Chapter 5. Oxygen Therapy
5.2 Oxygenation
In order for the nurse to understand the need to administer supplemental oxygen to a patient, a brief review of how we breathe and factors that affect oxygenation might be helpful. Supplemental oxygen is considered a medication and therefore requires continuous monitoring of the dose, concentration, and side effects to ensure its safe and effective use (Alberta Health Services, 2015). Oxygen therapy may be indicated for hypoxemia and hypoxia.
The air we breathe is made up of various gases, 21% of which is oxygen (Alberta Health Services, 2015). Therefore, a patient who is receiving no supplemental oxygen therapy is still receiving oxygen from the air. This amount of oxygen is adequate provided that the patient’s airway is not compromised and there is sufficient hemoglobin in the blood. The cardiovascular system must also be intact and able to circulate blood to all body tissues. If any of these systems fail, the patient will require supplemental oxygen to increase the likelihood that adequate levels of oxygen will reach all vital body tissues necessary to sustain life.
McCance, Huether, Brashers, and Rote (2014) describe four functional components of the respiratory system, all of which work in a concerted effort with the circulatory system to help our bodies maintain oxygenation. Table 5.1 outlines the four components and provides some explanation and health conditions that might present challenges with each.
Table 5.1 Four Functional Components of the Respiratory System & Health Conditions that Might Present Challenges in Terms of Increasing Risk of Impaired Oxygenation |
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| Neurochemical control of ventilation (respiratory center, chemoreceptors) | Ventilation is the movement of gases into and out of the lungs (Astle & Duggleby, 2019). The central nervous system controls ventilation by responding to signals from different neurochemicals (blood pH, PaCO2, PaO2 and others) at chemoreceptor sites located in different blood vessels. Some patients rely on the CO2 drive to breathe, and as such too much supplemental oxygen can cause them to stop breathing (Abdo & Heunks, 2012). |
| Mechanics of breathing (accessory muscles, lung elasticity, airway resistance, surface tension in alveoli, work of breathing) | Breathing is often effortless. Many people don’t realize that muscles like our diaphragm and intercostals muscles play a role in our ability to adequately oxygenate. Anything that can affect chest wall movement has the potential to affect ventilation. Some examples include pregnancy, obesity and chest wall trauma. Neuromuscular disorders can affect the diaphragm and intercostals muscle function as can medications like anesthetics (Astle & Duggleby, 2019). Certain lung diseases result in lost lung elasticity, and as such the work of breathing is increased as the lungs expand during inspiration and try to return to resting volume during expiration. Airway resistance might be caused by airway swelling, obstruction, and bronchospasm. Surfactant, a type of protein in our alveoli, prevents the alveoli from collapsing when we exhale. When surfactant is inadequate, the work of breathing is increased as the body tries to open the alveoli during inspiration to achieve adequate oxygenation. (McCance et al., 2014). Such is the case with many premature babies. If the mechanics of breathing is challenged, so is the work of breathing. |
| Gas transport (distribution of ventilation and perfusion, O2 and CO2 transport) | Oxygen is delivered to the cells and CO2 taken away. When caring for individuals nurses must consider if ventilation (exchange of air from the environment to the lungs) and perfusion (the amount of oxygen getting to the lungs) are adequate to meet the oxygen demands. O2 and CO2 exchange are impeded when the alveolar capillary membranes are thickened from conditions like pulmonary edema, pulmonary infiltrates, emphysema, and pneumothorax (McCance et al., 2014). Likewise persons with low Hgb may be challenged to meet their oxygen requirements (Astle & Duggleby, 2019). Some persons with chronic lung disease have abnormally high Hgb levels as a compensatory mechanism to help them achieve normal oxygen levels (McCance et al., 2014). |
| Control of pulmonary circulation (distribution of pulmonary blood flow) | Vasoconstriction of the vessels in pulmonary circulation can be the result of alveolar hypoxia. Possible causes of this hypoxia include obstruction, metabolic and respiratory acidosis, and other biochemical factors (histamine, prostaglandins, bradykinin, and others) (McCance et al., 2014). |
| Data sources: Abdo & Heunks, 2012; Astle & Duggleby, 2019 |
Hemoglobin
We rely on hemoglobin (Hgb) for gas transport. It holds oxygen in reserve until the metabolic demands of the body require more oxygen. The Hgb then moves the oxygen to the plasma for transport to the tissues. The body’s demand for oxygen is affected by activity, metabolic status, temperature, and level of anxiety. The ability of Hgb to move the oxygen to the tissues depends on a number of factors, such as oxygen supply, ventilatory effectiveness, nutrition, cardiac output, hemoglobin level, smoking, drug use, and underlying disease. Any one of these factors can potentially impede the supply and transport of oxygen to the tissues.
