Mister Ronald McDonald (MR RM) is a helpful flowchart for interpreting acid-base disorders, specifically for determining whether a patient’s condition is due to a metabolic (M) or respiratory (R) cause:

1. Check the pH (7.4 is the cutoff)

• pH > 7.4 → Alkalosis

• pH < 7.4 → Acidosis

2. Assess Carbon Dioxide (CO₂) Levels (PaCO₂)

• The key threshold is 40 mmHg:

• If CO₂ > 40 mmHg, this suggests respiratory acidosis or metabolic alkalosis.

• If CO₂ < 40 mmHg, this suggests respiratory alkalosis or metabolic acidosis.

3. Determine the Cause

• In Alkalosis:

• If CO₂ > 40, the cause is Metabolic (M) Alkalosis.

• If CO₂ < 40, the cause is Respiratory (R) Alkalosis.

• In Acidosis:

• If CO₂ > 40, the cause is Respiratory (R) Acidosis.

• If CO₂ < 40, the cause is Metabolic (M) Acidosis.

Key Takeaway

• The chart helps determine whether a patient’s acid-base imbalance is due to a metabolic or respiratory cause by analyzing pH and CO₂ levels.