ABG Interpretation Method

Simple Method of Acid Base Balance Interpretation

A FOUR STEP METHOD FOR INTERPRETATION OF ABGS

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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-

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Step 1. Use pH to determine Acidosis or Alkalosis.

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

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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

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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.

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Step 4. Use HC03 to verify metabolic effect

• Normal HCO3- is 22-26

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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).

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This page was last modified on January 10, 2015.