Oxygen Therapy

Clinical signs of hypoxia include dyspnea, cyanosis, tachycardia, tachypnea, postural changes, anxiety, and/or central nervous system depression. If clinical signs, arterial blood gases, pulse oximetry, or the patient's disease suggest hypoxia, supplemental oxygen may be administered via mask, tent, flow-by, nasal catheter, or the animal may be placed in an oxygen cage or tent.

Note: Remember that patients can be hypoxic without showing signs of cyanosis because greater than 5 g/dl of deoxygenated hemoglobin must be present in the circulation before cyanosis can be detected. The patient must have a PCV of approximately 15% to have 5 g hemoglobin/dl.

Flow-by oxygen may be the easiest way to provide supplemental oxygen in an emergency situation (Table 1). The oxygen line is placed within 1 to 3 cm of the patient's nose and mouth, which creates a small area where the fraction of inspired oxygen (F_IO₂) is increased. However, because it requires a care provider to be present to hold the oxygen line and to ensure that the patient does not move away from it, and a high oxygen flow rate is required, it is not always practical or the best choice. Furthermore, it is not nearly as effective as the other methods described below.

Table 1. Methods for supplementing oxygen.

Face mask delivery of oxygen is a useful short-term method to provide supplemental oxygen. With an oxygen flow rate of 6 to 10 L/min and a well-fitted mask, an F_IO₂ of 0.35 to 0.55 may be achieved (see Table 1). Be aware that face masks may not be tolerated (especially in severely dyspneic animals) and are often difficult to fit well to the faces of cats and brachycephalic dogs. An alternative is to use an Elizabethan collar covered with plastic wrap to create an oxygen-enriched environment. The end of the oxygen tube should be fed up through the collar and secured. To allow elimination of CO₂, make a small hole in the plastic wrap.

Nasal catheters may be used when more prolonged oxygen delivery is desired than can be achieved with flow-by or face mask techniques (Table 2). Other advantages of nasal catheter delivery of oxygen is that it permits access to the patient without loss of the oxygen-rich environment (vs. an oxygen cage) and it is well tolerated in most patients. When necessary, bilateral catheters can be placed. The appropriate oxygen flow rate is based upon assessment of the degree of respiratory distress, the patient's respiratory rate and pattern, and the patient's size. The recommended initial therapeutic dose for unilateral nasal oxygen supplementation is approximately 50 to 100 ml/kg/min. These flow rates can achieve a tracheal F_IO₂ of approximately 50%. Although high gas flow rates can be administered through a single nasal catheter, these high flow rates may be associated with patient discomfort. In such cases, administering oxygen through bilateral nasal catheters may be justified. A recent study showed that although F_IO₂ and PaO₂ could be increased with higher total oxygen flow rates, the increase is the same whether the higher flow is delivered through one nasal catheter, or two. Thus, the use of bilateral nasal catheters appears to be beneficial primarily in improving patient comfort. When oxygen is administered via nasal catheter for prolonged periods (i.e., greater than 6 to 12 hours), it should be humidified. Intratracheal catheters may be used in animals that will not tolerate an intranasal catheter (see Table 1).

Table 2. Nasal oxygen insufflation.

1.  Select a small, red rubber feeding tube (3.5-5 Fr for cats; 5-8 Fr for dogs) to serve as a catheter and lubricate the tip with lidocaine gel.

2.  Place one or two drops of local anesthetic (e.g., 2% lidocaine or proparacaine) in the nostril.

3.  Pre-measure the catheter to the medial canthus of the eye or the caudal ramus of the mandible.

4.  Elevate the dorsal aspect of the nose and feed the lubricated catheter into the nostril the predetermined distance.

5.  Suture or glue (e.g., VetBond) the catheter to the external naris and muzzle and over the frontal sinus or along the jaw. With cats, do not allow the tube to touch the whiskers.

6.  Place an Elizabethan collar on the animal.

7.  Attach the tube to an oxygen source and administer humidified oxygen to maintain oxygen saturation at greater than 90%; typically start at 50 ml/kg/minute and adjust as needed.*

* Gastric distention may occur if the flow rate is too high.

An oxygen cage provides a sealed environment in which the F_IO₂, ambient temperature, and humidity can be controlled. An ambient temperature of 70°F and relative humidity of 40% to 50% is desired. The major disadvantage of an oxygen cage is that is isolates the patient from the clinician because each time the oxygen cage door is opened there is a loss of the oxygen rich environment.