Multiplane rotation from the ME AV SAX to 120° to 160° brings into view the ME AV long-axis (LAX) view (Fig. 2B). In this view, the RCC is always in the distal sector field, while the posteriorly displayed cusp (in the proximal sector, adjacent to the base of the anterior mitral leaflet) may be either the LCC or NCC depending on imaging plane orientation. In the ME AV SAX, the sector can be centered in either the NCC or LCC and then rotated by 90° to visualize the respective cusp in the ME AV LAX view. The AV annulus diameter should be measured in the ME AV LAX during midsystole at the hinge points of opposing cusps, inner edge to inner edge, in a plane that is perpendicular to the long axis of the LVOT. The sinus of Valsalva and STJ should be evaluated (and measured in similar fashion) at end-diastole, as measurements are less likely to be affected by aortic compliance and loading conditions (Table 2).5,6
The deep transgastric LAX view (deep TG LAX) is obtained by probe advancement to 50 to 60 cm (at 0°) into the stomach, adjacent to the LV apex (Fig. 3A), followed by probe anteflexion toward the heart base and slow withdrawal. In this view, the LVOT and AV are in the far-field, and systolic blood flow is directed away from the transducer (diastolic flow toward). The cusp positioned adjacent to the mitral valve is the NCC or LCC, and the cusp to the left of the sector (adjacent to the interventricular septum) is the RCC.
The TG LAX view is obtained from the TG mid-SAX view by multiplane angle rotation (120°) until the AV appears in the far-right (distal) sector field (Fig. 4A). Turning the probe slightly to the right helps align the LVOT, AV, and proximal ascending aorta. The cusp toward the left of the far sector is the RCC, and the cusp adjacent to the mitral valve is either the NCC or LCC. Blood flow velocities across the LVOT and AV are assessed in these TG views because of the parallel alignment of the blood flow and Doppler beam.
TWO-DIMENSIONAL IMAGING EVALUATION
The AV should be examined for congenital and acquired abnormalities, and individual cusps should be evaluated for thickening, asymmetry, and coaptation defects. Examination of the anatomic integrity of the body of the cusps should be aided by color flow Doppler (CFD) (at “normal” Nyquist limit of 50–60 cm/s).
Planimetry can be used to measure the valve area (normal 3–4 cm2) in the ME AV SAX view. The minimal orifice of the AV should be traced along the edges of the AV cusps at the point of maximal opening during midsystole. Alternatively, the 3 commissures are considered the apices of an equilateral triangle, and the AV area (AVA) is its surface (AVA = ¼ × (inter-commissural distance)2 √3. Although these methods are simple and rapid, they can prove inaccurate since the plane of measurement may be oblique to the actual orifice plane, or valve calcification can obscure the true orifice or make identification of the cusp edge difficult.7 Either technique provides a measurement of the anatomic (geometric) AVA, in contrast to the functional (or effective) AVA calculated with Doppler.
CFD across the AV can be used to evaluate the direction, timing, and location of blood flow. Blood flow across a normal AV is laminar during systole and is absent across the AV or in the LVOT in diastole. A turbulent, mosaic pattern flow during systole indicates acceleration across a narrowed orifice, while diastolic flow is seen across the AV and inside the LVOT with aortic insufficiency. Systolic acceleration proximal to the AV indicates subaortic narrowing, such as in hypertrophic cardiomyopathy, systolic anterior motion of the mitral valve, and/or LVOT obstruction.
Blood flow velocity across the AV can be assessed by continuous wave Doppler (CWD), which typically reveals a laminar pattern with a quick upstroke that peaks in early systole. The CWD cursor should be placed along the area of fastest flow across the AV (may be imaged with CFD) and should be moved side to side to record the maximum velocity. This can be assisted by turning the spectral Doppler volume up to identify the highest pitch velocity. The height of the velocity waveform varies with valve area, amount of flow, and the Doppler angle of incidence. Normal peak Doppler velocities across the left LVOT and AV are <1.5 m/s, and there is minimal acceleration across a normal AV.
The peak pressure gradient is estimated from the peak velocity using the simplified Bernoulli equation:
Pressure gradients are flow dependent, and as volume flow rate through the AV increases and decreases, the pressure gradient changes respectively. Conditions that decrease AV flow rate include LV dysfunction, mitral valve pathology, and left-to-right intracardiac shunts. Increased transvalvular flow is associated with hyperdynamic LV function, sepsis, and severe aortic regurgitation. Integration of the velocity signal will produce the velocity time integral (VTI); this requires manual tracing of the digital signal using the on-board measurement package of the individual echocardiographic system (Fig. 3B).
