Venous Blood Gas
what this result means
| value | normal |
|---|---|
| pH | 7.35 to 7.45 |
| pCO₂ | 33 to 45 mmHg |
| HCO₃ | 22 to 28 mEq/L |
| base excess | −2 to +2 |
| lactate | < 2 mmol/L |
stepwise
- pH: <7.35 acidosis, >7.45 alkalosis
- HCO₃: low = metabolic acidosis, high = metabolic alkalosis
- pCO₂: high = respiratory acidosis, low = respiratory alkalosis
- compensation: appropriate? under or over = a mixed disorder
- anion gap (if metabolic acidosis): Na − (Cl + HCO₃); normal ~8–12 mEq/L, adjust for albumin if low
calculator Winters’ formula · MDCalc ↗︎
interpret a gas
acute vs chronic
optional, for the anion gap
a reading aid, not a diagnosis. always cross-check the full clinical picture.
when a VBG can mislead
- severe shock or CPR: venous stasis widens the V–A gap, trust an ABG
- prolonged tourniquet or difficult draw (>1 min): local stasis falsely shifts pH and pCO₂
- in critical illness, V–A offsets are approximate: get an ABG when precision changes management
rule-out tip: venous pCO₂ <45 mmHg (~6.0 kPa) makes significant arterial hypercapnia very unlikely (high NPV): normal VBG CO₂ can spare an ABG in stable patients.
immediate red flags
- airway compromise · respiratory distress
- shock · poor perfusion · altered mental status
- severe metabolic acidosis or hyperkalemia
- lactate >4 mmol/L or rising → think sepsis
- don’t miss: DKA, salicylate toxicity, mixed disorder
interpret the pattern
- metabolic acidosis: DKA, lactic acidosis, AKI, diarrhea, toxins
- metabolic alkalosis: vomiting, NG losses, diuretics, excess bicarbonate
- respiratory acidosis: COPD, opioid / CNS depression, neuromuscular disease, obesity hypoventilation
- respiratory alkalosis: sepsis, PE, hypoxia, pain, anxiety
high anion gap, MUDPILES
- Methanol, Uremia, DKA, Propylene glycol, Iron / INH, Lactate, Ethylene glycol, Salicylates
what to check next
- vitals, SpO₂
- ECG if electrolyte abnormality (esp. hyperkalemia)
examination
- general: respiratory distress, altered consciousness, shock
- respiratory: work of breathing, wheeze, reduced air entry
- circulatory: hypotension, poor perfusion, sepsis signs
initial workup
- CBC, U&E, glucose
- lactate, ketones
if indicated
- blood cultures if sepsis suspected
- troponin, CXR
- CT if indicated
what to do
treat the underlying disturbance
- DKA → DKA protocol: fluids + fixed-rate insulin 0.1 U/kg/h, replace K⁺
- sepsis → sepsis bundle: cultures, antibiotics, fluids, lactate-guided
- COPD with respiratory acidosis → controlled O₂, NIV if pH <7.35 with hypercapnia
- metabolic alkalosis → correct volume, K⁺ and Cl−, stop the driver
then
- repeat VBG, lactate trend, electrolytes, clinical response
when to escalate: urgent ICU / senior
- escalate severe acid-base abnormalities early
- pH <7.2 or severe / refractory acidosis
- lactate not clearing
- rising CO₂ or tiring despite NIV
- hemodynamic instability
common pitfalls / special situations
trust the VBG for
- pH, HCO₃, base excess, lactate, K⁺, Na⁺, glucose, Hb
not for oxygenation
- venous pO₂ is meaningless for hypoxia: use SpO₂, ABG for accurate O₂/CO₂
- rough venous vs arterial: pH ~0.03 to 0.04 lower, pCO₂ ~4 to 8 mmHg higher
- high venous CO₂ → ABG to quantify (rule-out tip above)
get an ABG when
- true oxygenation needed (hypoxia, O₂ titration)
- borderline or rising CO₂ in type 2 respiratory failure
- critical illness where precision changes management
MOC pearl
most VBGs read with three questions: acidotic or alkalotic? metabolic or respiratory? compensation appropriate?
evidence
| source | type |
|---|---|
| Winters’ formula, expected respiratory compensation | common practice / validated formula |
| MUDPILES high anion-gap mnemonic | common practice |
Verify indication, dose, allergies, interactions, renal/hepatic function and local protocols before prescribing.
free the on-call checklist covers the whole shift on one printable page.
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reviewed Jun 2026updated Sep 2026file lab/venous-blood-gas