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CRITICAL CARE AND TRAUMA

Change in Expiratory Flow Detects Partial Endotracheal Tube Obstruction in Pressure-Controlled Ventilation

Rafael Kawati, MD, PhD^*, Laszlo Vimlati, MD^*, Josef Guttmann, PhD, Göran Hedenstierna, MD, PhD, Ulf Sjöstrand, MD, PhD^*, Stefan Schumann, PhD, and Michael Lichtwarck-Aschoff, MD, PhD^*

From the Departments of *Surgical, and Medical Sciences, University Hospital, Uppsala, Sweden; Department of Anesthesiology and Critical Care Medicine, University Hospital, Freiburg, Germany; Department of Anesthesiology and Critical Care Medicine, Klinikum Augsburg, Augsburg, Germany.

Address correspondence and reprint requests to Rafael Kawati, MD, Department of Surgical Sciences/Anesthesiology & Intensive Care, Uppsala University Hospital, SE-751 85 Uppsala, Sweden. Address e-mail to r.kawati{at}comhem.se.

Only extreme degrees of endotracheal tube (ETT) narrowing can be detected with monitoring of tidal volume (V_T) during pressure-controlled ventilation (PCV). To assess the degree of ETT obstruction in PCV and to compare it to V_T monitoring, we produced 3 levels of partial ETT obstruction in 11 healthy anesthetized piglets using ETTs of 4 different inner diameters (IDs 9.0, 8.0, 7.0, and 6.0 mm). An expiratory flow over volume (_e–V) curve was plotted and the time constant (_e) at 15% of expiration time (T_e) was calculated. We also calculated the fractional volume expired during the first 15% of T_e (V_{ex fract,15}) and compared those variables to full expiratory V_T for each of the 3 obstructions. V_T monitoring failed to detect ETT narrowing. By contrast, V_{ex fract,15} decreased and _e increased significantly with increasing ETT narrowing (for IDs 9.0, 8.0, 7.0, and 6.0, mean V_{ex fract,15} was 195, 180, 146, and 134 mL respectively and mean _e was 380, 491, 635, 794 ms for IDs 9.0, 8.0, 7.0, and 6.0 respectively). We conclude that when the elastic recoil that drives _e is appropriately considered, analysis of _e and V_{ex fract,15} detects partial ETT obstruction during PCV.