Adam Auckburally BVSc, Cert VA, DipECVAA, PGCAP, FHEA, MRCVS
Derek Flaherty BVMS, DVA, DipECVAA, MRCA, FHEA, FRCVS
Endotracheal intubation is a vital skill that must be mastered by veterinarians and veterinary nurses. It is important to be able to correctly choose an appropriate tube and to be able to deal with situations of difficult intubation. Veterinary staff should be familiar with intubation aids, such as stylets, bougies and laryngoscopes. A basic understanding of tube design and construction will aid in the development of this skill.
Indications for Endotracheal Intubation
1. Gaseous anaesthesia
3. Airway obstruction e.g. anaphylaxis; laryngeal paralysis; BOAS
4. Obtundation e.g. following head trauma
5. Deep sedation
6. Muscular weakness e.g. myaesthenia gravis
7. Other disease processes requiring mechanical ventilation e.g. hypoxaemia
Tube Design & Construction
The placement of a tube into the airway will always add resistance and increase the work of breathing. This may lead to hypoventilation during anaesthesia. The important qualities of a tube that may affect resistance are:
- Tube internal diameter – the smaller the internal diameter of the tube, the greater the resistance will be. If you imagine that an endotracheal tube inside the trachea is essentially a tube within a tube → the diameter of the airway must be reduced. The wall thickness of the tube influences the internal diameter and smaller tubes are affected more (the ratio of diameter to wall thickness is greater).
- Tube length – the longer the tube, the greater the resistance. It is important that tubes of the correct length for the patient are selected to reduce resistance. Most commercially available tubes for humans are supplied with excessive length. The tube should be cut to the desired length. Specific veterinary tubes may not be so easily modified.
- Connector – the internal diameter of the connector will affect resistance as well as its configuration e.g. t-connector, swivel connector.
In addition to these factors, anything within the tube will also increase resistance e.g. secretions, endoscope, suction devices etc.
Apparatus Dead Space – As the tube is always smaller than the airway, dead space arising from the tube is usually less. In small patients where the tube may be excessively long, dead space may be increased. Connectors, filters and other devices between the tube and the breathing system may also increase dead space (e.g. capnograph).
Construction materials – There are a variety of materials used to make endotracheal tubes. The ideal material would be:
4. Non-reactive to anaesthetic agents
5. Easily cleaned/sterilised
6. Conformable but have inherent strength to withstand kinking
7. Resistant to the build-up of secretions
8. Latex free
Rubber – Although this material has fallen out of favour in human anesthesia, red rubber tubes are still manufactured for veterinary patients and remain in general use. The advantage of rubber tubes is that they can be re-used and re-sterilised multiple times. There are numerous disadvantages:
- They become hardened over time
- They are prone to kinking
- They are not transparent
- The wall of the tubes is generally quite thick
- They do not soften at body temperature and therefore do not conform to anatomy well
Polyvinyl chloride (PVC)
- Very common in disposable tubes used in humans.
- Highly suitable for use in veterinary patients
- Resist kinking but soften with body temperature
- Smooth surface which prevents the build up of secretions
Silicone – Many modern veterinary endotracheal tubes are made from silicone
- Have no preformed curve and are therefore more difficult to place
- Have a cuff that can be repaired if damaged
- Can be sterilised and re-used
Some different tube types
- Murphy tubes – or Murphy-type tubes are tubes with a hole in the wall opposite to the bevel. The function of the eye is facilitate passage of gas should the bevel become occluded.
- Magill tubes – or Magill-type tubes are those tubes without a Murphy eye. The absence of the eye means that the cuff can be positioned closer to the bevel and may reduce the risk of inadvertent endobronchial intubation.
- Cole tubes – not frequently seen in veterinary medicine. Are uncuffed and have a patient end that has a smaller diameter than the rest of the shaft. They are used for neonatal resuscitation in human medicine because resistance is lower. In veterinary medicine, Cole pattern tubes have been used in small exotic patients.
