Arterial Blood Gases
Interpret any ABG in four steps
Normal values
| pH | 7.35–7.45 | <7.35 acidosis · >7.45 alkalosis |
| PaCO₂ | 35–45 mmHg | Lungs. CO₂ acts as an acid |
| HCO₃⁻ | 22–26 mEq/L | Kidneys. Bicarbonate is a base |
| PaO₂ | 80–100 mmHg | <80 = hypoxemia |
| SaO₂ | 95–100% | Hemoglobin saturation |
Four steps
- pH: acidosis or alkalosis? (If normal, which side of 7.40 is it on?)
- PaCO₂: does it match the pH problem? If yes → respiratory.
- HCO₃⁻: does it match the pH problem? If yes → metabolic.
- Compensation: is the other value moving the opposite way?
| Uncompensated | pH abnormal; other value normal |
| Partially compensated | pH abnormal; both values abnormal |
| Fully compensated | pH normal (7.35–7.45); both values abnormal |
Memory hook: ROME
Respiratory Opposite: in respiratory problems, pH and PaCO₂ move in opposite directions (pH ↓, CO₂ ↑).
Metabolic Equal: in metabolic problems, pH and HCO₃⁻ move in the same direction (pH ↓, HCO₃⁻ ↓).
Causes and clues
| Imbalance | Common causes | Clues |
|---|---|---|
| Respiratory acidosis | Hypoventilation: COPD, opioid/sedative overdose, neuromuscular weakness, airway obstruction, severe asthma (tiring) | Drowsy, confused, headache; slow or shallow breathing |
| Respiratory alkalosis | Hyperventilation: anxiety, pain, fever, early sepsis, early PE, high altitude, ventilator rate too high | Light-headed, tingling around mouth and fingers |
| Metabolic acidosis | DKA, lactic acidosis (shock, sepsis), kidney failure, severe diarrhea, salicylate toxicity | Kussmaul breathing (deep, rapid), hyperkalemia |
| Metabolic alkalosis | Vomiting, NG suction, loop/thiazide diuretics, too much antacid or bicarbonate, hypokalemia | Slow shallow breathing, muscle twitching, hypokalemia |
Practice (cover the answers)
| pH 7.30 · CO₂ 55 · HCO₃ 24 | Uncompensated respiratory acidosis |
| pH 7.48 · CO₂ 30 · HCO₃ 23 | Uncompensated respiratory alkalosis |
| pH 7.28 · CO₂ 30 · HCO₃ 14 | Partially compensated metabolic acidosis |
| pH 7.37 · CO₂ 58 · HCO₃ 32 | Fully compensated respiratory acidosis |
| pH 7.52 · CO₂ 44 · HCO₃ 34 | Uncompensated metabolic alkalosis |
Oxygen targets
- Most acutely ill adults: SpO₂ 92–96%.
- At risk of CO₂ retention (COPD, obesity hypoventilation): 88–92%.
- Pulse oximeters can over-read in darker skin, and read falsely normal in carbon monoxide poisoning. Check an ABG when it matters.
- Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻); normal ~8–12. High gap → DKA, lactate, kidney failure, toxins.