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Dead Space Fraction as a Prognosticator in ARDS
Abstract & Commentary
By David J. Pierson, MD, Editor, Professor, Pulmonary and Critical Care Medicine, Harborview Medical Center, University of Washington, Seattle, is Editor for Critical Care Alert.
Synopsis: In this study of 80 patients with ARDS, an elevated dead space fraction in the first 3 days of lung injury, and also later in the illness at 8-10 days, was associated with increased mortality.
Source: Raurich JM, et al. Prognostic value of the pulmonary dead-space fraction during the early and intermediate phases of acute respiratory distress syndrome. Respir Care 2010;55:282-287.
Raurich and colleagues studied 80 patients with acute respiratory distress syndrome (ARDS) to determine whether the alveolar dead-space fraction (VD/VT, the proportion of each breath that does not participate in gas exchange) was a predictor of ultimate survival. The patients were ages 18 years or older, acutely ill in the investigators' ICU, and met the current international diagnostic criteria for ARDS.1 Patients with obstructive lung diseases, pulmonary vascular disease, and other conditions that might affect VD/VT were excluded. Using the traditional Enghoff modification of the Bohr equation [VD/VT = (PaCO2 − PECO2)/PaCO2] and measuring PECO2 directly from expired air collected in a Douglas bag, the authors determined VD/VT within 3 days of the onset of ARDS, and once again on days 8-10 in patients who remained mechanically ventilated. They then correlated the findings with clinical outcomes.
At both time periods after onset of ARDS, VD/VT correlated with mortality. In the first 3 days its value was 0.53 ± 0.11 among patients who survived, as compared to 0.64 ± 0.09 among patients who died (P < 0.001). In the intermediate phase of ARDS (on day 8-10) the corresponding numbers for VD/VT were 0.50 ± 0.10 vs 0.62 ± 0.09 (P < 0.001). For every dead-space fraction increase of 0.05 the odds of death increased by 59% in the early phase and by 186% in the intermediate phase, with both results statistically significant.
The alveolar dead-space fraction, the ratio of physiologic dead space to tidal volume (VD/VT), is a measure of the efficiency of ventilation. Depending on the delivered tidal volume, it is typically about 0.3 or 0.4 in supine ventilated patients with normal lungs, and is markedly elevated in conditions such as severe chronic obstructive pulmonary disease and pulmonary thromboembolism. Several methods are currently available for determining VD/VT in ventilated patients, and its assessment is useful in the differential diagnosis of patients who are unable to be weaned because of a persistently high minute ventilation.
As reviewed by Kallet and Siobal in the editorial accompanying this article,2 previous studies have shown that high dead-space fractions correlate with worse outcomes in patients with ARDS. The present study corroborates the previous findings of others that a higher alveolar dead-space fraction in early and intermediate phases of ARDS is associated with a greater risk of death.