4 Echocardiographic Parameters Important for Decision Making
Anatomic Imaging
LV Cavity Size and Systolic Function
Step 1: Determination of LV Cavity Size by 2D Echocardiography in Parasternal Long-Axis View
• By convention, M-mode specifies that each point of the measurement be placed on the near edge (closest to the transducer) of the bright echo corresponding to the appropriate interface (“leading edge–to–leading edge” technique). For LV cavity measurements, these interfaces are located between the interventricular septum and the ventricular cavity and between the LV cavity and the posterior wall. While this technique has decades of data supporting its use, it gives information only along the lines that originate from the transducer. Unless the desired chamber size is exactly parallel to one of these lines, the measurements will overestimate the chamber sizes as compared with those obtained from 2D echocardiography. These differences are particularly notable in patients with “vertical” hearts (hearts with a more vertical axis in the parasternal long-axis view).
• 2D echocardiographic LV dimensions are measured at the interface between the bright myocardium and the echo-lucent cavity space. 2D imaging provides the flexibility of measuring along any line within the image plane. Improved image resolution and easier determination of precise timing during viewing have increased the use of 2D echocardiography for the determination of the same chamber measurements that can be obtained from M-mode. Measurements made from 2D echocardiography provide an accurate representation of the chamber size.
• In the parasternal long-axis view in either M-mode or 2D, LV dimension is obtained at end-diastole (LVEDD) and end-systole (LVESD) (Figures 4-1 and 4-2 and Table 4-1)
TABLE 4-1 RECOMMENDATIONS FROM THE AMERICAN SOCIETY OF ECHOCARDIOGRAPHY FOR THE QUANTIFICATION OF LEFT VENTRICULAR SIZE AND VOLUME
Step 2: Estimation of LV Volumes and EF by 2D by Simpson’s Method
• By making the geometric assumption that the LV is a stack of thin disks (Figure 4-3), LV volumes can be estimated. Outlines of endocardial walls in 4-chamber (4C) and 2-chamber views are combined with a linear measurement of the long axis to create a stack of individual volumes of multiple “disks” within the LV cavity.
• In the apical 4C view, obtain an image optimizing visualization of the endocardial borders of the LV, paying particular attention to include the true apex. Next trace an outline of the endocardial walls of the LV still frame images at end-diastole and end-systole (Figure 4-4).
• Estimates of the LV volumes are made by combining measurements in both views at end-diastole and at end-systole. Most modern echocardiography machines contain software that automatically combines these measurements. Stroke volume (SV; difference between the LV end-diastolic volume and the LV end-systolic volume) and EF (SV divided by the LV end-diastolic volume) are then estimated (Table 4-2).
TABLE 4-2 RECOMMENDATIONS FROM THE AMERICAN SOCIETY OF ECHOCARDIOGRAPHY FOR THE QUANTIFICATION OF LEFT VENTRICULAR FUNCTION
Key Point
• Foreshortening of the apex may significantly underestimate LV volumes in both diastole and systole. Because EF is a relative estimation, the magnitude of the error on estimated LVEF is diminished. Thus, estimates of LVEF correlate with other modalities, such as cardiac magnetic resonance imaging (MRI), better than do estimates of SV, which is an estimation of an absolute value.
Step 3: Determination of Estimated LV Volumes and EF by 3D
• Images are obtained in the apical view. Current technology requires combining data from four cardiac cycles during a breath hold in order to obtain a full volume set.
• Although 3D echocardiography has been shown to estimate LV volume more accurately compared with MRI than does 2D echocardiography, care must still be taken to obtain images of the true apex for accurate volumes.
• In addition to estimated volume at end-diastole and end-systole only, volumes can be estimated at various points (typically 16 time points) throughout the cardiac cycle, enabling display of volume over time. The left ventricular cavity can be divided into 16 or 17 volumes corresponding the 16 or 17-wall-segment model and displayed over time (Figure 4-5).