- Reinforced (spiral embedded or armoured) tubes – These are available in a variety of diameters but cannot be cut to length since the connector is moulded to the machine end. The tubes have a metal or nylon spiral embedded into the wall to resist kinking or compression during anesthesia and are useful for patients who must be positioned with the neck in flexion (e.g. cervical dorsal laminectomy). Care must be taken that the patient does not bite the tube since this may cause a permanent occlusion resulting in airway obstruction.
Manufacturers must state the internal diameter (ID) in millimetres of the tube on the body of the tube. For cuffed tubes, this will be printed somewhere between the cuff and the point where the inflation balloon exits the body of the tube. For uncuffed tubes, the figure is printed towards the patient end. French gauge may still be used on some tubes and you can determine the size of the tube by dividing it by 3 to give the outer diameter (OD) in millimetres e.g., a tube with a diameter of 30 Fr will have an OD of 10mm.
Endotracheal tubes less than 6mm ID must also have the OD stated on the tube. Some manufacturers print the size on the pilot balloon so that the tube size can be identified once it is in the trachea.
There is a minimum length of tube that increases as ID increases. However, all manufacturers produce tubes longer than the minimum to enable the anaesthetist to cut the tube to size
From patient end to machine end:
- Radio-opaque marker (may be down the entire length of the tube)
- The word oral or nasal
- ID in mm
- OD in mm if present
- Manufacturer name
- Single use only if present
- Graduations show the distance from the patient end in centimetres
Some tubes may have dark markings at the patient end. These are used to ensure proper placement with respect to the vocal cords and are not applicable to veterinary patients.
The cuff is an inflatable sleeve near the patient end of the tube. A cuff system also includes the inflation tube, inflation valve and a pilot balloon. The cuff has 3 main functions:
- To prevent oral or regurgitated fluid passing from the oesophagus into the lung
- To prevent gas passing around the tube, especially during mechanical ventilation (MV) – this also ensures minimal environmental contamination with volatile anaesthetics.
- To centre the tube within the trachea
The cuff should:
- Be tear resistant
- Be soft and pliable
- Be tested to the same standards as the tube e.g. non-irritant
- Not encroach on the Murphy eye if present
- Not herniate into the bevel opening when inflated
- Inflate symmetrically
The cuff may be:
- High pressure; low volume – these cuffs require a high pressure to overcome cuff compliance and form a seal in the trachea over a small area. They are present on red rubber tubes. The advantage of this type of cuff is that it offers better visibility during placement and better protection against aspiration. However, the pressure exerted against the tracheal mucosa will be higher than perfusion pressure and the tracheal mucosa may become ischaemic. Use the largest size of tube possible to minimise how much air is needed to inflate the cuff. These tubes should not be used for anaesthetics of long duration.
- Low pressure; high volume – these cuffs are present on most PVC tubes and are large and compliant. The volume of air required to form a seal is large and the cuff forms a tracheal seal over a large area. Consequently the risk of tracheal mucosal ischaemia is reduced since the pressure inside the cuff is lower and the pressure is spread over a greater surface area. However, because the cuff is large, visualisation is reduced. The cuff also becomes wrinkled if not fully inflated and this can allow regurgitated ingesta to pass from the oesophagus and into the lung. The cuff should be lubricated with a sterile water based lubricant to fill the wrinkles and reduce the incidence of aspiration. If MV is performed, the cuff will distort during inspiration (positive pressure phase) and the pressure inside the cuff will increase. The incidence of sore throat in humans may be greater with this type of cuff.
Do not assume that tracheal mucosa ischaemic injury cannot occur with this type of cuff – any cuff can be overfilled and cause a problem.
Cuff pressure should not be greater than 25 – 34 cmH2O at the end of expiration in adult patients with normal blood pressure. Techniques whereby the cuff is inflated whilst squeezing the reservoir bag until no leak is heard have been found to be unsatisfactory. This is in part due to the fact that intracuff pressure will change over time, with temperature and with position of the patient. However, the measurement of cuff pressure with a manometer is uncommon in veterinary practice, and many anaesthetists use this minimal leak technique. Long term intubation e.g. in patients being ventilated in an intensive care setting, should have the cuff deflated periodically and tube repositioned or replaced.
