Measurement of respiratory impedance by Impulse Oscillometry - effects of endotracheal tubes

Impulse Oscillometry is a new, noninvasive method to measure respiratory im pedance, i.e. airway resistance and reactance at different oscillation freq uencies. These parameters are potentially useful for the monitoring of resp iratory mechanics in the critically ill patent with respiratory dysfunction . The endotracheal tube, used to mechanically Ventilate these patients, how ever? represents an additional nonlinear impedance that introduces artifact s into the measurements. The objective of this work was therefore to invest igate the effects of clinically available endotracheal tubes on resistance and reactance of an in vitro analogue of the respiratory system. Additional ly, the effects of decreasing the compressible gas volume in this experimen tal model, as a simulation of decreased lung capacity and compliance, was i nvestigated. Impulse oscillometric measurements of the test analogue gave h ighly reproducible results with and without an endotracheal tube. The tubes had significant influence on the measurement of the test object at all fre quencies investigated. Changes of low frequent reactance were negligible - at least if repetitive measurements of the same system are pet-formed - for realistic measurement of airway resistance, a correction of the tube imped ance or measurement of the pressure distal of the tube is required. Resista nce increased and low frequent reactance decreased significantly with decre asing gas volume. These changes were of magnitudes higher than the variatio ns due to the introduction of the endotracheal tubes. Our results suggest t hat changes of respiratory reactance measured with impulse oscillometry may be used as a monitoring parameter in intubated patients.