Cheat Sheet Blood Gas Interpretation

Cheat Sheet Blood Gas Interpretation

Arterial Blood Gas Quick Reference

Tap card Β· scan Β· interpret in seconds

1 Normal Ranges
Parameter Normal Range Abnormal
pH 7.35 – 7.45 <7.35 Acidosis >7.45 Alkalosis
PaCO2 4.6 – 6.0 kPa
(35 – 45 mmHg)		 High = Resp. Acidosis Low = Resp. Alkalosis
HCO3βˆ’ 22 – 26 mmol/L Low = Met. Acidosis High = Met. Alkalosis
Base Excess βˆ’2 to +2 mmol/L <βˆ’2 Acidosis >+2 Alkalosis
PaO2 10.6 – 13.3 kPa
(80 – 100 mmHg)		 Low = Hypoxaemia
Lactate 0.5 – 1.0 mmol/L High = Tissue hypoperfusion
2 What Each Abnormality Means
Low pH (Acidaemia)Blood is too acidic. Caused by CO2 retention (respiratory) or HCO3βˆ’ loss / acid gain (metabolic).
High pH (Alkalaemia)Blood is too alkaline. Caused by CO2 loss via hyperventilation (respiratory) or HCO3βˆ’ gain / acid loss (metabolic).
High PaCO2Hypoventilation, e.g. COPD, sedation, respiratory depression, neuromuscular weakness. Drives pH down.
Low PaCO2Hyperventilation, e.g. anxiety, pain, sepsis, PE, compensating for metabolic acidosis. Drives pH up.
Low HCO3βˆ’Metabolic acidosis, e.g. DKA, lactic acidosis, renal failure, diarrhoea (bicarbonate loss).
High HCO3βˆ’Metabolic alkalosis, e.g. vomiting, diuretics, excess antacid or bicarbonate use.
3 5-Step Interpretation Method
  1. Oxygenation: check PaO2. Is the patient hypoxaemic?
  2. pH: acidaemic (<7.35), alkalaemic (>7.45), or normal?
  3. Respiratory component: does PaCO2 move in the direction that explains the pH change?
  4. Metabolic component: does HCO3βˆ’ / base excess move in the direction that explains the pH change?
  5. Compensation: is the opposite system pushing pH back to normal? Fully compensated pH is near normal; partially compensated pH is abnormal but moving that way; uncompensated means the other system hasn't responded yet.
pH↓ + CO2↑Respiratory Acidosis
pH↑ + CO2↓Respiratory Alkalosis
pH↓ + HCO3↓Metabolic Acidosis
pH↑ + HCO3↑Metabolic Alkalosis
4 Key Formulas
Winter's Formula – expected respiratory compensation for metabolic acidosis Expected PaCO2 (mmHg) = (1.5 Γ— HCO3βˆ’) + 8 Β± 2 If measured PaCO2 is higher than expected β†’ concurrent respiratory acidosis. If lower β†’ concurrent respiratory alkalosis.
Anion Gap – helps identify the cause of a metabolic acidosis AG = Na&plus; βˆ’ (Clβˆ’ + HCO3βˆ’) Normal range: 8 – 16 mmol/L. Raised AG suggests DKA, lactic acidosis, uraemia, or toxins (e.g. methanol, ethylene glycol, salicylate).
Henderson–Hasselbalch (concept) pH ∝ HCO3βˆ’ Γ· PaCO2 pH is governed by the ratio of metabolic to respiratory components, not either value alone.
5 Interactive Interpreter
Enter values and tap Interpret.
Open the full MDCalc ABG Analyzer β†’
Quick reference only. Always interpret ABGs alongside clinical context and local protocol.
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