Direct tracheal airway pressure measurements are essential for safe and accurate dynamic monitoring of respiratory mechanics. A laboratory study

Background: All monitoring of respiratory mechanics should depend on trache al pressures (Trach-P) as endotracheal tube resistance (ETT-Res) will other wise distort them. The aim of this study was to investigate factors that ma y vary ETT-Res, causing difficulties in ETT-Res estimation clinically, and to evaluate a method for direct Trach-P measurements to obviate these probl ems. Methods: In a model we studied: 1) The influence on ETT-Res caused by diffe rent connectors and secretions; 2) Direct Trach-P measurements with a cathe ter (o.d. 2 mm, i.d. 0.9 mm) with either end or side hole, filled with eith er air or liquid, introduced through the ETT lumen and evaluated regarding response time and position. Results: The pressure drop between trachea and V-piece increased by 15% whe n respectively a swivel connector and a humidification device were connecte d to the ETT. When injecting 1 mi and 2 mi gel into the ETT lumen the inspi ratory resistance increased 100% and 600% respectively. The response time o f all catheters was less than or equal to 12 ms. During constant flow in in spiratory and expiratory directions the pressure difference between an end hole catheter positioned from 2 cm above the ETT tip to 4 cm below and a re ference pressure in the artificial trachea was less than 1.5 cmH(2)O. Conclusions: ETT connections and secretions cause a variance in resistance. Tracheal pressure can be measured with high precision with an air- or liqu id-filled catheter. An end hole catheter placed within 2 cm above or below the ETT tip will give sufficiently precise measurements for clinical purpos es.