Endotracheal tubes are usually made from silicone, polyvinyl chloride (PVC) plastic or red rubber. The silicone tubes are
the most expensive of the three materials, but the silicone resists deterioration better than PVC or red rubber. PVC plastic
tubes are much stiffer than the silicone tubes. Silicone tubes easily bend to conform to the trachea while PVC tubes are manufactured
with a curve already in them. Once the PVC tubes have been in the trachea for a while, the plastic softens and becomes more
pliable. The curve can be an advantage or a disadvantage. When intubating with a PVC plastic tube it's important to follow
the patient's natural anatomy in regards to the curve so that the tube doesn't get dragged along the tracheal wall and cause
damage. The stiffness of the plastic tubes sometimes aids in intubation because they are strong enough to push redundant tissue
or other structures gently out of the way. The curve can facilitate visibility of the larynx. Silicone tubes are softer and
therefore don't tend to cause damage to the tracheal wall unless the cuff is overinflated or the tube moves in the trachea
during certain procedures. Sometimes, especially with smaller silicone tubes, a stylet is needed to stiffen the tube because
they can be extremely flexible.
Red rubber endotracheal tubes have fallen out of favor in veterinary medicine because the opague lumen makes it impossible
to detect obstructions within the tube from debris or mucous. The rubber on these tubes breaks down very quickly and causes
cracks that are impossible to clean and disinfect.
Table 1
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Endotracheal tube lengths are measured in centimeters from the distal end (patient end) to the adaptor end. On silicone and
PVC tubes, these measurements are listed along the outside of the tube. These measurements are universal among tubes so that
one kind of tube should have the same exact measurements as a different brand of tube. This is useful when measuring the depth
of a tube once it is already in the trachea. A second tube can be measured on the outside of the pet, tracing the airway and
lining up the measurements with the visible ones on the inserted tube. From this the position of the distal end of the tube
can be determined. Tubes that are placed beyond the cervical trachea and thoracic inlet risk being placed into one of the
main stem bronchi. This is endobronchial intubation and it can have detrimental side effects. The unintubated lung can not
contribute to ventilation which can cause hypoxia and cyanosis. Most endotracheal tubes list both the internal and outer diameters
of the tube. These are listed as I.D. and O.D millimeters on the outside of the tube. The actual tube size is determined by
the I.D. or internal diameter. Some silicone tubes also come in French sizes. Equivalent sizes are listed in Table 1.
There are three common types of endotracheal tubes used in veterinary medicine. Murphy tubes, Magill tubes and Cole tubes.
Murphy tubes are the most common and have an oval opening across from the bevel of the tube called the Murphy eye that acts
as an emergency opening if the distal end of the lumen of the tube becomes clogged with mucous, excessive saliva, blood or
debris. Magill tubes do not have the Murphy eye but are otherwise very similar to Murphy tubes. Cole tubes have a smaller
diameter lumen at the distal end of the tube compared to the proximal end of the tube. This smaller lumen is the only part
of the tube that should actually fit into the trachea. The tube is designed with this "shoulder" slope so that this change
in diameter actually seals the airway at the laryngotracheal opening. Cole tubes are never cuffed. Cole tubes are the best
choice for intubating avians because they have complete tracheal rings that could potentially be ruptured by inflating a cuff.
The Cole tubes are also useful for intubating other species such as reptiles and for intubating tiny pediatric patients whose
delicate tracheas would not tolerate cuff inflation.
Many Magill and Murphy endotracheal tubes come with an inflatable cuff. The purpose of the cuff is to provide a seal between
the tracheal mucosa and the cuff wall so that saliva, blood, and other debris cannot be aspirated. The seal also protects
the staff from potential waste gas exposure.
