation published in the NEJM in 2000, protecting the injured lung with low tidal volumes has been widely adopted. In case you missed it, that trial showed that low tidal volume ventilation (6 ml/kg IBW) improved mortality from 40% to 31% in patients with established lung injury (PaO2/FiO2<300 mm Hg).  The tidal volumes utilized were 6 ml/kg in the low tidal volume group and 12 ml/kg in the conventional tidal volume group and the study utilized standardized PEEP/FiO2 tables.  Although many topics continue to be hotly debated in the ICU, including glycemic control and steroids for septic shock, lung protective ventilation with low tidal volumes has been widely accepted and forms the cornerstone of care for patients with established acute respiratory distress syndrome (ARDS).

However, critical care physicians frequently ponder the optimal tidal volume for patients without acute lung injury.  Should they err on the small side by emulating the ARDSnet protocol, or should they avoid the dangers of higher PEEP and the potential for atelectasis by using higher tidal volumes?  Furthermore, critical care physicians often wonder if the development of lung injury might be averted by utilizing low tidal volumes in patients at risk for lung injury, such as patients with severe sepsis or pancreatitis.  Serpo Neto et al address this important clinical question in the October 24th issue of JAMA.

What They Did

Serpo Neto et al performed a meta-analysis of 20 prior studies that tested higher vs. lower tidal volumes in patients requiring mechanical ventilation who did not have ARDS or acute lung injury at the time of intubation.  Importantly, the studies included in this meta-analysis were conducted across many clinical locations, including MICUs, SICUs and NICUs as well as operating rooms.  In total, more than 2,000 patients were included in this meta-analysis. Randomized trials and observational trials were included.

The average tidal volume was 6.5 ml/kg IBW in the lung-protective ventilation group and 10.6 ml/kg IBW in the conventional tidal volume group.   Thus, the conventional tidal volume group in this study was lower than in the 2000 NEJM ARDSnet trial.  This is an important distinction because one of the few criticisms of the multicenter RCT conducted by the ARDSnet group was that the conventional tidal volumes were higher than usual practice.  The average duration of per-protocol mechanical ventilation was short, only 6.9 hours in the lung protective group and 6.56 hours in the conservative group.  After combing key data from the different studies, the authors then measured the following end-points in patients ventilated with higher vs. lower tidal volumes:  1) development of ARDS 2) mortality 3) pulmonary infection 4) atelectasis.

What They Found

In this meta-analysis of a heterogeneous group of mechanically ventilated patients, patients who received lower tidal volumes showed a statistically significant decrease in all of the key outcomes measured in this trial.  Thus, the relative risk of the following outcomes was reduced in patients who received low tidal volume ventilation:

• The development of acute lung injury

• The development of pulmonary infection

• The development of atelectasis

• Mortality

What This Means

For those who have always wondered if they should be using a lung protective ventilation strategy in patients with soft signs of acute lung injury, for patients at risk for lung injury or even for patients without lung injury, this study helps inform your decision.  Bigger is not always better.  In fact, bigger (meaning high tidal volumes) does not even reduce the risk of developing atelectasis, which may be the last physiologic rationale for selecting higher tidal volumes.

Does this study justify the implementation of a definitive one size fits all ventilation strategy?  The answer is a resounding NO.  Not because this study is not statistically sound, but because the studies utilized in this meta-analysis were relatively small, some were not randomized controlled trials, and they were conducted in many different clinical settings.

However, this study certainly provides justification for a large randomized trial evaluating a lung protective, low-tidal volume ventilation strategy in all patients without lung injury admitted to critical care units.  Furthermore, future studies will be needed to identify which patient population will benefit most from this strategy and to identify the optimal reduction in tidal volumes.

Update: Low tidal volume ventilation was shown in a NEJM randomized trial to improve outcomes after elective abdominal surgery.

Clinical Takeaway: Until a more definitive, large randomized trial is performed, this study provides an evidence-based rationale for instituting a low-tidal volume ventilation strategy in patients without ARDS who do not have a clear contraindication to low tidal volumes.

Ary Serpa Neto, et al. Association Between Use of Lung-Protective Ventilation With Lower Tidal Volumes and Clinical Outcomes Among Patients Without Acute Respiratory Distress Syndrome: A Meta-analysis. JAMA. 2012;308(16):1651-1659.

Niall D. Ferguson. Editorial: Low Tidal Volumes for All? JAMA. 2012;308(16):1689-1690.