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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 - normal range 7.35-7.45)
PaCO2 - normal range 4.7-6.0 kPa / 35–45  mmHg
HCO₃^- - normal range 22-26 mmol/L

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

Step3. 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

Step 4. Use HCO₃^- to verify metabolic effect (Normal HCO₃^- is 22-26)

Please note: 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 o 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).

Reference

CM, N., Lel, C., Tello, G., Oluchi, Otiende, G. M., Abb, Vera, M., Tina, Idris, F., Angela, Ade, Menen, Mimie, Sonrisa, Shameer, S., Isuru, Lem, Sali, Vaji, … Rebecca. (2024, May 31). Arterial blood gas analysis made easy with tic-tac-toe method. Nurseslabs. https://nurseslabs.com/arterial-blood-gas-abgs-interpretation-guide/