Exotic companion mammal intubation and anesthesia (Proceedings)

Endotracheal intubation is the placement of a tube that extends from the oral cavity into the trachea. It is indicated for the administration of oxygen and inhalation anesthesia, to ensure a patent airway in unconscious patients, to provide ventilatory assistance, and to provide a conduit into the trachea to permit diagnostic and therapeutic measures (e.g. endoscopy, tracheal wash, direct instillation of medications). Intubation provides better airway control than a face mask and minimizes the risk of aspiration of foreign materials. This is especially important for complex and prolonged procedures, when complications such as respiratory obstruction and hypoventilation are more likely to occur.

Intubation methods

Blind intubation

By properly positioning the head and neck, the pathway from the oropharynx to the trachea is straightened so that an endotracheal tube can be placed without direct visualization of the larynx. This is possible with the aid of laryngeal palpation, patient response (i.e. coughing, gagging), and listening for patient respiration through the endotracheal tube itself. Under special circumstances, a tube may need to be inserted using a transtracheally-placed catheter as a guide.

Direct visualization

Visualization of the larynx is aided by hyperextension of the head and neck. Usually an assistant must open the mouth an oral speculum or gauze placed around the upper and lower incisors. A small-bladed (e.g. Miller 0 neonatal) laryngoscope is used to depress the tongue and elevate the soft palate. Once the vocal folds are visualized, the tube is placed. An atraumatic stylet (polypropylene catheter) can be placed through the tube so its tip extends beyond the end of the endotracheal tube in order to guide the tube through the vocal folds.

A canine otoscope can be used instead of a laryngoscope in smaller patients. After adequate visualization is achieved, a 5-fr polypropylene urinary catheter is guided down the otoscope between the vocal folds and advanced into the trachea. At this point, the otoscope is removed and the tracheal tube is threaded over the catheter and into the larynx. The catheter guide is then removed.

Direct visualization of the trachea can also be achieved using an endoscope. The endoscope is positioned so the larynx is in view, and an endotracheal tube is passed parallel to the scope and into the trachea. Further, with some scopes it is possible to put the endoscope directly inside the endotracheal tube like a stylet, and to visually guide the scope/tube assembly into the trachea.

Comments on individual species

Ferrets

Direct placement of an endotracheal tube in ferrets usually requires two people. Over-the-endoscope intubation of ferrets simplifies intubation because it does not require the jaws to be opened wide or the tongue to be pulled forward. The endoscope/tube combination is rigid enough force the tongue forward at its base, exposing the glottis. The tube and scope are advanced over the epiglottis and into the trachea. Either a 2 mm Cole or 2.5 mm straight endotracheal tube is recommended with this technique.

Rabbits

Blind placement in the rabbit is possible with the rabbit in sternal position. The operator uses one hand to elevate the head, hyperextending the neck, while the other hand directs the tube. Then the operator puts his ear to the end of the tube, and listens to the respiratory sounds in order to place the tube. A modification of this blind technique is described by Conlon et al (1990). A 3.5 mm ID uncuffed endotracheal tube is attached to a standard clinical stethoscope. A 4 mm X 2 mm elliptical hole is made in the stethoscope tubing just proximal to the endotracheal tube. Using the stethoscope to accurately place the end of the tube over the glottis, the endotracheal tube is advanced into the trachea during early inspiration. Proper placement results in a cough reflex, which is vented through the elliptical hole in the stethoscope tubing. The endotracheal tube is removed from the stethoscope and the endotracheal tube adapter is reattached. Conlon et al used this method successfully on over 50, 8-12 week old New Zealand white rabbits without complications or evidence of trauma at necropsy.

Bertolet and Hughes (1980) describe a blind method of endotracheal intubation that involves percutaneous preplacement of a polyethylene urinary catheter retrograde via a transtracheal guide needle. A 17-gauge is inserted into the trachea 2-3 cm below the larynx and directed cranially. A 19-gauge polyethylene catheter is threaded through the needle into the trachea, through the oropharynx and out the mouth. An endotracheal tube is then placed using the catheter as a guide. Bertolet and Hughs found only mild hemorrhage in rabbits necropsied immediately following the procedure, and no pathologic changes in rabbits at 7-14 days.

Visual placement of an endotracheal tube in rabbits is usually described as a two-person operation: one operator holds open the mouth and hyperextends the rabbit's neck while the other places the tube. However, several solo methods are also described. By placing the rabbit in dorsal recumbency with a rolled towel under the neck, the head will sit back at a 45 angle without a second operator. Using a self-retaining oral speculum, it is possible for one operator to hold the tongue and pediatric laryngoscope in one hand, and to visually place an uncuffed tracheal tube during inspiration with the other. Using this technique, Schuyt and Leene (1977) used slightly curved endotracheal tubes with the "point" of the tube situated on the concave side of the tube, and successfully intubated 114 rabbits using this method. Davis and Malinin (1974) described a similar method, but used a positioning device constructed of Styrofoam to stabilize the head and to keep the mouth, larynx and trachea in a linear alignment.

