Measurement of total respiratory impedance via the endotracheal tube; a model study.

The feasibility of applying the forced oscillation technique to determine the impedance of the respiratory system in intubated subjects was investigated on a model. This showed that the losses in kinetic energy occurring at the end of the endotracheal tube are not important when the flow is oscillatory (frequencies of 2 Hz and higher). Also, if the respiratory system is sufficiently linear, the alinearity of the tube can be dealt with if its mechanical characteristics, as well as the size of the oscillatory flow, are known. The tube impedance at that flow can then be subtracted from the total impedance of the tube connected to the respiratory system. When the driving signal is a complex wave containing several frequencies, the impedance of the tube can still be determined (and thus subtracted) by performing the measurements at various amplitudes of the driving flow and by extrapolating the resistance values (corresponding to those amplitudes) to zero flow. The latter is possible only if the measurements are performed during an apnea. To avoid these complexities, pressure can be measured at the outlet, rather than at the entrance of the tube. Accurate measurements of respiratory impedance can thus be obtained, whether the flow is determined at the entrance or outlet of the tube, even with a complex driving signal.