Enter pH, PaCO₂, and HCO₃⁻ and get an instant acid–base classification with compensation appropriateness, albumin-corrected anion gap, and oxygenation grade. Switch to Explore to drag a point around the interactive acid–base map, or Practice to drill random generated gases with scored feedback.
| Parameter | Normal range | Notes |
|---|---|---|
| pH | 7.35–7.45 | Reflects net H⁺ balance |
| PaCO₂ | 35–45 mmHg (4.7–6.0 kPa) | Respiratory component; driven by ventilation |
| HCO₃⁻ | 22–26 mEq/L (= mmol/L) | Metabolic component; regulated by kidneys |
| PaO₂ (room air) | 80–100 mmHg (10.7–13.3 kPa) | Declines with age; P(A-a)O₂ more reliable |
| SaO₂ | ≥ 95% | Pulse oximetry cannot distinguish COHb |
| Anion gap | ≤ 12 mEq/L | Correct for albumin in critically ill patients |
| Disorder | pH | PaCO₂ | HCO₃⁻ | Common causes |
|---|---|---|---|---|
| Resp. acidosis | ↓ | ↑ (primary) | ↑ (comp.) | COPD exacerbation, opioids, obesity hypoventilation |
| Resp. alkalosis | ↑ | ↓ (primary) | ↓ (comp.) | Anxiety, hypoxia, PE, pregnancy, mechanical overventilation |
| Met. acidosis | ↓ | ↓ (comp.) | ↓ (primary) | DKA, lactic acidosis, renal failure, diarrhoea, toxic ingestion |
| Met. alkalosis | ↑ | ↑ (comp.) | ↑ (primary) | Vomiting, diuretics, hyperaldosteronism, massive transfusion |
| Mixed acidosis | ↓↓ | ↑ | ↓ | Cardiac arrest, severe sepsis |
| Mixed alkalosis | ↑↑ | ↓ | ↑ | Diuretics + hyperventilation, liver failure on ventilator |
| Disorder | Expected compensation | Tolerance |
|---|---|---|
| Met. acidosis | PaCO₂ = 1.5 × HCO₃⁻ + 8 (Winters') | ±2 mmHg |
| Met. alkalosis | PaCO₂ = 40 + 0.7 × (HCO₃⁻ − 24) | ±5 mmHg |
| Acute resp. acidosis | HCO₃⁻ rises 1 mEq/L per 10 mmHg ↑ PaCO₂ | ±2 mEq/L |
| Chronic resp. acidosis | HCO₃⁻ rises 3.5 mEq/L per 10 mmHg ↑ PaCO₂ | ±2 mEq/L |
| Acute resp. alkalosis | HCO₃⁻ falls 2 mEq/L per 10 mmHg ↓ PaCO₂ | ±2 mEq/L |
| Chronic resp. alkalosis | HCO₃⁻ falls 5 mEq/L per 10 mmHg ↓ PaCO₂ | ±2 mEq/L |
A normal arterial blood gas has pH 7.35–7.45, PaCO₂ 35–45 mmHg (4.7–6.0 kPa), HCO₃⁻ 22–26 mEq/L, PaO₂ 80–100 mmHg on room air, and SaO₂ ≥ 95%. All five values must fall within range for the gas to be truly normal — a normal pH with abnormal CO₂ and HCO₃⁻ suggests full compensation, not a normal gas.
Winters' formula predicts the expected PaCO₂ in a pure metabolic acidosis: PaCO₂ = 1.5 × HCO₃⁻ + 8 (±2 mmHg). If the measured PaCO₂ is higher than expected, a concurrent respiratory acidosis is likely. If it is lower, a concurrent respiratory alkalosis (or very aggressive hyperventilation) should be considered. The formula assumes the metabolic acidosis is the primary disorder.
The kidney's renal compensation takes 2–5 days to fully develop. In acute respiratory acidosis, HCO₃⁻ rises approximately 1 mEq/L per 10 mmHg rise in PaCO₂. In chronic respiratory acidosis (> 2–3 days), HCO₃⁻ rises approximately 3.5 mEq/L per 10 mmHg. If the measured HCO₃⁻ falls between these predictions, consider an acute-on-chronic process.
The anion gap (Na − [Cl + HCO₃⁻]) estimates unmeasured anions in plasma; a normal value is ≤ 12 mEq/L. Albumin is the dominant unmeasured anion, so hypoalbuminaemia artificially lowers the gap and can mask a true high-gap acidosis. Correction adds 2.5 mEq/L to the gap for every 1 g/dL that albumin falls below 4 g/dL. In critically ill patients with albumin of 2 g/dL, uncorrected gaps can be 5 mEq/L lower than the true value.
A mixed disorder occurs when two separate primary processes act simultaneously rather than one primary process with compensation. The clue is that the secondary value moves in the wrong direction for simple compensation — for example, pH low with both PaCO₂ high and HCO₃⁻ low means both respiratory and metabolic forces are pushing the pH down. A normal or near-normal pH with markedly abnormal PaCO₂ and HCO₃⁻ often signals full compensation from one side, not a mixed disorder; look at the pH direction to tell which is primary.
The acid–base map (or Davenport diagram) plots pH on the x-axis against PaCO₂ on the y-axis. Each shaded band is the zone where an appropriately compensated single disorder lands — derived directly from the compensation formulas above. Dashed curves are lines of constant HCO₃⁻ (Henderson–Hasselbalch isopleths). A point falling inside a band is consistent with that single disorder; a point landing between bands or outside all bands suggests a mixed disorder or an unusual pattern that warrants clinical review.
Discussion
Sign in to join the discussion.
Loading comments…