Ventilator Use in Acute Lung Injury and Acute Respiratory Distress Syndrome
abstract & commentary
Source: The Acute Respiratory Distress Syndrome Network. Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. N Engl J Med 2000;342:1301-1308.
The mortality rate associated with acute lung injury and the acute respiratory distress syndrome (ARDS) has been estimated to be between 40% and 50%. During the last decade, researchers have pinpointed some of the mechanisms of acute lung injury. However, little if any progress has been made in treating its effects. This multicenter, randomized trial from the 10 university hospitals comprising the Acute Respiratory Distress Syndrome Network attempted to determine the effects of decreased ventilator tidal volumes in patients with acute lung injury and ARDS. Patients were randomized to one of two treatment arms. The first was "traditional" ventilation using an initial tidal volume of 12 mL/kg of predicted body weight, maintaining plateau pressures below 50 cm water. The "low tidal volume ventilation" arm used an initial tidal volume of 6 mL/kg of predicted body weight, keeping plateau pressures below 30 cm water. Bicarbonate and increased respiratory rates were used as necessary to help control acidosis. The primary study end points included patient death prior to discharge home with unassisted breathing and the number of ventilator-free days between study days one and 28. Secondary end points included days of nonpulmonary organ or organ system failure between days one and 28 and plasma interleukin-6 concentrations (a marker of inflammatory response).
This trial was stopped after interim analysis of the first 861 patients because of significantly lower mortality in the low tidal volume group than in the traditional tidal volume group (31.0% vs 39.8%, P = 0.007). The number of ventilator-free days was significantly increased using the low tidal volume strategy (12 + 11 vs 10 + 11, P = 0.007). The number of days with nonpulmonary organ or organ system failure was less in the low tidal volume group, and interleukin-6 levels decreased more in the low tidal volume group.
Comment by Jacob W. Ufberg, MD
Following traditional guidelines for mechanical ventilation, we use tidal volumes that are larger than the tidal volumes of normal humans at rest (7-8 mL/kg). We do this in order to normalize the arterial blood gas values of pH and pCO2. However, the last decade of research has demonstrated that decreasing tidal volumes in order to reduce "lung stretch" and lower end-inspiratory pressures may reduce mortality, despite the resultant hypercapnia and respiratory acidosis.
This large, randomized study from the Acute Respiratory Distress Syndrome Network showed that lowering tidal volumes reduced mortality by 22% and increased the number of ventilator-free days in patients with acute lung injury and ARDS. These benefits occurred despite the need for higher PEEP and FIO2 levels during the first several days of care. This well-done study should cause us to re-evaluate our approach to mechanical ventilation in these patients, keeping an eye on preventing lung stretch injury.