Technical tricks to interpreting ABGs

ABGs are indicated when you want to know a patient’s pH and acid-base status. It can be hard to remember how to read them, so here are some tips:

Use this handy mnemonic to remember the order of blood gases given in the ABG: PCO2. The letters correspond to PH, CO2, O2.
Sometimes, you will also see bicarbonate on the ABG. But beware: this value is calculated so it’s not as accurate as the chem-7 value.

Sometimes getting an ABG is impossible, so you will be asked to convert an VBG to an ABG. In the VBG, the pH is 0.03-0.05 points lower and the CO2 will be 6-8 mmHg higher. Be careful about how the VBG is collected: if the tourniquet is left on for more than 1 minute, it can make the pH seem much more acidotic than it really is.

You might be told that sometimes converting from a VBG is not appropriate because in patients whose respiratory status you’re concerned about, the PaO2 from the ABG is preferred. However, if their O2 sat is high enough (>95%), this would correspond with a high level on the Hgb-O2 dissociation curve. So in these cases, it would be okay to use the VBG values.

ABGs are not infallible either. Arterial values may not reflect true acid-base status at the tissue level when cardiac output is reduced (such as shock or sepsis) and when lactic acidosis is present.

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