[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Cardiovascular](https://durgical.com/en/physiology/areas/cardio/) › Cardiac work, oxygen use and efficiency

# Cardiac work, oxygen use and efficiency

- Concept
- For completeness

The heart spends energy to push blood against pressure. Pumping against high pressure costs far more oxygen than pumping more volume.

Updated Nov 13, 2026

Also called: Stroke work, Myocardial oxygen consumption, Tension-time index, Rate–pressure product

**See it in 3D:** [Left ventricle](https://durgical.com/en/anatomy/left-ventricle/) · [Ascending aorta](https://durgical.com/en/anatomy/ascending-aorta/)

## Key points

- Stroke work ≈ stroke volume × mean ejection pressure; it is the area inside the pressure–volume loop.
- Kinetic energy is normally ≈ 1% of left ventricular work, but can exceed half of it in aortic stenosis (very fast jet).
- Right ventricular work is about one sixth of left, because its pressures are about one sixth.
- Myocardial O₂ use tracks wall tension × time (tension-time index); rate–pressure product (HR × systolic BP) is the bedside index.
- Pressure work costs more O₂ than the same amount of volume work: afterload is expensive, preload is cheap.
- Mechanical efficiency is ≈ 20–25% normally and can fall to ≈ 5–10% in a failing, dilated heart.

## What it is

With each beat the ventricle does external work, mostly pressure–volume work (raising blood to arterial pressure) and a small share as kinetic energy. Its oxygen use depends mainly on wall tension and how long that tension lasts, plus heart rate and contractility. Only about a fifth to a quarter of the chemical energy becomes work; the rest becomes heat.

## How it works, step by step

- Energy comes from oxidation: ≈ 70–90% from fatty acids at rest, the rest from lactate and glucose; reserves are tiny, so O₂ use equals energy use.
- Most O₂ is spent building and holding systolic tension, so higher pressure (afterload) raises O₂ use steeply.
- A larger radius needs more tension for the same pressure (Laplace), so a dilated heart spends more O₂ per unit of work.
- Faster rates add more tension periods per minute; positive inotropy raises the cost of each beat.
- Increasing volume at the same pressure uses the Frank–Starling mechanism with little extra tension, so it is cheap.
- Coronary O₂ extraction is already ≈ 70–75% at rest, so extra demand must be met by more coronary flow.
- As arterial pressure rises, the healthy ventricle keeps output nearly constant (it stretches a little and beats harder) until pressure is very high, then output falls.

### Left-ventricular pressure–volume loop

**EDV** 120 mL **ESV** 50 mL **SV** 70 mL **EF** 58% **ESP** 105 mmHg

#### Normal

Normal loop: EDV about 120 mL, ESV about 50 mL, SV about 70 mL and EF about 60%. The loop's width is the stroke volume and its area is the stroke work.

Tap a corner of the loop (MC, AO, AC, MO) to see the valve event.

- Current loop
- Normal loop
- ESPVR (contractility, Ees)
- EDPVR (passive filling)
- Ea (arterial afterload)
- Stroke volume (SV)

The area inside the loop is the stroke work. Raise afterload and watch the loop grow taller.

## Equations

**Stroke work**SW ≈ SV × (MAP − LVEDP)

**SV**
stroke volume

**MAP**
mean arterial pressure

**LVEDP**
left ventricular end-diastolic pressure

Equals the area of the pressure–volume loop; ≈ 1 J per beat for the left ventricle at rest.

**Laplace law (wall stress)**σ = P × r / (2h)

**σ**
wall stress

**P**
ventricular pressure

**r**
chamber radius

**h**
wall thickness

Dilation raises stress at the same pressure; hypertrophy (larger h) brings it back down.

**Rate–pressure product**RPP = HR × SBP

**HR**
heart rate

**SBP**
systolic blood pressure

A practical index of myocardial O₂ demand; ≈ 70 × 120 = 8 400 at rest.

**Mechanical efficiency**Efficiency = external work / O₂ energy used

Normal ≈ 20–25%.

