Monitoring Oxygen Status

Although part of a common "blood gas" test panel with pH and pCO₂, the pO₂, %O₂Hb, and related parameters are independently used to detect and monitor oxygen deficits from a variety of causes. Measurement of blood gases and cooximetry may be done by laboratory analyzers, point of care testing, noninvasive pulse oximetry, and transcutaneous blood gases. The specimen type and mode of monitoring oxygenation that are chosen may be based on a combination of urgency, practicality, clinical need, and therapeutic objectives. Because oxygen concentrations in blood are extremely labile, there are several highly important preanalytical practices necessary to prevent errors in oxygen and cooximetry results. Effective utilization of oxygen requires binding by hemoglobin in the lungs, transport in the blood, and release to tissues, where cellular respiration occurs. Hydrogen ion (pH), CO₂, temperature, and 2,3-DPG all play important roles in these processes. Additional measurements and calculations are often used to interpret and locate the cause and source of an oxygen deficit. These include the Hb concentration, Alveolar-arterial pO₂ gradient, pO₂:FIO₂ ratio, oxygenation index, O₂ content and O₂ delivery, and pulmonary dead space and intrapulmonary shunting. The causes of hypoxemia will be covered and, to illustrate how the oxygen parameters are used clinically in the diagnosis and management of patients with abnormal oxygenation, two clinical cases will be presented and described.