Venous Blood Gas
what this result means
escalate now if
- airway compromise, respiratory distress, CO₂ rising or tiring on NIV → hypercapnia, ICU
- shock, poor perfusion, altered mental state → hypotension
- pH < 7.2, severe or refractory acidosis, hyperkalemia → potassium, acute kidney injury
- lactate > 4 mmol/L or not clearing → sepsis
- don’t miss → DKA, salicylate toxicity, mixed disorder
| 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 |
- pH < 7.35 acidosis, > 7.45 alkalosis
- HCO₃ moved → metabolic; pCO₂ moved → respiratory; both, or compensation off → mixed
- metabolic acidosis → expected pCO₂ = 1.5 × HCO₃ + 8 (± 2); anion gap = Na − (Cl + HCO₃), normal 8–12, add 2.5 per 10 g/L albumin below 40
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 makes significant arterial hypercapnia very unlikely (high NPV): normal VBG CO₂ can spare an ABG in stable patients.
calculator ABG analyzer · MDCalc ↗︎
interpret the pattern
- metabolic acidosis → DKA, lactic acidosis, AKI, diarrhea, toxins; high anion gap = MUDPILES (methanol, uremia, DKA, propylene glycol, iron / INH, lactate, ethylene glycol, salicylates); normal gap = HARDASS (hyperalimentation, acetazolamide, RTA, diarrhea, Addison’s, spironolactone, saline)
- metabolic alkalosis → vomiting, NG losses, diuretics, bicarbonate
- respiratory acidosis → COPD, opioids / CNS depression, neuromuscular disease, obesity hypoventilation
- respiratory alkalosis → sepsis, PE, hypoxia, pain, anxiety
what to check next
- check vitals, SpO₂, mental state, work of breathing, perfusion
- CBC, RFT, glucose, ketones, lactate; ECG if K⁺ abnormal; cultures if sepsis; CXR, troponin if the picture asks
what to do
- DKA → protocol: NaCl 0.9%, insulin 0.1 U/kg/h IV, replace K⁺
- sepsis → cultures, antibiotics within the hour, crystalloid 30 mL/kg if hypotensive or lactate > 4, repeat lactate
- COPD, high CO₂ → controlled O₂ to SpO₂ 88–92%; ABG to decide NIV: arterial pH ≤ 7.35 with PaCO₂ > 45 mmHg → NIV (a venous pH alone does not decide it)
- metabolic alkalosis → volume, K⁺, Cl⁻, stop the driver
- repeat gas in 1–2 h with lactate and K⁺
- hand over → last gas + trend, next gas due, escalation trigger
VBG or ABG
- trust the VBG for pH, HCO₃, base excess, lactate, K⁺, Na⁺, glucose, Hb
- venous pO₂ says nothing about oxygenation: SpO₂ or ABG
- venous pCO₂ < 45 mmHg rules out significant hypercapnia; higher → ABG to quantify (venous runs ~4–8 mmHg higher, pH ~0.03 lower)
- ABG when → hypoxia or O₂ titration, rising CO₂ in type 2 failure, shock or CPR, tourniquet on > 1 min
evidence
| source | type |
|---|---|
| Winters’ formula, expected respiratory compensation | common practice / validated formula |
| MUDPILES high anion-gap mnemonic | common practice |
| ERS/ATS 2017, NIV for acute respiratory failure: NIV in COPD when arterial pH ≤ 7.35 with PaCO₂ > 45 mmHg | guideline |
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reviewed Jun 2026updated Oct 2026file lab/venous-blood-gas