# Critical Care Blood Gas, Oxygenation, and Hemodynamics Calculators

Connect arterial blood gas, acid-base, oxygenation, cardiac output, and vascular resistance calculations in one focused critical-care workflow.

> Canonical page: https://elysiatools.com/en/hubs/critical-care-blood-gas-oxygenation-hemodynamics

- **Category:** calculate

- **Keywords:** critical care calculators, blood gas calculator, ABG interpretation, acid base calculator, oxygenation calculator, A-a gradient, P/F ratio, cardiac output, vascular resistance, hemodynamics

## Overview

This hub follows the physiology questions that often arrive together in respiratory and critical-care review: start with alveolar gas exchange and acid-base balance, then assess oxygenation, shunt, dead space, cardiac output, and vascular resistance. The calculators expose their formulas and assumptions so clinicians, students, and developers can check a result without treating it as a diagnosis or treatment order.

## Tools

- A-a Gradient (Alveolar-Arterial O₂): Calculate the alveolar-arterial O₂ gradient to distinguish hypoxemia causes. PAO₂ = FiO₂×(P_atm−47) − PaCO₂/0.8; A-a = PAO₂ − PaO₂. Normal ≈ (Age+10)/4. Elevated → V/Q mismatch, shunt, diffusion impairment. Derived from StatPearls, LITFL, Cornell PICU, and Martin 1999. Not medical advice.
- Alveolar Gas Equation (PAO₂ Calculator): Calculate PAO₂ = FiO₂ × (P_atm − 47) − PaCO₂ / RQ. Defaults: P_atm 760 mmHg (sea level), RQ 0.8. Gives a directional hint vs expected ~100 mmHg. For full A-a gradient interpretation use a-a-gradient-calculator. Derived from StatPearls, LITFL, Cornell PICU, and West. Not medical advice.
- Alveolar Ventilation Calculator (VA): Calculate VA = (VT − VD) × RR in L/min. VD may be supplied or estimated as ~2 mL/kg body weight. Normal resting ~4–6 L/min; low → hypercapnia, high → hypocapnia. Derived from West, LITFL, StatPearls, and Radford 1955. Not medical advice.
- Serum Anion Gap (AG) Calculator: Calculate the serum Anion Gap (AG) to classify metabolic acidosis. Standard formula AG = Na⁺ − Cl⁻ − HCO₃⁻ (mmol/L), or AG = Na⁺ + K⁺ − Cl⁻ − HCO₃⁻ with optional potassium. A high AG (≈ >12, or >20 with K⁺) indicates accumulation of unmeasured anions (lactic acidosis, ketoacidosis, renal failure, and toxins such as methanol/ethylene glycol, salicylates — mnemonic GOLD-MARK). A normal AG (8–12) during metabolic acidosis points to a hyperchloremic (normal-anion-gap) acidosis: diarrhea, renal tubular acidosis, saline resuscitation. A low AG (<3–6) is usually hypoalbuminemia, also hypercalcemia/hypermagnesemia, lithium, cationic IgG paraproteins, or bromide pseudo-hyperchloraemia. Optional albumin correction AG_corrected = AG + 2.5 × (4.0 − albumin g/dL) avoids missing a high-AG acidosis in hypoalbuminemia. Derived from Kraut CJASN 2007, Figge 1998, and MDCalc. Interpret with blood gas and full clinical context. Not medical advice.
- Base Excess Calculator (Metabolic Component): Calculate base excess (BE) using the Siggaard-Andersen formula. BE < −3 metabolic acidosis, > +3 metabolic alkalosis. HCO₃⁻ auto-calculated from pH + PaCO₂ if omitted. Derived from Siggaard-Andersen 1974, Cornell PICU, Medscape, and Langer 2022. Not medical advice.
- Henderson-Hasselbalch Equation (pH / HCO₃⁻ / PCO₂): Solve any one of pH/HCO₃⁻/PaCO₂ from the other two via pH = 6.1 + log10(HCO₃⁻/(0.03×PaCO₂)). Reports linear Henderson \[H⁺\] cross-check. Derived from Henderson 1908, Hasselbalch 1916, Siggaard-Andersen 1974, LITFL, StatPearls, Berend 2016. Not medical advice.
- Blood Gas Acid-Base Interpretation (AG / ΔAG): Complete ABG analysis: pH, primary disorder, compensation (Winter's formula), anion gap ± albumin correction, delta ratio for mixed disorder detection. Derived from Emmett 2016, Kraut 2007, Rastegar 2007, LITFL, MDCalc, and Adrogué 1998. Not medical advice.
- Cardiac Index Calculator (CI): Compute CI = CO / BSA with BSA from the Mosteller formula √(height × weight / 3600). Normal 2.5–4.0 L/min/m². Derived from Mosteller NEJM 1987, Du Bois 1916, StatPearls, LITFL. Not medical advice.
