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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. The alveolar air equation is not widely used in clinical medicine, probably because of the complicated appearance of its classic forms.
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.
The equation for calculating the A–a gradient is: = [4] Where: PAO 2 = alveolar PO 2 (calculated from the alveolar gas equation) = P a O 2 = arterial PO 2 (measured in arterial 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.
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]
The alveolar oxygen partial pressure is lower than the atmospheric O 2 partial pressure for two reasons.. Firstly, as the air enters the lungs, it is humidified by the upper airway and thus the partial pressure of water vapour (47 mmHg) reduces the oxygen partial pressure to about 150 mmHg.
Example calculation After drawing an arterial blood gas sample from a patient the P a O 2 is found to be 100 mmHg. Since the patient is receiving oxygen-saturated air resulting in a F I O 2 of 50% oxygen his calculated P a O 2 / F I O 2 ratio would be 100 mmHg/0.50 = 200 mmHg.
In respiratory physiology, the ventilation/perfusion ratio (V/Q ratio) is a ratio used to assess the efficiency and adequacy of the ventilation-perfusion coupling and thus the matching of two variables: