Calculating Stroke Volume Index (SVI)
To calculate Stroke Volume Index (SVI), divide the stroke volume (SV) by the body surface area (BSA): SVI = SV / BSA. 1, 2
Direct Calculation for Your Values
Using your provided measurements with BSA of 1.77 m²:
- For SV of 40.9 ml: SVI = 40.9 ml ÷ 1.77 m² = 23.1 ml/m² 1, 2
- For SV of 47.0 ml: SVI = 47.0 ml ÷ 1.77 m² = 26.6 ml/m² 1, 2
Clinical Interpretation of These Values
Both calculated SVI values indicate low-flow states, as they fall well below the normal threshold of 35 ml/m². 1
Classification Based on SVI Thresholds:
- Normal-flow: SVI > 35 ml/m² 1, 3
- Low-flow: SVI < 35 ml/m² 1
- Paradoxical low-flow, low-gradient: SVI < 35 ml/m² with preserved ejection fraction (≥50%) 1
Prognostic Significance:
- SVI < 30 ml/m² (which includes your first value of 23.1 ml/m²) carries worse prognosis in patients with preserved ejection fraction, with 1-year and 3-year mortality hazard ratios of 1.80 and 1.38 respectively compared to SVI > 35 ml/m² 3
- SVI 30-35 ml/m² (which includes your second value of 26.6 ml/m²) shows similar survival to SVI > 35 ml/m² in preserved ejection fraction patients, but worse outcomes in reduced ejection fraction patients 3
Important Measurement Considerations
The accuracy of your SVI calculation depends entirely on accurate measurement of the stroke volume. 1, 2
Key Technical Points for SV Measurement:
- LVOT diameter must be measured in mid-systole from inner edge to inner edge in a zoomed parasternal long-axis view 1, 2
- Small measurement errors in LVOT diameter are magnified when calculating cross-sectional area; a 1mm error in LVOT diameter (1.8 cm) results in 19% error in calculated stroke volume 1
- Pulsed-wave Doppler sample volume should be positioned approximately 0.5 cm proximal to the valve, with the signal containing a clear spectral envelope and closing click 1, 2
Common Pitfalls to Avoid:
- Never substitute the labeled prosthesis size for LVOT diameter in continuity equation calculations, as this is not a valid method 1
- Ensure measurements are obtained at end-expiration or averaged over several respiratory cycles 4
- In patients with concomitant regurgitation, the continuity equation method may not be reliable for mitral prostheses but remains valid for aortic prostheses 1