Veterinary nurses (VNs) hold a wide and varied skillset in order to provide the best possible care to their patients. VNs regularly assist with anaesthesia and have many skills in this area, including assisting with the induction of anaesthesia by incremental intravenous agents. Intubation of the trachea is routinely performed immediately after this and because of the positioning of the veterinary surgeon (VS), the VS often performs this skill. For this reason, the skill of endotracheal intubation can be under-developed, or minimally practiced by VNs even though they are legally allowed to perform this skill in the UK. Endotracheal intubation may also need to be performed in an emergency situation, where a VS is not present, as it is a key step of providing cardiopulmonary resuscitation (CPR) and so VNs should be confident to perform this skill (Fletcher et al, 2012). As the advanced VN role develops in the UK, it may be common for VNs to be more involved with anaesthesia induction and the skill of endotracheal intubation may be more regularly required. In some states in the USA veterinary technicians already have a greater role in anaesthesia induction: indeed, nurses must master the skill of endotracheal intubation in order to become credentialed as a Veterinary Technician Specialist (VTS) in Anaesthesia and Analgesia (AVTAA, 2022).
Purpose of intubation
Intubation of the trachea is one of many ways of ensuring the provision of oxygen to the respiratory system but is more usually used primarily to ‘secure the airway’; meaning to prevent foreign material from entering the trachea and the lower airways. Normal physiology prevents this occurrence through swallowing and the gag reflex.
Intubation to secure the airway is used where there is a disruption or depression of normal physiology. It is not only performed during anaesthesia, but also in patients that are deeply sedated, unconscious, or have neurological impairments affecting the swallow and gag reflexes.
Endotracheal intubation has a secondary function during general anaesthesia of allowing complete removal of waste anaesthesia gases (WAG) from the immediate environment, and a third function of facilitating positive pressure ventilation.
The fourth function of endotracheal intubation is to allow closer respiratory monitoring. Capnography, spirometry or gas analysis can be attached to the distal end of an endotracheal tube and inspired and expired carbon dioxide, oxygen and inhalant anaesthetic agents can be measured accurately, as can the measurement of airway pressure and compliance.
Comparisons of the function of endotracheal intubation versus laryngeal mask airway and use of a facemask are shown in Table 1.
Table 1. Comparison of endotracheal tubes, laryngeal mask devices and facemasks
| Cuffed endotracheal tube | Laryngeal mask or supraglottic airway devices | Face mask | |
|---|---|---|---|
| Increases fractional inspired oxygen percentage (FiO2) | Yes | Yes | Yes |
| Secures the airway | Yes | Yes (Prasse et al, 2016) | No |
| Allows for scavenging of waste anaesthetic gases | Yes | Yes | No |
| Allows positive pressure ventilation | Yes | Yes (Prasse et al, 2016) | No |
| Allows capnography readings | Yes | Yes | No |
| Likelihood of displacement | Low | Medium | High |
| Risk of tracheal damage | Medium | None | None |
Anatomy and physiology review
In order to master a clinical skill a VN should have an understanding of the associated anatomy and physiology.
The oral cavity is divided from the nasal cavity by the hard palate. Within the oral cavity the tongue is a large muscle that is responsible for moving food to the back of the throat in a co-ordinated and controlled manner. Consequently, diseases affecting the tongue can also compromise the patient's airway.
Caudal to the hard palate the soft palate divides the oropharynx from the nasopharynx and caudal to the soft palate is the pharynx (or common pharyngeal area). At the junction of the pharynx the lymphatic tonsils are located, which can become enlarged, reducing visualisation during intubation.
The epiglottis is located at the base of the tongue; this triangular cartilaginous structure is part of the larynx. Its relaxed position is laying rostro-ventral to the entrance to the trachea — the glottis. In the act of swallowing the epiglottis moves dorsal-caudally to cover the glottis and prevent swallowed material entering the trachea (Aspinall and Cappello, 2020).
The soft palate should terminate rostrally to the epiglottis, these two structures are not normally impinging in dogs and cats, but an over-long soft palate can catch on the epiglottis keeping it from returning ventrally after swallowing.
The oesophageal entrance lies just dorsal to the larynx and its proximity is cause for accidental oesophageal intubation.
The larynx is a group of cartilaginous structures that essentially form a box-like swing, hanging the glottis caudal to the pharynx and allowing it to move during swallowing (Aspinall and Cappello 2020).
Laryngeal cartilages move laterally with each breath to increase airflow into the trachea. Paralysis of the nerves enervating these structures reduces airflow, which can result in respiratory distress (Burbidge, 1995).
