Prognostic Value of Different Dead Space Indices in Mechanically Ventilated Patients With Acute Lung Injury and ARDS

Study objectiveThe aim of this prospective observational study was to evaluate the utility of derived dead space indexes to predict survival in mechanically ventilated patients with acute lung injury (ALI) and ARDS.Study populationThirty-six patients with ALI (Murray score, ≥1; Pao2/fraction of inspired oxygen [Fio2] ratio, < 300) in critical care departments at two separate hospitals entered the study.MeasurementsAt ICU admission, 24 h, and 48 h, we measured the following: simplified acute physiologic score II; Pao2/Fio2 ratio; respiratory system compliance; and capnographic indexes (Bohr dead space) and physiologic dead space (Enghoff dead space [Vdphys/Vt]), expired normalized CO2 slope, carbon dioxide output, and the alveolar ejection volume (Vae)/tidal volume fraction (Vt) ratio.ResultsThe best predictor was the Vae/Vt ratio at ICU admission (Vae/Vt-adm) and after 48 h (Vae/Vt-48 h) [p = 0.013], with a sensitivity of 82% and a specificity of 64%. The difference between Vae/Vt-48 h and Vae/Vt-adm show a sensitivity of 73% and a specificity of 93% with a likelihood ratio (LR) of 10.2 and an area under the receiver operating characteristic (ROC) curve of 0.83. The interaction between the Pao2/Fio2 ratio and Vae/Vt-adm predict survival (p = 0.003) with an area under the ROC curve of 0.84, an LR of 2.3, a sensitivity of 100%, and a specificity of 57%. The Vdphys/Vt after 48 h predicted survival (p = 0.02) with an area under the ROC curve of 0.75, an LR of 8.8, a sensitivity of 63%, and a specificity of 93%. Indexes recorded 24 h after ICU admission were not useful in explaining outcome.ConclusionsNoninvasive measures of Vae/Vt at ICU admission and after 48 h of mechanical ventilation, associated with Pao2/Fio2 ratio provided useful information on outcome in critically ill patients with ALI.