Introduction:

The cat is more difficult to intubate than the dog for several reasons including a small oropharynx and susceptability to laryngospasm. The degree of difficulty for feline intubation is similar to human babies and it is no coincidence that the cat is often used as a model for intubation of small children. However, with patience and proper preparation by the veterinarian or veterinary technician anesthetist, the cat can be efficiently and safely intubated.

A. Equipment     (Click on laryngoscope to enlarge.)

The primary materials and equipment required for efficient intubation of the feline include induction agent, laryngoscope, topical anesthetic, endotracheal tube, and gauze to secure the tube.

Thiopental      

is a good induction agent yet makes the larynx susceptible to laryngospasm.

A fifty-fifty mixture by volume of ketamine/diazepam

is also a good induction agent and has the added benefit of reducing laryngospasm. Mask induction with one of the inhalation agents, halothane, sevoflurane or isoflurane, provides good relaxation and a short window of opportunity for intubation. A laryngoscope is advantageous but not required if ample light is available. A pediatric size "O" Miller blade is a popular size for feline intubation. Two percent lidocaine is used topically to facilitate relaxation of the arytenoid cartilages. Administer directly on the arytenoid cartilages with a tuberculin syringe containing 0.2 ml of lidocaine.  There are two types of endotracheal tubes which are adaptable to the cat, the Murphy and the Cole tubes. The Murphy tube is a cuffed tube with a side hole opposite the bevel at the distal end of the tube. The Cole tube is a cuffless tube characterized by a "shoulder" near the distal end. New Murphy tubes are unnecessarily long for the cat and require trimming at the machine end. The tube when in place extends from just outside the incisors to just in front of the thoracic inlet. The Cole tube is designed with the patient end of the tube smaller than the remainder of the tube. When used properly, the patient end is close to the diameter of the trachea thereby preventing leaks and aspiration around the tube. To avoid laryngeal dilatation the shoulder of the tube should not put pressure on the larynx. Gauze or equivalent is used to secure the tube using a bow tie in back of the cat’s head.

B. Anesthesia Technique

After suitable sedative premedication, ketamine/diazepam is administered at dose of 1 ml/20 lb IV giving the first 1/2 in a bolus and the remainder to effect until the jaw is relaxed. Two and one-half percent thiopental may be substituted for ketamine/diazepam and administered at a rate of 2-3 mg/lb IV the first bolus followed by 1 mg/lb increments until the jaw is relaxed.

C. Positioning

After suitable relaxation from the anesthetic, position the cat in sternal recumbency with the head and neck extended towards the ceiling.

Use your thumb and index finger to hold the maxilla immediately posterior to the canine teeth. The little finger of the hand holding the maxilla can be placed on the posterior skull/neck area. This provides excellent support for the skull and allows the assistant to easily extend the head and neck. By rolling the upper lips around the teeth, the mandible will drop open allowing the assistant to insert the index finger of the opposite hand between the lower canine teeth to pull the tongue out of the mouth between the canine teeth. Using this method of positioning the tongue, the assistant is not likely to be bitten due to the cat’s lips rolled over the maxillary teeth. This method should only be used on cats that are relaxed from the induction dose of anesthesia. When holding the tongue, do not pull strongly as support tissues at the base of the tongue may be injured resulting in swelling of the tongue due to edema. In addition do not pull the tongue over the incisors at a strong downward angle as the incisors may lacerate the frenulum area. This image illustrates positioning of the tongue, which if pulled downward strongly, will cause trauma the ventral surface of the tongue. The tongue could also become intrapped by the endotracheal tube tie or the weight of a pulse oximeter probe could cause compression of the lingual veins over the lower arcade of teeth resulting in lingual edema.

D. Local Anesthesia

After the cat is positioned in sternal recumbency with the tongue pulled out and the head and neck pointed toward the ceiling, place one drop of 2% lidocaine on each arytenoid cartilage using a tuberculin syringe without a needle. To do this you will need to place the tip of the syringe behind the epiglottis and directly over the larynx.

Remember that one drop of 2% lidocaine is equivalent to 1-2 mg and that 1 mg/lb is considered to be the safe limit for total dose. If dose becomes a concern, 1% lidocaine can be substituted. This concern may not be real as systemic absorption of lidocaine from mucous membranes may not be efficient. Allow the lidocaine at least 1 minute to take effect before attempting intubation.

Technique:

With the cat in sternal position, place the blade of the laryngoscope on the tongue with the tip of blade as far posterior as the molars. Keep the blade off the epiglottis. When the blade is carefully depressed, the epiglottis should flatten allowing a good view of the larynx.

The endotracheal tube should be positioned past the epiglottis toward the glottis. Direct the tube to the ventral part of the glottis gently advancing through the opening. If the arytenoid cartilages are closed, gently rotate the tube using the bevel of the tube to open the cartilages. If the cartilages will not open, the lidocaine was not applied correctly. An alternative method for difficult intubations involves a 5 or 8 French canine polypropylene or polyethylene urinary catheter used as a guide tube. Place the guide tube through the endotracheal tube slightly past the patient end of the tube so that it enters the trachea first. Then slide the endotracheal tube off the guide tube into the trachea. Once the tube is in place, connect the adapter of the endotracheal tube to the anesthesia machine. Tie the tube in place using one throw of a square knot around the tube. Make sure this knot is snug so that the endotracheal tube will not slip through the knot. Then tie the ends of the gauze around the head using a bow tie. Make sure this knot is easily untied so that the tube can be quickly removed at the completion of surgery. After closing the pop-off valve, gently squeeze the rebreathing bag of the anesthesia machine to a pressure of 10-15 cm of water. Listen for leaks around the cuff of the endotracheal tube. If a leak is sensed, put a small amount of air into the cuff via the pilot balloon valve.

Try to adjust the volume in the cuff of the tube so that a leak is absent up to a pressure of 15 cm water. Over 15 cm of water pressure a leak should occur.   This technique of inflating the cuff will provide a safety valve to release excessive pressure preventing barotrauma. Do not rotate or otherwise move the cat unless the endotracheal tube is first detached from the anesthesia machine. This will prevent trauma to the epithelium of the trachea from the twisting action or movement of the tube.

F. Extubation

Cats awaken quickly from inhalation anesthesia. As the cat is recovering, untie the gauze holding the endotracheal tube and deflate the cuff. Monitor breathing until movement or swallowing attempts are noticed. Then remove the tube from the trachea. The cat may cough a few times immediately after removal of the tube but should begin to breathe with a normal pattern within seconds.

G. Review of important points

    1. Adequate muscle relaxation is necessary for atraumatic endotracheal intubation.

    2. Roll the cats upper lips around the maxilary teeth to open the mouth.

    3. Do not pull strongly on the tongue.

    4. Do not place the laryngoscope on the epiglottis.