June 2026
In this VETgirl online veterinary continuing education blog, Amanda M. Shelby, RVT, VTS (Anesthesia & Analgesia) continues the discussion surrounding endotracheal tubes (ETT). In Part 1, we reviewed the anatomy of an ETT, provided examples of various types of endotracheal tubes commonly utilized in veterinary species, and detailed common methods of ETT cuff inflation. In part 2 of this 2-part series, we will explore selecting the optimal-sized ETT, investigate the benefits of lubricating the ETT cuff before use, and introduce key concepts regarding securing a patent airway!

Did you miss reading Part 1? Check it out HERE!

Secrets to Securing a Patent Airway Through Orotracheal Intubation: Part 2

By Amanda M. Shelby, RVT, VTS (Anesthesia & Analgesia), VETgirl Senior CE Specialist

In part 2 of this 2-part blog series, we take a deeper dive into elements that help secure patent airways in dogs and cats: endotracheal tube (ETT) size selection, lubrication, and depth of insertion.

Selecting the ETT

Most domesticated cats’ tracheas are similar in size, making ETT size selection easier, but dog sizes vary greatly! Adult cats can generally be intubated with 3.5 – 5 mm internal diameter (ID) ETTs. Dogs’ tracheas can vary greatly depending on a variety of factors, including size and breed, making linear association (i.e., bigger dog means bigger ETT) with selection based on lean body weight more challenging in dogs than in cats. I am sure you have heard of a variety of tricks for selecting the appropriate size ETT for dogs. Check out Figure 1 for common canine breeds where the ETT is larger or smaller than expected compared to similarly sized dogs.

Most experienced anesthetists have tricks for estimating the expected size of the ETT that will successfully fit in a patient. Some select an ETT based on a subjective assessment of the space between the nares as a guide to the appropriate-sized ETT (see Figure 2). However, I would encourage you to palpate the trachea (see Figure 3).

Figure 2. Estimating ETT size to width between a patient’s nares. (Image courtesy of Amanda M. Shelby)

 

Figure 3. Palpation of the trachea in a dog. (Image courtesy of Amanda M. Shelby)

By placing my thumb and index finger on either side of the patient’s trachea, I can clearly establish a guide to best select an ETT. The next key to successful intubations includes always having multiple ETT sizes available. Proper use of a laryngoscope will often allow you to visualize the larynx and tracheal opening (see Figure 4).

Figure 4. Using a laryngoscope to visualize the larynx in a cat (Image courtesy of Amanda M. Shelby)

Then it’s all about placement and knowing optimal fit. If the lumen of the ETT is excessively small, the patient will experience increased work of breathing via increased inspiratory and expiratory resistance. When extreme, increased work of breathing can be seen on a capnogram in a ‘teepee’ shape, indicating resistance to both expiration and inspiration (see Figure 5). The ETT fit should be comfortable and easily placed without resistance. Large ETT can cause irritation to tracheal mucosa (or worse—cough, tracheal mucosal ischemia, stricture, or tear), especially when ETT cuffs are overinflated.

Figure 5. Capnogram with severe inspiratory and expiratory resistance produced by an excessively small ETT. (Image courtesy of Amanda M. Shelby)

Depth of Insertion

The depth of insertion of the ETT is important and easy to establish prior to induction and intubation. Notice the ETT has graduations along its length to assist with confirmation for depth of insertion (see Figure 6). These graduations are measured in centimeters from the patient’s end. Simply hold an ETT next to the patient from the thoracic inlet or desired depth to the tip of the nose. Observe the measurement mark on the ETT or make a physical or mental mark for this location to be visible at the tip of the nose or incisors following intubation.

Figure 6. Confirming appropriate depth of insertion of ETT using a secondary tube (Image courtesy of Amanda M. Shelby)

The goal for the depth of insertion is to comfortably pass the cuff, if present, caudal to the larynx while the patient end of the ETT remains cranial to the bronchial bifurcation (see Figure 7). Excessively long ETTs extending beyond the incisors or tip of the patient’s nose add to airway resistance and dead space. Many will elect to trim the ETT if possible, to reduce airway resistance and dead space.