In dogs and cats the trachea is made from incomplete C-shaped cartilaginous rings, which allow the oesophagus to expand around it as food passes. The trachea is lined with ciliated mucosa that is easily bruised, necrosed, or ruptured by the endotracheal tube causing compression or abrasions (Mitchell et al, 2000; Alderson et al, 2006). Figure 1 shows the main anatomy associated with endotracheal intubation.
Equipment
Essential items of equipment for safe endotracheal intubation are an endotracheal tube (ETT), some suitable material to secure the tube (known commonly as an ‘ETT tie’), a laryngoscope, sterile water-based lubrication and local anaesthesia spray.
ETTs share many identical features, although they can be made from different materials. The distal tip of an ETT is bevelled, allowing added accuracy when positioning the tube. The proximal end attaches to a standard 15 mm connector, allowing universal attachment to breathing systems and other ventilation apparatus, such as AMBU bags. Most ETTs are formed into a curved shape, to mimic the angle of the trachea. Silicone tubes are too flexible to maintain a curve, but the use of a wire stylet will increase rigidity.
‘Cuffed’ ETTs have an inflatable balloon a few centimetres proximal to the distal tip that are used to create a secure seal of the airways. The cuff is connected via a separate lumen to a satellite-, or pilot-, balloon that reaches past the distal end of the ETT. Air can be injected into the cuff, via the satellite with a standard tip syringe. The purpose of the satellite is to give a tactile and visual indicator of the amount of air in the cuff. ETT cuffs can be either ‘low pressure–high volume’ or ‘high pressure–low volume’. The former is less likely to create a high-pressure point on the tracheal mucosa (leading to necrosis), while the latter creates a more trustworthy seal as low pressure-high volume ETTs can create channels that allow fluid to pass (Hughes, 2016). High pressure–low volume cuffs also have less bulk on the outer surface of the ETT, potentially improving visualisation (Mosley, 2015). The argument of which type is safer is multifactorial and should be decided based on the risk/benefit of a patient potentially aspirating fluid when the tube is in situ versus likelihood of tracheal damage from over inflation of the cuff.
Additional features of some ETTs include a Murphy eye and a radiopaque line. The Murphy eye is to reduce the risk of blockage if the distal tip sits against the side of the tracheal wall (Al-Shaikh and Stacey, 2013). Figure 2 shows a labelled endotracheal tube.
When ETTs are reused, they should be manually cleaned of all organic material, before being disinfected, rinsed thoroughly and dried. ETTs should be stored in a closed container to prevent them becoming contaminated or blocked with foreign debris (AORN, 2005). A comparison of different ETTs is shown in Table 2.
Table 2. Comparison of endotracheal tube (ETT) materials
| Red Rubber | PVC | Silicon | |
|---|---|---|---|
| Durability | Perish over time | Disposable | Reusable |
| Repairable | No | No | Yes |
| Available Sizes | 2–15 mm ID | 2–11 mm ID | 2–30 mm ID |
| Autoclavable | Yes | No | Yes |
| Mould to shape when used | No | Yes | Yes |
| Withstands kinks | No | Better than rubber | Better than rubber |
| Irritant | Yes | No | No |
| Expense | ++ | + | +++ |
| Blockages visible | No | Yes | Yes |
| Pre-formed curve | Yes | Yes | No |
| Ease of insertion | Easy | Easy | May need stylet |
| Self-sealing pilot balloon | No | Yes | Yes |
| Types of cuff available | LV/HP — prone to herniation | HV/LP or LV/HP | LV/HP |
(adapted from Hughes, 2016)
Laryngoscopes are battery powered, hand-held devices for examination of the oropharynx, larynx and surrounding structures. They comprise two parts: a handle and a blade. The handle contains the batteries and comes in standard or narrow widths. There are two international standards for fitting blades to handles, generally shown as red or green coloured dots (Al-Shaikh and Stacey, 2013). The light source of a laryngoscope is a small bulb and it may be sited within the handle or the blade. Bulbs in the handle tend to be brighter LED, and light is fed to the blade via fibreoptics. There are many styles of laryngoscope blades available, but the Miller and the Macintosh are most common in veterinary medicine, with the former usually preferred for dogs and cats. It is worth noting that most laryngoscopes are designed for right-handed medical use, where intubation is performed ‘upside-down’ and so the light source is on the correct side for medical use, but the incorrect side for veterinary use (Mosley, 2015). Veterinary specific, or left-handed medical laryngoscopes eliminate this problem. Laryngoscopes aid intubation by depressing the tongue and providing adequate light for correct placement of the ETT. They should be considered essential rather than complementary during routine endotracheal intubation.