Nitrous oxide and cuff pressure – the intracuff pressure in a cuff inflated with air and a patient breathing a nitrous mixture will increase over time. This occurs because nitrous oxide diffuses into the cuff. Once the nitrous is turned off then the cuff pressure rapidly decreases. The amount of pressure change is dependant on the cuff wall permeability, the temperature and the duration of nitrous oxide anaesthesia.
The inflation tube – take care not to tie this in place with the tube as deflation of the cuff may become impossible at the time of extubation. Many modern tubes now have a valve that must be depressed by the syringe tip to allow inflation. Once the syringe tip is removed, gas cannot pass back out of the cuff. Red rubber tubes have inflation tubes that must be capped after filling – these often fail during the procedure and can be made more secure by folding the inflation tube over and fixing it in this position by placing a needle cap over the folded portion.
Tube connector – ensure you choose the right size of connector for the right tube. A common mistake in large red rubber tubes is to insert the connector the wrong way around. In these circumstances, connection to the breathing system will be impossible because the connector is back to front. The use of superglue to fix metal connectors to tubes is a useful technique, although this may affect the lifespan of the tube.
Connectors are available in a variety of forms: they may be straight or have a 90° bend. Some may swivel. In addition there may be suctioning ports, sampling ports or administration ports incorporated into the connector.
Choosing the Right Tube
- Does the patient need to be intubated? Consider that placement of an endotracheal tube in the trachea will always cause some morbidity, whether or not this is detectable in the patient postoperatively. Intubation may even increase mortality in some cases (e.g. cats).
- Cuffed or uncuffed? Traditionally it has been advocated that cats should be intubated with an uncuffed tube. This has the advantage that tracheal mucosal damage is less likely in this species. However, this may lead to problems in performing MV in that leaks may be present, and result in inaccuracies in capnography. If MV is anticipated (e.g. a cat requiring thoracotomy, diaphragmatic rupture repair) then a cuffed tube used carefully is a good option to facilitate ventilation. Studies in children have not detected a higher incidence of complications when using cuffed endotracheal tubes. The disadvantages associated with cuffed tubes are highlighted above, but in addition a smaller tube must be chosen which will increase the work of breathing. Cuffed tubes may cause problems on extubation if the cuff becomes caught in the larynx. For dogs, always choose a cuffed tube.
- Size – it is important to choose a size of tube appropriate for the patient. Remember that the intubated trachea is really a tube within a tube. Therefore to reduce the affect of airway resistance and work of breathing, choose a tube that fits comfortably in the trachea and does not require overinflation of the cuff to produce a gas tight seal. If you find you are filling the cuff with excessive amounts of air, then tube you have chosen is likely to be too small. Conversely, a tube that is very tight is likely to result in post-anaesthetic sore throat (will you recognise this?) and associated morbidity. There are studies which have used weight, width of the nasal septum and palpation of the trachea to judge the size of tube, but they are not failsafe and experience is a better tool. Always have a good selection of leak tested tubes ready as tracheal size is sometimes impossible to predict. If an uncuffed tube is used in a cat, then a suitable size for most adults is ID 5.0 – 6.0 mm. Some large cats may take an ID 7.0mm.
- Length – presize the tube by measuring the length from the nares to the thoracic inlet or point of the shoulder. Tube lengths beyond this may be inadvertently inserted into a mainstem bronchus. Cut the tube to the correct length and reinsert the connector. If the tube is not trimmable to length then apparatus deadspace should be monitored using a capnograph.
The majority of veterinary practices do not sterilise tubes between patients. However, the tubes should be cleaned with e.g. chlorhexidine or other suitable disinfectant, and thoroughly rinsed and dried prior to re-use. Note that chlorhexidine may act as a tracheal irritant and tubes must be rinsed correctly. Check patency of the lumen by looking through the tube from one end to the other. This is made easier if the tube is made of a transparent material. Plugs of mucus can be difficult to dislodge and may be missed during the cleaning process. It is useful to have a selection of bottle brushes to ensure patency during the cleaning process.