Although cuffed endotracheal tubes are overall the safest tubes to use for both patient and veterinary staff, these are by
no means benign pieces of equipment. Two types of cuffs have historically been used on endotracheal tubes. The traditional
high pressure, low volume cuff operates by distending a rubber balloon around the tip of the tube, such as with the old red
rubber tubes. When the cuff is deflated it lies flat against the tube. As the name implies, these cuffs require a low volume
of air to establish a seal in the trachea. The disadvantage of these cuffs is that they only contact a small surface area
of the trachea and thus can exert high pressure along a small part of the tracheal mucosa. Most of the silicone tubes have
this type of cuff. When this type of cuff remains inflated in the trachea for an extended period of time, tracheitis, rupture
of the tracheal wall or pressure necrosis can occur. This can lead to tracheal strictures. Over inflation of this type of
cuff can actually cause the tube lumen to collapse inwardly, causing obstruction of air. To protect the trachea from trauma
from the endotracheal tube and cuff it is highly recommended that patients are disconnected from the breathing circuit whenever
a change in recumbency is needed. This is true any time a patient is moved, but extra care should be taken on dental cases
where head and neck movement and recumbency changes take place frequently.
The more modern cuffs are called high volume, low pressure cuffs. These cuffs require more air to provide a seal, but the
pressure exerted on the tracheal wall is distributed over a wider surface area so that when the cuff is inflated properly,
and the correct sized tube is chosen there is less damage to the tracheal mucosa. That being said, it is also possible to
over inflate these cuffs and the side effects and damage can be the same as listed above for the low volume, high pressure
cuffs. These cuffs are seen on most of the PVC tubes used in medicine today. When these cuffs are deflated they do not lie
flat against the tube but "wrinkle" up, especially on the PVC type tubes. Occasionally the cumbersome, yet safer, cuffs can
occlude visualization of the larynx during intubation of narrow mouthed patients. If this is the case, use of a guide wire
is recommended.
The procedure for properly inflating an endotracheal tube cuff to avoid overinflation is ideally done with two people because
cuff inflation is a two handed job. One person can do it as long as they make opening the pop-off valve at the end of the
procedure a priority. To inflate the cuff, a syringe with air in it needs to be attached to the cuff inflation valve. For
small tubes and cats, a 3ml syringe should be sufficient. In bigger tubes and dogs a 6 to 10 ml syringe can be used. The pop-off
valve needs to be closed, either by the assistant holding down the pop-off button (ideal for safety reasons) or by closing
the pop-off valve and the reservoir bag should be squeezed. While the bag is being squeezed (being careful not to exceed 20
cm H2O peak inspiratory pressure) the anesthetist should place their ear by the mouth of the patient and listen for leaks. If leaks
are heard (usually a hissing sound) during the inspiratory phase of the given breath air should be injected into the cuff.
Only enough air should be injected to stop the leak sound. If two people are cuff checking, it is customary for the bag squeezer
to say "breathing" during the bag squeeze and "release" during relaxation of the bag. This is because it is normal to hear
air escaping the lungs back through the tube during the exhalation phase of positive pressure ventilation. This exhaled air
should not be confused with a leak. If the leak is sealed, the rebreathing bag will remain full and firm while being squeezed.
If there is still a leak, the bag will empty as air rushes past the tube into the oral cavity. If the inhalant anesthetic
is on it will be easy to smell the waste gas. Remember that the cuff valve is a two way valve. A common mistake when inflating
the cuff is for the anesthetist to push air into the cuff and then let go of the syringe plunger. If the syringe plunger is
not held in place while attached to the valve, all of the air that was pushed in will escape back into the syringe once the
plunger is released. Once the cuff is properly inflated the syringe must be disconnected from the valve so that the air stays
in place within the cuff. The anesthetist should note how much air was used to seal the cuff. The amount of air should not
be excessive. If the cuff inflation is not going well and a seal cannot be formed with a reasonable amount of air, the tube
should be inspected for proper placement and if it is in the trachea, it should be removed, replaced and inspected for holes
in the cuff later. Holes in the cuff can be found by inflating the cuff and submerging the cuffed end of the tube into a bowl
of water. Holes will be apparent by the formation of air bubbles in the water. The tube should be discarded if it has a leaky
cuff. Larger silicone tube cuffs can be replaced. For silicone tubes sized 11 and up (definitely equine sized tubes) it may
be more cost effective to replace the cuff. Cuff replacement kits are available from Surgi-Vet and the repairs are relatively
easy to do. Some pilot lines can be replaced as well.
