Just like standard ET tubes, armored or reinforced tubes have the typical left-facing bevel tip and Murphy eye. Their distinctive feature is a metal wire coil embedded in the wall of the tube shaft. The fact that these tubes contain a metal wire coil means there is no need for a radio-opaque line.
Tip of an armored/ reinforced tube with the metal coil clearly visible
In contrast to ‘standard’ ET tubes, the tube connector of armored tubes is firmly fixed to the tube shaft and not detachable.
A further difference is that armored tubes are bendier and therefore not as well preformed as standard tubes. Because of this they are more likely to require a stylet for successful intubation.
Due to the embedded wire coil, armored tubes have a slightly thicker wall. The difference between internal and external diameter is therefore greater compared to regular tubes. However, this difference might be small and clinically insignificant, depending on the manufacturer.
|ET tube size (I.D)||Regular ET tube (O.D.)||Armored ET tube (O.D.)|
Comparing outer diameter (O.D.) sizes between regular and armored ET tubes (Mallinckrodt type tubes by Covidien)
Although armored tubes are less likely to occlude through kinking they are not a substitute for a bite block. Whilst it is significantly more difficult for a patient to totally occlude an armored compared to a regular tube by biting onto it, the armored tube has the disadvantage of having a ‘memory’, meaning it won’t expand back to a round diameter once the bite is released (see picture below).
Picture demonstrating the effect of patient bite
With regards to MRI safety, armored endotracheal tubes are classified as MR-conditional, which means they are generally safe to use in an MRI environment, with no increased risk of clinically significant heating in the magnetic field. Actually, most endotracheal tubes are only classed as MRI-conditional and not MRI-safe, because the pilot balloon usually contains a metal spring-loaded valve. For any medical equipment to be MRI-safe, it must not contain any metal at all. It might still be worth to avoid using armored tubes in the MRI environment as the metal coil can adversely affect picture quality, especially if the scanned area is in close vicinity to the tube, i.e. in c-spine and brain MRIs.
MR safety symbols; a) MR safe, b) MR conditional, c) MR unsafe
Reinforced tubes have certain advantages over standard endotracheal tubes in several situations due to their resistance to occlusion when bent and their overall greater flexibility:
- They can be advantageous in fiberoptic intubation via either the oral or nasal route since they are usually easier to 'railroad' off the scope due to their superior flexibility.
- They might be useful for intubation through a tracheotomy. Again, the greater flexibility of these tubes makes for an easier navigation of the angle between the tracheotomy site at the skin and the trachea.
- Many practitioners prefer using armored tubes in head & neck and neurosurgical cases, i.e. when access to the airway is limited and bulky equipment in front of the patient's mouth and face can get in the way of the surgeon. An armored tube can easily replace an oral RAE tube.
- Reinforced tubes can be useful in patients positioned prone.