Microlaryngoscopy tube (MLT®)
A microlaryngoscopy tube (also generally called 'MLT', although this is a registered trademark) is in essence a pediatric-sized standard ETT with an adult length or an adult standard ETT with a pediatric-sized diameter, whichever way you want to look at it.
MLT' also
stands for 'massively long tube'!
The MLT®
comes in three sizes, 4.0, 5.0 and 6.0mm. The size 5.0mm tube is the most
commonly used.
Comparing
a 5.0 MLT® and a 5.0 'standard' pediatric endotracheal tube side by side, two
other differences apart from the obvious one in length noticeable:
Different
cuff size/ length: The MLT® has a cuff size/ length that would be typical for
an adult-sized 'standard' ETT. A 'standard' pediatric 5.0 enndotracheal tube
has a smaller cuff made for a pediatric-sized trachea (see picture below).
Distance
of cuff from tube tip: In an MLT® the cuff is further away from the tube tip
which is acceptable as the adult trachea is obviously longer than the pediatric
one (see picture below).
For
pediatric ET tubes the length of the cuff and the distance of the cuff from the
tip matter, because tracheas of pediatric patients are shorter, so the risk of
accidentally intubating a main bronchus is higher.
The size
5.0 MLT® will just about accomodate an adult stylet, the 4.0 one won't.
USES
Microlaryngoscopy
tubes are used, as the name suggests, in laryngeal surgery. Particularly in
vocal chord surgery, the smaller diameter of these tubes allows the surgeon an
easier view and access to the vocal chords and adjacent structures.
SPECIAL CONSIDERATIONS
One thing
to keep in mind when using ETTs of such small size in adults is the relatively
high resistance to gas flow through these tubes, meaning that comparatively
high airway pressures are required to deliver the desired tidal volumes. Airway
pressure is significantly higher proximal to the tube where it is measures than
in the alveoli. Similarly, gas flow is slow during expiration, meaning that
lower I:E ratios are often necessary to allow for complete expiration.
