Head position and oral vs nasal route as factors determining endotracheal tube resistance

We performed this in vitro study to determine the following: (1) if there is any significant difference in resistance between comparably sized endotracheal tubes (ETTs) in simulated anatomic oral and nasal conformations: (2) if neck flexion would increase the resistance of the ETT; and (3) if a wire-reinforced tube in simulated oral conformation would minimize the resistance increase at bends in the tube.

The pressure drops (the change in pressure the flow through the ETT) at the proximal end of three sizes of tubes (6-, 7-, and 8-mm inner diameter) were measured in anatomic conformations at flows ranging from 20 to 100 L/min with the tubes warmed to 37 degrees C.

There were no significant differences in pressure drops between comparably sized ETTs in the nasal vs oral conformation at any flow tested. Maximal head flexion produced a small increase in pressure drop for the standard 6-mm ETT but not for the larger tubes. The wire-reinforced tubes, when compared with the standard 7- and 8-mm ETTS, actually had greater pressure drops across the tubes at high flows.

(1) Pressure differences between ETTs in nasal and oral conformations with comparable size and length are insignificant, even at high flow rates. (2) Maximal neck flexion does not deform the tube enough to make a clinically significant difference in resistance in vitro, although the 6-mm ETT had a slightly higher resistance in flexion. (3) A size 7- or 8-mm reinforced ETT has a higher resistance compared with a standard ETT.