Acid Base Balance

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 (H2CO3).
• As previously discussed, under the heading Respiratory Mechanism, carbon dioxide is an acid when dissolved in water.
  • 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 buffer system.

The two headed arrows indicate that the process is reversible

CO2 + H2O <--> H2CO3 <--> (HCO3-) + (H+)

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 correct it.

• "Compensation" is the body's normal and healthy attempt to normalize pH by neutralizing the opposite mechanism.
• 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 alkalosis."
    • The metabolic compensation should stop when the pH begins to normalize.

Points to remember:

1. Either mechanism (respiratory or metabolic) can cause either alkalosis or acidosis.
2. It is normal for each mechanism to compensate the other.
3. You have to find the cause.
4. Treat the cause not the compensation.

Click here to enter the Tulane Univ. School of Medicine, Dept. of Anesthesia Acid-Base balance tutorial.

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