Oxygen Therapy

Chapter 19 Oxygen Therapy



Elisa M. Mazzaferro, MS, DVM, PhD, DACVECC



KEY POINTS



• Hypoxemia occurs in a variety of critical illnesses. Oxygen supplementation can improve oxygen delivery and decrease the incidence of lactic acidosis.

• Supplemental oxygen administration should be provided whenever a patient’s partial pressure of arterial oxygen (PaO2) is less than 70 mm Hg, oxygen saturation (SaO2) is less than 93% on room air, and in cases of significant anemia or cardiovascular instability.

• Noninvasive means of oxygen supplementation, including flow-by, mask, hood, and oxygen cage, are a simple means of providing an oxygen-enriched environment to a critically ill patient.

• Nasal and nasopharyngeal oxygen supplementation require minimal equipment and are well tolerated by many hypoxemic patients.

• Tracheal oxygen supplementation provides a higher fraction of inspired oxygen (FiO2) than nasal or noninvasive means of oxygen therapy, but it is technically more difficult and has higher inherent risks to the patient.


INTRODUCTION AND INDICATIONS FOR OXYGEN THERAPY


Hypoxemia is defined as a deficiency of oxygen in the arterial blood.1,2 Hypoxemia can occur as a result of hypoventilation, ventilation-perfusion mismatch, diffusion impairment, decreased oxygen content of inspired air, and intrapulmonary or cardiac shunting. Global hypoxemia can occur in a variety of critical illnesses, including pulmonary parenchymal, neuromuscular, pleural cavity, chest wall, and cardiac disease. Hypoxemia results in inadequate delivery of oxygen to the tissues and subsequent cellular hypoxia. Systemic illnesses such as sepsis, systemic inflammatory response syndrome (SIRS), anemia, and acid-base imbalances such as a metabolic or respiratory alkalosis or acidosis can also contribute to inadequate tissue oxygen delivery.


Oxygen supplementation is one of the most important management tools for a variety of critical conditions, including cardiopulmonary disease, sepsis, SIRS, and head trauma. Delivery of oxygen to tissues is affected by the patient’s arterial oxygen content (CaO2) and cardiac output (Q). The formula to calculate arterial oxygen content is as follows:



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where (SaO2) is arterial oxygen saturation and (PaO2) is partial pressure of arterial oxygen.


Arterial oxygen content depends on the concentration of hemoglobin as well as the binding affinity and the SaO2 of the hemoglobin. Most of the arterial oxygen is carried to tissues while bound to hemoglobin. A small fraction is carried dissolved (or unbound [0.003 × PaO2]) in plasma and subsequently diffuses into the tissues. Providing supplemental oxygen to increase the fractional concentration of oxygen in inspired gas (FiO2) above 21% is an effective means of increasing both bound and unbound oxygen in arterial blood, provided that a cardiac or pulmonary parenchymal shunt is not present.1


Supplemental oxygen should be provided whenever a patient’s PaO2 is less than 70 mm Hg or SaO2 is less than 93% on room air.1 Oxygen administration can be divided into noninvasive and invasive techniques. The method of supplementation is dependent on each patient’s needs and tolerance, patient size, degree of hypoxemia, desired FiO2, anticipated duration, clinical experience and skill, and the equipment and monitoring means available.2



HUMIDIFICATION


Oxygen supplementation can be accomplished in a number of ways. All methods require some kind of humidification if used for more than several hours. Otherwise the patient is likely to experience drying and dehydration of the nasal mucosa, respiratory epithelial degeneration, impaired mucociliary clearance, and an increased risk of infection.2 A supplemental oxygen source can be humidified easily by bubbling the oxygen through a tube that is submerged in a bottle of sterile saline.2 The humidified oxygen then accumulates above the surface of the solution. The gas that collects can then be delivered through a length of tubing to the patient’s oxygen delivery device, whether it is a mask or tube into some component of the respiratory tract.



NONINVASIVE METHODS OF OXYGEN SUPPLEMENTATION



Flow-By Oxygen


Flow-by oxygen supplementation is one of the simplest techniques to use in an emergency setting. A length of oxygen tubing connected to an oxygen source is held adjacent to or within 2 cm of a patient’s nostril or mouth. An oxygen flow rate of 2 to 3 L/min generally provides an FiO2 of 25% to 40%.3 This technique is well tolerated by most patients and can be used during the initial triage and patient assessment. Because this technique also delivers a large quantity of oxygen to the surrounding environment, it is wasteful and not appropriate or economical for long-term use.



Face Mask


Short-term oxygen supplementation can be accomplished by placing a face mask over a patient’s muzzle, then delivering oxygen (humidified if oxygen administration of more than a few hours is required) or tank oxygen in a circle rebreathing or a nonrebreathing circuit. With a tight-fitting face mask, flow rates of 8 to 12 L/min can provide an FiO2 of up to 50% to 60%.3 With loose-fitting face masks, higher flow rates of 2 to 5 L/min are recommended, depending on the size of the patient and degree of hypoxemia. With a tight-fitting face mask, rebreathing of carbon dioxide can occur, as well as dangerous increases in temperature and humidity. The face mask should be vented periodically, or changed to a looser face mask or other means of oxygen supplementation as soon as possible.


Awake and coherent patients may not tolerate oxygen delivered by face mask for long periods. An attendant must be present to ensure that the mask does not become detached and that the patient does not struggle or damage its eyes with the edge of the mask.4 Advantages of this technique are that minimal equipment is required and that the patient can be simultaneously treated and evaluated in emergency situations.



Oxygen Hood


Several varieties of oxygen hood are available from commercial manufacturers, or they can be made easily in the hospital with clear plastic wrap such as Saran Wrap, tape, and a rigid Elizabethan collar (or recycled x-ray film of appropriate size). To create an oxygen hood, the front of a rigid Elizabethan collar is covered with lengths of clear plastic wrap taped in place. A small portion of the front is left open to room air to allow the hood to vent. The collar is then placed around the patient’s neck and secured snugly. A length of oxygen tubing is inserted through the back of the collar and taped to the inside of the collar so that it will not be dislodged with patient movement. Once the hood has been flooded with oxygen (1 to 2 L/min), flow rates of 0.5 to 1 L/min typically will deliver an FiO2 of 30% to 40%,5 depending on the size of the patient and how tightly the collar fits. With extremely small patients, such as toy breeds or neonates, the entire patient can be placed in the collar for a homemade oxygen tent or minicage. Some patients will not tolerate the collar and can become hyperthermic. If the hood is left unvented, carbon dioxide and moisture can accumulate within the hood and contribute to patient distress. Overall, an oxygen hood is an economical and practical means of supplemental oxygen administration and is well tolerated by most patients.

Related posts:

  1. Deteriorating Mental Status
  2. Hyperthermia and Fever
  3. Ventilator-Associated Lung Injury
  4. Pulmonary Contusions and Hemorrhage

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Sep 10, 2016 | Posted by in SMALL ANIMAL | Comments Off on Oxygen Therapy

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