View Abstract

A7233 - Chest Wall Elastance During Passive Mechanical Ventilation: An Alternative Hypothesis
Author Block: G. Hedenstierna1, S. Lundin2, A. Pesenti3, D. Chiumello4, A. S. Larsson5, O. Stenqvist6; 1Medical Sciences, University Hosp, Uppsala, Sweden, 2Anesthesia and intensive care, University Hosp, Göteborg, Sweden, 3Department of Pathophysiology and Transplantation, University of Milan, Milan, Italy, 4Anesthesia and Intensive Care, Milan, Italy, 5Surgical Sciences, Uppsala University, Uppsala, Sweden, 6Anaesthesiology and Intensive Care, Sahlgrenska University Hospital, Goteborg, Sweden.
RATIONALE: Esophageal pressure during mechanical ventilation is considered to reflect chest wall elastance. We hypothesized that esophageal pressure swings during dynamic conditions, i.e. passive tidal ventilation with no or short respiratory holds, are influenced by frequency dependent behavior of abdominal content. If so, static esophageal pressure, i.e. after long respiratory holds, is needed to calculate chest wall elastance. METHODS: Retrospective analysis of airway (Paw) and esophageal (Pes) pressures and lung volumes in mechanically ventilated, supine patients in different published studies. RESULTS: Pes - volume curves obtained 1/during a minute-long expiration with intermittent halts in anesthetized, mechanically ventilated children (1) and 2/ between different levels of positive end-expiratory pressure in patients with acute respiratory failure (2-4) showed very steep Pes curves, i.e. small or no change in pressure despite large volume change. The “static” Pes change was less than 10% of the Paw change. This suggests a low chest wall elastance that may even come close to zero. In these patients, considerable tidal variation in esophageal pressure was seen, in line with results from other studies. The tidal Pes - volume ratio was 4 - 15 times higher than the “static” Pes - volume ratio. Thus, tidal Pes changes do not reflect, or only to a minor extent, static chest wall elastance. Rather, they reflect forces to displace abdominal tissue. This displacement seems to continue after inflation of the lung is finished and, most likely, reflects a stress-relaxation phenomenon. CONCLUSIONS: This retrospective analysis suggests that changes in esophageal pressure during ongoing mechanical ventilation do not primarily reflect a chest wall elastance but displacement of abdominal tissue. This opens a possibility to estimate static lung compliance without the recording of esophageal pressure. However, this is only true if chest wall elastance is negligible. It shall also be emphasized that this is mainly a theoretical analysis of the recording of respiratory mechanics and that a prospective study is needed for the validation of the hypothesis that has been presented here. References 1. Ingimarsson J, et al. Am J Respir Crit Care Med 2000; 162: 412-417. 2. Pelosi P, et al. Am J Respir Crit Care Med 1995; 152: 531-537. 3. Chiumello D, et al. Intensive Care Med 2014; 40: 1670-1678. 4. Lundin S, et al. Acta Anaesthesiol Scand 2015; 59: 185-196.