OXYGEN MASKS

Oxygen masks are useful for short-term supplementation. The animal experiences minimal stress, and manipulations such as venous catheter placement and thoracocentesis can be performed. A snug fit is desirable to decrease the volume of dead space, and a relatively high flow rate is necessary (Table 27-1). Sterile eye ointment is applied to prevent desiccation of the corneas.

TABLE 27-1 Maximum Achievable Oxygen Concentrations and Associated Flow Rates for Various Methods of Supplementation

* After cage is filled, flow is adjusted based on oxygen concentration as measured by oxygen sensor.

From Court MH et al: Inhalation therapy: oxygen administration, humidification, and aerosol therapy, Vet Clin North Am Small Anim Pract 15:1041, 1985.

OXYGEN HOODS

Oxygen hoods that can be placed over the animal’s head are available. With some, the animals must be laterally recumbent and still, limiting the use of hoods to animals recovering from anesthesia, those that are severely depressed, and those that are heavily sedated (Fig. 27-1). Others are designed to completely surround the animal’s head and are attached around the neck. One design is an adaptation of an Elizabethan collar (OxyHood, Jorgensen Laboratories, Inc.). In some situations oxygen hoods may be better tolerated than oxygen masks, and it may take less manpower to care for an animal for which one is being used than an animal with an oxygen mask. A means for escape of exhaled air must always be provided to prevent the buildup of CO₂ within the hood.

FIG 27-1 An oxygen hood can be used for recumbent animals as a substitute for an oxygen mask. In this patient oxygen is being delivered through an opening in the top of the hood, and the light blue opening that will accommodate standard anesthesia tubing is left open for circulation of air. Regardless of the method used to increase the oxygen in inspired air, a means for escape of expired CO₂ is essential. (Disposa-Hood, Utah Medical Products, Inc., Midvale, Utah.)

NASAL CATHETERS

Nasal catheters can be used for long-term oxygen supplementation (Fig. 27-2). The animal is relatively free to move and is accessible for evaluation and treatment. Most animals tolerate the catheter well. Catheters can become obstructed with nasal secretions, however. Soft red rubber or infant feeding tubes or polyurethane catheters can be used. Tube size is based on patient size. In general, a 3.5 to 5 French tube is used for cats, and a 5 to 8 French tube is used for dogs.

FIG 27-2 Dog with intranasal catheter in place for delivery of oxygen. The catheter is sutured to the muzzle less than 1 cm from its exit from the naris and is further anchored with sutures to the face so that it exits behind the animal’s head. An Elizabethan collar is routinely used to prevent the animal from removing the catheter.

The method of placement has been described by Fitzpatrick et al. (1986). First, the length of tubing to be inserted into the nasal cavity is measured against the head of the animal. The tubing should reach the level of the carnassial tooth. Sedation is rarely necessary. A water-soluble lubricant or 0.2% lidocaine jelly is applied to the length of the catheter that will be within the nasal cavity. Next, 0.2% lidocaine is dripped gently into the nasal cavity through the naris with the animal’s nose pointed upward. The catheter is then passed through the naris, initially aimed dorsomedially through the naris, then immediately ventromedially. Once the correct length of catheter has been inserted, it is gently bent beneath the lateral cartilage and sutured to the muzzle no farther than 1 cm caudal to the exit from the naris. The catheter can be further anchored to the face with sutures, traveling between the eyes to behind the animal’s head. An Elizabethan collar is placed on the patient to prevent the animal from removing the catheter.

A sterile intravenous set can be connected to the catheter. The intravenous line can be attached to a half-filled bottle of sterile saline solution and positioned above the fluid level. Oxygen is then delivered through the bottle, below the fluid level, providing some moisture as the oxygen bubbles through the saline.

TRANSTRACHEAL CATHETERS

Oxygen can be administered through a jugular catheter placed with a sterile technique through the trachea. This approach is particularly useful for the emergency stabilization of animals with an upper airway obstruction. Branditz et al. (1989) have described a method for cardiopulmonary resuscitation that can be performed by one person by administering oxygen at a high flow rate of 15 L/ min through a tracheal catheter. In this method a large jugular catheter is placed as described for transtracheal washing (see Chapter 20).