Inflate the cuff and leave inflated for 5 minutes to ensure that the cuff system is not leaking. Remember to deflate the cuff prior to placement
Check that the connector is secured to the machine end of the tube, and in the case of metal connectors for large tubes, ensure that it is placed in the tube correctly.
Lubricate the patient end of the tube immediately prior to intubation with a water based lubricant gel.
Securing the Tube
Securement is essential following intubation to prevent inadvertent extubation, or advancement into a bronchus. Gauze bandage is suitable for securing tubes and secure the tube itself rather than the connector. In this way, if the connector becomes disconnected from the tube, the tube itself is still secure and inhalation of the tube is prevented. In addition, a disconnected tube can be quickly reconnected if tape or bandage is not obscuring the connector. The bandage is wrapped around the tube where it meets the connector and pulled tight. If the bandage is wrapped further down the tube then the tube may be distorted as the bandage is pulled tight. Once the tube is secured, then bandage ends are then passed around the maxilla or mandible using the canine teeth to secure it in place. Alternatively the bandage can passed around the back of the animals head and tied behind the ears. Placement on the animals head is sometimes dictated by the surgery.
Confirmation of Correct Tube Placement
- Intubation of the trachea should be performed under direct laryngoscopy. This goes a long way to confirm correct placement. Patients that are lightly anaesthetised may also cough on correct placement of the tube.
- Connection to a capnograph – the presence of carbon dioxide confirms placement of the tube without doubt. However, absence of CO2 does not necessarily indicate oesophageal placement. Cardiac arrest, pulmonary embolism and tube obstructions may all prevent CO2 being detected.
- Movement of the bag – can be unreliable as the oesophagus may have a tidal volume in association with breathing.
- Palpation of the bag – again unreliable but may give an indication of oesophageal intubation.
- Chest wall movement – unreliable. If the anaesthetist squeezes the bag, the chest wall may rise even if oesophageal intubation has been performed.
- Abdominal distension – may indicate oesophageal placement but occurs over time and oesophageal intubation requires early detection.
- Moisture inside the tube – unreliable, but if no moisture is seen then the tube is likely to be in the oesophagus.
- Oxygenation – hypoxaemia is a late indicator and cyanosis may not be detected.
- Palpation of the throat – can be useful. Often the tube can be felt to run over the tracheal rings during placement. The tube can be manipulated once placed and the neck palpated for oesophageal intubation.
- Air in cuff required to create a seal – excessive amounts of air will be needed if oesophageal intubation has been performed.
- Trauma – usually associated with poor technique, rough intubations or repeated attempts at intubation. Injuries can include haematoma formation, lacerations, contusions, cartilage damage, tracheal avulsion (esp. cats), tracheal rupture and puncture wounds. Never use more than gentle pressure during intubation, always lubricate the tube and if using a stylet, never allow the stylet to protrude beyond the end of the tube. Cats appear to be more prone to problems compared with dogs as they are prone to laryngospasm and have delicate laryngeal structures. The larynx of cats should be sprayed with local anaesthetic prior to intubation and adequate time elapsed prior to placement of the tube. The intubation technique in all species should be careful, but probably more so in cats. The trachea of cats is also more prone to cuff-related injuries.
- Brachycephalics – can be challenging to intubate. These breeds can have much redundant tissue in the oropharynx making visualisation of the larynx difficult. The trachea may also be hypoplastic so a good selection of tubes should be ready. A laryngoscope is very useful. Vagal stimulation may also occur during intubation and heart rate should be monitored or an anticholinergic included at the time of premedication.
- Oesophageal intubation – can occur even with the most experienced anaesthetist. Recognition is essential (see above) to ensure the patient receives adequate oxygenation and anaesthetic gas.
- Endobronchial intubation – can occur relatively easily if the endotracheal tube is too long. Recognition can be difficult; changes in the capnogram may occur but is not reliable. Endobronchial intubation is easily recognised on a lateral thoracic radiograph. The non-intubated lung becomes atelectatic and patients may experience reduced oxygenation. In healthy patients, one lung is able to compensate for the work of both if the inspired oxygen concentration is 100%. If MV is performed, the intubated lung may suffer volutrauma due to overinflation. Endobronchial intubation may occur subsequent to a normal placement of the tube, especially if the patient’s position is changed. The problem is easily avoided by pre-measuring the length of tube (see above) and cutting the tube to the correct size.