- Cardiac Output by Fick Principle (CO): Compute CO = VO₂ / (CaO₂ − CvO₂) / 10 (L/min) with the Fick principle. Requires PA-catheter mixed venous samples. VO₂ defaults to 250 mL/min. Derived from Fick 1870, StatPearls, LITFL, West, Leach 2002. Not medical advice.
- Dead Space Fraction (VD/VT, Bohr & Enghoff): Calculate VD/VT using both the Bohr equation (PACO₂-based) and the Enghoff modification (PaCO₂-based). Normal ~0.20–0.40; >0.60 signals severe dead space in ARDS/PE. Derived from Bohr 1891, Enghoff 1938, Tusman 2012, LITFL, StatPearls. Not medical advice.
- Left Ventricular Ejection Fraction Estimator (LVEF): Compute LVEF = (EDV − ESV) / EDV. Normal 50–70%; <40% HFrEF; >70% hyperdynamic. Derived from Lang JASE 2015, Ponikowski ESC 2016, StatPearls, LITFL, Braunwald. Not medical advice.
- Mean Arterial Pressure (MAP): Calculate MAP = DBP + (SBP − DBP)/3. Normal 70–100; ≥65 sepsis target; <65 hypoperfusion. Derived from Geddes 1984, SSC 2021, BTF 2017, StatPearls, LITFL. Not medical advice.
- Blood Oxygen Content Calculator (CaO₂ / CvO₂): Compute CaO₂ = 1.34×Hb×SaO₂ + 0.003×PaO₂ and (if mixed-venous values supplied) CvO₂, a-v difference, extraction ratio, DO₂, VO₂. Derived from Hüfner 1894, West, StatPearls, LITFL, Leach 2002. Not medical advice.
- PaO₂/FiO₂ Ratio (P/F Ratio, ARDS Severity): Calculate the PaO₂/FiO₂ ratio and classify ARDS severity per the Berlin Definition (2012). Derived from ARDS Definition Task Force JAMA 2012, LITFL, and Rice 2007. Not medical advice.
- Pulmonary Vascular Resistance Calculator (PVR): Compute PVR = (mPAP − PCWP) / CO in Wood units and dyn·s/cm⁵. Normal < 3 Wood; elevated in pulmonary hypertension. Derived from Grossman 1991, Simonneau 2019, Humbert ESC/ERS 2022, StatPearls, LITFL. Not medical advice.
- Rate-Pressure Product Calculator (RPP): Compute RPP = HR × SBP, a non-invasive index of myocardial O₂ demand. Normal resting 6,000–10,000; >12,000 angina threshold. Derived from Gobel 1978, StatPearls, LITFL, Braunwald, Fletcher 2013. Not medical advice.
- Pulmonary Shunt Fraction (Qs/Qt): Calculate Qs/Qt = (CcO₂ − CaO₂)/(CcO₂ − CvO₂). PAO₂ from alveolar gas equation. Normal <5%; >20% severe shunt refractory to O₂. Defaults SvO₂=0.75, PvO₂=40 when mixed-venous values unavailable. Derived from Berggren 1942, StatPearls, LITFL, West. Not medical advice.
- Stroke Volume Calculator (SV): Compute SV = CO × 1000 / HR (mL/beat). Normal 60–100 mL; low in shock, hypovolemia, tachycardia; high in bradycardia and AR. Derived from Guyton & Hall, StatPearls, LITFL, Braunwald. Not medical advice.
- Systemic Vascular Resistance Calculator (SVR): Compute SVR = 80 × (MAP − CVP) / CO (dyn·s/cm⁵). Normal 800–1440; high in cardiogenic shock and hypertension; low in distributive shock and vasodilators. Derived from Grossman 1991, StatPearls, LITFL, Vincent NEJM 2013, Braunwald. Not medical advice.
- Winter's Formula (Expected PaCO₂ in Metabolic Acidosis): Calculate expected respiratory compensation PaCO₂ for metabolic acidosis: 1.5×HCO₃⁻ + 8 ± 2 mmHg. Classifies compensation as appropriate or flags mixed disorders. Derived from Albert 1967, Emmett 2016, LITFL, MDCalc. Not medical advice.

## Frequently asked questions

### What does this hub calculate?

It covers blood-gas and acid-base relationships, oxygenation and pulmonary gas exchange, cardiac output, cardiac index, blood pressure, and systemic or pulmonary vascular resistance.

### Can these tools diagnose a patient?

No. They implement stated equations for education and checking calculations. Clinical interpretation requires the complete patient context, measurement quality, local protocols, and a qualified clinician.

### Which tools should I start with for an ABG?

For a respiratory or metabolic question, start with the blood-gas and anion-gap tools, then use the A–a gradient, P/F ratio, oxygen content, or Winter's formula when the available measurements support them.

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