Suspect a mixed disorder when compensation is outside the expected range, the delta ratio is abnormal, or the pH is normal with abnormal PaCO2 and HCO3-. A venous gas is adequate for pH, HCO3-, lactate and electrolytes, but not for PaO2 or (often) PaCO2.
At the bedside
Timed steps- 1
Mixed clues
SpotNormal pH with abnormal values; compensation off by > 2-3 mmHg; delta ratio < 0.8 or > 2.
- 2
Common combos
ExamplesSepsis with vomiting (HAGMA + metabolic alkalosis); DKA + diarrhoea (HAGMA + NAGMA); COPD + diuretics (respiratory acidosis + metabolic alkalosis); salicylate (respiratory alkalosis + HAGMA); cardiac arrest (respiratory + metabolic acidosis).
- 3
Agreement
VBGVenous pH ≈ arterial − 0.03; HCO3- ≈ arterial + 1; venous PCO2 ~4-6 mmHg higher but wide limits; venous PO2 is not useful. Normal venous PCO2 (< 45) reliably excludes arterial hypercapnia.
- 4
When
Use ABGNeed PaO2 (oxygenation targets, P/F), shock with poor perfusion (VBG diverges), suspected hypercapnia needing precise values, and ventilator adjustments.
ABG or VBG?
Step 1 of ~3
Decision: Need precise oxygenation (P/F, PaO2 targets) or in shock?
Based on Kelly AM. Educational use; follow local protocol.
Evidence
- Kelly 2010: venous pH and bicarbonate agree closely with arterial values; PCO2 agreement is too wide for substitution but normal venous PCO2 screens out hypercapnia.
🇮🇳 India notes
- Arterial gas syringes and analyser cartridges are costly; VBG strategies reduce cost and patient discomfort.
References
Test yourself
Module quiz · score 70% to complete the module
Educational reference for trained clinicians. Check doses against your hospital protocol and the patient in front of you.