This tutorial helps you uncover acid base disorders that you may see in your patients. These are metabolic acidosis, metabolic alkalosis, respiratory acidosis, and respiratory alkalosis, or a combination thereof.

Step 1: get your bloodwork and define your normal values

You will need to order the following bloodwork:

  • Serum electrolytes (Na, Cl, K, HCO3)
  • Arterial blood gas
  • Additional bloodwork to uncover the cause as needed (e.g. lactate, ketones, drug levels, etc)

Normal values for your bloodwork

  • Normal anion gap (calculated from your electrolytes) is 12 mmol/L or less
  • Normal HCO3 is 24 mmol/L

Normal values for your arterial blood gas (ABG)

  • Normal pH is 7.4 (alternatively, between 7.35 and 7.45)
  • Normal HCO3 is 24 mmol/L
  • Normal pCO2 is 40 mmHg

Step 2: what is the emia?

  • If pH is less than 7.4, there is an acidemia
  • If pH is greater than 7.4, there is an alkalemia
  • If pH is exactly 7.4, decide with clinical context:
    • If the patient is intoxicated or in respiratory failure, there is probably an acidemia
    • If the patient has vomiting, diarrhea, or hyperventilation, there is probably an alkalemia

Step 3: what is the osis?

In acidemic patients

  • If HCO3 is low, there is a metabolic acidosis
  • If pCO2 is high, there is a respiratory acidosis

In alkalemic patients

  • If HCO3 is high, there is a metabolic alkalosis
  • If pCO2 is low, there is a respiratory alkalosis

Step 4: what is the anion gap?

  • Anion gap = Na – Cl – HCO3 (normal anion gap is 12 mmol/L or less)
  • If elevated (e.g. 16), there is an anion gap metabolic acidosis
  • If not elevated and the patient has acidosis, there is a non anion gap metabolic acidosis

Step 5: what is the delta delta?

TLDR version

  • In ketoacidosis and most other anion gap metabolic acidoses, a 1 mmol/L increase in the anion gap should be counterbalanced with a 1 mmol/L decrease in HCO3
  • In lactic acidosis, each 1.6 mmol/L increase in the anion gap should be counterbalanced with a 1 mmol/L decrease in HCO3
  • If there is an excessively large decrease in HCO3, there is a concurrent non-anion gap metabolic acidosis
  • If there is a excessively small decrease in HCO3, there is a concurrent metabolic alkalosis or (and) a concurrent chronic respiratory acidosis

The full explanation

The concept of delta delta simply states that excess anions should be counterbalanced with a decrease in HCO3. The ratio is typically for every 1 mmol/L increase in the anion gap, there should be a 1 mmol/L decrease in HCO3. So if the anion gap is 16, your HCO3 should be 20. If the counterbalance is lop-sided, then there is another concurrent acid base abnormality that you need to uncover.

Patients can have any combination of these acid base disorders:

  • Metabolic acidosis
  • Metabolic alkalosis
  • Respiratory acidosis
  • Respiratory alkalosis

The only combination that is practically not possible is for a patient to have both a respiratory acidosis and a respiratory alkalosis at the same time.

There are exceptions to the 1:1 rule. In ketoacidosis and most other causes of anion gap metabolic acidosis, the 1:1 rule applies. Each 1 mmol/L increase in the anion gap should correspond to a 1 mmol/L decrease in HCO3. However, in lactic acidosis, each 1.6 mmol/L increase in the anion gap results in only a 1 mmol/L decrease in HCO3.

Step 6: is the patient compensating appropriately?

In metabolic acidosis

In metabolic acidosis, each 1 mmol/L drop in HCO3 should correspond to a 1.2 mmHg drop in pCO2.

  • If the pCO2 drops too much, there is a concurrent respiratory alkalosis
  • If the pCO2 drops too little, there is a concurrent respiratory acidosis

In metabolic alkalosis

In metabolic alkalosis, each 1 mmol/L rise in HCO3 should correspond to a 0.7 mmHg rise in pCO2.

  • If the pCO2 rises too much, there is a concurrent respiratory acidosis
  • If the pCO2 rises too little, there is a concurrent respiratory alkalosis

In respiratory acidosis

In acute respiratory acidosis, each 10 mmHg rise in pCO2 should correspond to a 1 mmol/L rise in HCO3.

In chronic respiratory acidosis, each 10 mmHg rise in pCO2 should correspond to a 4 mmol/L rise in HCO3.

  • If the HCO3 rises too much, there is a concurrent metabolic alkalosis
  • If the HCO3 rises too little, there is a concurrent metabolic acidosis

In respiratory alkalosis

In acute respiratory alkalosis, each 10 mmHg drop in pCO2 should correspond to a 2 mmol/L drop in HCO3.

In chronic respiratory alkalosis, each 10 mmHg drop in pCO2 should correspond to a 5 mmol/L drop in HCO3.

  • If the HCO3 drops too much, there is a concurrent metabolic acidosis
  • If the HCO3 drops too little, there is a concurrent metabolic alkalosis