Lung-Protective Ventilation Linked to Improved Survival

Steven Fox

March 27, 2012

March 27, 2012 — Reducing breath size and pressure settings for mechanical ventilators used in intensive care units can substantially improve long-term survival in patients with acute lung injury (ACI), according to results from the most comprehensive study to date evaluating the long-term effects of mechanical ventilation on patients with ACI.

The prospective cohort study was conducted by Dale Needham, MD, associate professor, Division of Pulmonary and Critical Care Medicine, Johns Hopkins University School of Medicine, Baltimore, Maryland, and colleagues. The study was published online March 27 in the British Medical Journal.

"Average tidal volume showed a linear relation with two year survival, such that even a relatively small decrease in average tidal volume over stay in the intensive care unit was independently associated with an important decrease in risk of mortality," the authors write. "Greater use of lung protective ventilation in routine clinical practice could reduce long-term mortality in patients with acute lung injury."

Previous studies have shown that using volume-limited and pressure-limited mechanical ventilation (so-called lung-protective ventilation) in patients with acute lung injury can significantly reduce short-term mortality. One earlier randomized trial found that using such an approach resulted in an absolute 8.8% reduction in short-term mortality.

Recommended settings for lung-protective ventilation can be found on the National Heart, Lung, and Blood Institute's Acute Respiratory Distress Syndrome Network (ARDS) Web site.

In the current study, Dr. Needham and colleagues wanted to find out whether similarly favorable results could be obtained during the longer term.

They studied 2-year survival rates in 485 consecutive mechanically ventilated patients with acute lung injury who were treated in the intensive care units of 4 hospitals in Baltimore. Data on the 485 patients included 6240 ventilator settings, as measured twice each day, for a median of 8 ventilator settings per patient.

A ventilator setting was considered adherent to lung-protective ventilation if it satisfied 2 criteria:

  • It had a tidal volume equal to or less than 6.5 mL/kg of predicted body weight, which is the threshold used in a seminal study of reduced tidal volumes that was conducted by ARDS to designate study sites' adherence to the goal tidal volume of 6.0 mL/kg predicted body weight.

  • The setting had a plateau pressure equal to or less than 30 cm of water, which was based on documentation of respiratory therapy. When pressure-regulated modes of ventilation were used, in which plateau pressure is not measured, the researchers used peak pressure or the sum of positive end expiratory pressure and the prescribed increment in inspiratory pressure.

Dr. Needham and colleagues report that only 41% of the more than 6200 total ventilator settings adhered to recommended settings for lung-protective ventilation. About 37% of patients never received lung-protective ventilation, the researchers say.

Of the 485 patients included in the study, 311 (64%) died within 2 years, the authors say. For each 1 mL/kg predicted body weight increase in average tidal volume, there was an associated 18% increase in risk for death during the subsequent 2 years.

Further, after adjusting for the total length of time patients were on ventilation, along with other relevant factors, each additional ventilator setting that was adherent to lung-protective ventilation was associated with a 3% decrease in the risk for mortality over 2 years (hazard ratio, 0.97; 95% confidence interval [CI], 0.95 - 0.99; P = .002), the authors note.

When those figures were compared with no adherence, the authors add, estimated absolute risk reduction in mortality for a typical patient with 50% adherence to lung-protective ventilation was 4.0% (95% CI, 0.8% - 7.2%; P = .012). With full, 100% adherence, risks were reduced even further, by 7.8% (95% CI, 1.6% - 14.0%; P = .011).

The authors stress that settings for ventilators should be based on patients' height, sex, and predicted weight, rather than their actual weight, because lung size remains essentially the same, regardless of how much a patient weighs, the researchers say.

Limitations include that the study is observational and lacked, at times, measurement or recording of plateau pressures.

"Given the study's findings that patients with acute lung injury often did not receive lung protective ventilation, greater efforts to implement lung protective ventilation in routine clinical practice should be undertaken to reduce patients' long term mortality," the authors conclude.

The study was supported by the National Institutes of Health, and 1 author received a clinician-scientist award from the Canadian Institutes of Health Research. The authors have disclosed no relevant financial relationships.

BMJ. Published online March 27, 2012.