Canine and Feline Respiratory Medicine. Lynelle R. Johnson

the catheter. When preparing to enter between the tracheal rings, the trachea is firmly stabilized against the neck muscles to prevent it from moving away from the needle. The tracheal rings and annular ligament can often be felt with the tip of the needle, allowing entry directly between the cartilage rings. The trachea is almost directly below the skin and only a minor amount of forward motion is required to enter the lumen. A distinct pop is usually felt when the needle enters the tracheal lumen, and the dog often coughs. With the needle in the airway, the angle between the needle or catheter and the trachea is decreased to 60° to facilitate passage of the catheter down the center of the airway (Figure 2.7b). The needle is withdrawn at this stage to pass the sampling catheter through the short catheter to the level of the carina (Figure 2.8).

Figure 2.7 (a) To initiate a transtracheal wash, the needle‐through‐catheter is directed into the trachea at a perpendicular angle. (b) After it has penetrated between the tracheal rings, the hub of the needle‐through‐catheter is moved toward the neck to create a more parallel angle to feed the catheter off the needle in preparation for passing the urinary catheter.

Figure 2.8 The short catheter has been seated between the tracheal rings and is grasped firmly while the long catheter is passed down the trachea for collection of an airway sample.

      The urinary catheter should move freely down the airway and passage usually stimulates coughing. If neither of these occurs, the sampling catheter could be traveling in subcutaneous tissue and both catheters should be removed in order to repeat the approach to the trachea.

      When the sampling catheter is fully deployed, the three‐way stopcock and syringe can be attached to perform the airway wash. Individual aliquots of saline (4–10 ml) followed by 2–3 ml of air are instilled into the airway, and suction is used to retrieve the fluid and cells from the lower airway, as detailed earlier.

Respiratory Endoscopy

      Rhinoscopy

A full rhinoscopic examination includes evaluation of the caudal nasopharynx to visualize nasal openings at the choanae and inspection of the rostral nasal cavity. The best equipment to use in evaluating the caudal nasopharynx is an endoscope capable of 180° flexion that is ~5 mm in outer diameter (Figure 2.9). Smaller flexible endoscopes can be used, but are at greater risk for damage if the animal gags and the pharynx closes down on the scope. Examining the nasopharynx is a highly stimulatory procedure. Local anesthesia of the oropharynx with lidocaine gel on long cotton swabs can facilitate placement of equipment around the soft palate. Two mouth gags are in place to hold the mouth wide open during placement of the endoscope or other equipment. Caution is warranted when performing this examination in cats, because over‐extension of the jaw can reduce retinal blood flow through the maxillary artery (the continuation of the external carotid as it crosses the mandible), resulting in blindness (Barton‐Lamb et al. 2013). Therefore, excess opening of the jaw should be avoided and time spent completing the examination kept to a minimum. Extending the neck and pulling the tongue forward will also help place the instruments in the proper position. To achieve the appropriate view with an endoscope, the non‐flexed scope is passed into the oral cavity beyond the soft palate and then flexed until the light can be seen above the soft palate. The scope is then pulled forward (with the flex in place) until the choanae are visualized. Note that the image is upside down and backward (Figure 2.10).

Figure 2.9 Visualization of the choanae is obtained by inserting a flexible endoscope into the oral cavity and flexing the scope maximally (180°) around the soft palate.

      If an endoscope is unavailable to complete the retroflexed exam, a bright light source and dental mirror can be used, along with a spay hook to retract the soft palate cranially, although this can be quite challenging in smaller animals. The dental mirror is placed in the back of the pharynx and angled above the soft palate while a light source is used to illuminate the view of the choanae in the mirror.

Foreign material can be removed from the nasopharynx or choanae using the endoscope and samples can be obtained for histopathology. With the endoscope removed from the patient and the tip of the scope in a non‐flexed position, forceps are passed through the biopsy channel until they are at the end of the scope. The scope is then inserted into the mouth and flexed about the soft palate, as previously described. The degree of flexion that the scope can achieve is reduced by the presence of forceps in the channel, making visualization of any lesion more challenging. Forceps are passed forward to grab a foreign body or mass lesion and firm traction is applied. It is most efficient and safest to unflex the scope, withdraw it from the patient, and extrude the forceps to remove the sample, rather than pulling the sample and instrument back through the scope.

Figure 2.10 Image of the normal nasopharynx in (a) a dog and (b) a cat. The soft palate is dorsal in these images.

After the caudal nasopharynx is examined and samples are obtained for histopathology as indicated, a moistened surgical lap pad is used to pack the throat in order to prevent aspiration during rostral rhinoscopy. Equipment available for rostral rhinoscopy includes rigid endoscopes (with or without an external sheath), otoscopes, and small flexible endoscopes. Rigid scopes have better optics, are easier to maneuver, and come in smaller sizes, while flexible scopes allow greater access to the nasal cavity and can permit examination of the frontal sinuses when there is marked turbinate destruction (such as in nasal aspergillosis). Rigid scopes are available with viewing angles from 0 to 30°. Rhinoscopy can be performed using the telescope portion of the rigid scope alone (∼2.8 mm outer diameter) or by using the sheathed scope (∼5 mm outer diameter), which has flush and suction ports as well as a biopsy port available (Figure 2.11). The biopsy port of the sheath will accept standard endoscopic biopsy or foreign body retrieval instruments.

Before entering the nasal cavity, the endoscope is placed against the skull and measured to the level of the medial canthus of the eye to approximate the position of the cribriform plate. A piece of tape is applied to the instrument at this length, and the equipment should not be passed further than this point to avoid penetrating through the cribriform plate into the brain case. The normal nasal cavity is made up of scrolls of turbinates comprising the dorsal, middle, and ventral concha. The mucosa is generally pink, with a pale sheen of serous secretions coating the smooth epithelial surface (Figure 2.12). The primary changes to look for during rhinoscopy are mucosal hyperemia, mucus accumulation, epithelial irregularities, turbinate destruction that is visualized as increased space between turbinates, or a mass effect that reduces space in the nasal cavity. After the initial visual inspection and when the cribriform plate is intact, nasal drops containing oxymetazoline or phenylephrine can be dropped or sprayed into the nasal cavity to create