Right Ventricle
Step 1: Determination of Right Ventricular Size by 2D
• Although a general sense of RV size can be obtained from the parasternal long- and short-axis views, because of the complex shape of the RV, neither definitive nor quantitative assessment of RV size is recommended in the parasternal view.
• The apical 4-chamber (4C) view provides a more complete and reliable perspective. Initially, a qualitative assessment is performed by noting RV apical location and overall size in comparison with the LV. The RV apex is normally closer to the base of the heart than the LV. With RV enlargement, the apex of the RV may be closer to or participate in forming part of the apex of the heart. Normally, the RV is approximately two-thirds the area of the LV in the 4C view. Qualitatively the degree of RV enlargement can be classified as: mild when the RV size is smaller than the LV but larger than three-fourths the size of the LV, moderate when the RV and LV are approximately equal in size, and severe when the RV is larger than the LV.
• A quantitative assessment can be obtained in the 4C view by measuring the minor axis at the tricuspid annulus, the minor axis at the midventricular level, and/or a longitudinal axis from the mid-tricuspid plane to the apex at end-diastole. In addition, the endocardial borders of the right ventricular cavity can be traced at end-diastole in the apical 4C view to obtain an RV diastolic area (Figure 4-6A and Table 4-3). Similar to LV measurements, it is important to avoid foreshortening, which may underestimate values.
Step 2: Determination of RV Systolic Function
• Assessment of RV size and systolic function can be obtained from the parasternal and subcostal views. However, assessment of RV systolic function is optimally obtained in the 4C view.
• Qualitative judgment remains critically important in assessment of RV systolic function. Typical categorization includes hyperdynamic, normal, mildly decreased, moderately decreased, and severely decreased.
• Because of the complex geometry of the RV, estimates of volume by 2D are relatively unreliable. Thus, quantitative estimation of right ventricular ejection fraction (RVEF) is less common than LVEF. One means of quantitative assessment of RV systolic function is the change in fractional area in the 4C view, which can be obtained using Simpson’s method as with the LV (see Figure 4-6B).
• A second quantitative assessment of RV systolic function can be obtained by measuring the excursion of the tricuspid annulus during the cardiac cycle. A sample volume is obtained from the lateral tricuspid annulus by tissue Doppler imaging (TDI) in the apical 4C view. Normal tricuspid annular plane systolic excursion (TAPSE) is greater than 2.0 cm. Of note, TAPSE has been shown to be decreased in patients with normal RV systolic function but decreased LV systolic function.
Key Points
• 3D echocardiography provides a more complete and reliable assessment of RV size. Quantitative assessment of RV size and RVEF by 3D has been shown to correlate better than that by 2D with other modalities, such as MRI.
• Assessment of RV diastolic function has not been validated to the degree that it has in determination of LV diastolic dysfunction. However, assessment of tricuspid inflow and diastolic parameters of TDI may become increasingly useful, particularly with improvements in 3D speckle tracking, which allows measurement of motion in all directions, not just in the direction of the probe.
LA Volume (LAV)
Step 1: Obtaining 2D Images
• LAV is best obtained in the apical 4C and 2-chamber views using the area-length method. In the final frame before mitral valve (MV) opening, approximating end-systole, endocardial borders of the left atrium are traced. Of note, the pulmonary veins and left atrial appendage should be carefully excluded, and a straight line should be drawn from one mitral annulus to the other (excluding the area immediately under the MV leaflets).
Step 2: Calculating LAV
• LAV is then calculated using the formula:
where A1 is the area in the 4C view, A2 is the area in the 2-chamber view, and L is the shorter of the major axes obtained (Table 4-4).
• Alternatively, LAV can be estimated using Simpson’s method. The LA is divided into a “stack of disks,” each with a set height and an area calculated from diameters obtained in the apical 4C and 2-chamber views.
Physiologic Data
LV Diastolic Function
Step 1: LV Inflow
• Normal LV inflow during diastole consists of a predominant early diastolic wave (“E wave”), caused by ventricular relaxation, and a smaller late diastolic wave (“A wave”), caused by atrial contraction. Thus, measurement of the peak velocity of each of these waves yields an E/A ratio greater than 1. Note that normal E/A ratios in adolescents and young adults can exceed 2. Normal E/A ratios in adults older than 40 years typically are in the 1 to 2 range.
