An acid-base disorder is named by its direction (acidosis or alkalosis) and its source (respiratory or metabolic). Finding the cause matters more than the number itself.
Respiratory disorders
Respiratory acidosis comes from breathing too little, as in COPD, opioid overdose or exhaustion. Respiratory alkalosis comes from breathing too much, as in anxiety, pain or early sepsis.
Metabolic acidosis
Metabolic acidosis comes from added acid or lost bicarbonate. The patient breathes deep and fast to blow off CO2.
The anion gap
The anion gap is sodium minus chloride and bicarbonate, normally about 8 to 12 mmol/L. A high gap points to added acid such as lactate, ketones or toxins; a normal gap points to bicarbonate lost through diarrhea or the kidney.
Metabolic alkalosis
Metabolic alkalosis comes from lost acid or excess bicarbonate, most often from vomiting or diuretics. It usually comes with low potassium and low volume.
| Disorder | Primary change | Common causes |
|---|---|---|
| Respiratory acidosis | PaCO2 up | COPD, opioids, weak breathing |
| Respiratory alkalosis | PaCO2 down | Anxiety, pain, sepsis, pulmonary embolism |
| Metabolic acidosis | HCO3- down | DKA, lactic acidosis, kidney failure, diarrhea |
| Metabolic alkalosis | HCO3- up | Vomiting, diuretics |
At the bedside
Deep, fast Kussmaul breathing in a patient with high blood glucose suggests diabetic ketoacidosis. Check the blood gas, anion gap and ketones.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).