ETTs should be tied in place to prevent accidental extubation or further internalisation of the tube. The tie should fasten around the ETT, ideally between the satellite and the connector, and then exit the mouth caudal to the canine teeth, wrap around the patient's head, mandible or maxilla and be tied in a bow or quick release knot. Any non-stretchy material can be used: wide-open-weave bandage, shoelaces and braided string can all be used, or purpose made iM3 Anaesthesia Tube Ties have the additional benefits of being water resistant, easy to see and not getting tangled in fur (iM3 Dental Limited, 2022).
Technique
In preparation for endotracheal intubation, all materials should be collected and be close at hand. At least three sizes of ETTs are often prepared, allowing for a precise selection once the glottis is visualised. Haider et al (2020) have created a clinical tool to aid size selection. Tubes should be prepared by completing a 10-minute inflation of the cuff to ensure no leakage, a visual inspection of the lumen, and assess the length of ETT against the patient.
The patient should be suitably anaesthetised to facilitate intubation, or in an emergency situation, unconscious with reduced muscle tone. The patient should not react when the tongue is grasped, or the back of the throat is stimulated. Loose jaw tone is a good indicator of suitable anaesthetic depth for intubation.
Additional anaesthetic induction agent can be administered, if necessary, at any time during the intubation and efforts to intubate should be paused while the agent takes effect. 1.5 mg/kg intravenous lidocaine has been shown to reduce the incidence of coughing in dogs and can be considered (Panti et al, 2016). Lidocaine spray should be applied to reduce the chances of laryngeal spasm in cats.
As standard, the patient should be positioned in sternal recumbency. Lateral recumbency is also possible and is required for intubation during CPR. A detailed step-by step description of intubation can be found in Box 1.
Box 1.Step by step technique for intubation of dogs and cats
- An assistant should lift the patient's maxilla using the endotracheal tube (ETT) tie. This prevents accidental injuries of the assistant's fingers should the patient suddenly clamp jaws. For large dogs the patient's dorsal neck may need to be lifted. The head and neck should be straight, aligned with the spine.
- Hold the laryngoscope in your non-dominant hand. Use the tip of the blade to push the tongue out of the mouth (again to prevent accidents to fingers).
- Grasp the tongue between your first and second fingers on your non dominant hand, using your first finger and thumb to hold and position the laryngoscope (Figure 3).
- Using the tongue to pull downwards on the mandible, open the patient's mouth wide.
- Position the tip of the laryngoscope on the base of the tongue, ventral to the epiglottis.
- Push the tip of the laryngoscope down, which should move the epiglottis ventrally and make the glottis visible. Do not press on the epiglottis as it is a fragile structure.
- For cats, apply a single spay of lidocaine to the larynx to avoid laryngospasm. Consider appropriate dosage for smaller cats (an insulin syringe may need to be used). Wait for 30–60 seconds, while providing oxygen before proceeding.
- After visualising the glottis (Figure 4), select a suitably sized ETT. Use the biggest size possible without having to use excessive force.
- Hold the ETT in a ‘pencil grip’ to have maximum control over the bevelled tip.
- Apply some sterile lubrication to the body of the tube, avoiding the tip or Murphy eye.
- Insert the tube over the tongue and epiglottis towards the glottis. Use the tip of the bevel to enter the glottis (requiring slight rotation of the tube).
- In cats, if the larynx is closed, wait for an inspiration and the widening of the larynx before entering.
- As the tube enters and advances, rotate the tube back to match the natural curve of the patient's neck.
- Once advanced, the tip of the tube should sit approximately midway along the trachea. To avoid bronchial intubation, it should be no further than the thoracic inlet.
- The tube should be secured as soon as it is placed to avoid it moving. Inflation of the cuff should follow accordingly.
Tips for learning
As with all new clinical skills, intubation can be daunting to learn. After expressing a wish to learn the skill to the supervising VS, it is help-ful to observe others performing the skill while they ‘think out loud’ to explain their actions and decisions. Once a VN can describe the skill they can begin to perform it under close supervision.
It is prudent to pre-select suitable cases. A good candidate would be a mesocephalic small or medium breed dog with no respiratory or cardiovascular disease, and no enhanced risk of regurgitation, undergoing pre-planned anaesthesia. Pre-oxygenation is always beneficial before induction of anaesthesia, as it allows a longer period before hypoxia should severe respiratory depression occur as a side-effect to the induction drugs. Additional pre-placed monitoring, such as pulse oximetry and electrocar-diogram (ECG) will allow adequate monitoring of the patient's vital signs during a potentially extended period while intubation takes place. Once a VN is confident with this type of case, they can advance to other sizes and shaped patients. Cats may be similarly approached.
