What is Stroke Volume Index (SVI)?
Stroke Volume Index (SVI) is stroke volume normalized to body surface area, calculated as stroke volume divided by body surface area (BSA), with normal values typically >35 mL/m² in adults. 1
Definition and Calculation
SVI represents the volume of blood ejected by the left ventricle per cardiac cycle, adjusted for body size to allow meaningful comparison across patients of different sizes. 1
The calculation is straightforward:
- SVI = Stroke Volume (mL) / Body Surface Area (m²) 1
- Stroke volume itself is calculated as: SV = Left Ventricular End-Diastolic Volume - Left Ventricular End-Systolic Volume 1
Measurement Methods
Echocardiographic Doppler Method
The most common clinical approach uses Doppler echocardiography to measure flow through the left ventricular outflow tract (LVOT): 2
- Measure LVOT diameter in mid-systole from parasternal long-axis view (inner edge to inner edge) 2
- Calculate LVOT cross-sectional area = π × (LVOT diameter/2)² 2
- Measure LVOT velocity-time integral (VTI) using pulsed-wave Doppler positioned just proximal to the aortic valve 2
- Calculate stroke volume = LVOT CSA × LVOT VTI 2
- Index to BSA to obtain SVI 1
Alternative Methods
SVI can also be measured using 2D or 3D echocardiography to calculate ventricular volumes, though these methods yield different absolute values and are not interchangeable with Doppler-derived measurements. 3
Normal Reference Values
Normal SVI is generally considered >35 mL/m² in adults, though this varies by measurement technique, age, sex, and ethnicity. 1, 4
Key Variations:
- Women have significantly lower SVI than men across all measurement techniques 3
- SVI decreases with aging regardless of measurement method 3
- Asian populations have smaller SVI compared to white and black populations, even after BSA normalization 3
- Measurement technique matters: 2D echocardiography yields significantly lower values than Doppler or 3D methods 3
Clinical Significance
Aortic Stenosis Risk Stratification
SVI is particularly important in evaluating patients with aortic stenosis, especially those with low-gradient disease:
- In low-gradient severe AS with preserved EF (≥50%): SVI <35 mL/m² defines "low-flow" physiology and indicates worse prognosis 1
- The mortality threshold differs by ejection fraction: In preserved EF patients, the critical threshold is <30 mL/m², while in reduced EF patients it remains at <35 mL/m² 4
- Each 5 mL/m² reduction in SVI is associated with a 20% increase in adjusted mortality risk in low-gradient severe AS 5
Hemodynamic Assessment
- Low SVI (<35 mL/m²) with preserved LVEF suggests a small, hypertrophied left ventricle with thick walls and diastolic dysfunction 1
- SVI helps distinguish true severe AS from pseudo-severe AS in patients with low gradients 1
- In high-gradient AS patients undergoing TAVR, low SVI (≤35 mL/m²) identifies those at increased risk for adverse outcomes, particularly when combined with reduced LV function 6
Other Clinical Applications
- Acute pulmonary embolism: SVI <20 mL/m² is associated with in-hospital death or cardiopulmonary decompensation in intermediate-risk PE patients 7
- Cardiac output calculation: Cardiac Output = SVI × BSA × Heart Rate 2
- Valvular regurgitation quantification: SVI is used in continuity equation methods to calculate regurgitant volumes 1
Important Clinical Caveats
When interpreting SVI, always consider:
- Measurement technique used - values are not interchangeable between Doppler, 2D, and 3D methods 3
- Patient demographics - age, sex, and ethnicity significantly affect normal ranges 3
- Clinical context - the prognostic threshold varies by underlying condition (e.g., 30 vs 35 mL/m² in different AS populations) 4
- Accuracy of LVOT diameter measurement - even small errors (1.8 mm) result in 19% error in calculated values 1