Blood Gases
Assessing Lung Function

 Blood Gases............................... \$49

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.
For example, consider this blood gas, drawn in Salt Lake City on a seizure patient breathing room air:

pH = 7.225
paO2 = 54
PaCO2 = 51

Pretty bad lungs, huh? Probably aspirated, right? No. These lungs are perfectly normal. Calculation of the arterial-alveolar (Aa) gradient shows that no significant pulmonary problem is present. The Aa gradient is 8. (The barometric pressure in SLC is 647.)
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.
The Aa gradient may help you decide whether a hyperventilating patient is simply upset, or has a pulmonary embolism. In this case a “normal” oxygen may turn out to be abnormal considering the low CO2 caused by hyperventilation.
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. It can be looked at as a measure of how well oxygen gets from air into blood. The higher the A-a gradient, the more problem there is with oxygen passage into the blood.
Calculating this “efficiency” of oxygen passage allows an accurate picture of overall lung health, because the effects of hyper- or hypoventilation on PaO2 are eliminated.
A calculation is necessary because alveolar air doesn’t have the same oxygen pressure as outside air. Some of the oxygen is displaced by water vapor, and by carbon dioxide exiting the blood into the alveolus. The partial pressure of all gases must add up to atmospheric pressure. If the CO2 goes down, the oxygen proportion will go up. Conversely, if the CO2 elevates, there can be less oxygen in the alveolus. Back to Top

The arterial-alveolar gradient is the difference between the measured pressure of oxygen in the blood stream and the calculated oxygen in the alveolus. The oxygen pressure in the alveolus can be calculated by:

1) Subtracting the partial pressure of water vapor at 37 degrees

2) This result is multiplied by the oxygen percentage in the
remaining air (it’s the same as the outside air before it was humidified).
This gives us the oxygen pressure in totally humidified air.

3) Because carbon dioxide displaces oxygen in the alveolus,
the estimated alveolar CO2 must be subtracted. The alveolar
CO2 is estimated by multiplying the arterial PaCO2 by a
“respiratory quotient” fudge factor of 1.25.

Subtracting arterial oxygen from alveolar oxygen, the formula for calculating the Aa gradient is:

Aa = (BP - pH2O) x FiO2 - (1.25 x PaCO2) - PaO2

BP is barometric pressure, pH2O is the partial pressure of water at body temperature (47 mm Hg at 37 degrees centigrade), FiO2 is the fraction of inspired oxygen.

At sea level and room air, the formula simplifies to:

Aa = 150 - (1.25 x PaCO2) - PaO2

In Salt Lake City (home of the 2002 Winter Olympic Games), athletes breathing room air would have their Aa gradient calculated with the formula: to:

Aa = 126 - (1.25 x PaCO2) - PaO2

The A-a gradient merely reflects the gross difference between alveolar oxygen and blood oxygen. It says nothing about what caused that difference. An atrial septal defect that shunts unoxygenated blood through the heart can also elevate the A-a gradient. Like everything else in medicine, the A-a gradient must be evaluated while looking at the entire clinical picture. Back to Top

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