As a rule, PAOĢ by at least 5–10 mmHg, even in a healthy person with normal ventilation and perfusion. However even though the partial pressure of oxygen is about equilibrated between the pulmonary capillaries and the alveolar gas, this equilibrium is not maintained as blood travels further through pulmonary circulation. In a perfect system, no A-a gradient would exist: oxygen would diffuse and equalize across the capillary membrane, and the pressures in the arterial system and alveoli would be effectively equal (resulting in an A-a gradient of zero). However, in states of ventilation perfusion mismatch, such as pulmonary embolism or right-to-left shunt, oxygen is not effectively transferred from the alveoli to the blood which results in an elevated A-a gradient. ![]() ![]() For example, in high altitude, the arterial oxygen PaOĢ is low but only because the alveolar oxygen ( PAOĢ) is also low. The A–a gradient helps to assess the integrity of the alveolar capillary unit. It is a useful parameter for narrowing the differential diagnosis of hypoxemia. Pathophysiology sample values BMP/ ELECTROLYTES:Ģ, or A–a gradient), is a measure of the difference between the alveolar concentration ( A) of oxygen and the arterial ( a) concentration of oxygen.
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