The patient is not breathing. You attempt to intubate but there seems to be no space for the endotracheal tube (ETT). It bounces back or deflects upwards and repeatedly passes into the oesophagus. It will not pass into the trachea. The tongue is becoming cyanotic; you grasp it firmly and try to open the mouth wider, struggling to see inside. The clock is ticking. What can you do?
Why do we need to secure an airway?
In order to provide safe anaesthesia, the anaesthetist must be able to maintain adequate respiratory function (Grimm et al, 2015). Part of this entails securing a clear airway and endotracheal intubation is probably the most effective way to do this. Endotracheal intubation is the process of placing an airway device (ETT) via the mouth into the trachea. This process not only helps to secure the airway, but also enables the delivery of oxygen and volatile anaesthetic agents, while facilitating effective scavenging of waste gases and reducing environmental contamination. Endotracheal intubation can also be used to facilitate positive pressure ventilation (PPV).
What equipment do you need?
Endotracheal tubes
ETTs are manufactured from a variety of materials, although modern tubes are usually polyvinyl choride (PVC) or silicone. They are available in a range of diameters and may need to be cut to prevent excess length protruding from the patient's mouth, which increases apparatus dead space and can cause rebreathing (Grimm et al, 2015). At least three potentially suitable ETTs should be prepared before induction of anaesthesia. If a difficult intubation is anticipated, a range of smaller tubes should also be ready to ensure that the airway can be secured rapidly. The widest diameter tube that will pass comfortably through the cricoid ring into the trachea should be chosen to reduce resistance to breathing; the ETT should not be forced through the larynx or into the trachea, as this may cause serious damage (Grimm et al, 2015).
Most ETTs have a cuff with a pilot balloon attached to it. Cuffed ETTs prevent the leakage of waste anaesthetic gases, aspiration of fluid or debris into the lungs, and dilution of inspired anaesthetic gases with room air (Seymour and Duke-Novakovski, 2007). It is important to determine the correct cuff inflation and so keep the airway secure and reduce trauma, but in depth discussion of this is outside the scope of this article. A properly inflated ETT cuff will also facilitate adequate PPV.
Cats are more prone to laryngeal spasm, which can lead to airway obstruction (Mitchell et al, 2000). They also have a delicate dorsal tracheal ligament which can be easily ruptured and many veterinary surgeons therefore prefer to use uncuffed tubes when intubating cats.
Laryngeal masks
Laryngeal mask airways (LMAs) are designed for human use, but can be used in veterinary patients. The mask sits over the patient's larynx and has an inflatable cuff which helps to lock it in the oropharynx. Because they do not pass through the larynx, there is less risk of laryngospasm; but they have been shown to be less secure than ETTs and can rotate within the pharynx (Grimm et al, 2015). They may not be appropriate if the architecture of the oropharynx is disrupted as it may be difficult to position them properly and maintain a patent airway. They can compress the mucosa causing vagal stimulation, leading to bradycardia and hypotension. They can also be too bulky to use for dentals and other airway surgeries, but if it is not possible to place an ETT, they can be very useful.
V-gel
The v-gel® (Docsinnovent®) is a descendent of the i-gel, a second generation supraglottic airway device designed for use in humans. It provides a safe way to deliver and maintain anaesthesia without having to intubate and risk the potential trauma to the airways (Nasir, 2014). These are made specifically for the veterinary market and cover a smaller part of the laryngopharyngeal area. but are more likely to be displaced if the wrong size is used (Toman et al, 2015). These devices deliver anaesthetic gases/oxygen to a patient above the level of the vocal cords, and they may be a safer way of administering these agents to patients where endotracheal intubation may be viewed as being traumatic, i.e. cats and rabbits, and where poor technique during endotracheal intubation may potentially increase the patient's anaesthetic risk (O'Dwyer et al, 2013). A full discussion of these devices is beyond the scope of this article and the authors suggest that the reader looks at the original article in The Veterinary Nurse.
Lidocaine spray
In animals that are more prone to laryngeal spasm (such as rabbits and cats), lidocaine should be sprayed onto the larynx before intubation. It should be sprayed directly onto the larynx and 60–90 seconds should pass before attempting to intubate. Use of human preparations, which are more concentrated, should be avoided and the manufacturer's data sheet should be consulted to avoid delivering a toxic dose.
Tube ties
The ETT should be secured in place using an appropriate tie such as a white open weave (WOW) bandage, umbilical tape or proprietary ETT ties (available from Ace Veterinary Supplies). This should be tied around the tube, tightly enough to secure it into place but not so tight that it obstructs the ETT. Obstruction of the ETT with ties can happen when using smaller tubes such as those for cats, rabbits and other smaller species. The tie can either be secured behind the ears or above the nose. Care must be taken not to cause damage to either the nose or the lips while tying these in place, especially for prolonged anaesthetics.
Laryngoscope
Use of a laryngoscope greatly increases visualisation for its user, increasing the chances of a smooth intubation. The authors believe that all staff should become familiar with its use, as it is often needed in an emergency situation or to aid in difficult intubation. A suitable length of blade should be used, so that the tip of the blade is resting on the base of the tongue. Once pressure is placed on the base of the tongue, the epiglottis should flip down, clearly showing the opening to the larynx (Figure 1) (Grimm et al, 2015). Blades can be curved (Macintosh) or straight (Miller), but the authors favour a straight blade as a curved one can interfere with laryngeal visualisation (Dugdale, 2010) (Figure 2).
