Background: Intratracheal pressure (Ptrach) should be the basis for analysis of lung mechanics. If measured at all, Ptrach is usually assessed by introducing a catheter into the trachea via the lumen of the endotracheal tube (ETT). The authors propose a computer-assisted method for calculating Ptrach on a point-by-point basis by subtracting the flow-dependent pressure drop delta PETT(V) across the ETT from the airway pressure (P(aw)), continuously measured at the proximal end of the ETT.
Methods: The authors measured the pressure-flow relationship of adult endotracheal tubes with different diameters (ID, 7-9 mm) at different lengths and of tracheostomy tubes (ID, 8-10 mm) in the laboratory. The coefficients of an approximation equation were fitted to the measured pressure-flow curves separately for inspiration and expiration. In 15 tracheally intubated patients under volume-controlled ventilation and spontaneous breathing, the calculated Ptrach was compared with the measured Ptrach.
Results: The authors present the coefficients of the "nonlinear approximation": delta PETT = K1.VK2, with delta PETT being the pressure drop across the ETT and K1 and K2 being the coefficients relating V to delta PETT. An important result was an inspiration/expiration asymmetry: the pressure drop caused by the inspiratory flow exceeds that of the expiratory flow. A complete description of the pressure-flow relationship of an ETT, therefore, requires a set of four coefficients: K1I, K2I, K1E, and K2E. The reason for this asymmetry is the abrupt sectional change between ETT and trachea and the asymmetric shape of the swivel connector. Comparison of calculated and measured Ptrach in patients gives a correspondence within +/- 1 cmH2O (mean limits of agreement). The mean root-mean-square (rms) deviation is 0.55 cmH2O.
Conclusions: Ptrach can be monitored by combining our ETT coefficients and the flow and airway pressure continuously measured at the proximal end of the ETT.