45 Increased Dead Space Fraction in Patients with Inhalation Injury

Abstract

Inhalation injury increases morbidity and mortality in burn patients. The pathology involves injury to large airways and V/Q mismatch, and differs from ARDS and pneumonia. We noted large differences between end-tidal CO2 (ETCO2) and PaCO2 in patients with inhalation injury. This difference is an indirect measure of dead space. We reviewed our experience with inhalation injury and the correlation of increased dead space with outcomes: mortality, ventilator associated pneumonia (VAP), and ventilator days (VNTd.) Following IRB approval we identified 51 adult patients with burns and inhalation injuries in our admission records from 2011–2015. Data collected includes: demographics, burn size, length of stay (LOS), ventilator days (VNTd), blood gas results, ETCO2, presence of VAP, and mortality. We calculated the dead space fraction (DSF) using the Enghoff-Modified Bohr Equation (difference between PaCO2 and ETCO2, divided by PaCO2.) We also calculated the Modified Baux Score (MBS.) We used Student’s T-tests to compare DSF between survivors and non-survivors, and between patients with and without VAP. We used bivariate correlation analysis to explore the relation between DSF and VNTd, DSF and MBS, and DSF and burn size. Values expressed as mean± SD. The 51 patients had a mean age of 52 ± 16.1 years and a mean burn size of 17.5 ± 25.2% resulting in a mean MBS of 87 ± 26.8. Mean LOS was 11.9 ± 15.0 days. In this sample 10 patients died (19.6%) and 6 developed VAP (11.8%). Mean DSF was 0.38 ± 0.22. Student’s T-tests found the DSF to be significantly lower in survivors (mean 0.34 ± 0.21) compared to non-survivors (mean 0.52 ± 0.24), p=0.027) but not between patients with VAP (mean 0.47 ± 0.31) and without (mean 0.36 ± 0.20, p=0.259.) Bivariate correlation analysis identified a weak correlation between DSF and VNTd, but this was not statistically significant (Pearson correlation coefficient 0.281, p=0.075.) Bivariate analysis identified a significant correlation between DSF and MBS (Pearson coefficient 0.524, p<0.001) and a stronger correlation between DSF and burn size (Pearson coefficient 0.606, p<0.001.) The dead space fraction (DSF) is often increased in patients with inhalation injury. DSF was significantly higher in patients who died with inhalation injury compared to survivors. DSF tended to be higher in patients with VAP, but the difference was not statistically significant. DSF correlates with the MBS and burn size. Further study of DSF and its role in inhalation injury is warranted. Dead space fraction (DSF) is easy to calculate from PaCO2 and ETCO2 and may be useful in measuring the severity of inhalation injury, the patient’s prognosis, and response to treatment.