Left Ventricular Outflow Tract (LVOT) Assessment on Echocardiography
The Left Ventricular Outflow Tract (LVOT) is assessed on echocardiography through standardized measurements of its diameter, velocity, and velocity time integral, which are critical for accurate calculation of stroke volume, cardiac output, and aortic valve area in patients with aortic stenosis. 1
Anatomical Assessment
LVOT Diameter Measurement
- Location: Measured in parasternal long-axis view at mid-systole
- Technique: Inner edge to inner edge measurement from septal endocardium to anterior mitral leaflet, parallel to aortic valve plane 1
- Timing: Preferably measured during mid-systole
- Recommended position: At the level of insertion of the aortic valve leaflets or 1-5 mm below the aortic annulus 1
Common Pitfalls in LVOT Diameter Measurement:
- Calcium extending from aortic annulus to anterior mitral leaflet may lead to underestimation
- In patients with sigmoid septum, measurements taken too far apically may underestimate true flow area
- LVOT is actually elliptical rather than circular in most patients, which may lead to underestimation of LVOT area when using standard 2D measurements 1
Hemodynamic Assessment
LVOT Velocity Measurement
- Technique: Pulsed-wave Doppler from apical approach
- Views: Anteriorly angulated four-chamber view (five-chamber view) or apical long-axis view
- Sample volume position: Just proximal to the aortic valve 1
- Normal value: Peak velocity typically 0.8-1.0 m/s in healthy individuals
LVOT Velocity Time Integral (VTI)
- Definition: Integral of instantaneous flow velocity curve during left ventricular ejection 1
- Measurement: Area under the velocity curve during systole
- Normal value: Approximately 18-22 cm
Flow Pattern Characteristics
- LVOT flow is not spatially uniform but skewed with higher velocities along the subaortic ventricular septum 2
- Peak velocities are approximately:
- 98 ± 16 cm/s in the medial margin (near septum)
- 89 ± 14 cm/s in the midline
- 79 ± 14 cm/s in the lateral margin (near mitral valve) 2
- Center LVOT measurements closely approximate spatial average velocities 2
Clinical Applications
Stroke Volume Calculation
- Formula: SV = LVOT area × LVOT VTI
- LVOT area calculation: π × (LVOT diameter/2)² assuming circular shape
- Clinical use: Assessment of cardiac output, valvular stenosis severity
Aortic Valve Area Calculation (Continuity Equation)
- Formula: AVA = (LVOT area × LVOT VTI) / Aortic valve VTI
- Simplified version: AVA = LVOT area × (LVOT velocity / Aortic valve velocity) 1
- Alternative: Dimensionless velocity index = LVOT velocity / Aortic valve velocity (normal >0.25) 1
Advanced Techniques to Improve Accuracy
- 3D Echocardiography: Allows direct planimetry of LVOT area, avoiding underestimation from assuming circular shape 1
- MSCT (Multi-Slice Computed Tomography): Can provide accurate LVOT area measurements, particularly important in low-gradient aortic stenosis 1
Special Considerations
- During stress echocardiography, only apical window measurements of LVOT can be reliably used during exercise 1
- In patients with abnormal sub-aortic flow (e.g., hypertrophic cardiomyopathy, subaortic membrane), standard LVOT measurements may be inaccurate 1
- When serial measurements are performed, using the same LVOT diameter is recommended as LVOT size rarely changes over time in adults under stable conditions 1
By following these standardized approaches to LVOT assessment, clinicians can obtain accurate measurements critical for diagnosis and management of various cardiac conditions, particularly valvular heart disease.