Oxygen Supplementation

Background

The practice of oxygen supplementation for hospitalized patients generally does not follow any standardized protocols. It is often a hospital norm that patients are provided supplemental oxygen regardless of their blood oxygen saturations (SaO ₂ ). Until recently, many healthcare professionals believed that oxygen had little or no deleterious effects. However, recent publications indicate that supplemental oxygen use in patients with normal SaO ₂ increases mortality.

It is widely recognized that severe hypoxemia leads to rapid organ failure and death. Supplemental oxygen, when used appropriately to treat hypoxemia, is an essential aspect of resuscitation and will improve mortality. However, there is little clinical evidence that hyperoxia, or supraphysiologic levels of oxygen, has any clinical benefit.

Supplemental oxygen is widely available and frequently used in the hospital setting, and as such, there is some terminology that is important to be aware of when discussing oxygen use. The purpose of oxygen supplementation is to maintain global oxygen delivery. Oxygen delivery describes the amount of oxygen delivered to tissues in each minute and is a product of the cardiac output and arterial oxygen content. Oxygen content of arterial blood is the sum of the oxygen bound to hemoglobin and the oxygen dissolved in plasma in each 100 mL of blood. Oxygen extraction is the fraction of oxygen delivered to the tissues that is actually utilized by those tissues. It is the ratio of oxygen consumption to oxygen delivery. In a healthy adult, about 20%–30% of oxygen delivered is utilized by the tissues and can be noted in the difference in pressure of oxygen present in an arterial blood gas (ABG) compared with a central venous blood gas. A full understanding of oxygen delivery, consumption, and extraction is important for pulmonary physiology but too vast a topic for this chapter and can be found in pulmonary physiology textbooks such as West’s Respiratory Physiology: The Essentials . Lastly, oxygen supplementation is used to treat hypoxia. Hypoxia is defined as low oxygen content and pressure within cells. It is caused by dysregulated oxygen delivery, poor cellular oxygen utilization, and hypoxemia, which is a low partial pressure of oxygen within the blood.

Hypoxia is evaluated by the partial pressure of dissolved oxygen in arterial blood (PaO ₂ ) and is measured via an ABG. However, repeated arterial blood draws are often not feasible due to difficulty of blood draw, pain and discomfort, and lack of an arterial line for continuous blood monitoring outside of an intensive care unit setting. Therefore, pulse oximetry can provide oxygen saturation via absorption of specific wavelengths of light to compare oxyhemoglobin with deoxyhemoglobin. Oxygen saturation is the ratio of oxygenated hemoglobin to total hemoglobin and thereby provides an indirect measure of PaO ₂ . This is less invasive and easier to follow. Pulse oximetry has the added benefit of being immediate, continuous (if necessary), inexpensive, and does not cause the patient any pain. In addition, these devices can be purchased easily and be used at the point of care both by physicians in the hospital and patients at home.