• With abnormal LV relaxation, the early diastolic E wave decreases in velocity. To compensate, atrial contraction increases, yielding a higher velocity A wave. Once the E/A ratio decreases below 1.0, assessment is generally considered mild diastolic dysfunction (grade I). Note that, in people over the age of 65, the E/A ratio is frequently less than 1.0 and many echocardiographers do not consider it true diastolic dysfunction (Figure 4-8).
• As LV diastolic dysfunction progresses, left atrial pressure will increase to compensate, causing increase in the early diastolic E wave. Diastolic dysfunction can be considered moderate (grade II) once the E/A ratio increases above 1.0.
• As the LV becomes more restricted, left atrial pressure rises and left atrial contraction diminishes, resulting in further increases in the E/A ratio. Diastolic dysfunction can be considered severe when the E/A ratio exceeds 2.0. Typically, a decrease in preload will cause the E/A ratio to decrease below 1.0. If the E/A ratio reverses below 1.0 with the Valsalva maneuver, then the restriction is considered reversible (grade III). If it does not, it is considered irreversible (grade IV).
Key Points
• While this system of diastolic function assessment can occur regardless of LV function, clinical application of diastolic function should be viewed in terms of coexisting reduced or preserved LVEF. In particular, given that the E/A ratio is greater than 1.0 in both normal and moderate diastolic dysfunction, relying solely on LV spectral Doppler flow velocities and pattern can be misleading.
• Nonetheless, in patients with significantly abnormal systolic dysfunction, some degree of diastolic dysfunction is likely to be present. Thus, in patients with reduced EF, prognosis has been shown to be lower for patients with E/A ratios greater than 1.0.
• For patients with preserved systolic function, typically determined by an EF greater than 50%, the LV inflow pattern may possess diagnostic value. The differential diagnosis of patients with preserved systolic function presenting with dyspnea is wide. An abnormal LV inflow pattern increases the likelihood of diastolic heart failure.
Step 2: Deceleration Time (DT) of Early Diastolic Filling
• In addition to the peak velocity, the DT of the E wave can yield valuable information. The DT is measured from the peak E wave to the baseline (see Figure 4-8).
Step 3: Isovolumic Relaxation Time (IVRT) and Myocardial Performance Index (MPI)
• The sample volume of the pulsed wave spectral Doppler determination is placed within the LV cavity to include both LV inflow during diastole and LV outflow during systole. The time between the end of outflow and the beginning of inflow represents the IVRT (Figure 4-9). The MPI incorporates the isovolumic contraction time (IVCT), the IVRT, and the LV systolic ejection time (LVET) into one assessment with the formula:
Step 4: Color Doppler M-Mode of LV Inflow to Obtain Propagation Velocity (Vp)
• In the apical 4C view, align the M-mode cursor in the center of the mitral annulus, as parallel as possible to the LV inflow.
• Obtain color Doppler images flow-superimposed upon M-mode with the color sector as narrow as possible. The Nyquist limit should be set at approximately 75% of the peak E-wave velocity. Sweep speed should be 150 mm/s to optimize measurements (Figure 4-10).
• The Vp is the slope of the change in color (at the first aliasing velocity) of the mitral inflow from the mitral annulus to 4 cm into the LV cavity. Normal values are greater than 50 cm/s.
Step 5: Spectral Doppler of Pulmonary Veins
• The superior right pulmonary vein from the apical 4C view provides the most reliable assessment of flow into the left atrium. The sample volume is obtained approximately 1 cm within the pulmonary vein. Flow consists of a forward systolic wave (S), corresponding to LV contraction and atrial relaxation; a second forward diastolic wave (D), corresponding to early mitral inflow; and a reverse late diastolic wave (A), corresponding to atrial contraction (Figure 4-11).
