Anion gap

AG = Na⁺ − (Cl⁻ + HCO₃⁻)

Normal reference: 8–12 mmol/L (assumes albumin ≈ 4 g/dL).

Albumin-corrected anion gap

Corrected AG = AG + 2.5 × (4 − Albumin)

Accounts for hypoalbuminemia; used as contextual information, not as the primary diagnostic driver.

Compensation formulas

Winter's formula (metabolic acidosis)

Expected PaCO₂ = (1.5 × HCO₃⁻) + 8 ± 2

Respiratory compensation in metabolic alkalosis

Expected PaCO₂ = 40 + 0.7 × (HCO₃⁻ − 24) ± 5

Respiratory acidosis compensation

  • Acute: PaCO₂ ↑10 → HCO₃⁻ ↑1
  • Chronic: PaCO₂ ↑10 → HCO₃⁻ ↑3–4

Respiratory alkalosis compensation

  • Acute: PaCO₂ ↓10 → HCO₃⁻ ↓2
  • Chronic: PaCO₂ ↓10 → HCO₃⁻ ↓4–5

Mixed-disorder analysis

Adjusted bicarbonate (delta-gap)

Adjusted HCO₃⁻ = HCO₃⁻ + (AG − 12)

Delta ratio

Delta Ratio = (AG − 12) ÷ (24 − HCO₃⁻)

VBG–ABG relationship

  • VBG pH ≈ ABG pH − 0.03
  • PvCO₂ ≈ PaCO₂ + 4–6 mmHg
  • Oxygenation indices are calculated from arterial (ABG) samples only

Base excess and Henderson–Hasselbalch

Live bicarbonate preview and base-excess calculation use the Henderson–Hasselbalch relationship between pH, PaCO₂ and HCO₃⁻.

Oxygenation and O₂ transport

  • Oxygenation index (OI), P/F ratio, and A–a gradient using standard West/Nunn respiratory physiology formulas
  • Berggren shunt equation for corrected end-capillary oxygen content (assumes full capillary saturation)
  • Hüfner constant of 1.34 mL O₂/g Hb for oxygen-carrying capacity
  • Cardiac output pattern classification per Shoemaker criteria

Ventilation-mode intelligence

  • ROX Index for HFNO escalation guidance — ROX = (SpO₂% / FiO₂%) / RR, threshold 4.88, validated in non-COPD, non-immunocompromised pneumonia patients (Roca JC, et al. Intensive Care Med, 2016). Not validated in COPD or immunocompromised patients.
  • NIV driving pressure (IPAP − EPAP), analogous to pressure support on invasive ventilation.

Temperature and perfusion

Central–peripheral temperature gradient thresholds for perfusion assessment follow Creteur J, et al., Intensive Care Med, 2010.

Renal and CRRT context

Acute kidney injury staging references KDIGO staging concepts. AVBG notes that staging properly requires a baseline creatinine trend, and that single values may over- or under-stage AKI without one.

Interpretation hierarchy

  1. Anion gap
  2. Adjusted HCO₃⁻
  3. Delta ratio
  4. Compensation formulas
  5. Corrected anion gap (contextual only)

Limitations

AVBG supports clinical interpretation and does not replace laboratory verification, comprehensive assessment or clinician judgment. Internal self-tests confirm the calculation engine behaves as designed; they are not regulatory certification or formal clinical validation.

The formulas above are population-derived reference points from the cited literature, not patient-specific predictions — individual physiology can deviate from them. Several thresholds carry their own documented limits: the ROX Index is not validated in COPD or immunocompromised patients, and AKI staging from a single creatinine value can over- or under-stage injury without a baseline trend. This reference list documents the methodology already implemented in AVBG's calculation engine, for transparency and clinician review — it is not a substitute for the primary literature or your institution's own protocols.