One of several measurements of the efficiency of respiration, especially of the extent of intrapulmonary shunting in respiratory failure. Among the commonly used gas exchange indices is the alveolar-arterial oxygen tension difference (a measurement derived from an analysis of the oxygen tension of an arterial blood gas, compared with the atmospheric oxygen content).
The process known as respiration involves the extraction of oxygen from inhaled air, which then enters the bloodstream, while simultaneously eliminating carbon dioxide from the blood and exhaling it out of the body.