Four-Step Guide
to ABG Analysis
- Is the pH normal, acidotic or alkalotic?
- Are the pCO2 or HCO3 abnormal? Which one appears to influence the
pH?
- If both the pCO2 and HCO3 are abnormal, the one which deviates most from
the norm is most likely causing an abnormal pH.
- 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.