Acid-base & electrolytes · 6 min
Acid-base — last-night quick guide
ABG interpretation in 30 seconds, anion gap, electrolyte fixes.
Rule
ABG interpretation in 4 steps
(1) pH: <7.35 acidosis, >7.45 alkalosis. (2) Primary: PaCO2 high → respiratory; HCO3 low → metabolic. (3) Compensation: respiratory compensates fast (mins), renal slow (days). (4) Anion gap = Na − (Cl + HCO3); normal 8-12. Wide AG → MUDPILES. Closed AG metabolic acidosis → diarrhea, RTA, NS-induced hyperchloremic acidosis.
Mnemonic — MUDPILES
MUDPILES — Wide-anion-gap metabolic acidosis
MUDPILES wide-AG metabolic acidosis
- MMethanol
- UUremia
- DDiabetic ketoacidosis (DKA)
- PPropylene glycol / paraldehyde
- IIsoniazid / iron
- LLactic acidosis (sepsis, ischemia, shock — most common in OR)
- EEthylene glycol
- SSalicylates
Modern alternative: GOLDMARK (Glycols, Oxoproline, L-lactate, D-lactate, Methanol, Aspirin, Renal failure, Ketoacidosis). MUDPILES is the classic boards-friendly version.
Compensation rules
| Disorder | Expected |
|---|---|
| Metabolic acidosis | PaCO2 = (1.5 × HCO3) + 8 ± 2 (Winter's formula) |
| Metabolic alkalosis | PaCO2 ↑ 0.7 mmHg per 1 mEq HCO3 above 24 |
| Acute respiratory acidosis | HCO3 ↑ 1 per 10 mmHg ↑ PaCO2 |
| Chronic respiratory acidosis | HCO3 ↑ 4 per 10 mmHg ↑ PaCO2 |
| Acute respiratory alkalosis | HCO3 ↓ 2 per 10 mmHg ↓ PaCO2 |
| Chronic respiratory alkalosis | HCO3 ↓ 5 per 10 mmHg ↓ PaCO2 |
Electrolyte derangements — quick fix
| Issue | Threshold | Treatment |
|---|---|---|
| HyperK | >6 + EKG changes | Calcium gluconate 1g, insulin 10u + D50, bicarb if acidosis, albuterol, kayexalate, dialysis |
| HypoK | <3.0 | K replacement 10-20 mEq/hr peripheral max; check Mg (often co-low) |
| HyperNa | >155 | Free water (D5W); correct slowly <10 mEq/24h to avoid central pontine |
| HypoNa | <125 + symptomatic | 3% saline 100 mL bolus; correct <8-12 mEq/24h |
| HypoCa | ionized <1.0 | Ca gluconate 1 g IV (large transfusion citrate) |
| HypoMg | <1.5 | Mg sulfate 2 g IV over 30 min |
Watch out
Don't correct sodium too fast
Hyponatremia correction >12 mEq/24h → central pontine myelinolysis (osmotic demyelination). Always check serum Na q4-6h during correction. Same caution for hyperNa correction (cerebral edema).