Acid–Base Basics
Respiratory vs Metabolic Acidosis: A Quick Clinical Guide
One of the most common acid-base questions is whether a patient has respiratory acidosis or metabolic acidosis. The answer comes from a structured blood gas approach: first look at the pH, then decide whether the primary driver is CO₂ or HCO₃⁻, and then check whether compensation fits the pattern.
Also see our ABG Interpretation Guide, review ABG Analysis basics, or practice cases with ABGenius.
Quick Answer
If the patient is acidotic and the PaCO₂ is high, think respiratory acidosis. If the patient is acidotic and the HCO₃⁻ is low, think metabolic acidosis.
Respiratory acidosis: pH ↓, PaCO₂ ↑
Metabolic acidosis: pH ↓, HCO₃⁻ ↓
Respiratory Acidosis
Respiratory acidosis happens when the lungs are not removing enough CO₂. As CO₂ rises, carbonic acid rises, and pH falls. This is usually a ventilation problem.
- COPD exacerbation or severe obstructive disease
- Hypoventilation from sedatives, opioids, or CNS depression
- Neuromuscular weakness
- Airway obstruction
- Inadequate ventilator settings or ventilatory failure
Metabolic Acidosis
Metabolic acidosis happens when bicarbonate falls due to acid accumulation or bicarbonate loss. This is usually a metabolic or perfusion problem, not primarily a ventilation problem.
- Lactic acidosis from shock or tissue hypoperfusion
- Diabetic ketoacidosis
- Renal failure
- Diarrhea or GI bicarbonate losses
- Toxin ingestion or severe sepsis
Side-by-Side Comparison
| Feature | Respiratory Acidosis | Metabolic Acidosis |
|---|---|---|
| Primary abnormality | PaCO₂ high | HCO₃⁻ low |
| Main system involved | Ventilation / lungs | Metabolic / renal / perfusion |
| Typical compensation | HCO₃⁻ rises over time | PaCO₂ falls via hyperventilation |
| Classic examples | COPD, opioid overdose, hypoventilation | DKA, lactic acidosis, renal failure |
Compensation Clues
Compensation helps confirm whether the disorder is primary and whether an additional mixed disorder may be present.
- In metabolic acidosis, the lungs should lower PaCO₂ through compensatory hyperventilation. Use Winter's Formula to estimate the expected PaCO₂.
- In respiratory acidosis, the kidneys retain bicarbonate over time. Acute and chronic respiratory acidosis have different expected bicarbonate responses.
When to Suspect a Mixed Disorder
Think about a mixed disorder when the compensation does not fit the expected pattern. For example, a patient with metabolic acidosis whose PaCO₂ is not low enough may also have a concurrent respiratory acidosis.
Example: pH 7.20 / PaCO₂ 50 / HCO₃⁻ 18 suggests metabolic acidosis, but the CO₂ is higher than expected for compensation — so think mixed metabolic + respiratory acidosis.
Practice and Related Tools
FAQ
How do you tell respiratory acidosis from metabolic acidosis?
Look at the pH first. If the patient is acidotic, determine whether the primary abnormality is a high PaCO₂ or a low HCO₃⁻. High CO₂ points toward respiratory acidosis, while low bicarbonate points toward metabolic acidosis.
What causes respiratory acidosis?
Common causes include COPD exacerbation, hypoventilation, opioid or sedative effect, neuromuscular weakness, airway obstruction, and ventilatory failure.
What causes metabolic acidosis?
Common causes include lactic acidosis, diabetic ketoacidosis, renal failure, diarrhea, toxin ingestion, and other acid-producing or bicarbonate-losing states.