The Science Journal of the American Association for Respiratory Care

2008 OPEN FORUM Abstracts

VALIDATION OF ELEVATED ESTIMATED DEAD-SPACE FRACTION AS A POOR PROGNOSTIC FEATURE IN MECHANICALLY VENTILATED PATIENTS WITH ACUTE LUNG INJURY

Marija Kojicic1, Hassan A. Siddiki1, Guangxi Li1, Murat Yilmaz2, Taylor B. Thompson3, Rolf D. Hubmayr1, Ognjen Gajic1



Background: Dead space fraction (Vd/Vt) is a powerful predictor of mortality in acute lung injury (ALI) but the requirement for additional equipment limits its routine measurement. In a recent single center study Vd/Vt calculated from alveolar gas equation (estimated Vd/Vt) correlated well with Vd/Vt based on Bohr equation with the help of a noninvasive metabolic device which detects CO2 in expired air (measured Vd/Vt). We aimed to determine the prognostic value of estimated Vd/Vt in a large sample of patients with ALI enrolled into four previous ARDS-net clinical trials.

Methods: Estimated Vd/Vt was calculated using the rearranged alveolar gas equation: Vd/Vt = 1- [(0.86 x VCO2) / (VE x PaCO2)] where VE is minute ventilation and VCO2 is the estimated CO2 production calculated from the Harris Benedict equation adjusted for hypermetabolic factors. PaCO2 was determined from the arterial blood gas analysis. Univariate and multivariate logistic regression were created to determine the prognostic value of estimated Vd/Vt with and without adjustment for known prognostic factors.

Results: A total of 1636 patients, 909 male (55,6%), were included. Median age was 49,5 years (IQR 38-63), median APACHE III score 89 (IQR 70-110). Median hospital length of stay was 21 days (IQR 14-33) with mortality of 30%. Estimated Vd/Vt on day 1 and day 3 were predictive of hospital mortality of patients with ALI in a dose-response manner (Figure). When adjusted for APACHE III predicted mortality, shock, and PaO2/FIO2 (or Oxygenation index), estimated Vd/Vt was associated with hospital mortality (odds ratio 1.12, 95% CI 1.05-1.18, for each 0.05 increase in Vd/Vt).

Conclusion: stimated Vd/Vt predicted hospital mortality in patients with ALI in a dose-response manner. Calculated from readily available routine data estimated Vd/Vt can be used as a simple bedside prognostic tool in patients with ALI.