The alveolarĬO2 is estimated by multiplying the arterial PaCO2 by a The estimated alveolar CO2 must be subtracted. This gives us the oxygen pressure in totally humidified air.ģ) Because carbon dioxide displaces oxygen in the alveolus, Remaining air (it’s the same as the outside air before it was humidified). The oxygen pressure in the alveolus can be calculated by:ġ) Subtracting the partial pressure of water vapor at 37 degreesĢ) This result is multiplied by the oxygen percentage in the The arterial-alveolar gradient is the difference between the measured pressure of oxygen in the blood stream and the calculated oxygen in the alveolus. Conversely, if the CO2 elevates, there can be less oxygen in the alveolus. If the CO2 goes down, the oxygen proportion will go up. The partial pressure of all gases must add up to atmospheric pressure. Some of the oxygen is displaced by water vapor, and by carbon dioxide exiting the blood into the alveolus. The higher the A-a gradient, the more problem there is with oxygen passage into the blood.Ĭalculating this “efficiency” of oxygen passage allows an accurate picture of overall lung health, because the effects of hyper- or hypoventilation on PaO2 are eliminated.Ī calculation is necessary because alveolar air doesn’t have the same oxygen pressure as outside air. It can be looked at as a measure of how well oxygen gets from air into blood. What is the A-a gradient? It’s the difference between the measured pressure of oxygen in the blood stream and the calculated pressure of oxygen in the alveolar sacs.
In this case a “normal” oxygen may turn out to be abnormal considering the low CO2 caused by hyperventilation. The Aa gradient may help you decide whether a hyperventilating patient is simply upset, or has a pulmonary embolism. Simply put, calculating the Aa gradient allows you to determine whether a measured oxygen value is normal for the patient’s altitude, inspired oxygen percentage, and rate of respirations. Calculation of the arterial-alveolar (Aa) gradient shows that no significant pulmonary problem is present. Pretty bad lungs, huh? Probably aspirated, right? No. There are times when the primary question to be answered from the blood gases is: are the lungs normal? Yet the values of oxygen and carbon dioxide, taken alone, can be misleading.įor example, consider this blood gas, drawn in Salt Lake City on a seizure patient breathing room air:
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