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 (P_trach),P_trach 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 (P_trach,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 P_trach from P_trach,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 P_trach. The mean maximum deviation of P_trach from P_trach,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 P_trach is thus comparable to the control of airway pressure afforded by the unmodified demand-flow ventilator.