PubMed · 6017430
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M Livingston, R Durell. Epiglottic elevator.. https://doi.org/10.1097/00000542-196701000-00026
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Turbulent jets in endotracheal tubes induce air entrainment and airway pressure changes. We attempted to understand the physical explanation for these effects, which open up to a wide range of applications in intubated patients. An in vitro study was performed on standard size endotracheal tubes with diameters of 8, 7, and 3 mm and several capillaries molded into the wall (less than 1 mm diam) allowing gas injection at approximately 1-2 cm from the tracheal end of the endotracheal tube. This produced a jet velocity-dependent gain in tracheal pressure (Ptr) during inspiration. Data have been interpreted with a theory, based on the classic momentum theorem, which indicates that the mechanisms involved resemble those of axisymmetrical confined jets: air entrainment by turbulent friction with a longitudinal increase in lateral pressure. The difference with axisymmetrical systems lies in the nonconservation of the total thrust in our system because, secondary to wall friction and to the nonaxial incidence of the jets, only a fraction of the jet momentum flux is transformed into pressure. This suggests faster mixing in the present lateral jet system, as shown by 1) the independence of Ptr on tracheal geometry and 2) the very rapid increase in lateral pressure. The present study supports the idea that pressure changes in the airways, which are potentially beneficial in intubated patients, can be satisfactorily generated by turbulent jets.
A comparative study was undertaken of the physical and flow characteristics of "streamline" cuffed tracheal tubes (SCT; Medishield) and "normal" cuffed tracheal tubes (NCT) of sizes 6.0, 6.5, and 9.0 mm. The embedding of the pilot tube within the wall of SCT imparts a D-shaped lumen which is more marked in 6.0- and 6.5-mm tubes. As a result, there is a 15% reduction in the lumen of a 6.5-mm SCT. All sizes of SCT studied had a smaller outer diameter compared with the corresponding sizes of NCT and thus occupied less space in the trachea. In sizes 6.0 and 6.5 mm, the SCT was found to have distinct disadvantages, such as increased resistance to ventilation of up to twice that of NCT, and turbulent flow even at a minute volume of 5 litre min-1. SCT, size 9.0 mm, however offered marginally less resistance compared with NCT of the same size.
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