Over-the-endoscope intubation provides direct visualization of the glottis during intubation, and can be accomplished by a solo operator. The author uses a 1.9mm x 6" semi-rigid fiber optic endoscope (Focuscope, MDS Inc.), which is compatible with endotracheal tubes having an internal diameter (ID) of 2.0mm and greater. The endoscope acts as a stylet for the endotracheal tube. The rabbit is placed in lateral or sternal recumbency. A 3.5 mm to 2.5 mm uncuffed oral-nasal endotracheal tube or a 2 mm Cole endotracheal tube is recommended. The endoscope/endotracheal tube combination is advanced over the base of the tongue until the tip of the epiglottis is visible through the soft palate. The tip of the scope is advanced gently in a dorsocaudal direction, lifting the soft palate and thus allowing the epiglottis to fall forward. The scope is withdrawn approximately 1cm, the tip of the endoscope is rested on the epiglottis, and the glottis is visualized. The tube/scope combination is advanced through the laryngeal opening, where positioning is confirmed by the presence of tracheal rings.

Prairie dogs

Prairie dogs can be intubated in a manner similar to that described for rabbits. They are generally more difficult to intubate as the glottal opening is slightly smaller than in rabbits. The soft palate of the prairie dog appears to be longer and under less tension than in the rabbit and it is more difficult to keep soft palate from obscuring the glottis. The author prefers to place prairie dogs in sternal recumbency for intubation, and uses either of the endoscopic methods described above to visualize the laryngeal opening. A 2.5 mm uncuffed oral-nasal or a 2 mm Cole endotracheal tubes are recommended.

Guinea pigs/chinchillas

Cavies and chinchillas are generally harder to intubate than the species discussed above. This is due in part to the small size of their laryngeal openings. Guinea pigs and, to a lesser extent, chinchillas have a narrow palatial ostium formed by the soft palate, palatoglossal arches, and tongue. Atropine or glycopyrrolate may be indicated to decrease salivary secretions, especially in guinea pigs, which tend to exhibit profuse hypersalivation under anesthesia.

Gilroy (1981) reported successfully intubating guinea pigs (and smaller rabbits) using a canine otoscope for visualization of the larynx, and a 5-fr urinary catheter as a guide for the endotracheal tube (see Direct Visualization above). The author had success with over-the-endoscope intubation using a 2 mm Cole or pediatric straight endotracheal tube, however a tube constructed from an 8-fr red rubber urinary catheter may work better. A 1.9 mm endoscope is inserted down into the catheter as far as it will go, and the catheter is cut at an angle across the tube at this point, creating a beveled tip. The flared end of the catheter is trimmed to make it fit onto an endotracheal tube adapter. Then the 1.9 mm endoscope is inserted to within 1-2 mm from tip of the tube for placement.

Rats/mice

Jaffe and Free (1973) describe a blind technique for intubating the rat in which the anesthetized patient is placed in dorsal recumbency on the edge of a table with its head dependent. The rat's tongue is retracted to one side, and a stylet/endotracheal cannula combination is blindly advanced through the other side at a 45° angle below horizontal. Gagging by the patient is an indicator that the cannula assembly is properly positioned. Moderate resistance by the laryngeal folds may be met however gentle, persistent, forward pressure allows the cannula to enter the trachea. A weighing spatula is sometimes pre-placed in the patient's esophagus to (1) displace the soft palate and cause the patient to breath through its mouth, and (2) to serve as a guide for cannula placement.

Medd and Heywood (1970) devised a method for direct visualization of the trachea in rats that uses a tilting platform. The rat is placed in dorsal recumbency and affixed to a horizontal platform by his upper incisors and the front legs. The platform is then tilted vertically, the tongue is withdrawn to the side of the lower incisors, and fine laryngoscope with a magnifying lens is used to illuminate the larynx for intubation. A similar tilting platform and laryngoscope system is commercially available from Hallowell (Hallowell EMC, Pittsfield, MA). A modified otoscope is used to visualize the glottis, and lidocaine is applied to the larynx. An endotracheal cannula is advanced into the trachea with the aid of a stylus and then tied into place. Proper placement of the tube is confirmed by observing condensation on a dental mirror.

Confirming proper positioning

With blind intubation, proper positioning is assessed indirectly: the corrugations of the trachea may be felt as the tube is passed; the patient may cough; condensation can be seen inside the endotracheal tube or on glass or metal placed at the end of the tube; air movement can be detected by listening for breath sounds and watching for movement of the non-rebreathing bag or a hair placed at the end of the tube.