## Reference values (adult at rest)

| Parameter | Value | Note |
| --- | --- | --- |
| Myocardial O₂ use | ≈ 8–10 mL O₂/min per 100 g | |
| Coronary O₂ extraction | ≈ 70–75% | |
| Left ventricular stroke work | ≈ 1 J per beat | |
| Kinetic share of LV work | ≈ 1% | > 50% possible in aortic stenosis |
| Mechanical efficiency | ≈ 20–25% | ≈ 5–10% in heart failure |
| Rate–pressure product | ≈ 7 000–10 000 mmHg/min | angina often at a reproducible value |

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

## Clinical notes

- Hypertension and aortic stenosis load the ventricle with pressure work: hypertrophy, high O₂ demand and angina even with normal coronaries.
- Beta blockers lower heart rate, contractility and systolic pressure, cutting O₂ demand; this is why they help stable angina.
- In stable angina, chest pain often appears at the same rate–pressure product each time: the supply ceiling of a narrowed artery.

## Memory aid

Squeezing hard is costly, filling more is cheap: the heart pays for pressure, not for volume.

## Test yourself

Think of the answer, then tap the question to see it.

## Two hearts double their stroke work, one by doubling stroke volume and one by doubling pressure. Which uses more O₂, and why?

The pressure-loaded heart. O₂ use follows wall tension × time; raising pressure raises tension in every beat, while raising volume uses the cheap Frank–Starling mechanism.

## Why does the kinetic-energy share of left ventricular work rise so much in severe aortic stenosis?

Kinetic energy grows with the square of velocity; blood must pass the narrow valve at very high speed, so a large part of the work goes into accelerating it.

## Why does a dilated failing ventricle use more O₂ for the same work?

By Laplace's law, a larger radius needs more wall tension to generate the same pressure, so efficiency falls (to ≈ 5–10%).

## A patient's angina starts at a heart rate of 110/min with a systolic pressure of 150 mmHg. What is the rate–pressure product and what does it mean?

16 500 mmHg/min. It estimates the O₂ demand at which supply through the narrowed coronary artery is exceeded; lowering rate or pressure delays angina.

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## Connections

## See it in 3D 2

The structures of Durgical's 3D anatomy where it happens.

- [Left ventricle](https://durgical.com/en/anatomy/left-ventricle/)
- [Ascending aorta](https://durgical.com/en/anatomy/ascending-aorta/)

## Related topics 4

- [The ventricular pressure–volume loop](https://durgical.com/en/physiology/the-ventricular-pressure-volume-loop/)
- [Coronary circulation and myocardial ischemia](https://durgical.com/en/physiology/coronary-circulation-and-myocardial-ischemia/)
- [Preload, afterload and contractility](https://durgical.com/en/physiology/preload-afterload-and-contractility/)
- [Cardiac output and what sets it](https://durgical.com/en/physiology/cardiac-output-and-what-sets-it/)

## Related conditions 4

- [Aortic Valve Stenosis](https://durgical.com/en/conditions/aortic-valve-stenosis/)
- [Stable angina](https://durgical.com/en/conditions/stable-angina/)
- [Hypertensive heart disease](https://durgical.com/en/conditions/hypertensive-heart-disease/)
- [Heart failure](https://durgical.com/en/conditions/heart-failure/)

## In Biochemistry 2

- [ATP (adenosine triphosphate)](https://durgical.com/en/biochemistry/molecules/atp/)
- [Fatty acid β-oxidation](https://durgical.com/en/biochemistry/pathways/fatty-acid-oxidation/)

## In Pharmacology 1

- [Beta blockers](https://durgical.com/en/pharmacology/classes/beta-blockers/)

[← All of physiology](https://durgical.com/en/physiology/)

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: [Stable Angina](https://www.ncbi.nlm.nih.gov/books/NBK559016/) (StatPearls (NCBI Bookshelf), 2025); [Aortic Stenosis](https://www.ncbi.nlm.nih.gov/books/NBK557628/) (StatPearls (NCBI Bookshelf), 2025); [Heart Failure (Congestive Heart Failure)](https://www.ncbi.nlm.nih.gov/books/NBK430873/) (StatPearls (NCBI Bookshelf), 2025).

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/cardiac-work-oxygen-use-and-efficiency/