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An ABG test measures the blood gas tension values of the arterial partial pressure of oxygen (PaO2), and the arterial partial pressure of carbon dioxide (PaCO2), and the blood's pH. In addition, the arterial oxygen saturation (SaO2) can be determined. Such information is vital when caring for patients with critical illnesses or respiratory disease.
Blood gas tension refers to the partial pressure of gases in blood. [1] There are several significant purposes for measuring gas tension. [2] The most common gas tensions measured are oxygen tension (P x O 2), carbon dioxide tension (P x CO 2) and carbon monoxide tension (P x CO). [3]
However, it is not practical to take a sample of gas from the alveoli in order to directly measure the partial pressure of oxygen. The alveolar gas equation allows the calculation of the alveolar partial pressure of oxygen from data that is practically measurable. It was first characterized in 1946. [1]
2, [1] or A–a gradient), is a measure of the difference between the alveolar concentration (A) of oxygen and the arterial (a) concentration of oxygen. It is a useful parameter for narrowing the differential diagnosis of hypoxemia. [2] The A–a gradient helps to assess the integrity of the alveolar capillary unit.
The body maintains a stable level of oxygen saturation for the most part by chemical processes of aerobic metabolism associated with breathing.Using the respiratory system, red blood cells, specifically the hemoglobin, gather oxygen in the lungs and distribute it to the rest of the body.
The oxygen–hemoglobin dissociation curve, also called the oxyhemoglobin dissociation curve or oxygen dissociation curve (ODC), is a curve that plots the proportion of hemoglobin in its saturated (oxygen-laden) form on the vertical axis against the prevailing oxygen tension on the horizontal axis.
Hypoxemia refers to the low level of oxygen in arterial blood. Tissue hypoxia refers to low levels of oxygen in the tissues of the body and the term hypoxia is a general term for low levels of oxygen. [2]
This is calculated by dividing the PaO2 by the FiO2. Example: patient who is receiving an FiO2 of .5 (i.e., 50%) with a measured PaO2 of 60 mmHg has a PaO 2 / FiO 2 ratio of 120. In healthy lungs, the Horowitz index depends on age and usually falls between 350 and 450.