Respiratory acidosis vs Metabolic acidosis
How to tell Respiratory acidosis and Metabolic acidosis apart when a board question makes them look alike.
Respiratory acidosis is CO2 retention; metabolic acidosis is bicarbonate loss or acid gain.
Side by side
Primary PaCO2 elevation from hypoventilation, COPD exacerbation, sedatives, neuromuscular weakness, or airway obstruction, with renal bicarbonate retention as compensation.
Primary bicarbonate decrease from acid gain or bicarbonate loss, with hyperventilation lowering PaCO2 as compensation.
Alveolar hypoventilation retains CO2, increasing carbonic acid and lowering pH.
Nonvolatile acid accumulation or bicarbonate loss lowers serum bicarbonate and pH.
Respiratory acidosis: pH below 7.35, primary PaCO2 ELEVATED above 45 mmHg, secondary bicarbonate compensation rises 1 mEq/L per 10 mmHg rise in PaCO2 acutely (chronic 3 to 4 mEq/L per 10 mmHg). Causes: hypoventilation from CNS depression (opioids, sedatives, stroke), neuromuscular disease (myasthenia, ALS, Guillain-Barre), chest wall problems (kyphoscoliosis, obesity hypoventilation), and severe lung disease (COPD exacerbation, severe asthma exacerbation, pneumonia, ARDS).
Metabolic acidosis: pH below 7.35, primary bicarbonate REDUCED below 22 mEq/L, secondary respiratory compensation lowers PaCO2 according to Winter formula (expected PaCO2 = 1.5 times bicarbonate + 8 ± 2); if measured PaCO2 is higher than expected, respiratory acidosis is also present; if lower, respiratory alkalosis is also present. Then calculate anion gap to split MUDPILES (high gap) from HARDASS (normal gap).
The trap
Both can have dyspnea and low pH; decide which variable moved first, PaCO2 or bicarbonate.
Watch it explained
The repair move
For acidemia, identify the primary abnormal value before interpreting compensation.
Try a free question
CoreA patient with COPD exacerbation has acidemia, elevated PaCO2, and a compensatory rise in bicarbonate. Which primary disorder is most likely?
Show answer and explanation
Answer: A. Respiratory acidosis. Respiratory acidosis is primary CO2 retention from hypoventilation; the kidney compensates by retaining bicarbonate.
Why the other choices are tempting
- B. Metabolic acidosis is primary bicarbonate loss or acid gain with respiratory compensation.
- C. Respiratory alkalosis has low PaCO2.
- D. Metabolic alkalosis has primary elevated bicarbonate.
- E. Acidemia and high PaCO2 are not normal.
Flashcards
Tap a card to flip it.
What is the fastest separator for Respiratory acidosis vs Metabolic acidosis?
Respiratory acidosis is CO2 retention; metabolic acidosis is bicarbonate loss or acid gain.
What clue should make you choose Respiratory acidosis?
Primary PaCO2 elevation from hypoventilation, COPD exacerbation, sedatives, neuromuscular weakness, or airway obstruction, with renal bicarbonate retention as compensation.
Quick answers
What is the fastest way to tell Respiratory acidosis from Metabolic acidosis?
Respiratory acidosis is CO2 retention; metabolic acidosis is bicarbonate loss or acid gain.
What is the key clue for Respiratory acidosis?
Primary PaCO2 elevation from hypoventilation, COPD exacerbation, sedatives, neuromuscular weakness, or airway obstruction, with renal bicarbonate retention as compensation.
What is the key clue for Metabolic acidosis?
Primary bicarbonate decrease from acid gain or bicarbonate loss, with hyperventilation lowering PaCO2 as compensation.
What is the common trap with Respiratory acidosis vs Metabolic acidosis?
Both can have dyspnea and low pH; decide which variable moved first, PaCO2 or bicarbonate.