Measurement of Oxygen in the Blood
The vast majority of oxygen carried in the blood is attached to hemoglobin and can be assessed by monitoring the oxygen saturation through pulse oximetry (SpO2).The target range for oxygen saturation as measured by blood analysis (SaO2), such as arterial blood gas, is 92% to 98% for a normal adult. Arterial blood gas (ABG) is the analysis of an arterial blood sample to evaluate the adequacy of ventilation, oxygen delivery to the tissues, and acid-base balance status and is measured as SaO2 (Simpson, 2004). For patients with COPD, the target SaO2 range is 88% to 92% (Alberta Health Services, 2015; O’Driscoll, Howard, & Davison, 2008; Kane et al., 2013). Only about 3% of the oxygen carried in the blood is dissolved in the plasma, which can be assessed by looking at the partial pressure of oxygen in the blood through blood gas analysis (PaO2). The normal PaO2 of a healthy adult is 80 to 100 mmHg. The SpO2 is more clinically significant than the PaO2 in determining the oxygen content of the blood.
Oxygen is considered a medication and therefore requires continuous monitoring of the dose, concentration, and side effects to ensure its safe and effective use (Alberta Health Services, 2015). Oxygen therapy may be indicated for hypoxemia and hypoxia.
Understanding Hypoxemia and Hypoxia
Although the terms hypoxemia and hypoxia are often used interchangeably, they do not mean the same thing. Hypoxemia is a condition where arterial oxygen tension or partial pressure of oxygen (PaO2) is below normal (<80 mmHg). Hypoxemia is the inadequate supply of oxygen in the arterial blood. Hypoxia is the reduction of oxygen supply at the tissue level, which is not measured directly by a laboratory value (Meštrović, 2014), but by pulse oximetry (SpO2) (O’Driscoll et al., 2008).
Generally, the presence of hypoxemia suggests that hypoxia exists. However, hypoxia may not be present in a patient with hypoxemia if the patient is able to compensate for a low PaO2 by increasing oxygen supply. This is usually achieved by increasing cardiac output (by raising the heart rate) or by decreasing tissue oxygen consumption. Conversely, patients who do not show signs of hypoxemia may be hypoxic if oxygen delivery to the tissues is diminished or if the tissues are unable to adequately use the oxygen.
Hypoxemia is the most common cause of tissue hypoxia, and if the correct diagnosis is made, it is readily treatable. Hypoxemia has a number of causes including:
- Breathing air at pressures less than atmospheric pressure, such as at high altitudes or in an enclosed space with inadequate ventilation (Karius, nd).
- Hypoventilation such as happens with brain stem injury, neuromuscular impairment, atelectasis, and opioid overdose (Henderson & Bonsall, 2014).
- When the ventilation / perfusion ratios are mismatched. In other words the ratio of air coming in and going out does not match perfusion in the lungs (Mestrovic, 2014).
Examples of medical conditions that have the potential to cause hypoxemia include:
- Asthma
- COPD
- Heart failure
- Pleural effusions
- Pneumonia
- Pneumothorax
- Pulmonary edema
- Pulmonary emboli
With hypoxia, there is inadequate transport of oxygen to the cells or tissues, either because of obstruction, secretions, or tumours in the lungs; hypoventilation due to disease, injury to the respiratory system, or medications; or poor blood flow due to a compromised circulatory system (O’Driscoll et al., 2008). Hypoxia related to anemia or circulatory system compromise, such as decreased cardiac output, will respond poorly to oxygen therapy, and other appropriate interventions should be considered.
Oxygen Therapy Will:
- Decrease the work of breathing in patients with respiratory or cardiovascular conditions, which may prevent respiratory and muscle fatigue (Jardins & Burton, 2011).
- Decrease cardiopulmonary workload by reducing high cardiopulmonary demand (Perry et al., 2014). For example, patients with left ventricular failure benefit from additional oxygen to the tissues because the heart cannot provide enough oxygen to the tissues due to decreased cardiac output.
- Support post-operative recovery, and may be ordered for a specific time frame at a specific rate while the patient recovers from the surgical procedure.
It is important for the nurse to recognize early signs of respiratory compromise which might include shortness of breath, changes in mental status, anxiety, tachypnea (increase respiratory rate), and decreasing SpO2 despite increasing amounts of supplemental oxygen (Fournier, 2014). Hypoxia is a medical emergency (Alberta Health Services, 2015). Untreated hypoxia can result in anaerobic metabolism, acidosis, cell death, and organ failure (Considine, 2007). It is important for the nurse to build competence in recognizing hypoxia and to work within their scope of practice and agency policies and guidelines to provide treatment.
Critical Thinking Exercises
- Explain how you might know if your patient is hypoxic or hypoxemic?
- Why might a post-surgical patient require supplemental oxygen?