Capture that airway: Everything you need to know about endotracheal tubes and difficult intubations (Proceedings)

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.

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.

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.

Guarded, armored or wire reinforced tubes are Magill or Murphy tubes that come with wire embedded within the walls of the tube. This helps prevent kinking of the tube in the airway when the head and neck are flexed. They are useful for procedures such as ophthalmic surgeries, oral surgeries, neurosurgeries involving the head and neck or for cervical myelograms. These tubes should be used during any procedure where patient positioning compromises the airway. Guarded tubes are quite expensive and so the use of a "bite guard" is recommended. Bite guards protect the tube from punctures from the teeth of awakening patients. It is also more cost effective to repair the damaged cuffs of these tubes than to discard them, regardless of size.

Successful intubation depends largely on the positioning of the patient during the procedure. For this reason, if an assistant is used to position the pet for intubation, it is extremely important that the patients head and neck are maintained in a straight line to facilitate visualization of the larynx. The best way to do this is to have the assistant place her thumb and forefinger just behind the canine teeth on the maxilla and pull the head up and out. The assistant's fingers should remain outside of the oral cavity because chewing reflexes can result in a puncture wound otherwise. The lips of the patient should be held back and out of the field of vision. The tongue should be pulled out of the mouth (either by grasping the tongue with a gauze sponge or if the patient is light, using a laryngoscope to push it out to the side) and over the lower incisors. The mouth should be opened as wide as possible. A laryngoscope can facilitate visualization because of its light and it can also act as a tool to push redundant tissue out of the way or to free up an epiglottis that is trapped behind the soft palate. This can be done by pressing ventrally on the base of the tongue. The laryngoscope should not ever touch the glottis as this can cause laryngospasm. Intubation can be achieved by one person if they have a strong overhead light source and are somewhat handy with their non-dominant hand. The canine tooth can be hooked with the index finger and the tongue held with the middle finger and thumb of the non-dominant hand. Intubation is achieved with the dominant hand. One person intubation tends to be more traumatic on the patient's tongue and is generally much easier and faster with two people as long as the assistant has some experience.

Choosing the appropriate sized tube for a patient can be done a couple of different ways. With experience choosing an endotracheal tube becomes almost automatic depending on the breed and weight of the patient. But breeds of dogs can come in many different sizes and it's easy to over or under estimate sometimes. One way to choose a size is to first palpate the trachea to get a feel for the size of it. Another way of measuring for tube size is to measure the space between the pet's nares with the approximated tube, bevel end. If the tube diameter fits between the nares pretty closely, then the tube chosen is thought to be the right size. This method involves excessive handling of the ET tubes which can cause contamination. Ideally, since the ET tube is being placed directly into the tracheal mucosa which is highly susceptible to infection, it is best to limit handling of the tubes, especially the patient end of the tube. For some reason, many people feel the need to squeeze the cuff balloon when it is inflated. This should be discouraged, again because of the potential for contamination. The only way to get proficient at choosing ET tubes for pets is to do it often and to look at a lot of tracheas. Ideally, when setting up for a procedure, 3 tubes should be set up on the induction tray: the tube size that you think you will use and one a size smaller and one a size bigger. Brachycephalic breeds can be very deceiving and usually require a much smaller tube than one would normally anticipate. A thirty kg Bulldog for instance, may only fit a 6 mm tube. Tubes should also be assessed for length. Endotracheal tubes should not extend beyond the thoracic inlet and the proximal end should be right at the patient's incisors. Extra long tubes add to dead space and also risk being pushed too far into the trachea (endobronchial intubation). All three tubes should have their cuffs inflated and left inflated for a while to check for leaks. Prior to induction, all tubes should be deflated and ready to use.

Difficult intubations can be those patients that have oral masses or other pathology that prevents good positioning during intubation. Extremely small, neonatal patients may be difficult to intubate due to their size. Any time a difficult intubation is suspected, the patient should be preoxygenated for at least 5 minutes to help "buy some time" for the intubation. Many times the laryngoscope can be very helpful during difficult intubations. Patients with small oral openings and much distance from the incisors to the larynx (such as small ruminants) or patients whose jaws cannot be opened very wide can often be intubated with the help of a relatively long and narrow laryngoscope blade. Again, positioning is key as well. If a difficult intubation is expected an experienced assistant can make all the difference in the world. They understand the importance of proper positioning and can understand your directives regarding small positioning changes needed to improve visualization. With these patients it is often possible to get the narrow blade through the molars and down at the base of the tongue by going through the side of the mouth. Using the laryngoscope to push down gently at the base of the tongue often reveals the arytenoids and larynx. Sometimes it takes a few seconds to get your bearings in the oral cavity. If placing the endotracheal tube within the oral cavity blocks the view completely, use of a guide wire or tube is recommended. A guide wire is a long, somewhat sturdy tube that can be placed through the endotracheal tube. Polypropylene urinary catheters size 8-14 french can be used. Usually a small gage wire is placed for stability into the catheter and then two catheters are firmly secured together. Once this guide wire is placed through the ET tube, the ET tube can remain out of the oral cavity while the guide wire is placed well in between the arytenoids. The guide wire is small enough that it does not impede vision of the oral structures. Once placement is confirmed, the guide wire is held in place as the ET tube is slid over it and down into the trachea. The guide wire is immediately pulled out of the ET tube and the patient hooked up to oxygen inhalant and the cuff properly inflated. Ideally visualization of the tube between the arytenoids will confirm placement of the ET tube but in cases where it is difficult to visualize proper placement can be determined by watching the patient's chest and the reservoir bag move simultaneously, feeling breath come out of the end of the ET tube, seeing "fog" inside the tube, using the bottom of the silver laryngoscope to check for fog, and by holding a tuft of hair in front of the proximal end of the tube and watching the hair move with breaths. Most of these methods are not fool proof however. Air can move out of the stomach and mimic breaths with these methods. Besides visualization, the absolute best way to confirm placement of the ET tube in the trachea is to use capnography. The presence of CO2 will confirm tracheal placement.

For patients that have a completely obstructed view of the glottis due to masses or tumors in the oral cavity or airway, or due to severe trauma perhaps, the retrograde method of intubation can be used. This involves placing a hypodermic needle through the second and third tracheal rings. A guide wire can then be passed through the needle and maneuvered into the oral cavity. The ET tube can then be placed over the guide wire and passed down into the trachea. The cuff on the ET tube should be placed lower than the puncture site by the hypodermic needle so that subcutaneous emphysema or pneumothorax does not occur.

References

Dorsch JA, Dorsch SE. 2008. "Tracheal tubes and associated equipment" In Understanding Anesthesia Equipment, 5th ed, edited by Dorsch JA, Dorsch SE, pp. 563-584. Philadelphia: Lippincott Williams & Wilkins.

Hartsfield SM. 2007. "Airway management and ventilation" In Lumb and Jones' Veterinary Anesthesia and Analgesia, 4th ed, edited by Tranquilli WJ, Thurmon JC, Grimm KA, pp. 495-512. Ames: Blackwell Publishing.

Mitchell SL, McCarthy R, Rudloff E, Pernell RT. 2000. Tracheal rupture associated with intubation in cats: 20 cases (1996-1998). J Am Vet Med Assoc 216(10):1592-1595.