- Inhalation of foreign bodies – ensure that the tube patency is checked prior to use. Even so, dislodgement of teeth has occurred during intubation resulting in inhalation of the tooth.
- Leaks – are common – thoroughly check the tube prior to use, inflate the cuff to the appropriate volume to ensure a leak-free fit inside the trachea, and check this at regular intervals, especially during long procedures and after patient repositioning. If the leak cannot be resolved, it is prudent to check that the tube is actually still within the trachea. Leaks can normally be identified by loss of the capnogram alveolar plateau. If the leak continues then consider replacing the tube as the cuff may have become damaged during the intubation process.
- Tube obstruction – recognition is imperative to patient safety. The patient may show evidence of increased respiratory effort, paradoxical chest movement or if MV is being performed, the unit may alarm. Increased resistance to gas flow can be manually ‘felt’ by squeezing the reservoir bag. Obstruction may occur because of:
- Foreign material – e.g. a plug of mucus, especially problematic in small ID tubes.
- Kinking – usually encountered if the head of the patient is hyperflexed e.g. during CSF aspiration from the cisterna magna. However, instruments placed in the patient’s mouth e.g. during dentistry, may also kink the tube. Tubes vary in their proneness to kinking – red rubber tubes kink more easily than PVC or silicone, and smaller tubes are more likely to kink. Consider replacing the tube with an armoured one.
- Cuff problems – the cuff may inflate asymmetrically and push the bevel of the tube against the tracheal wall; it may herniate over the end of the bevel; over-inflation of the cuff may compress the lumen of the tube.
- External compression
- Patient re-positioning
- Obstruction of equipment between the tube and the breathing system.
In the first instance check the position of the tube and re-position the patient if kinking may be the cause. Deflate the cuff to see if this resolves the problem. If the tube is transparent, the visible portion of it should be checked for foreign material. Eliminate other causes of increased resistance or reduced lung/chest wall compliance e.g. bronchospasm. If the problem continues then consider removal of the offending tube.
- Difficult extubation – this may occur if the cuff fails to deflate. The pilot balloon may give no indication that there is a problem as the inflation tube may be sealed further down. If this is suspected then a needle and syringe may be used to puncture the inflation tube further down the tracheal tube and aspiration of air attempted. If this doesn’t work, then the tube should be withdrawn until the cuff is visible within the larynx and carefully punctured using a sharp object. The animal should remain anaesthetised until this is performed. The tube may also be caught in suture if surgery has involved the trachea or larynx e.g. laryngeal tieback procedures.
- Post-operative sore throat – very difficult to recognise in veterinary patients although very common in humans postoperatively. Reduce the incidence by careful intubation technique and choosing tubes of a suitable size.
- Laryngeal oedema – again, probably occurs more often than realised. Animals may evidence stridorous or stertorous breathing. In its most severe form, complete airway obstruction will occur. Avoid by careful intubation and extubation technique, ensure an adequate plane of anaesthesia prior to intubation, clean tubes with no residual disinfectant and head movement once intubated should be kept to a minimum.
- Tracheal stenosis – a rare postoperative problem but may be associated with traumatic intubation or tracheal mucosal damage.
Animals should be extubated once active swallowing has returned. Cats may be extubated earlier to prevent laryngospasm on extubation, and brachycephalics extubated later to ensure that they are conscious enough to maintain their airway. However, late extubation may result in the tube being chewed so it is essential to be vigilant as the sheared portion may be inhaled with disastrous consequences. The oral cavity should be examined prior to extubation and suctioning performed if deemed necessary. The cuff should be completely deflated just prior to extubation. The tube may be removed with the cuff partially inflated if it is suspected that there may be fluid or ingesta accumulation above the cuff. Note though that this practice may result in injury. An alternative technique is to suction the trachea above the cuff prior to extubation.