Velocity Time Integral (VTI) is a bedside echocardiography measurement that track blood flow speed over time to measure the distance blood travel in a single heart beat.
- Stroke Volume (SV) in TTE is obtained as the product of the LVOT cross sectional area (LVOT-CSA) with LVOT Velocity-time integral.
- SV = LVOT CSA * LVOT VTI
- LVOT CSA is derived from the formula πr2
- LVOT CSA = 3.1416 * (LVOTd/2)2 in cm2
- Where LVOTd is LVOT diameter
- LVOTd is acquired from parasternal long axis at mid systolic frame, measured from inner edge to inner edge of LVOT.
- The LVOTd is acquired from the parasternal long axis view, at a mid-systolic frame, measured from the inner edge to inner edge of the LVOT or eventually between the site of insertion of the right- and
- non-coronary aortic leaflets.
- LVOT VTI is obtained by racing envelope of the Doppler spectrum of LVOT systolic flow from the apical five- or three chamber view using pulsed-wave Doppler (PWD), with the sample volume placed within the LVOT, approximately at 1 cm distance to the aortic valve.
- Alignment of the PWD sample volume should be parallel to the subaortic flow and minimal spectral broadening is considered as optimal VTI.
- CO = HR * SV in L/min
Normal Range
- Studies have shown normal range of LVOT VTI is 20± 3 cm
Practical Application
- Increase in VTI > 10-15% after a treatment intervention suggest a concrete response.
- Studies have shown that increase in VTI >12% after PLR test demonstrates fluid responsiveness.
- Mini-fluid challenge : 100 ml in 1 min, a increase in > 10% VTI is predictive in patients at risk of fluid overload like ARDS or septic shock.
- LVOT VTI < 10 cm in acute decompensated heart failure and LVOT VTI < 15cm in pulmonary embolism is assosciated with poor prognosis.
Limitation
- It is not reliable in moderate to severe AR.
- Dynamic LVOT obstruction may be observed in extreme hypovolemia, anterior wall MI, Takotsubo syndrome.
- Dynamic LVOTO and AR can over estimate VTI.
- VTI in AF will show beat to beat variability which can be overcome by taking average of 5 values.
Reference
- Rationale for using the velocity–time integral and the minute distance for assessing the stroke volume and cardiac output in point‑of‑care settings.Blanco Ultrasound J (2020) 12:21. https://doi.org/10.1186/s13089-020-00170-x
