Interpretation of Arterial Blood Gases by American Thoracic Society

Interpreting an arterial blood gas (ABG) is a crucial skill for physicians, nurses, respiratory therapists, and other health care personnel. ABG interpretation is especially important in critically ill patients.

The following six-step process helps ensure a complete interpretation of every ABG. In addition, you will find tables that list commonly encountered acid-base disorders.

Many methods exist to guide the interpretation of the ABG. This discussion does not include some methods, such as analysis of base excess or Stewart’s strong ion difference. A summary of these techniques can be found in some of the suggested articles. It is unclear whether these alternate methods offer clinically important advantages over the presented approach, which is based on the “anion gap.”

6-step approach:
Step 1:  Assess the internal consistency of the values using the Henderseon-Hasselbach equation
Step 2:  Is there alkalemia or acidemia present?

pH < 7.35  acidemia 

pH > 7.45  alkalemia

Step 3:  Is the disturbance respiratory or metabolic?  What is the relationship between the direction of change in the pH and the direction of change in the PaCO₂? In primary respiratory disorders, the pH and PaCO2 change in opposite directions; in metabolic disorders the pH and PaCO₂ change in the same direction.
Step 4:  Is there appropriate compensation for the primary disturbance?  Usually, compensation does not return the pH to normal (7.35 – 7.45).
Step 5:  Calculate the anion gap (if a metabolic acidosis exists)
Step 6:  If an increased anion gap is present, assess the relationship between the increase in the anion gap and the decrease in [HCO₃-].
Link to Interpretation of Arterial Blood Gases (ABGs) by David A. Kaufman, MD:
http://www.thoracic.org/professionals/clinical-resources/critical-care/clinical-education/abgs.php