Evaluation of pressure/volume loops based on intratracheal pressure measurements during dynamic conditions

Background: The aim of this study was to evaluate and compare information a bout lung mechanics obtained by dynamic pressure/volume loops based on Y-pi ece and intratracheal airway pressure. Methods: Airway pressure was measured simultaneously at the Y-piece and tra chea. Flow/tidal volume was measured by sidestream spirometry at the Y-piec e. The effect of an intraluminal catheter in the tube was evaluated in a lu ng model. Ten adults with acute lung injury and mechanical Ventilation were studied. Measurements were performed during volume-(VC) and pressure-contr olled (PC) ventilation at different ventilator settings. Results: Lung model: There was a statistically significant difference (P<0. 001) between trachea and Y-piece pressure/volume loop areas during both VC and PC ventilation. The ratio trachea area/Y-piece area decreased with incr eased endotracheal tube resistance (r=0.96). Patients: The difference betwe en trachea and Y-piece P/V-loops was statistically significant at all 21 ve ntilatory settings (P<0.05-0.001). The tracheal loop revealed clearly intri nsic PEEP and lowered compliance during overinflation, which was difficult or impossible to see in the Y-piece pressure/volume loop. Conclusion: By measuring airway pressure at the trachea the effect of endot racheal tube resistance during inspiration is excluded while it is included during expiration, yielding correct end-points of inspiration and expirati on. This makes it possible to calculate accurately total compliance of the respiratory system during dynamic conditions. By monitoring of airway press ure in the trachea, respiratory mechanics can be assessed more accurately a nd ventilatory settings adjusted to attenuate ventilator induced lung injur y. (C) Acts Anaesthesiologica Scandinavica 4 (2000).