Stroke volume (product of VTI and LVOT area) can be accurately calculated by pulsed wave Doppler (PWD) analysis of the LVOT in the TG LAX or deep TG LAX views. The angle of the Doppler beam should be as parallel to blood flow as possible (aided with CFD) to yield the greatest amplitude spectral waveforms. The PWD sample volume is moved 0.5 to 1 cm away from the AV annulus, until a smooth velocity curve with a “closing artifact” is obtained.7 The average of 3 (in normal sinus rhythm) or 5 (in atrial fibrillation) velocity envelopes are used for VTI measurements to be used in the calculation.8 Angles of incidence >30° lead to underestimation of the peak velocity and VTI. Errors may also be introduced via inaccurate assessment of LVOT area by diameter measurements. The LVOT area is considered a circle and calculated from an LVOT diameter assessed in the ME AV LAX view, parallel and about 0.5 to 1 cm proximal to the site of measurement of the aortic annulus. However, there is a discrepancy between anatomic (2-dimensional planimetry) and physiologic (2-dimensional CWD VTI continuity calculation) area. The PWD sample for the assessment of LVOT velocity should be placed in the same subvalvular location where the LVOT diameter is measured. This is more important if there is flow acceleration proximal to the AV.
Determination of AVA is then accomplished via the continuity equation: AVA = LVOT stroke volume/VTI of aortic velocity. This AVA measurement is based on velocities and is therefore the “effective” AV orifice. Major limitations of the continuity equation are errors in measuring the LVOT diameter and flow dependency of the VTIs.
The high temporal resolution of M-mode is used to image the motion of the AV cusps. With the M-mode cursor placed across the cusps in the ME LAX view, a normal AV is shown with a “box” waveform (Fig. 4B). Premature opening of the AV (before the QRS complex) is seen with increased LV pressure. Premature closure of the AV (before the T wave) may be indicative of dynamic outflow obstruction due to hypertrophic cardiomyopathy or systolic anterior motion of the mitral valve. This may cause the M-mode “box” to “tremble” as it prematurely collapses. An eccentric closure line of the M-mode box is often seen with a bicuspid AV.
LV ejection time (LVET), a flow-independent index of LV systolic function, can be derived from M-mode across the AV in the ME AV LAX view (Fig. 4B) or CWD in the TG views. With M-mode, LVET is the time from opening to closure of the AV cusps. LVET may also be assessed via spectral Doppler (duration of LV ejection). Normal LVET is 362 ± 42 milliseconds and is obtained by determining the mean of 3 consecutive waveforms.9 A shortened LVET (LV failure or rapid heart rate) indicates decreased stroke volume, whereas a prolonged LVET (compensated aortic stenosis or slow heart rate) reflects increased stroke volume.
- A complete examination of the aortic valve (AV) should include assessment of valvular and aortic root anatomy and function, using 4 views: midesophageal AV short-axis, midesophageal AV long-axis (LAX), transgastric, and deep transgastric LAX.
- Measurement of the left ventricular outflow tract and AV annulus should be performed in the midesophageal AV LAX view in midsystole and of the sinuses of Valsalva and sino-tubular junction during end-diastole.
- The normal AV has a smooth, filled-in spectral waveform with a laminar pattern that peaks in early systole. Velocity is flow dependent.
- Determination of AV area is done with planimetry (tracing the 2D image), the equilateral triangle technique, or calculated with the continuity equation. A source of error with the continuity equation is accurate determination of left ventricular outflow tract diameter. With planimetry, the valve orifice is traced in midsystole or calculated as the area of an equilateral triangle.
- Systolic flow acceleration with color flow Doppler indicates a narrowed orifice, and diastolic flow across the AV indicates regurgitation. Attention to location, timing, and direction of flow elucidates pathology.
Name: Joseph Devin Roberts, MD.
Contribution: This author helped write the manuscript.
Attestation: Joseph Devin Roberts approved the final manuscript.
Name: Richa Dhawan, MD.
Contribution: This author helped write the manuscript.
Attestation: Richa Dhawan approved the final manuscript.
Name: Roberto M. Lang, MD.
Contribution: This author helped write the manuscript.
Attestation: Roberto M. Lang approved the final manuscript.
Name: Mark A. Chaney, MD
Contribution: This author helped write the manuscript
Attestation: Mark A. Chaney approved the final manuscript.
This manuscript was handled by: Martin J. London, MD.
We would like to thank Andrea H. Worthington, Technical Director of Intraoperative Echocardiography, at Brigham and Women’s Hospital, Boston, MA, for her assistance with graphic design.
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