Doppler Echo Cardiac Output Calculator
Enter your LVOT diameter, LVOT VTI, heart rate, height and weight to calculate cardiac output (CO), cardiac index (CI), stroke volume (SV), and left ventricular outflow tract cross-sectional area (CSA). Results update instantly with clinical normal-range guidance. This tool uses the standard pulsed-wave Doppler echocardiography method accepted in cardiology and critical care.
Formula
Worked example
LVOT diameter 2.0 cm, VTI 20 cm, HR 70 bpm, height 170 cm, weight 70 kg. CSA = π × 1.0² = 3.14 cm². SV = 3.14 × 20 = 62.8 mL. CO = 62.8 × 70 / 1000 = 4.40 L/min. BSA = √(170 × 70 / 3600) = 1.82 m². CI = 4.40 / 1.82 = 2.42 L/min/m².
What this calculator computes
This tool uses the pulsed-wave Doppler echocardiography method to estimate cardiac output non-invasively. You supply three echo-derived measurements - the left ventricular outflow tract (LVOT) diameter, the LVOT velocity time integral (VTI), and the heart rate - plus height and weight for body surface area. The calculator then returns cardiac output (CO), cardiac index (CI), stroke volume (SV), and LVOT cross-sectional area (CSA), all with normal-range context. No blood sampling or invasive catheterisation is required.
The Doppler echo method and its formulas
The calculation follows three sequential steps. First, the LVOT cross-sectional area is derived from the diameter: CSA (cm²) = π × (diameter/2)². Because the LVOT is approximately circular, this converts the single diameter measurement into an area. Second, stroke volume is found by multiplying CSA by the VTI: SV (mL) = CSA (cm²) × VTI (cm). The VTI is the integral of blood velocity over one cardiac cycle - geometrically, it equals the stroke distance (how far a column of blood travels in one beat), so multiplying by the cross-sectional area gives the volume of that column. Third, cardiac output is SV scaled by heart rate: CO (L/min) = SV (mL) × HR (bpm) / 1000. Cardiac index is CO divided by body surface area, which is calculated using the Mosteller formula: BSA (m²) = √((height cm × weight kg) / 3600). Indexing by BSA allows meaningful comparison across patients of different body sizes.
How to acquire the measurements
Obtain the LVOT diameter from the parasternal long-axis (PLAX) view at mid-systole, measuring the inner edge-to-inner edge distance just below the aortic valve. This is typically 1.8 to 2.2 cm in adults. Measure in the same location each time - a 1 mm difference in diameter changes the CSA, and therefore CO, by roughly 10 percent because the diameter is squared. The LVOT VTI is recorded from the apical five-chamber view using pulsed-wave (PW) Doppler, with the sample volume placed 5-10 mm below the aortic valve in the LVOT. Align the Doppler beam as parallel to flow as possible (within 20 degrees) to minimise the angle-dependent underestimation of velocity. Trace the modal velocity envelope (not the maximum) to compute the VTI. Heart rate should be documented at the time of the Doppler acquisition.
Clinical interpretation and limitations
A normal adult cardiac output at rest is 4 to 8 L/min. Values below 4 L/min may suggest cardiogenic shock, severe left ventricular dysfunction, or tamponade, and typically prompt urgent assessment. Values above 8 L/min occur in hyperdynamic states such as sepsis, significant anaemia, thyrotoxicosis, or arteriovenous fistulas. Cardiac index is more useful for cross-patient comparisons and is the standard in critical care; a CI below 2.2 L/min/m² is the classic threshold for cardiogenic shock. An LVOT VTI below 18 cm is a widely used bedside predictor of low stroke volume. The main sources of error are: (1) inaccurate LVOT diameter, since this is squared in the CSA formula; (2) suboptimal Doppler angle causing underestimation of velocity; (3) irregular rhythms such as atrial fibrillation, where averaging five or more consecutive beats reduces variability. Serial measurements under consistent conditions are far more informative than any single value.