How to check for proper placement
There are numerous ways to confirm correct placement of an ETT:
Things that may help during a difficult intubation
A stylet (a thin, plastic coated wire which is longer than the tube used) can be inserted into the ETT lumen, and is particularly helpful when using either very small or silicone tubes as they make the ETT more rigid. Once the ETT is placed, the stylet is removed to restore its flexible nature.
If the airway is very small, a urinary catheter can be attached to a 2.5 ml syringe with the plunger removed, which is then plugged into to a 7 mm ETT connector (Figure 3), attached to a capnograph and then to a breathing system to deliver oxygen. The catheter is then threaded into the airway and correct placement is confirmed by a CO2 trace detected by the capnograph. Once the catheter is in the trachea, the syringe can be removed and an appropriately sized ETT can be threaded over the catheter and into the airway. The catheter is removed once the ETT is in place.
A bougie is a flexible device made of braided polyester with a resin coating (Figure 3); it can be used instead of a urinary catheter to guide an ETT into a difficult airway. Its advantage is that it is more rigid than a urinary catheter and may be easier to place. It also has a uniform diameter so that it is easier to thread an ETT over it once correctly placed. A disadvantage is that it may occlude the airway if too large.
Anticipation of difficult intubation
Some patients can be more challenging to intubate; for example:
When faced with these situations, the best approach is to be prepared for all eventualities. Where possible, preoxygenate the patient before induction of anaesthesia. If tolerated, oxygen is best delivered via a face mask. However, if this causes distress, holding the end of the breathing system close to the nose will provide a small increase in the percentage of oxygen in the inspired gas. Doing this can delay the onset of hypoxaemia following obstruction (McNally et al, 2009).
Sedation can lead to airway obstruction as the patient's pharyngeal muscles relax. Atrisk animals should therefore be premedicated in the induction area so that they can be observed. Laryngeal swelling and dyspnoea can be worsened by panting or stress, so the area should be kept as calm as possible. Sedated patients should not be left unattended in case airway obstruction develops, and equipment for rapid induction and intubation should be prepared.
The RVN should be aware of the equipment needed to perform an emergency tracheotomy and prepare it in advance. A wide-bore catheter or needle can also be inserted into the tracheal lumen via a midline approach, and between the tracheal cartilage rings. The catheter/needle can then be connected to a 2 ml syringe barrel and attached to a 7 mm ETT adaptor to allow oxygen delivery through a standard breathing system (Figure 4).
If available, endoscopically-guided intubation can be performed (Grimm et al, 2015). If intubation is not possible, it may be safer to let the patient regain consciousness, as repeated attempts to intubate may be damaging rather than beneficial (Welsh, 2003).
When a normal airway can become difficult
If it is necessary to dramatically flex the neck, for example, for some ocular surgeries and for the collection of cerebrospinal fluid, a regular ETT may kink and occlude the airway. To avoid this, armoured tubes, which contain spiral nylon or wire reinforcement in the tube wall, are available and will not kink (Figure 5) (Grimm et al, 2015). However, they have a thicker wall to accommodate the armour so their outer diameter will be greater; consequently, a smaller internal diameter tube is usually required (Figure 6).
Extubation
The recovery phase of anaesthesia is associated with the most complications and mortality (Brodbelt, 2006). Patients should therefore be monitored very closely during recovery, until they are able to breathe on their own and have regained an adequate level of consciousness.
Cats may suffer laryngospasm and should be extubated when there is a strong palpebral reflex or ear flick, but before they start to swallow (Nutt et al, 2014). Brachcephalic breeds should remain intubated for as long as they will tolerate the ETT. It is safe to wait until they can lift their heads, and many dogs are quite happy sitting up and looking around with the ETT still in place. However, they must be watched closely and the tube removed if they start to chew or bite down onto it.
All species may benefit by being placed in sternal recumbency, which facilitates breathing (Welsh, 2003). Raising the head with a rolled-up towel may also help to maintain the airway, but care must be taken not to apply upward pressure to the pharynx and larynx, which could make breathing more difficult. Equipment should be available for rapid re-intubation should this be necessary; for example:
Suction equipment can also be invaluable if blood or excessive secretions are present as they may block the patient's airway.
Overheating can lead to panting, which increases the work of breathing through a small airway. Therefore, the authors do not advise actively warming brachycephalic patients unless they are extremely cold. If patients are warmed, they must be monitored very closely and the warming stopped before they are normothermic.
Drugs
Many surgeons advocate the use of dexamethasone before recovery in patients at risk of laryngeal swelling and obstruction. If this is used, the patient should not concurrently receive non-steroidal anti-inflammatory drugs (NSAIDs) as they increase risk of gastrointestinal ulceration (Gaynor and Muir, 2014).
In the past, atropine was given to reduce secretions. However, it can cause mucous to become more viscous and perhaps increase the risk of airway obstruction. It also increases heart rate and blood pressure so it is not recommended (Grimm et al, 2015).
Terbutaline is a selective beta-2 adrenergic receptor agonist used to manage bronchospasm in human asthmatics. It is unlicensed for use in animals, but may be used under the cascade to treat bronchospasm. It is unlikely to cause tachycardia unless given at high doses, but it may lead to cardiac arrhythmias, especially if the patient is hypoxaemic.
Conclusion
There will always be patients that are challenging to intubate and manage during anaesthesia. However, it should be possible to identify many of them before they are anaesthetised and make a plan to manage their airway. If well prepared to manage a potentially difficult airway, in the majority of cases, it is possible to provide a safe anaesthetic and a smooth recovery.