It is also sensible to practice intubation in lateral recumbency, as this is the position required for endotracheal intubation during CPR (Fletcher et al, 2012). It can be practiced in any patient that is not at high risk of regurgitation during the induction period. When intubating in lateral recumbency it is important to change your position, so your own alignment matches the patient's (Figure 5).
Troubleshooting
An important step in mastering a clinical skill is to be able to troubleshoot atypical presentations; or cope with things when they go wrong. This is a particularly valid concern with endotracheal intubation as the ability to ventilate is essential for life in a very short time period. A VN should be prepared for certain problems and should be able to make rapid decisions to improve the situation. As with any difficult situation, colleagues should be made aware of concerns as soon as possible so they can offer assistance and act appropriately. The two most common problems with endotracheal intubation are not being able to visualise the glottis, and not being able to enter the glottis with the ETT; they may occur simultaneously or individually. When faced with these problems, the priority is to consider if the patient is suitably oxygenated and if they are able to ventilate; if they are, the situation can be dealt with in due course without need for ‘emergency’ action.
Poor visualisation
To improve visualisation the following can be attempted:
- Adjust positioning — ensure head is straight on the midline, the neck is not unnaturally bent
- Consider if the epiglottis is caught on the soft palate — if it is, use the ETT to gently dislodge it
- Check and adjust positioning of the laryngoscope and tongue — ensure tongue is fully pulled out and the laryngoscope tip is pushing down on the tongue base
- Ask the assistant to provide some ventral traction on the neck
- If there is additional soft tissue, a second laryngoscope or some tongue depressors held by a second assistant may be required
- Swabs or suction can be used to remove excess saliva or fluid material in the oral cavity.
Unable to advance tube through the glottis
If the glottis can be visualised but is unable to be entered by the ETT the following can be attempted:
- Use a smaller sized ETT
- Make use of the bevel of the tube, aiming for towards the ventral glottis
- Insert a stylet, bougie or rigid urinary catheter through the ETT and intubate initially with that, before sliding the ETT over.
Continuing problems
If intubation continues to be unsuccessful, a more experienced VN or VS should take over the intubation; Langton and Blevins (2021) recommended a novice handing over intubation after a total of 55 seconds to mitigate risks of hypoxaemia occurring. If there is a problem intubating and a patient is hypoxic, the most experienced person should take over the intubation immediately.
If intubation remains unsuccessful, but the patient is breathing and remaining well saturated (via SpO2), they should be allowed to recover from anaesthesia under close monitoring, and a revised plan made for intubation at a later occasion.
A situation where intubation is proving difficult and a patient is hypoxic (or rapidly becoming so), is known as a ‘can't intubate, can't ventilate’ (‘CICV’) situation and is an emergency event. Emergency tracheostomy should be considered, and the patient's vital signs closely monitored as they may progress rapidly to cardiopulmonary arrest.
A summary of troubleshooting actions is provided in Figure 6.
Brachycephalic obstructive airway syndrome
A specific example of a difficult intubation in veterinary patients, is that of a brachycephalic patient suffering from brachycephalic obstructive airway syndrome (BOAS). In this situation, initial visualisation may be obstructed by the soft palate or surrounding pharyngal tissue, and it may be harder than usual to enter the glottis to its reduced size and lack of space impeding visualisation. In this situation the patient should have been preoxygenated and additional supplies prepared, including smaller than expected ETTs. If the soft palate continues to impede visualisation after being disengaged from the epiglottis, a wooden tongue depressor can be used to hold it dorsally. A stylet or rigid urinary catheter can be fed through the ETT, and the patient can be intubated with this smaller diameter tube, before the ETT is fed over and into the trachea.
Conclusions
As with any clinical skill, endotracheal intubation requires a VN to be familiar with the associated anatomy, physiology, equipment, and technique. The VN should be confident in their ability to perform the required steps and troubleshoot a difficult situation. As the consequences of CICV are of the most serious nature, a VS should always be immediately reachable in case skills outside of a VN's remit are required. By taking the initiative to learn and practice endotracheal intubation, VNs can become an even more valuable resource in providing excellent anaesthesia and emergency care to patients.
KEY POINTS
- Endotracheal intubation is a useful skill for veterinary nurses to master.
- A thorough understanding of the associated anatomy is required to perform and troubleshoot endotracheal intubation.
- Essential equipment required for endotracheal intubation is an endotracheal tube, a laryngoscope, a tie, sterile water-based lubricant, and lidocaine spray.
- A veterinary nurse should know how to troubleshoot common problems such as not being able to view the glottis and not being able to enter the glottis.
- A ‘can't intubate, can't ventilate’ scenario is an emergency and veterinary surgeon backup should be available.