2025 – PAGE 364 – ACID-BASE DISORDERS

ABG RULES FOR A RESPIRATORY ACIDOSIS OR RESPIRATORY ALKALOSIS

• If you diagnose a RESPIRATORY DISORDER as your overarching acid-base disorder, you must calculate for metabolic compensation. Use the pCO2 to calculate the predicted bicarbonate level. The upcoming table will help with this.
  • If the bicarb level is higher than what your calculation predicted, then there’s an additional metabolic alkalosis present (more bicarb means more base).
  • If the bicarb level is lower than what your calculation predicted, then there’s less bicarb present, and thus a concurrent metabolic acidosis (less base means the relative existence of more acid). In this situation, simply use your anion gap (and albumin level) to help you decide if the additional primary metabolic acidosis is an anion gap metabolic acidosis or a non-anion gap metabolic acidosis.
• RESPIRATORY ACIDOSIS
  • (DOUBLE TAKE) PEARL: If you are given an ABG and you diagnose an overarching respiratory acidosis, ALWAYS calculate for metabolic compensation. You do this by looking at the increase in pCO2 from the baseline of 40, and checking to see if the compensatory increase in bicarbonate (com­pensatory metabolic alkalosis via bicarb retention) is appropriate. If the bicarbonate level is lower than expected, then you have an additional primary metabolic acidosis. If it’s higher than expected, then there is an additional primary metabolic alkalosis. Keep in mind that the compensatory increase in bicarbonate could be from 1 to 3.5mmol/L for every 10 mmHg increase in pCO2, depending on whether you are dealing with an acute or chronic respiratory acidosis. Consider acute to be anything that’s happened within 3 days.
    • ACUTE respiratory acidosis compensation: 1 mmol of bicarb per 10 mmHg of pCO2
    • CHRONIC respiratory acidosis compensation: 5 mmol of bicarb per 10 mmHg of pCO2
• RESPIRATORY ALKALOSIS:
  • (DOUBLE TAKE) PEARL: If you are given an ABG and you diagnose an overarching respiratory alkalosis, ALWAYS calculate for metabolic compensation. You do this by looking at the decrease in pCO2 from the baseline of 40, and checking to see if the compensatory decrease in bicarbonate (compensatory non-anion gap metabolic acidosis via bicarb excretion) is appropriate. If the bicarbonate level is lower than expected, then you have an additional primary metabolic acidosis on top of the compensatory acidosis. If it’s higher than expected, then there is an additional primary metabolic alkalosis. Keep in mind that the compensatory decrease in bicarbonate could be from 2 to 5 mmol/L for every 10mmHg decrease in pCO2, depending on whether you are dealing with an acute or chronic respiratory alkalosis. Consider acute to be anything that’s happened within 3 days.
    • ACUTE respiratory alkalosis compensation: 2 mmol of bicarb per 10 mmHg of pCO2
    • CHRONIC respiratory alkalosis compensation: 5 mmol of bicarb per 10 mmHg of pCO2

MNEMONIC: Use the table below anytime there is an overarching respiratory disorder. It is amazing! Just remember that everything goes in alphabetical order.

• When you diagnose an overarching respiratory disorder, first decide if it’s ACUTE or CHRONIC. That determines which set of rows you use. Alphabetical!
• The top row in each set of rows deals with ACIDS (acidosis, not BASES for alkalosis). Alphabetical!
• The top row in each set of rows deals with Ascending arrows, not descending. Alphabetical!
• The arrows for pCO2 and HCO3 always travel in the same direction for any given row
• The numbers used for compensation are sadly NOT 1, 2, 3 and 4. But they ARE 1, 2, 3.5 and 5. Think of the 5 as the number that falls into that last block from the 3.5 above it. That’s it!

Acute then chronic → Acid then base → up (Ascending) then down (Descending) → 1 > 2 > 3.5 > 5