CONTINUOUS CALCULATION OF INTRATRACHEAL PRESSURE IN TRACHEALLY INTUBATED PATIENTS

Background: Intratracheal pressure (P(trach)) should be the basis for analysis of lung mechanics. If measured at all, P(trach) is usually as sessed by introducing a catheter into the trachea via the lumen of the endotracheal tube (ETT). The authors propose a computer-assisted meth od for calculating P(trach) on a point-by-point basis by subtracting t he flow-dependent pressure drop DELTAP(ETT)(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 labor atory. The coefficients of an approximation equation were fitted to th e measured pressure-flow curves separately for inspiration and expirat ion. In 15 tracheally intubated patients under volume-controlled venti lation and spontaneous breathing, the calculated P(trach) was compared with the measured P(trach). Results. The authors present the coeffici ents of the ''nonlinear approximation'': DELTAP(ETT) = K1 . V(K2), wit h DELTAP(ETT) being the pressure drop across the ETT and K1 and K2 bei ng the coefficients relating V to DELTAP(ETT). An important result was an inspiration/expiration asymmetry: the pressure drop caused by the inspiratory flow exceeds that of the expiratory flow. A complete descr iption of the pressure-flow relationship of an ETT, therefore, require s a set of four coefficients: K1I, K2I, K1E, and K2E. The reason for t his asymmetry is the abrupt sectional change between ETT and trachea a nd the asymmetric shape of the swivel connector. Comparison of calcula ted and measured P(trach) in patients gives a correspondence within +/ - 1 cmH2O (mean limits of agreement). The mean root-meansquare (rms) d eviation is 0.55 cmH2O. Conclusions: P(trach) can be monitored by comb ining our ETT coefficients and the flow and airway pressure continuous ly measured at the proximal end of the ETT.