Special Situations To Be Aware of In Evaluating the PaO₂ on an ABG

There are a few special situations of which you should be aware when assessing the PaO₂ on an arterial blood gas.

The Hypothermic Patient

Hypothermia can affect the accuracy of the PaO₂ measurement by causing changes in the position of the Hb-O₂ dissociation curve. If you do not inform the laboratory of the patient’s body temperature at the time the sample was measured, you may measure an artificially high PaO₂. To understand how this happens, consider the following scenario. Suppose you see a patient in the emergency room with a temperature of 32°C and draw an ABG while he is still at that temperature. At this point in time, his Hb-O₂ dissociation curve has shifted to the left and, as a result, for any given oxygen content of the arterial blood, the PaO₂ is lower than it would be at a normal temperature. When the sample is sent to the lab and placed in the blood gas analyzer, however, the machine will warm the sample to 37°C before doing the measurement. This warming process shifts the Hb-O₂ dissociation curve back to the right. Because the oxygen content of the blood has not changed, this rightward shift will lead to a higher measured PaO₂ than if the measurement had taken place without warming the blood. For this reason, you should always notify the lab about the patient’s temperature at the time of the measurement if they are hypothermic so they can make the appropriate adjustments in the blood gas analyzer.

Leukocyte Larceny

This is a rare situation that may be seen when working on an oncology service with patients presenting with acute leukemia or other patients with extremely high white blood cell counts. White blood cells are metabolically active and consume oxygen. In the time between when a sample is drawn from the wrist and the measurement is made in the blood gas analyzer, the white blood cells may consume enough oxygen to decrease the PaO₂ from the actual value at the time the sample was drawn. The net result is you get a PaO₂ result that is much lower than what you would expect based on the oxygen saturation at the time the sample was drawn. This situation, which may be seen in leukemic patients with WBC counts of 50,000 or greater, can occur even if the sample is cooled immediately and analyzed within 10 minutes. This problem can be avoided by putting the sample on ice and adding potassium cyanide to halt oxygen consumption.

Be aware that there are other situations in which you will see a discrepancy between a high SpO₂ and a low PaO₂ on the ABG. These include:

• Spuriously high SpO₂value
• Inadvertent venous blood sampling
• Excessive time delay in sample analysis
• Air bubbles left in the blood gas sample

Methemoglobinemia

In leukocyte larceny, the PaO₂ is lower than you would expect given the SpO₂. In methemoglobinemia, the opposite occurs; the patient has a low SpO₂ (typically in the mid-upper 80% range) but the PaO₂ is much higher than you would expect from that value. In this case, the PaO₂ is actually a valid reading and the discrepancy reflects important pathology.

In this situation, there is some insult (usually a medication such as dapsone)  that leads to oxidation of the iron in the hemoglobin molecule from the 2+ valence to a 3+ valence. As a result, the hemoglobin molecule changes conformation and does not bind oxygen adequately. Blood with the altered hemoglobin molecule has different light absorption properties that lead you to measure a lower saturation using a pulse oximeter. Under normal circumstances, when the PaO₂ is high, you expect to see a very high saturation. However, in methemoglobinemia, even though you may have the patient on a lot of supplemental oxygen and the PaO₂ is very high, the oxygen will not bind to hemoglobin and, as a result, the saturation will continue to be low (Because of the absorption properties of methemoglobin, the saturation typically remains around 87-88%).

The tip-off to this diagnosis comes when you see a patient with a low saturation that remains low despite putting them on supplemental oxygen, but find a high PaO₂ on their blood gas. If you see this pattern and suspect the diagnosis, the next step is to ask the laboratory to check a methemoglobin level on the blood gas sample.

Once the diagnosis is confirmed, you must then identify and stop the offending agent and, in some case, initiate disease-specific therapies. A useful mnemonic for drugs that have the potential to cause methemoglobinemia is:

• L: Lidocaine (and other "-caine" local anesthetics)
• A: Anti-malarials
• N: Nitrites/Nitrates
• D: Dapsone (perhaps the most common offender)

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