π« Doppler Echo Cardiac Output
LVOT diameter, VTI, and heart rate to cardiac output.
Educational only. Hemodynamic decisions belong to the clinical team.
Echocardiography measures cardiac output without a catheter: the LVOT diameter gives the cross-sectional area, the Doppler velocity-time integral gives the stroke distance, and heart rate does the rest. The method is standard in every echo lab and validated against thermodilution. Enter the three measurements above.
How to Use This Calculator
Enter LVOT diameter (parasternal long axis), LVOT VTI (apical pulsed-wave Doppler), and heart rate.
Enter LVOT diameter in cm, measured in mid-systole at the aortic annulus.
Enter the LVOT VTI in cm from the pulsed-wave Doppler tracing.
Read stroke volume and cardiac output with the normal ranges.
The Formula
The physics is elegant: the LVOT is treated as a cylinder, the VTI is how far the blood column travels per beat (the area under the Doppler velocity curve), and area times distance is volume. Multiply by heart rate for output per minute. The circular-LVOT assumption is the known simplification; the LVOT is slightly elliptical, so the formula modestly underestimates area, and since diameter is squared, a 1-2 mm measurement error swings the result substantially. Measure the diameter carefully: it is the most influential number in the equation.
Validation is solid: Doppler-derived cardiac output correlates well with invasive thermodilution (r ~0.9 in most studies) when measurements are careful. It is the standard for serial assessment: tracking CO through fluid challenges, inotrope titration, and valve disease follow-up. Stroke volume variation across the respiratory cycle even predicts fluid responsiveness in ventilated patients.
Worked Example
LVOT diameter 2.0 cm, VTI 20 cm, HR 70: CSA = 0.785 x 4.0 = 3.14 cm2. SV = 3.14 x 20 = 62.8 mL/beat, comfortably normal. CO = 62.8 x 70 / 1000 = 4.40 L/min, normal resting output. Now a failing ventricle: same diameter, VTI 12 cm, HR 95: SV = 37.7 mL, CO = 3.58 L/min, low despite tachycardia, the classic low-output pattern where rate cannot compensate for lost stroke volume. The numbers tell the physiology story at a glance.
Pitfalls and Refinements
Diameter error dominates: measure inner-edge to inner-edge in mid-systole, zoom the image, average beats in atrial fibrillation. Malaligned Doppler underestimates VTI: keep the angle under 20 degrees to flow. Aortic regurgitation inflates forward SV (the LVOT VTI includes regurgitant volume); significant mitral regurgitation means forward output is less than total LV stroke volume. For indexed values, divide by body surface area: our cardiac index calculator does that step. And remember what echo CO cannot do: it measures left-sided forward flow, not right-sided output or tissue perfusion, which is why it complements rather than replaces the clinical exam. Serial measurements matter more than single snapshots: trending stroke volume through a fluid challenge or after starting an inotrope shows whether the intervention worked, which one number alone cannot tell you.
Related Calculators
Keep exploring: try our cardiac output (Fick) for the next piece of the picture.
For informational purposes only, not medical advice. Consult a healthcare professional.
Frequently Asked Questions
How is cardiac output calculated on echo?
CO = (0.785 x LVOT diameter^2) x LVOT VTI x heart rate / 1000. The LVOT cross-sectional area times the Doppler velocity-time integral gives stroke volume; times heart rate gives cardiac output in L/min.
What is LVOT VTI?
The velocity-time integral of pulsed-wave Doppler at the left ventricular outflow tract: the area under the velocity curve, in cm, representing how far blood travels per beat. Normal is roughly 18-22 cm.
What is normal cardiac output?
About 4-8 L/min at rest; stroke volume 50-100 mL/beat. Indexed to body size (cardiac index), normal is 2.5-4.0 L/min/m2. Values vary with body size, fitness, and metabolic demand.
How accurate is echo cardiac output?
Well-validated against invasive thermodilution (correlation ~0.9) with careful technique. The LVOT diameter is the main error source since it is squared; small measurement errors swing the result.
Why does aortic regurgitation affect the calculation?
The LVOT VTI then includes blood that regurgitates back: total LV stroke volume overestimates effective forward output. Quantifying regurgitation separately corrects the interpretation.
What is stroke volume variation?
Beat-to-beat SV change across the respiratory cycle in ventilated patients: variation above ~13% predicts fluid responsiveness. It turns the echo CO method into a real-time resuscitation guide.
Can echo measure right-sided cardiac output?
Yes, via the RVOT (diameter and VTI) similarly, though it is technically harder. In the absence of shunts, right and left outputs are equal; discrepancies suggest measurement error or shunts.
What heart rate is used in echo CO?
The rate during the Doppler acquisition, ideally simultaneous with the VTI beats. In atrial fibrillation, average several beats because stroke volume varies with cycle length.
Does the circular LVOT assumption cause error?
Modest: the LVOT is slightly elliptical, so 0.785 x D^2 slightly underestimates true area. 3D echo can planimeter the true area, but the 2D method remains the validated clinical standard.
When is invasive cardiac output still needed?
Complex shock, pulmonary hypertension assessment, and transplant evaluation still use pulmonary artery catheters. Echo CO covers most routine blood-flow questions noninvasively.