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P a O 2 – Partial pressure of oxygen at sea level (160 mmHg in the atmosphere, 21% of standard atmospheric pressure of 760 mmHg) in arterial blood is between 75 mmHg and 100 mmHg. [4] [5] [6] Venous blood oxygen tension (normal) P v O 2 – Oxygen tension in venous blood at sea level is between 30 mmHg and 40 mmHg. [6] [7]
The rest of the difference is due to the continual uptake of oxygen by the pulmonary capillaries, and the continual diffusion of CO 2 out of the capillaries into the alveoli. The alveolar pO 2 is not routinely measured but is calculated from blood gas measurements by the alveolar gas equation .
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 alveolar gas equation is the method for calculating partial pressure of alveolar oxygen (p A O 2).The equation is used in assessing if the lungs are properly transferring oxygen into the blood.
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.
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.
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Type 1 respiratory failure is characterized by a low level of oxygen in the blood (hypoxemia) (PaO2) < 60 mmHg with a normal (normocapnia) or low (hypocapnia) level of carbon dioxide (PaCO2) in the blood. [1] The fundamental defect in type 1 respiratory failure is a failure of oxygenation characterized by: