Abstract
Background: Clinicians who manage respiratory diseases use patients' blood gas tensions to understand the severity of gas exchange impairment. They combine some values from the patients' blood gas results with mathematical equations that are commonly known as oxygenation indices. Many clinical decisions are based on the value and interpretation of these indices. Therefore, it is vital to understand the nature, indication, and limitation of the oxygen indices that are currently utilized in critical care settings. This review provides an assessment of the available oxygen indices that are commonly used in the critical care setting. We provide a review of the available oxygen indices that are commonly used to assess the oxygen exchange deficit. Broadly, there are two types of indices: the content-based index, which measures venous admixture Qs/Qt), and the tension-based indices. The latter includes the alveolar to arterial oxygen tension differences (P(A-a)O2), arterial oxygen tension to alveolar oxygen tension ratio (PaO2/PAO2), PaO2 to FiO2 ratio (PaO2/FiO2), and the respiratory index. Based on this review, no comprehensive index of oxygenation exists until now. The respiratory therapists and clinicians use many of the current indices because of their simplicity, still this does not indicate their validity. There is a need, therefore, to develop a more reliable oxygen index that can be correlated with the changes in the clinical condition.