Endotracheal tube configuration
There are a number of types and styles of ETTs designed for single use in humans , which can also be used for veterinary patients . In veterinary practice , ETTs are often used and then put through a cleaning process to be reused a number of times . Generally ETTs have similar design features and may be made from silicone , polyvinyl chloride ( PVC ) or red rubber . Some ETTs contain a guard ( a metal spiral wire that prevents the tube bending ); these are particularly useful , for example , for ophthalmology patients with their heads ventroflexed for cataract surgery ( Mosley , 2015 ).
ETTs are made up of a few components , including a connector that secures to the breathing circuit or Ambu ® bag , the main body of the tube , the cuff indicator , the cuff and a Murphy Eye . There are several numbers found on the main body of the ETT . The length measurements are in centimetres and identify the length of the tube . The large bold number or ID number is the internal diameter in millimetres . Tube diameter commonly ranges from 3 mm for small cats to 12 mm for large dogs , increasing in 0.5 mm increments . Larger tubes ( i . e . up to 16 mm ) are available for giant-breed dogs . The cuff indicator allows inflation of the cuff using an empty syringe and can be used to determine the pressure of the cuff on the trachea once air has been infused . The cuff is present to create a leak-proof seal between the ETT and the tracheal lumen ; this will help reduce the risk of aspiration of secretions or vomit / regurgitation while the patient is intubated . The cuff is also useful in preventing dilution of the volatile anaesthetic agent with room air and to prevent leakage of the exhaled anaesthetic gases into the theatre . Great care should be taken to avoid over-inflation , to prevent the incidence of ischaemic damage to the trachea ( Figure 1 ). A manometer can be used during inflation and a pressure of 25 cm H 2
O is suggested , to provide an adequate seal and avoid complications associated with over-inflation ( Tear , 2012 , Dugdale et al ., 2020 ). The Murphy Eye is a safety feature allowing gas to flow through the oval hole , should the bevel ( end of the tube ) become obstructed or somehow positioned against the tracheal wall ( Hughes , 2016 ).
Technique for endotracheal intubation
The first step is selecting the size of ETT required , to ensure minimal resistance to breathing , using an ETT with the largest diameter possible ( but avoiding causing trauma to the airway ) ( Dugdale et al ., 2020 ). A range of sizes can be gathered to ensure an appropriately sized ETT can be placed into the patient . The length of tube should be measured to reach cranial to bronchial bifurcation : hold the ETT against the side of the patient from the jaw to the thoracic inlet . If the tube is too long , it may slip into one bronchus , preventing oxygen delivery to the other lung ; if it is too short , displacement is possible ( Murrell & Ford-Fennah , 2020 ). Once inserted , the tube should sit at the entrance to the oral cavity ( approximately at the level of the incisors ); if the tube exits the mouth , this is considered dead space . In larger patients , a small degree of dead space can be tolerated and the tube may be secured exiting the oral cavity a little further from the incisors , rather than cutting tubes down to make them shorter . In smaller patients , the tube may be cut to length , avoiding the cuff inflation line ( Phillips , 2018 ).
The tube should be visually inspected to ensure it is clean , with no signs of damage . The cuff should be inflated at the earliest opportunity and left as long as possible ( ideally at least 10 minutes ) prior to intubation . This allows for an obvious leak to be detected immediately , or for a more insidious slow leak to be detected after some time . If a leak is found , the tube should be discarded and replaced – it is good practice to have a couple of spares of each size in stock to ensure a tube of every size is always available .
Following the injection of induction agent by the veterinary surgeon , the patient should be relaxed into lateral or sternal recumbency , with an assistant holding the patient ' s maxilla towards the operator , extending the head and neck straight and in line with the spine ( Figure 2 ).
Figure 1 . Over-inflation of the cuff .
Figure 2 . Presenting the patient for intubation .
Volume 37 ( 5 ) • November 2022
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