Velocity Time Integral (VTI) in Echocardiography
Velocity Time Integral (VTI) is defined as the area under the velocity time curve during forward flow across a cardiac valve, measured in centimeters, representing the distance blood travels during a specific time period. 1
Definition and Measurement
- VTI is calculated by tracing the outer edge of the spectral Doppler envelope, providing both the velocity-time integral for the continuity equation and the mean gradient 1
- It is measured in centimeters (cm) and represents the integral of the flow velocity curve during forward flow across a valve 1
- For accurate measurement, a smooth velocity curve with a dense outer edge and clear maximum velocity should be recorded 1
Clinical Applications
Valve Assessment
- VTI is used in the calculation of valve area using the continuity equation 1
- The continuity equation using VTI calculates valve area by dividing the pulsed wave Doppler-derived subvalve stroke volume by the VTI of the target valve 1
- VTI measurements are essential for assessing stenotic valves, particularly aortic stenosis 1
Volume Flow Assessment
- VTI is used to calculate stroke volume and cardiac output when multiplied by the cross-sectional area of the measured location 1, 2
- Left ventricular outflow tract (LVOT) VTI serves as an analog for stroke volume and can predict outcomes in heart failure patients 2
- Low LVOT VTI (≤15 cm) is associated with adverse outcomes in conditions like acute pulmonary embolism 3
Regurgitant Volume Calculation
- VTI is used in calculating regurgitant volume using both Doppler volumetric and flow convergence methods 1
- In the Doppler volumetric method, the difference in stroke volume at two locations is divided by the VTI of the maximal velocity through the valve 1
- In flow convergence method, regurgitant volume is calculated by multiplying the effective regurgitant orifice area times the VTI of the maximal velocity through the valve regurgitation 1
Normal Values and Variations
- Normal aortic valvular flow VTI is approximately 22 ± 4 cm, slightly higher than pulmonary valvular flow (20 ± 4 cm) 4
- LVOT, right ventricular outflow tract, and mitral valvular flow VTIs are similar (16 ± 3,15 ± 3, and 15 ± 3 cm, respectively) 4
- Tricuspid valvular flow VTI is typically lower at 10 ± 3 cm 4
- VTI is inversely related to heart rate and age, particularly after age 60 4
Technical Considerations
- For patients in sinus rhythm, averaging three or more beats is recommended; for irregular rhythms, at least 5 consecutive beats should be averaged 1
- When calculating the dimensionless index for aortic stenosis follow-up, using peak velocity ratio may provide less variability than using VTI ratio (coefficient of variation 12.6% vs 25.4%) 5
- The maximum velocity is measured at the outer edge of the dark signal; fine linear signals at the peak of the curve (transit-time effects) should not be included 1
Clinical Significance
- LVOT VTI outperforms ejection fraction and Doppler-derived cardiac output for predicting outcomes in advanced heart failure patients 2
- Flow time (systolic duration) is directly proportional to LVOT VTI and indirectly related to mean ejection velocity 6
- VTI measurements are crucial for calculating valve areas in stenotic conditions using the continuity equation 1