“Ventilation with Lower Tidal Volumes as Compared with Traditional Tidal Volumes for Acute Lung Injury and the Acute Respiratory Distress Syndrome”
by the Acute Respiratory Distress Syndrome Network (ARDSNet)
N Engl J Med. 2000 May 4;342(18):1301-8. [free full text]
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Acute respiratory distress syndrome (ARDS) is an inflammatory and highly morbid lung injury found in many critically ill patients. In the 1990s, it was hypothesized that overdistention of aerated lung volumes and elevated airway pressures might contribute to the severity of ARDS, and indeed some work in animal models supported this theory. Prior to the ARDSNet study, four randomized trials had been conducted investigating the possible protective effect of ventilation with lower tidal volumes, but their results were conflicting.
Population: patients with ARDS diagnosed within < 36 hrs
Intervention: initial tidal volume 6 ml/kg predicted body weight, downtitrated as necessary to maintain plateau pressure ≤ 30 cm of water
Comparison: initial tidal volume 12 ml/kg predicted body weight, downtitrated as necessary to maintain plateau pressure ≤ 50 cm of water
Outcomes:
primary
1) in-hospital mortality
2) ventilator-free days within the first 28 days
secondary
1) number of days without organ failure
2) occurrence of barotrauma
3) reduction in IL-6 concentration from day 0 to day 3
Results:
861 patients were randomized before the trial was stopped early due to the increased mortality in the control arm noted during interim analysis. In-hospital mortality was 31.0% in the lower tidal volume group and 39.8% in the traditional tidal volume group (p = 0.007, NNT = 11.4). Ventilator free days were 12±11 in the lower tidal volume group vs. 10±11 in the traditional group (n = 0.007). The lower tidal volume group had more days without organ failure (15±11 vs. 12±11, p = 0.006). There was no difference in rates of barotrauma among the two groups. IL-6 concentration decrease between days 0 and 3 was greater in the low tidal volume group (p < 0.001), and IL-6 concentration at day 3 was lower in the low tidal volume group (p = 0.002).
Implication/Discussion:
Low tidal volume ventilation decreases mortality in ARDS relative to “traditional” tidal volumes.
The authors felt that this study confirmed the results of prior animal models and conclusively answered the question of whether or not low tidal volume ventilation provided a mortality benefit. In fact, in the years following, low tidal volume ventilation became the standard of care, and a robust body of literature followed this study to further delineate a “lung protective strategy.”
Critics of the study noted that at the time of the study the standard of care/“traditional” tidal volume in ARDS was less than the 12 ml/kg used in the comparison arm. (Non-enrolled patients at the participating centers were receiving a mean tidal volume of 10.3 ml/kg.) Thus not only was the trial making a comparison to a faulty control, but it was also potentially harming patients in the control arm. Here is an excellent summary of the ethical issues and debate regarding this specific issue and regarding control arms of RCTs in general.
Corresponding practice point from Dr. Sonti and Dr. Vinayak and their Georgetown Critical Care Top 40: “Low tidal volume ventilation is the standard of care in patients with ARDS (P/F < 300). Use ≤ 6 ml/kg predicted body weight, follow plateau pressures, and be cautious of mixed modes in which you set a tidal volume but the ventilator can adjust and choose a larger one.”
PulmCCM is an excellent blog, and they have a nice page reviewing this topic and summarizing some of the research and guidelines that have followed.
Further Reading/References:
1. Wiki Journal Club
2. 2 Minute Medicine
3. PulmCCM “Mechanical Ventilation in ARDS: Research Update”
4. Georgetown Critical Care Top 40, page 6
Summary by Duncan F. Moore, MD