Objective measurement of ventilation and oxygenation is best obtained by evaluation of arterial blood gas measurements. However, pulse oximetry is much more readily available than blood gas analyzers and is therefore probably used more frequently in small animal practice. Pulse oximetry measures the percentage of hemoglobin that is saturated with oxygen, an indirect measure of arterial oxygen levels. As the pulse oximeter readings decrease below 91%, oxygen levels drop sharply, according to the oxyhemoglobin saturation curve (
Figure 1).
Pulse oximeters do have limitations, including erroneous readings with peripheral vasoconstriction/low tissue perfusion, pigmentation of mucous membranes, and increased ambient lighting. When a pulse oximeter reading is assessed, the heart rate measured by the machine should match the patient's true heart rate, and a good waveform and strong signal should be verified.
In addition, hemoglobin abnormality (ie, carboxyhemoglobin and methemoglobin) will result in inaccurate pulse oximetry readings and should not be assessed in these situations. Carboxyhemoglobin is most often due to smoke inhalation injury, resulting in elevated levels of carbon monoxide in the blood. Methemoglobin most often results from acetaminophen toxicity, especially seen in cats. Hemoglobin is oxidized from its ferrous state (Fe2+) to ferric (Fe3+) iron, which is incapable of carrying oxygen. Another drawback to the use of the pulse oximeter is the inability to assess blood carbon dioxide levels; thus, ventilation cannot be evaluated.
Finally, anemia may result in a pulse oximetry reading that is higher than the actual oxygenation status of the patient because of relatively fewer oxygen-binding sites available. Polycythemia might lead to an opposite reaction because there would be an abnormally increased number of oxygen-binding sites available for a normal number of oxygen molecules in a healthy patient.
Oxygen Therapy
As previously stated, when oxygen is administered to a patient in respiratory distress, the method used should increase the level of FiO2 while inducing the lowest amount of stress to the patient.
- Mask or flow-by O2 provides for a very high FiO2, but unless the patient is recumbent with decreased mentation, it is not a functional method for administering oxygen on a long-term basis. This method is primarily used for patients that present in respiratory distress and require immediate oxygen therapy while being assessed for the cause of the pulmonary dysfunction. Mask or flow-by O2 can also be used during procedures such as thoracic radiography, thoracocentesis, and transtracheal wash. When flow-by O2 is being administered, the oxygen flow should not be directed into the patient's face because many animals dislike the sensation. In addition, oxygen masks can induce undue stress and should not be used if the patient begins to resist. Flow-by O2 can be administered horizontally, in front of the animal's nose; this is usually tolerated very well in dyspneic patients.
- An enclosed oxygen cage or neonatal incubator is the preferred method for oxygen administration in cats with respiratory distress. An FiO2 of approximately 40% can be achieved by using this method. Many commercial oxygen cages provide the FiO2 value in the cage environment and also regulate humidity and temperature in the cage. The disadvantages of incubators or unregulated oxygen cages include an unknown FiO2 concentration, no humidification of the oxygen administered (unless the oxygen is run through a water source), and the risk for overheating due to the enclosed environment of the cage/incubator. For these reasons, a commercial oxygen unit is recommended. When an oxygen cage or incubator is used for patient oxygen support, treatments should be combined so that the number of times the doors are opened is minimized.
- Nasal oxygen administration through the use of unilateral or bilateral nasal cannulas is the recommended form of oxygen administration for dogs. Once they have been placed, nasal cannulas provide a consistent, high level of oxygen flow with minimal stress to the patient. In patients with extreme respiratory distress, nasal cannulation may be too stressful a procedure, so placement in an oxygen cage is preferred. With nasal cannulation, flow rates usually cannot exceed 2 L/min with one cannula and 4 L/min with bilateral cannulas, allowing for FiO2 levels of approximately 40% to 60%, depending on the number of cannulas and oxygen flow rates. Higher flow rates cause discomfort of the nasal mucosa from the forceful jet of oxygen through the tubing. Disadvantages of oxygen administration through nasal cannulas include the stress involved with placement, the tendency for easy removal through sneezing or pawing at the area, and clogging of the tube with nasal secretions after a prolonged period of use.
What You Will Need