With visual methods of intubation, proper positioning is determined using all of the methods discussed above. With direct visualization, however, the operator also has direct visual evidence that the tube has been properly placed. He sees the tube as it passes through the vocal folds and, with the over-the-endoscope method, actually sees the inside of the trachea.

Discussion

Intubation of small exotic mammals takes practice. Repeated attempts may prolong anesthesia and injure the larynx. Laryngeal edema, spasm, or perforation may result. Applying a small amount of lidocaine to the tip of the tube or directly onto the vocal folds will decrease gagging and laryngospasm by the patient, making intubation easier. Intubation should not be attempted for more than a few minutes; discontinue if there is evidence of trauma.

Patients need to be closely observed while they are intubated. Due to the design of some endotracheal tubes (i.e. Cole) it is easy for accidental extubation to occur. Small endotracheal tubes kink readily, produce greater resistance to air movement, and are easily obstructed by secretions and mucus. Because most of the tubes utilized with this technique are uncuffed, patients are not completely protected from aspiration. If the endotracheal tube gets bitten or the adapter becomes dislodged, aspiration of the endotracheal tube (or portion thereof) can occur. Inadequate ventilation may occur if the endotracheal tube is inadvertently placed too deeply, into a bronchus. Accidental intubation of the esophagus can result in acute gastric dilatation if positive pressure is applied and the stomach is inflated.

Over-the-endoscope endotracheal intubation is a versatile technique. That can allow practitioners to safely and efficiently intubate ferrets, rabbits, guinea pigs, prairie dogs, chinchillas, and many other species. The procedure enables intubation to be performed by a single person with relative ease however the procedure takes practice to perform quickly. The glottal opening of the patient ultimately limits which patients can be intubated in this manner. For those patients that require an endotracheal tube smaller than 2 mm, a 1 mm version of the MDS Focuscope is available, or the endotracheal tube can be introduced beside the endoscope rather than over it.

Conclusion

Endotracheal intubation during general anesthesia is the standard of care for most companion animals because it offers many advantages over the use of a face mask alone. Intubation provides control over ventilation and facilitates resuscitation should respiratory arrest occur. Failure to maintain adequate ventilation is a frequent cause of death in anesthetized small exotic mammals. Intubation makes mechanical ventilation possible and permits the use of a capnograph, apnea alarm, and other monitoring equipment. An endotracheal tube protects against aspiration of GI contents, saliva, blood, etc, and makes it possible to operate on the face or in the oral cavity.

In spite of these benefits many practitioners routinely maintain general anesthesia using a face mask, especially for obligate nasal breathers such as rabbits, guinea pigs, chinchillas, and prairie dogs. This is partly because these species are unable to or rarely vomit, and partly because intubation of these species is difficult. Intubation techniques vary, and few practitioners are ever taught how to do it. However, as additional methods are developed and perfected, instruction on the endotracheal intubation of small exotic mammals will become more widely available, and the practice will undoubtedly be accepted as the standard of care for these species as well.

Over-the-endoscope intubation simplifies the intubation of small exotic mammals by putting the operator's "eyes", light source, and endotracheal tube all in one place so they move in unison. The endoscope/endotracheal tube combination is rigid enough that it can be directed into the trachea, and the endoscope allows the clinician to see that the tube has been properly inserted.

References

Bertolet RD, Hughes HC. Endotracheal intubation: An easy way to establish a patent airway in rabbits. Lab Animal Sci 1980;30(2):227-230.

Briscoe JA, Syring R. Techniques for emergency airway and vascular access in special species. Semin Avian Exotic Pet Med 13:3. Philadelphia: WB Saunders, 2004;119-125.

Conlon KC, Corbally MT, Bading JR, et al: Atraumatic endotracheal intubation in small rabbits. Lab Animal Sci 1990; 40:221-222.

Davis NL, Malinin TI. Rabbit intubation and halothane anesthesia. Lab Animal Sci 1974;24(4):617-621.

Gilroy A. Endotracheal intubation of rabbits and rodents. J Am Vet Med Assn 1981;183:1295.

Quesenberry K, Carpenter J. Ferrets, Rabbits, and Rodents: Clinical Medicine and Surgery, 2nd Ed. St. Louis: Saunders, 2004.

Jaffe RA, Free MJ. A simple endotracheal intubation technique for inhalation anesthesia of the rat. Lab Animal Sci 1973;23(2):266-269.

Johnson DH. Over-the-endoscope endotracheal intubation of small exotic mammals. Exotic DVM 2005;7(2):18-23.

Medd RK, Heywood R. A technique for intubation and repeated short-duration anesthesia in the rat. Lab Animals 1970; 4:75-78.

Schuyt CH, Leene W. An improved method of tracheal intubation in the rabbit. Lab Animal Sci 1977; 27:690-693.