Automatic compensation of endotracheal tube resistance in spontaneously breathing patients.

The considerable additional ventilatory work needed to overcome the resistance of the endotracheal tube (ETT) is flow-dependent. In spontaneously breathing intubated patients this additional ventilatory work is therefore dependent on the flow pattern and cannot be adequately compensated for by support with a constant pressure. We propose a method to fully compensate for the ETT resistance during inspiration and expiration by regulating tracheal pressure (Ptrach),Ptrach is calculated at a rate of 500 Hz by measurement of flow and pressure at the outer end of the ETT and from coefficients describing the flow-dependent ETT resistance. The calculated tracheal pressure is fed into a modified demand-flow ventilator which can then control tracheal pressure to a target value (Ptrach,targ). Tracheal pressure can either be kept constant (automatic tube compensation, ATC), or changed in any chosen fashion. We tested our system on a laboratory lung model simulating a spontaneously breathing patient. Even under the simulation of extreme conditions the maximum deviation of Ptrach from Ptrach,targ was smaller than 2.5 mbar. We evaluated our system in 10 spontaneously breathing intubated patients breathing at ATC with or without volume proportional pressure support (VPPS) by measuring Ptrach. The mean maximum deviation of Ptrach from Ptrach,targ was 2.9 mbar. The rms-deviation was 1.1 mbar (inspiration and expiration considered) and 1.7 mbar (inspiration alone). The accuracy of the control of Ptrach is thus comparable to the control of airway pressure afforded by the unmodified demand-flow ventilator.