In-Vivo Comparison of Measured Tracheal Pressure and Tracheal Pressure Determined by a Simple Bedside Equation

BACKGROUND: High peak airway pressure (P_aw) has been associated with ventilator-induced lung injury. However, in intubated patients, a marked pressure drop can occur across the endotracheal tube (ETT) thereby making intra-tracheal airway pressures (P_trach) more accurate than peak P_aw as read from the ventilator. We sought to determine whether an equation based on in-vitro values for ETT resistance (R_ett) reported by Gottfried et al could be used at the bedside to accurately estimate P_trach in ventilated critical care patients. METHOD: P_trach was estimated by P_trach = (estimated R_pt)(flow) + P_plateau, (where R_pt is patient resistance determined as respiratory system resistance minus R_ett, and P_plateau is the pressure read from the ventilator manometer after a period of no flow). Estimated P_trach was compared to measured P_trach obtained by using a small-bore catheter placed distal to the tip of the ETT. In 10 patients (Group 1), ventilator peak P_aw, true proximal peak P_aw, P_trach, and P_plateau were measured and averaged over 3 breaths at the patient's set flow. In a second set of 10 patients (Group 2), the same measurements were made at 4 different flows. In both groups bias ± precision was used to compare estimated P_trach and measured P_trach. We defined limits of agreement of ±3.0 cm H₂O as clinically acceptable. In Group 2 patients, ANOVA was used to compare bias and precision of P_trach at 4 different flows. p < 0.05 was considered statistically significant. RESULTS: In Group 1, bias ± precision of P_trach was-8.8±6.87 cm H₂O when ventilator peak P_aw was substituted in the formula but only -2.0±3.5 cm H₂O when proximal peak P_aw was used. In Group 2 patients when ventilator peak P_aw was used, bias ± precision increased with increasing flows, with bias ± precision at 120 L/min (-10.2 ±3.83) significantly different from bias ± precision at 60 L (-1.3±1.58) and 80 L/min (-5.0±0.71). CONCLUSIONS: Estimates of peak tracheal pressure based on in-vitro deter-minations of endotracheal tube resistance may not accurately reflect directly measured values for tracheal pressure.