Figure 7. ETT tube (and feeding tube) depth confirmation on right lateral (Image courtesy of Amanda M. Shelby)

When endobronchial intubation occurs, patients can be difficult to transition smoothly to inhalant anesthetic and/or difficult to keep under an appropriate plane of anesthesia without increasing inhalant anesthetic excessively or supplementing with additional injectable induction or minimum alveolar concentration (MAC) sparing drugs. If you are utilizing a capnograph (great news!!), you will be able to appreciate endobronchial intubation quickly by observing unexpectantly low end-tidal carbon dioxide (ETCO2) following intubation. If endobronchial intubation is suspected, grab an extra ETT and confirm depth (see Figure 6). Deflate the ETT cuff and back the intubated ETT to the matching premeasured depth. Appropriately reinflate the ETT cuff to ensure a seal. (Not sure how to do this—check out part 1 of this blog series). Maybe it’s easiest to confirm depth prior to intubation!

Endotracheal Tube Lubrication

Don’t forget the importance of applying sterile water-soluble lubricant to the ETT cuff prior to intubation. This important step does not make it ‘easier’ to intubate, but does provide an improved ETT cuff seal at lower pressures than non-lubricated ETTs.1 This means the risk of causing irritation to the tracheal mucosa is reduced with the use of water-soluble lubricant and appropriate ETT cuff inflation. A frequent complaint in humans following orotracheal extubation is airway discomfort. It is likely our patients will experience this discomfort to a higher degree because our methods for inflating the endotracheal tube are less accurate in achieving optimal tracheal sealing pressure. (If you haven’t read Part-1 , do it! It explains this in detail.) Lubrication of the ETT takes very little time, and it doesn’t require much (see Figure 8)!

Figure 8. Lubricating the ETT tube cuff (Video courtesy of Amanda M. Shelby)

Rechecking the ETT Cuff

Let’s face it, the trachea relaxes after induction often from the potent muscle relaxant properties of the inhalant anesthetics used for maintenance and ETT cuffs can leak over time, especially if reusing tubes. Here are a few times it could be worth rechecking that ETT cuff:

  1. Reconfirm ETT cuff seal 10-30 minutes following intravenous induction and intubation.2,3 This is especially important following the transition to inhalant anesthetic from induction, especially with a dissociative agent (i.e., ketamine or tiletamine [Telazol®, Zoetis, TzedTM, Dechra, ZoletilTM, Virbac]).
  2. Following repositioning of the patient (i.e., moving from a pre-operative area to the operating room, rotating the patient to lateral recumbency during a dental prophylaxis, etc.)
  3. Following a noticeable change on the capnograph waveform, especially if there is a decrease in the slope of the inspiratory phase (see Figure 9)

Figure 9. Capnogram demonstrating a leak in the ETT (Image courtesy of Amanda M. Shelby)

In conclusion, what seems like a seemingly routine procedure, endotracheal intubation, is an exceptionally important step in securing a patent airway to provide supplemental oxygen and inhalant anesthetics, a means to support and monitor ventilation, and a method of protection of the airway from aspiration of fluids during states of unconsciousness. Routine doesn’t mean unimportant! These above-mentioned factors fall to the responsibility of the anesthetist. So, empower yourself to optimize the patient’s experience through appropriate ETT selection, placement, and cuff inflation.

Abbreviations
ETCO2: end-tidal carbon dioxide
ETT: endotracheal tube
ID: internal diameter
IPPV: intermittent positive pressure ventilation
MAC: minimum alveolar concentration
PIP: peak inspiratory pressure

References
1. Nishioka H, Usuda Y, Hirabayashi G, et al. Effects of lubrication on air-sealing performance of a pediatric cuffed tracheal tube. BMC anesthesiology. 2017;17(1):129.
2. Shin CW, Son WG, Jang M, et al. Changes in endotracheal tube intracuff pressure and air leak pressure over time in anesthetized Beagle dogs. Vet Anaesth and Analg. 2018;45(6):737-44.
3. Thomas JA, Lerche P. Canine and feline anesthesia. In: Anesthesia and Analgesia for Veterinary Technicians. Elsevier, USA; 2011:233-264.

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