ENDOTRACHEAL TUBES

Endotracheal tubes are used to administer oxygen during surgical procedures and cardiopulmonary resuscitation. They can be used to bypass most upper airway obstructions for emergency stabilization. Pure oxygen can be administered for short periods. Longer supplementation requires the mixing of 100% oxygen with room air. Ventilation can be provided with a cuffed endotracheal tube. Trauma to the trachea is decreased through the use of high-volume, low-pressure cuffs and by inflating the cuff with the least amount of pressure necessary to create a seal. If positive-pressure ventilation is not being used, the cuff can remain deflated.

Because endotracheal tubes are not tolerated by alert animals, tracheal tubes are preferred for long-term management. Conscious animals in which endotracheal tubes are used must be given sedatives, analgesics, paralyzing agents, or a combination of these drugs. The combination of hydromorphone and diazepam is adequate in some animals. Pentobarbital, administered intravenously to effect, can be added if necessary. The combination of ketamine and valium may be safer for the initial intubation of patients that are hypoxemic. Following intubation and improvement in hypoxemia, morphine and pancuronium can be given.

The cuff should be deflated when possible to minimize tracheal damage. The tube must be cleaned periodically to remove secretions (see the recommendations for tracheal tube cleaning), and frequent flushing of the oral cavity is performed. Moisture must be added to the inspired gases, as previously discussed.

TRACHEAL TUBES

Tracheal tubes are placed through the tracheal rings and are readily tolerated by conscious animals. It is rare that an animal requires an emergency tracheostomy. Nearly all such animals can be stabilized using other techniques. Thus tracheal tubes can be placed using a careful, sterile surgical technique. Tracheal tubes are generally used for the management of animals with an upper airway obstruction. Room air often contains adequate oxygen for use in animals with an upper airway obstruction once the obstruction has been bypassed.

The tube itself should have a diameter nearly as wide as the tracheal lumen and a length of 5 to 10 rings. It is necessary to use high-volume, low-pressure cuffs to prevent tracheal damage and subsequent stricture. Double-lumen tubes are ideal for this method. The inner tube can be removed for cleaning and replaced easily. Single-lumen tubes also work and may be necessary in small animals.

Tracheal tubes are usually placed with the animal anesthetized with a short-acting agent. The trachea is exposed through a ventral midline incision made just beneath the larynx. The trachea is entered through an incision made a few rings below the cricoid cartilage, parallel to the trachea and perpendicular to the rings, and through just enough rings to allow passage of the tube. Either end of the incision can be widened with a small transverse incision. Stay sutures are placed on each side of the incision to facilitate initial placement of the tube as well as later replacement if the tube is accidentally or intentionally removed. The tube is then inserted into the opening. With minimal pressure on the airway, it is tied with gauze around the neck of the animal. Few or no sutures are used to close the incision to prevent the collection of air subcutaneously. A gauze sponge with a slit cut in it and coated with antiseptic ointment can be placed over the incision and around the tube.

The tube must be monitored for obstruction and cleaned. The inner tube of double-lumen tubes can be easily removed for this purpose. The tube is cleaned every 30 to 60 minutes initially, with the interval increased as less secretions accumulate. Sterile technique is used when handling the tubes, and they must be replaced if they become contaminated.

Single-lumen tubes are difficult to remove and replace safely for the first few days unless stay sutures are left in place. Periodic cleaning can be performed with the tube in place. Sterile saline solution is instilled into the tube for this purpose. To perform suctioning, a sterile urinary catheter with several openings at the end is attached to a suction unit and passed through the tube. The trachea and tracheal tube are then suctioned to remove secretions. Suctioning is performed for short intervals to allow the lungs to reinflate. Cleaning is performed every few hours initially, then less frequently if secretions are not accumulating.

A smaller tube can be used once the animal is able to oxygenate adequately with room air. The tube can be removed when the animal can oxygenate by breathing around a small tube with the lumen obstructed. The incision is allowed to heal without suturing. The tip of the tube is cultured for bacteria.

Antibiotics are not administered prophylactically. Any existing infection or infections that occur during therapy are treated on the basis of culture and sensitivity information.

OXYGEN CAGES

Oxygen cages provide an oxygen-enriched environment with minimal stress to animals. However, the animal is isolated from direct contact, which can be a disadvantage. Other environmental factors, such as humidity, temperature, and carbon dioxide concentration, must be monitored and controlled or extreme stress and even death can occur. The animal is totally dependent on proper cage function. The ability of the cage to maintain the correct environment varies with the specific cage as well as with each animal. Commercial cages are available for veterinary use. Incubators from human hospitals can be modified for small animals.