Pv-aCO2 / Ca-vO2 Ratio Calculator

Bedside surrogate for global respiratory quotient. Distinguishes Type A (anaerobic) from Type B (non-hypoxic) lactic acidosis.

Arterial sample (ABG)

PaCO2 mmHg

SaO2 %

PaO2 mmHg

Hemoglobin g/dL

same value used for both samples

Central / mixed venous sample

PvCO2 mmHg

ScvO2 or SvO2 %

CVC = ScvO₂ | PA catheter = SvO₂

PvO2 mmHg

Lactate mmol/L

Calculated values

PCO₂ gap

—

mmHg (ref 2–6)

CaO₂

—

mL/dL

CvO₂

—

mL/dL

Ca-vO₂

—

mL/dL (ref 4–6)

Ratio
—
Pv-aCO₂ / Ca-vO₂

ScvO₂ / SvO₂

—

% (target ≥70)

Enter values above to interpret

Chart note

Enter values above to generate chart text.

Formulas

// step 1 — CO₂ tension gap (numerator) ΔCO₂ = PvCO₂ − PaCO₂ // step 2 — O₂ content (Hüfner equation) CxO₂ = (1.34 × Hb × SxO₂) + (0.003 × PxO₂) // step 3 — ratio (bedside RQ surrogate) Ratio = ΔCO₂ ÷ (CaO₂ − CvO₂)

Reference thresholds

Finding	Threshold	Implication	Source
Ratio — normal aerobic	≤ 1.0–1.4	Type A unlikely	Mekontso-Dessap 2002
Ratio — anaerobic signal	> 1.0	Anaerobic CO₂ production	Ospina-Tascón 2015
Ratio — Type A threshold	> 1.4	Type A likely — 90% ↓ lactate clearance odds	Mesquida 2015
Occult dysoxia (ScvO₂ ≥80%)	≥ 1.6	Occult anaerobic — ICU mortality predictor (Sn 83%, Sp 63%)	He et al. 2017
PCO₂ gap — normal	2–6 mmHg	Gap >6 suggests low CO or high VCO₂	—
Lactate — elevated	≥ 2.0 mmol/L	Triggers Type A vs B assessment	Surviving Sepsis

Caveats

No proven mortality benefit — Su et al. (2018, n=228) found no difference in 28-day or 60-day mortality with ratio-guided vs ScvO₂-guided resuscitation.
Severe metabolic acidosis — shifts CO₂-Hb dissociation curve; may overestimate anaerobic CO₂ at very low DO₂.
CVC vs PA catheter — ScvO₂ is a reasonable but imperfect surrogate for true mixed venous SvO₂.
Diagnostic only — an elevated ratio confirms dysoxia but does not identify the cause (macro flow, microvascular shunting, mitochondrial dysfunction).