by Tom Pietrantonio, BSRT, RRT-ACCS
MedCenter Air, Carolinas HealthCare System, Charlotte, NC
Oxygen is able to move throughout the body by the process of diffusion. Diffusion occurs because there is a partial pressure gradient between two points. When a partial pressure gradient exists, oxygen will readily diffuse from an area of higher concentration to an area of lower concentration. According to Fick’s Law, the rate of diffusion is directly proportional to the difference in pressures and inversely proportional to the thickness of the membrane to which oxygen must diffuse across; thus, the larger the pressure gradient, the greater the rate of diffusion. The thicker the membrane, the less the rate of diffusion.
Oxygen transport relies on a normal functioning respiratory and cardiovascular system. That is, there must be a sufficient amount of oxygen in the lungs (PAO2) and the blood (PaO2), adequate blood flow and hemoglobin availability to transport oxygen, and the ability of the body tissue cells to use and metabolize oxygen. Anything that disrupts the normal physiologic process of oxygen transport will create a condition of hypoxia
What Is Hypoxia
Hypoxia is the result of a deficient supply of oxygen to the body tissues. If hypoxia is not corrected and is allowed to manifest to a severe state, the ultimate outcome results in cell death (Guyton and Hall). The addition of supplemental oxygen is usually the necessary corrective action; however, depending on the type of hypoxia, supplemental oxygen can have little to no value (Guyton & Hall, 2008). It is important to understand the different types of hypoxia in order to determine the value of providing supplemental oxygen therapy.
Arguably the most common type of hypoxia encountered, hypoxic hypoxia is caused by inadequate oxygen at the tissues in response to a low partial pressure of oxygen in arterial blood (PaO2) (Des Jardins, 2007). This type of hypoxia occurs from a variety of respiratory complications such as a low partial pressure of oxygen in the lungs (PAO2), diffusion defects, V/Q mismatching, and pulmonary shunting. Hypoxic hypoxia results from a problem with the respiratory system. Compensation occurs naturally as vessels dilate to increase perfusion and oxygen delivery; however, the administration of supplemental oxygen is of high value and should be provided to correct the situation.
When oxygen diffuses across the alveolar-capillary membrane it binds to the hemoglobin molecules in the blood. If there is a reduction of hemoglobin availability, oxygen cannot be transported in sufficient quantity to the body tissues. Blood loss, anemia, hemoglobin abnormalities, and carbon monoxide (CO) poising are liable causes to induce anemic hypoxia, thus, the problem is related to the blood and not the respiratory system. Similar to hypoxic hypoxia, compensation is initiated when vessels dilate to increase perfusion and oxygen delivery. Providing supplemental oxygen is of high value when CO poisoning is suspected. CO has a greater affinity for hemoglobin than oxygen, thus oxygen will have to compete to bind to hemoglobin in the presence of CO. The administration of 100% oxygen is the recommended method of treatment in the presence of CO poisoning. In all other situations, the use of supplemental oxygen is of little value. Reason being, as oxygen diffuses across the alveolar-capillary membrane, a small amount of oxygen dissolves in the blood. It is thought that by providing supplemental oxygen, the amount of oxygen dissolved in the blood can be increased. This small amount of extra oxygen may be the difference between life and death (Guyton & Hall, 2008). Never-the-less, the primary corrective response is to administer blood. By administering blood, the oxygen-carrying capacity of hemoglobin is increased and oxygen delivery is restored.
Stagnant (Hypoperfusion) Hypoxia
Theoretically, the amount of blood pumped from the heart should be the same amount that returns to the heart. Cardiac arrest, pulmonary embolism, and even positive pressure ventilation can cause a condition of stagnant hypoxia where cardiac output is compromised. The primary culprit in this situation is the cardiovascular situation and all attempts to restore cardiac function should be of primary concern. The use of supplemental oxygen is of no value when stagnant hypoxia is present, and since the PaO2 is normal, increasing the fraction of inspired oxygen (FiO2) is not helpful to correct the problem (Pittman, 2011).
Sometimes it is not a respiratory or cardiovascular system abnormality that results in hypoxia, but rather a problem with the body tissue cells and their ability to metabolize oxygen. With histotoxic hypoxia, the cell is unable to use oxygen due to the inhibition of the cytochrome oxidase enzyme and the most common cause is cyanide poisoning. Cyanide poisoning can result from alcohol or drug ingestion, smoke inhalation, metabolic disorders, as well as the administration of certain pharmaceutical medications. If cyanide poisoning is suspected, a cyanide level should be obtained by sending a sample of blood to the lab for analysis. The use of supplemental oxygen in the presence of cyanide poisoning is of no value because the primary problem resides with the cells ability to use and metabolize oxygen; however oxygen supplementation could be debatable depending on the assessment and condition of the patient. The main concern is to treat the underlying cause. Antidotal remedies are recommended to correct histotoxic hypoxia to restore oxygen delivery.
Oxygen Therapy in Summary
In summary, the use of supplemental oxygen is of no value when treating stagnant and histotoxic hypoxia because the problem is due to impaired cardiac function and cell enzyme inhibition respectfully. Supplemental oxygen is of little value when treating anemic hypoxia because the problem is due to a lack of hemoglobin availability. The preferred method of treatment is to transfuse units of packed red blood cells (PRBCs). However, when CO poisoning is present, oxygen is of high value and should be provided without delay. When hypoxic hypoxia is present, the use of supplemental oxygen is of high value to restore oxygen in the lungs and blood. Understanding how hypoxia inhibits normal oxygen transport is useful when selecting the best corrective actions necessary to reverse hypoxia and to restore normal oxygen transport. The primary cause of hypoxia should be corrected, and the need to provide supplemental oxygen should be assessed.
Des Jardins, T. (2007). Cardiopulmonary Anatomy and Physiology: Essentials for Respiratory Care (5th ed.).: Cengage Learning.
Guyton, A. C., & Hall, J. E. (2008). Textbook of Medical Physiology (11th ed.).: Elsevier Health Sciences.
Pittman, R. (2011). Chapter 7: Oxygen Transport in Normal and Pathological Situations: Defects and Compensations. In Regulation of Tissue Oxygenation. Retrieved from http://www.ncbi.nlm.nih.gov/books/NBK54113/