Why do HR × SV and Fick's principle give the same cardiac output?

They're measuring the same physical quantity — total blood pumped per minute — through two independent methods. HR × SV counts it mechanically (beats per minute times volume per beat); Fick's principle counts it indirectly through oxygen delivery (how fast oxygen is consumed divided by how much oxygen each liter of blood gives up). Agreement between the two is a useful physiological sanity check.

What is a normal resting cardiac output?

About 5 L/min for an average adult at rest — roughly 70-75 beats/min times 65-75 mL/beat. It can rise to 20-25 L/min during intense exercise, mostly through increased heart rate and, to a lesser extent, increased stroke volume.

Why does Fick's principle use the arteriovenous (A-V) O2 difference?

CaO2 (arterial oxygen content) minus CvO2 (venous oxygen content) is how much oxygen each liter of blood gives up to the tissues on one pass. Total oxygen consumed per minute (VO2) divided by oxygen given up per liter tells you how many liters of blood must have passed through per minute — which is exactly cardiac output.

Cardiac Output Calculator — HR × SV or Fick's Method, Free

Heart rate (beats/min)

Stroke volume (mL/beat)

Cardiac output

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How to calculate cardiac output

Cardiac output (CO) is the volume of blood the heart pumps per minute. The direct, mechanical way: CO = HR × SV, heart rate in beats/min times stroke volume (blood ejected per beat) in mL/beat — divide by 1000 for L/min. The indirect way is Fick's principle: CO = VO₂ / (CaO₂ − CvO₂), where VO₂ is whole-body oxygen consumption and (CaO₂ − CvO₂) is the arteriovenous oxygen difference — how much oxygen each liter of blood delivers to tissue on one circuit. Both methods measure the same underlying quantity and should agree.

Typical resting values: HR ≈ 70-75 bpm, SV ≈ 65-75 mL/beat, giving CO ≈ 5 L/min. Units matter — this tool assumes SV in mL/beat and O₂ contents in mL/L; watch for the common alternate unit of mL/100 mL (vol%), which is 10× smaller.

Related tools: Osmotic pressure & tonicity · all biochem tools.

Worked example: resting cardiac output, both methods

A healthy adult at rest: HR = 72 bpm, SV = 70 mL/beat (the tool's HR×SV default).

CO = HR × SV = 72 × 70 = 5040 mL/min = 5.04 L/min

Now the same person via Fick's principle: VO₂ = 250 mL/min, CaO₂ = 200 mL/L, CvO₂ = 150 mL/L (the tool's Fick default).

CO = VO₂ / (CaO₂ − CvO₂) = 250 / (200 − 150) = 250/50 = 5.0 L/min

5.04 L/min and 5.0 L/min — the two independent methods agree almost exactly, which is exactly the point: cardiac output is one real physical quantity, measurable multiple ways.

Practice problems

1. During exercise, HR rises to 150 bpm and SV to 100 mL/beat. What's the new cardiac output?

Show answer

CO = HR × SV = 150 × 100 = 15,000 mL/min = 15.0 L/min Roughly 3× the resting value — mostly from the doubled heart rate, with a smaller contribution from increased stroke volume.

2. During that same exercise, VO₂ rises to 1200 mL/min, CaO₂ to 195 mL/L, and CvO₂ drops to 120 mL/L (tissues extracting more oxygen). Find CO via Fick's principle.

Show answer

CO = VO₂ / (CaO₂ − CvO₂) = 1200 / (195 − 120) = 1200/75 = 16.0 L/min Close to the 15.0 L/min from the HR×SV method above — real physiological measurements from two different methods rarely match to the decimal, but should land in the same range.

FAQ

Why do HR×SV and Fick's principle give the same answer?
They measure the same thing two ways — one mechanically (beats × volume/beat), one via oxygen delivery (consumption ÷ extraction per liter). Agreement is a useful physiological sanity check.

What's a normal resting cardiac output?
About 5 L/min for an average adult (70-75 bpm × 65-75 mL/beat). Can reach 20-25 L/min during intense exercise, mostly via increased heart rate.

Why the A-V O₂ difference specifically?
CaO₂ − CvO₂ is how much oxygen each liter of blood gives up to tissue on one pass. VO₂ (total O₂ consumed per minute) divided by that per-liter delivery tells you how many liters must have passed through per minute — cardiac output.

How does cardiac output relate to blood pressure?
MAP ≈ CO × TPR (total peripheral resistance) — the cardiovascular analog of Ohm's law (V=IR). Anything changing CO or TPR shifts blood pressure.