Normal haemodynamic ranges (adult at rest)
| Parameter | Normal range | Clinical note |
|---|---|---|
| Cardiac output (CO) | 4 to 8 L/min | Volume pumped per minute |
| Cardiac index (CI) | 2.5 to 4.0 L/min/m² | CO adjusted for body size |
| Stroke volume (SV) | 50 to 100 mL/beat | Volume per contraction |
| LVOT VTI | 18 to 25 cm | Stroke distance; <18 cm suggests low SV |
| LVOT diameter | 1.8 to 2.2 cm | Measured in parasternal long axis |
| LVOT CSA | 2.5 to 3.8 cm² | Derived from diameter |
| Heart rate | 60 to 100 bpm | Sinus rhythm at rest |
Reference values used in cardiology and critical care. Ranges may differ slightly by source and patient population.
Frequently asked questions
What is a normal cardiac output?
For adults at rest, a cardiac output between 4 and 8 litres per minute is generally considered normal. Cardiac index, which adjusts for body size, has a normal range of 2.5 to 4.0 L/min/m². A cardiac output below 4 L/min or a CI below 2.5 L/min/m² may indicate reduced cardiac function and warrants clinical evaluation.
What is LVOT VTI and what does it mean?
The left ventricular outflow tract velocity time integral (LVOT VTI) is the area under the pulsed-wave Doppler velocity curve recorded in the LVOT over one cardiac cycle. It represents the stroke distance - the distance a column of blood travels per heartbeat. Multiplied by the LVOT cross-sectional area, it gives the stroke volume. A normal adult VTI is approximately 18 to 25 cm. Values below 18 cm suggest reduced stroke volume, and values below 12 to 15 cm are often associated with haemodynamic compromise.
Why does a small error in LVOT diameter matter so much?
The LVOT diameter appears squared in the cross-sectional area formula (CSA = π × r²). This means a 5 percent error in the diameter measurement produces roughly a 10 percent error in CSA and, because SV and CO are directly proportional to CSA, a 10 percent error in the final cardiac output. For a 2.0 cm LVOT, a single millimetre difference changes the diameter by 5 percent and the CSA by about 10 percent. Careful, reproducible measurement technique is therefore the most important source of precision in this method.
Can this method be used in atrial fibrillation?
Yes, but additional care is required. In atrial fibrillation, stroke volume varies beat-to-beat because of irregular R-R intervals. Averaging the VTI over at least five consecutive beats (some guidelines suggest ten) reduces variability and gives a more representative value. Single-beat measurements should be avoided in irregular rhythms.
What is cardiac index and when is it used instead of cardiac output?
Cardiac index (CI) is cardiac output divided by body surface area, expressed in L/min/m². It normalises CO for body size, making it useful for comparing patients of different heights and weights. CI is the preferred metric in intensive care unit monitoring and in haemodynamic research. A CI below 2.2 L/min/m² is a commonly used threshold for cardiogenic shock, while CI above 4.0 L/min/m² suggests a hyperdynamic state.
How does this compare to other methods of measuring cardiac output?
Pulmonary artery catheter thermodilution is the historic reference standard, but requires invasive placement. Doppler echocardiography correlates well with thermodilution (reported correlations of 0.85 to 0.95 in most studies) and has the advantage of being non-invasive and repeatable. Other non-invasive methods include impedance cardiography and pulse contour analysis, which are less operator-dependent but may be less accurate in certain clinical scenarios. The echocardiographic method is now standard practice in cardiology and point-of-care ultrasound.
Sources
- Lang RM et al. Recommendations for cardiac chamber quantification by echocardiography. Journal of the American Society of Echocardiography, 2015.
- Nagueh SF et al. Recommendations for the evaluation of left ventricular diastolic function by echocardiography. Journal of the American Society of Echocardiography, 2016.
- POCUS 101: Measuring Cardiac Output with Echocardiography Made Easy.