Use Old Arterial Blood Gas Results to Your Advantage

There are certain clinical situations where a review of the patient’s prior arterial blood gas results can be of great value (if the patient did, in fact, have a sample drawn in the past). To see how prior ABG results may be of use, consider the following scenario.

A patient with very severe COPD was found ashen and pulseless on the wards and a Code 199 was called. The initial arterial blood gas revealed

• pH 7.32
• PCO₂ 50
• PO₂ 105
• HCO₃- 25
• on an FIO₂ of 1.0.

This patient was acidemic with a high PCO₂ and a normal bicarbonate and it was determined that he had a primary respiratory acidosis without metabolic compensation. That is the correct interpretation if all you have is the information from that one arterial blood gas. However, the patient did have an earlier arterial blood gas, well before his cardiac arrest whose results were as follows:

• pH 7.38
• PCO₂ 80
• PO₂ 72
• HCO₃- 48

These results are consistent with a primary respiratory acidosis and a compensatory metabolic alkalosis that reflects his history of very severe COPD and known carbon dioxide retention. Of note, his baseline bicarbonate is 48, well higher than the "normal" appearing value of 25 that was obtained during the code. If all you did in this situation were look at the ABG drawn during the code, you would see a "normal" bicarbonate and might not realize that he has developed a severe metabolic acidosis. If, however, you have his old blood gas values, you would correctly note that his bicarbonate has fallen dramatically. As a result, he likely has some problem (e.,g. sepsis) that caused a severe metabolic acidosis and ultimately led to his decompensation.

Top of Page

Oxygenation

The arterial blood gas also provides information about the oxygenation status of the patient. In general, there are two basic pieces of information you can obtain using the PO₂ on a blood gas: insight into the cause of the hypoxemia and an assessment of the adequacy of gas exchange.

Assessing the Cause of Hypoxemia

There are five broad categories of problems that cause hypoxemia:

1. Low inspired partial pressure of oxygen (eg. high altitude)
2. Hypoventilation
3. Shunt
4. Low ventilation relative to perfusion (low V/Q)
5. Diffusion limitation (rarely an issue at sea-level)

When a patient presents with unexplained hypoxemia, you can begin to sort through the differential diagnosis by calculating the alveolar-arterial oxygen difference (AaO₂ Difference). You begin this calculation by determining the alveolar PO₂ using the alveolar gas equation:

PaO₂ = (P_B – 47) FiO₂ – PaCO₂ /R

Whereby P_B = barometric pressure, FiO₂ = the inspired oxygen concentration and R = the respiratory quotient. If you have trouble remembering the full equation, you can simplify things by remembering the PIO₂ of room air where you live. The PIO₂ is the first term of the alveolar gas equation (PO₂ = [P_B – 47] FiO₂). For example, the PIO₂ while breathing ambient air at sea level is 150 mmHg whereas if you live in Boise, Idaho, where the average PB is 695 mmHg, the PIO₂ of room air is 135 mmHg.

Once you have calculated this value, the AaO₂ Difference is calculated as follows:

AaO₂D = PAO₂ – PaO₂

The normal value for the AaO₂ Difference is about 10-15 mm Hg. If you determine that the patient has a normal AaO₂ Difference, then the hypoxemia can be attributed to hypoventilation or a low inspired partial pressure of oxygen. If, however, you find that the patient has an elevated AaO₂ difference then they have some process going on that is causing shunt or low V/Q, such as pneumonia, that is contributing to the observed hypoxemia.

There are a few important points to be aware of regarding the use of the AaO₂ Difference.

• In order to use the alveolar gas equation you must have an accurate assessment of the FiO₂. The FiO₂ will be accurate when your patient is on room air, non-invasive or invasive mechanical ventilation or is on a high-flow facemask system. When patients are on supplemental oxygen using nasal cannula, regular face masks, Venturi masks or non-rebreather masks, the FiO₂ is not reliable; these systems often deliver less gas flow than the patient is requiring for their minute ventilation and, as a result, they end up entraining in a lot of room air which dilutes out the intended inspired oxygen concentration.
• The "normal" AaO₂ Difference increases with age. Some sources state that the upper limit of normal for the AaO₂ Difference is equal to: 2.5 + (0.21 x age).
• The "normal" AaO₂ Difference varies based on the FiO₂. On an FiO₂ of 1.0, the normal AaO₂ difference is 100 mmHg. Also, if you go to high altitude, where the barometric pressure is decreased and the PIO₂ falls, the normal AaO₂  Difference is lower than 10.
• Make sure you are using the correct value for R. In general, we use a value of 0.8. However, if your patient is on an FiO₂ of 1.0, then R is taken to be 1.

Top of Page

 < Previous: Generating Differential Diagnoses | Next: Assessing the Adequacy of Gas Exchange