Understanding the cause
of an acid-base imbalance is the key to treating it.
- The simplified approach
to understanding the relationship of acid and base starts with carbonic acid
- As previously discussed,
under the heading Respiratory
Mechanism, carbon dioxide is an acid when dissolved
- Carbon dioxide is
produced by metabolism.
- As long as cells
are functioning, there will be CO2 produced.
- The respiratory mechanism
affects the pH within minutes.
- Metabolic changes
can take days to affect pH.
So now, lets
combine the two formulae to describe the carbonic acid - bicarbonate
The two headed
arrows indicate that the process is reversible
CO2 + H2O
<--> H2CO3 <--> (HCO3-) + (H+)
takes only minutes for the kidneys to change the amount of bicarbonate in the
You see how either mechanism
(respiratory or metabolic) can cause an acidosis or an alkalosis.
- Normal pH is maintained
by balancing the H2CO3 (carbonic acid) and HCO3- (bicarb) .
The task of ABG interpretation
is to determine the cause of an acid-base (carbonic acid-bicarb) imbalance and
is the body's normal and healthy attempt to normalize pH by neutralizing the
- For example: If the pH
is high because of respiratory alkalosis (low CO2):
- Alkaline HCO3-
will decrease to neutralize the pH.
- In this case, the
abnormal bicarb is not a metabolic problem; it is a metabolic solution
to a respiratory problem.
- If you correct the
underlying respiratory problem, the compensatory metabolic abnormality
will correct itself.
- That's why it's important
to determine which is the cause and which is the effect.
- If you treat the
compensatory abnormality, you make the pH even more abnormal.
- In other words,
in this example, if you gave bicarbonate because the bicarb was low,
you would stop the metabolic compensation and increase the alkalosis
by adding another alkaline substance.
- The treatment should
be to decrease the rate and/or depth of ventilation to retain more CO2.
- This will "acidify
- The metabolic
compensation should stop when the pH begins to normalize.
Points to remember:
- Either mechanism (respiratory
or metabolic) can cause either alkalosis or acidosis.
- It is normal for each
mechanism to compensate the other.
- You have to find the
- Treat the cause not the
you treat the cause of the problem, the compensatory mechanism should correct