Educational tool — not a diagnosis. Stroke volume and cardiac output are interpreted by clinicians alongside the echocardiographic study, haemodynamics and the patient's condition.
Do not use this page to make clinical decisions. Accurate LVOT measurement and VTI tracing require trained echocardiography.
Quick answer: stroke volume = π × (LVOT diameter ÷ 2)² × VTI. It models the beat's ejected blood as a cylinder — the LVOT cross-section as the base, the VTI as its length. A 2.0 cm LVOT and 20 cm VTI give about 63 mL. Cardiac output is stroke volume × heart rate.
The formula
Stroke volume = LVOT area × VTI
Cardiac output = stroke volume × heart rate
Stroke volume index = stroke volume ÷ body surface area
The elegant idea is that a single heartbeat ejects a cylinder of blood through the left ventricular outflow tract: the cross-sectional area of the LVOT is the cylinder's base, and the velocity–time integral (VTI) — the distance the blood column travels in one beat — is its length. Multiply them and you have the volume.
Worked example — LVOT diameter 2.0 cm, VTI 20 cm, heart rate 70 bpm:
- LVOT area: π × (2.0 ÷ 2)² = π × 1.0 = 3.142 cm²
- Stroke volume: 3.142 × 20 = 62.8 mL
- Cardiac output: 62.8 × 70 = 4,398 mL/min ≈ 4.4 L/min
The LVOT VTI method is standard Doppler echocardiography for stroke volume and cardiac output, described in echocardiography guidelines from the American Society of Echocardiography and the European Association of Cardiovascular Imaging.
Reference figures
| Parameter | Commonly cited resting range |
|---|---|
| Stroke volume | ≈ 60–100 mL per beat |
| Stroke volume index | ≈ 35–65 mL/m² |
| Cardiac output | ≈ 4–8 L/min |
| LVOT VTI | ≈ 18–22 cm |
These are broad orientation figures for adults at rest. They vary with body size, heart rate, loading conditions and cardiac function, and are always interpreted within the full clinical and echocardiographic picture.
Why LVOT measurement dominates
Because the LVOT area uses the diameter squared, any error in the diameter measurement is roughly doubled in the result. Measure the LVOT diameter 10% too large and the stroke volume comes out about 21% too high (1.1² = 1.21). This is the single largest source of error in the method, which is why echocardiography protocols emphasise measuring the LVOT carefully, at the same point and in the same view each time — particularly when tracking change over time.
Frequently asked questions
How is stroke volume calculated by echocardiography?
Stroke volume = LVOT cross-sectional area × LVOT velocity–time integral (VTI). The area is π × (LVOT diameter ÷ 2)². With a 2.0 cm LVOT diameter and a VTI of 20 cm, area is 3.14 cm² and stroke volume is about 63 mL.
What is a normal stroke volume?
Resting stroke volume in adults is commonly around 60 to 100 mL per beat, though it varies with body size, heart rate and cardiac function. Stroke volume index, which adjusts for body surface area, is often cited around 35 to 65 mL/m².
How do I calculate cardiac output from stroke volume?
Cardiac output = stroke volume × heart rate. A 70 mL stroke volume at 70 beats per minute gives 4,900 mL/min, or about 4.9 litres per minute. Normal resting cardiac output is roughly 4 to 8 litres per minute.
Why is the LVOT diameter squared so important?
Because the area depends on the diameter squared, a small error in measuring the LVOT diameter has an outsized effect. A 10 percent error in diameter produces about a 21 percent error in area and therefore in stroke volume, which is why careful, consistent LVOT measurement is emphasised in echocardiography.
What is the velocity–time integral (VTI)?
VTI is the distance blood travels during one heartbeat, obtained by tracing the Doppler velocity envelope over one cardiac cycle. Multiplying it by the LVOT cross-sectional area gives the volume of blood ejected in that beat — the stroke volume.
References
- American Society of Echocardiography / European Association of Cardiovascular Imaging guidelines on quantification of cardiac chambers and Doppler stroke volume.
- Standard derivation: stroke volume = LVOT cross-sectional area × velocity–time integral.