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pH
CO2
HCO3


Normal Arterial Blood Gas Values

pH 7.35-7.45
PaCO2 35-45 mm Hg
PaO2  80-95 mm Hg
HCO3  22-26 mEq/L
O2 Saturation 95-99%
BE  +/- 1

Four-Step Guide to ABG Analysis

  1. Is the pH normal, acidotic or alkalotic?
  2. Are the pCO2 or HCO3 abnormal?  Which one appears to influence the pH?
  3. If both the pCO2 and HCO3 are abnormal, the one which deviates most from the norm is most likely causing an abnormal pH. 
  4. Check the pO2.  Is the patient hypoxic?

I used Swearingen's handbook (1990) to base the results of this calculator.  The book makes the distinction between acute and chronic disorders based on symptoms from identical ABGs.  This calculator only differentiates between acute (pH abnormal) and compensated (pH normal).  Compensation can be seen when both the PCO2 and HCO3 rise or fall together to maintain a normal pH.  Part compensation occurs when the PCO2 and HCO3 rise or fall together but the pH remains abnormal.  This indicates a compensatory mechanism attempted to restore a normal pH.  I have not put exact limits into the calculator.  For example, it will perceive respiratory acidosis as any pH < 7.35 and any CO2 > 45 (i.e. a pH of 1 and CO2 of 1000).  These results do not naturally occur.

pH PaCO2 HCO3
Respiratory Acidosis
Acute < 7.35 > 45 Normal
Partly Compensated < 7.35 > 45 > 26
Compensated Normal > 45 > 26

Respiratory Alkalosis
Acute > 7.45 < 35 Normal
Partly Compensated > 7.45 < 35 < 22
Compensated Normal < 35 < 22

Metabolic Acidosis
Acute < 7.35 Normal < 22
Partly Compensated < 7.35 < 35 < 22
Compensated Normal < 35 < 22

Metabolic Alkalosis
Acute > 7.45 Normal > 26
Partly Compensated > 7.45 > 45 > 26
Compensated Normal > 45 > 26

Mixed Disorders

It's possible to have more than one disorder influencing blood gas values.  For example ABG's with an alkalemic pH may exhibit respiratory acidosis and metabolic alkalosis.  These disorders are termed complex acid-base or mixed disorders.

*This table is able to classify most clinical blood gas values but not all.  In cases where blood gas values do not fall into any of the above classifications, an answer "unable to determine" will appear when using the interpreter.  For example a pH of 7.428, pCO2 43.6, and a HCO3 of 29.1 do not match any of the classifications (I found these results in someone's chart).  While the pH and pCO2 are normal, the HCO3 is abnormally high.

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