A simplified method for separate measurements of lung and chest wall mechanics in ventilator-treated patients

Background: Airway pressure measurements above the endotracheal tube will b e distorted because of endotracheal tube resistance. To separate lung and c hest wall compliance, esophageal pressure is conventionally measured with a n air-filled balloon catheter, which is difficult insert in unconscious pat ients; We have developed a methodology with fluid-filled catheters for intr atracheal and esophageal pressure measurements. Methods: Twelve anesthetized patients were studied. Tidal volumes were meas ured by side-stream spirometry. Airway pressures were measured at the Y-pie ce and in the trachea with fluid-filled pressure lines. Esophageal pressure was measured via the narrow lumen in a fluid-filled Salem(TM) double-lumen stomach tube, which was slowly retracted from the stomach up through the e sophagus until maximal respiratory pressure readings and minimal cardiac ar tifacts were obtained. Lung mechanics were measured at different tidal volu mes (TV) and positive end-expiratory pressure (PEEP). Results: There was a significant difference between airway pressures at the Y-piece and in the trachea. Total compliance significantly increased with increasing TV and decreased with increasing PEEP. Chest wall compliance inc reased significantly with increasing TV,TV, while lung compliance did not c hange significantly. Two patients showed repeatedly marked increase in lung compliance at one specific ventilatory setting, an increase the proportion of which was not reflected by changes in total compliance. Conclusions: Y-piece pressures are not representative of intratracheal pres sures, which can be measured by inserting a fluid-filled pressure line thro ugh the tube. Esophageal pressure is easily recorded with a fluid-filled Sa lem(TM) double-lumen catheter. Large changes in lung compliance may pass un noticed when only total compliance is monitored. Monitoring of lung complia nce may offer an improved basis for decisions about ventilator settings.