ABG Interpretation Method

Simple Method of Acid Base Balance Interpretation

A FOUR STEP METHOD FOR INTERPRETATION OF ABGS

Usefulness

This method is simple, easy and can be used for the majority of ABGs. It only addresses acid-base balance and considers just 3 values.

• pH,
• PaCO2
• HCO3-

Step 1. Use pH to determine Acidosis or Alkalosis.

ph
< 7.35	7.35-7.45	> 7.45
Acidosis	Normal or Compensated	Alkalosis

Step 2. Use PaCO2 to determine respiratory effect.

PaCO2
< 35	35 -45	> 45
Tends toward alkalosis Causes high pH Neutralizes low pH	Normal or Compensated	Tends toward acidosis Causes low pH Neutralizes high pH

Step 3. Assume metabolic cause when respiratory is ruled out.

• You'll be right most of the time if you remember this simple table:

High pH		Low pH
Alkalosis		Acidosis
High PaCO2	Low PaCO2	High PaCO2	Low PaCO2
Metabolic	Respiratory	Respiratory	Metabolic

• If PaCO2 is abnormal and pH is normal, it indicates compensation.
  • pH > 7.4 would be a compensated alkalosis.
  • pH < 7.4 would be a compensated acidosis.

These steps will make more sense if we apply them to actual ABG values. Click here to interpret some ABG values using these steps. You may want to refer back to these steps (click on "linked" steps or use "BACK" button on your browser) or print out this page for reference.

Step 4. Use HC03 to verify metabolic effect

• Normal HCO3- is 22-26

Please note:

• Remember, the first three steps apply to the majority of cases, but do not take into account:
  • the possibility of complete compensation, but those cases are usually less serious, and
  • instances of combined respiratory and metabolic imbalance, but those cases are pretty rare.
    • "Combined" disturbance means HCO3- alters the pH in the same direction as the PaCO2.
      • High PaCO2 and low HCO3- (acidosis) or
      • Low PaCO2 and high HCO3- (alkalosis).

Now, try some patient case studies

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