Critical Care Calculators
Critical care calculators for common ICU/ED maths — including MAP, A–a gradient, and quick ratio checks — with formulas, worked examples, and FAQs.
Mean Arterial Pressure (MAP)
Calculate MAP from systolic and diastolic blood pressure
A–a Gradient
Alveolar–arterial oxygen gradient for ABG interpretation
Shock Index
Quick haemodynamic assessment: HR ÷ SBP
How Critical Care Calculations Work
Critical care calculations support fast, consistent arithmetic for haemodynamics and lab relationships used in critical care documentation and quick clinical checks. These tools help clinicians assess perfusion, tissue oxygenation, and haemodynamic stability.
Inputs need correct units and clinical context. Calculators reduce arithmetic mistakes but do not replace clinical judgement, assessment, or institutional protocols.
Common Critical Care Calculations Explained
Mean Arterial Pressure (MAP)
MAP is an estimate of average arterial pressure calculated from systolic and diastolic BP.
Example question:
BP is 110/70 mmHg. What is the MAP?
Worked example:
Answer: MAP ≈ 83 mmHg
Tip: Diastolic counts twice in this estimate.
A–a Gradient (Alveolar–Arterial Oxygen Gradient)
A–a gradient compares estimated alveolar oxygen (PAO₂) to measured arterial oxygen (PaO₂). It's a structured calculation used alongside ABG interpretation.
Example question:
FiO₂ is 0.40, PaCO₂ is 40 mmHg, and PaO₂ is 90 mmHg. Estimate PAO₂ and then calculate A–a gradient (sea-level example).
Worked example:
Answer: A–a gradient = PAO₂ − PaO₂ (calculator provides exact).
Tip: Make sure units match what the calculator expects (mmHg vs kPa).
Shock Index
Shock index is a quick ratio check: heart rate divided by systolic blood pressure.
Example question:
HR is 120 bpm and SBP is 90 mmHg. What is the shock index?
Worked example:
Answer: Shock index ≈ 1.33
Tip: HR ÷ SBP. Keep it simple.
When These Calculators Are Used
- •Converting BP readings into MAP for documentation and quick checks
- •Performing structured ABG-related calculations (e.g., A–a gradient) when required
- •Quick ratio checks (e.g., shock index) during assessment
- •Double-checking arithmetic and unit conversions in high-acuity settings
- •Education/study for core critical care formulas
Clinical Safety Note
Critical care calculations depend on correct inputs and units (mmHg vs kPa, FiO₂ as a fraction vs %). These calculators support calculation checking but do not replace clinical assessment, protocols, or professional judgement. Examples on this page are for calculation practice only.
Related Calculator Categories
Frequently Asked Questions
Clinical reminder: Always follow local protocols and consult medication information sheets. These examples are for calculation practice only.