ACOUSTIC METHOD TO ESTIMATE THE LONGITUDINAL AREA PROFILE OF ENDOTRACHEAL-TUBES

A problem in mechanical ventilation is the accumulation of mucus secre tions in the endotracheal tube (ETT), which tends to reduce the patent cross-sectional area. Here we characterized the extent and locus of t he ETT obstruction using an acoustic reflection method recently modifi ed to be applied at bedside. Experiments were conducted both in vivo i n 10 intubated patients and in vitro in ETT with or without known cons trictions of 1 to 3 mm over 5 cm, located at various distances from th e ETT entry: 5, 10, 15, and 20 cm. Acoustic results were compared with the results obtained by an hydraulic reference method, which was the only method available to measure ETT obstruction in mechanically venti lated patients. In vivo acoustic results showed that area reductions w ere maximal near the tracheal extremity of the ETT, with a range from 2 to 36% (mean value 13 +/- 10%), when estimated relative to the area measured in an unused ETT of the same inner diameter (7 to 9 mm). Stat istical analysis of the differences between acoustic reflection data a nd hydraulic data showed that the two methods did not differ significa ntly. In vitro acoustic results obtained in constricted ETT showed a h ighly significant correlation with the actual area (r = 0.97, p = 0.00 01). Thus, reductions in ETT area may be detected, quantified, and loc ated by the present acoustic reflection method, which therefore provid es a means to avoid emergency extubation because of ETT obstruction.