[Acoustic reflection position monitoring of tracheal cannulae. A contribution to quality assurance?].

Non-invasive acoustic airway monitoring was evaluated in an experimental study. Recording amplitude and travel time of acoustic pulse response, an area-distance function of the cross sectional dimensions of the endotracheal tube and the adjacent airway was calculated to obtain an acoustic pattern of the airway. Measurements on models and excised human cadaver lungs were performed to discover whether displacement or obstruction of the artificial airway can be detected in the acoustic equivalent. Regression analysis revealed a close correlation between displacement of tracheostomy tubes and the shifting of the acoustic area-distance function (corr. coeff. 0.97-1). Dispersion analysis confirmed reasonable reliability (coeff. of variation 0.6-2.1%). Location and amount of obstruction could likewise be identified. Thus acoustic mapping provides an adequate approximation of the true geometry of tracheostomy and endotracheal tubes. We conclude that acoustic monitoring may provide a powerful tool to achieve primary prevention of airway disturbances in intubated and mechanically ventilated patients, as geometrical changes of airway configuration can be detected even before they cause substantial effects on respiratory metabolism.