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

^* From the Department of Perioperative Medicine, Intensive Care and Emergency (Drs. Lucangelo, Bernabè, Vatua, and Degrassi), Cattinara Hospital, Trieste University School of Medicine, Trieste, Italy; Critical Care Center (Drs. Villagrà, Fernandez, Saura, and Blanch), Hospital de Sabadell, Institut Universitari Fundació Parc Taulí, Universitat Autònoma de Barcelona, Barcelona, Spain; Laboratorio de Neumología Experimental (Dr. Romero), Hospital Universitario de Bellvitge, L’Hospitalet de Llobregat, Llobregat, Spain; and the Department of Mathematics and Computer Science (Mr. Borelli), Trieste University, Trieste, Italy.

Correspondence to: Umberto Lucangelo, MD, Ospedale di Cattinara, Dipartimento di Medicina Perioperatoria, Terapia Intensiva ed Emergenza, Strada di Fiume 447, I-34149 Trieste, Italy; e-mail: u.lucangelo{at}fmc.units.it

Abstract

Study objective: The 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 population: Thirty-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.

Measurements: At 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.

Results: The 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.

Conclusions: Noninvasive 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.

Key Words: acute lung injury • mechanical ventilation • prognosis • pulmonary dead space • volumetric capnography