Introduction Analysis of exhaled gas may provide useful insights into metabolic processes in certain diseases. The volatile constituents of human breath include acetone, n-pen- tane and isoprene [1, 2]. Acetone is formed by decar- boxylation of acetoacetate which derives from lipolysis or lipid peroxidation. High concentrations of acetone are found in uncontrolled diabetes mellitus [3]. N-pen- tane and ethane are regarded as markers of lipid perox- idation and have been demonstrated in a variety of pathological conditions [4±15]. Isoprene (2-methylbut- adiene-1,3) is thought to be formed along the mevalonic pathway of cholesterol synthesis [16]. There is experi- mental evidence that isoprene exhalation is related to oxidative damage of the fluid lining of the lung [17] and the body [18]. We therefore hypothesised that, in clinical conditions with high oxidative and inflammatory activity, metabol- ic changes should be mirrored in the composition of ex- haled gas. To test this hypothesis, we studied patients with acute respiratory distress syndrome (ARDS) and with a pulmonary infection, employing a new gas chro- matographic method. J. K. Schubert W. P. E. Müller A. Benzing K. Geiger Application of a new method for analysis of exhaled gas in critically ill patients Received: 17 June 1997 Accepted: 28 November 1997 J. K. Schubert ( ) ) × W. P. E. Müller × A. Benzing × K. Geiger Department of Anaesthesiology and Intensive Care Medicine, University Hospital of Freiburg, Hugstetter Str.55, D-79106 Freiburg, Germany Fax: + 49-7 61-2 70 26 84 email: schubert@ana1.ukl.uni-freiburg.de Abstract Objective: Application of a new method for analysis of ex- haled gas in critically ill patients. Design: Open study. Setting: Surgical intensive care unit of an university hospital. Patients: Thirty-seven consecutive, critically ill, mechanically ventilated patients. Interventions: None. Measurements and results: Chemical analysis of the patient's exhaled gas was based upon substance adsorp- tion and concentration onto activat- ed charcoal, microwave desorption and gas chromatographic separa- tion. Patients with acute respiratory distress syndrome (ARDS) exhaled less isoprene than those without ARDS [9.8 (8.2±21.6) vs 21.8 (13.9±41.4) nmol/m 2 per min [medi- an (95 % confidence interval)], p = 0.04]. In patients who developed pulmonary infection, pentane elimi- nation increased from 0.4 (0.0±5.4) to 2.7 (0.6±6.1, p = 0.05) nmol/m 2 per min and isoprene elimination decreased from 5.2 (0±33) to 5.0 (0±17, p = 0.05) nmol/m 2 per min, resulting in a significant increase in pentane/isoprene ratio from 0.1 (0±0.3) to 0.4 (0±15, p = 0.007) when compared to patients without pul- monary infection. Conclusions: The new method al- lows quantitative analysis of human gas samples with low substance con- centrations and is well suited for clinical studies which involve the in- vestigation of metabolic processes in the lung and the body. Key words Acetone × Gas chromatography × Isoprene × Lipid peroxidation × Microwave desorption × N-pentane Intensive Care Med (1998) 24: 415±421 Ó Springer-Verlag 1998 ORIGINAL