Key Point
• In young patients, the D wave can significantly exceed the S wave. In normal middle-aged adults, the S and D waves have similar peak velocities. With mild diastolic dysfunction, the D wave decreases in velocity, similar to the decrease in the mitral inflow E wave. With more advanced diastolic dysfunction, ventricular compliance worsens allowing less complete atrial emptying, the velocity of the S wave decreases and that of the D wave increases.
Step 6: TDI of Mitral Annulus
• Assessments can be obtained at the annulus of each wall in the three apical views (4-, 2-, and 3-chamber).
• In patients with normal diastolic function, the early diastolic TDI velocity (Ea) is higher than the late diastolic TDI velocity (Aa). Similar to mitral inflow, with mild diastolic dysfunction the early diastolic flow decreases below the late diastolic flow (i.e., Ea/Aa becomes less than 1.0). However, in contrast to mitral inflow, with worsening diastolic dysfunction E continues to decrease. Thus, the pseudonormal mitral inflow E/A ratio of moderate diastolic dysfunction can be distinguished from normal diastolic function by a low Ea (typically below 8 or 9 cm/s) (Figure 4-12).
Step 7: Overall Assessment of LV Diastolic Function
• From the information obtained from the LV inflow, pulmonary veins, and TDI, an overall assessment of diastolic function can be made.
Mitral Regurgitation
Step 1: Overall Assessment of Etiology
• Assessment of MR in patients with systolic heart failure begins with characterization of the anatomy of all components of the MV apparatus. Careful evaluation of the annulus, leaflets, chordae tendinae, and papillary muscle provides insight into the etiology and mechanism of MR, which will be critical toward determining optimal therapeutic intervention.
• The initial assessment in all patients with significant MR involves categorization into primary, due to abnormalities of the mitral apparatus, or functional, due to LV dilatation.
• Perhaps the most dramatic example of primary MR is a ruptured papillary muscle. While this scenario can be catastrophic, it is usually recognized acutely and, if addressed appropriately with timely surgical intervention, will not lead to chronic systolic dysfunction. Primary leaflet abnormalities, such as MV prolapse, can lead to systolic dysfunction. Patients with MV prolapse and significant MR should be evaluated clinically and echocardiographically serially for MV repair or replacement.
• Functional regurgitation frequently occurs in patients with significant LV dilatation. Myocardial infarction leading to regional wall remodeling can cause outward displacement of the papillary muscles. The chordae are stretched and tether the mitral leaflets, causing poor coaptation (Figure 4-13). An additional mechanism of functional MR is annular dilatation from regional or global LV dilatation. Functional MR can be addressed by decreasing afterload or limiting LV dilatation and assessment for mitral annular ring placement or MV replacement.
Step 3: Color Doppler Evaluation
• Color Doppler evaluation of MR remains the foundation for assessment of severity and can also give clues toward etiology.
• Assessment includes estimation of the area of the jet as a proportion of the area of the left atrium in the 4C view, and estimation of the diameter of the vena contracta in the parasternal long-axis view. Severe MR has been defined as a jet area greater than 40% of the left atrium or a vena contracta greater than 7 mm.
• In addition to the quantitative assessment of these parameters, jet direction is an important factor. Severity of wall-impinging jets is significantly underestimated using jet area. Thus, MR may be severe with any wall-impinging jet. Direction of the jet also gives clues toward etiology. A posteriorly directed jet may be from a prolapsed anterior leaflet or a restricted posterior leaflet. Similarly, an anteriorly directed jet may be from a prolapsed posterior leaflet or a restricted anterior leaflet. Functional regurgitation is usually centrally directed.
Step 4: Spectral Doppler Evaluation
• The intensity of the spectral Doppler of the mitral jet is directly related to severity. Often a poorly characterized jet suggests a milder MR, but a very intense spectral Doppler signal suggests a more severe regurgitation.
• Assessment of pulmonary venous flow: a 1- to 2- mm sample volume is obtained in the right upper pulmonary vein (described above). Significant MR will blunt or reverse the systolic wave in the pulmonary vein.
• In addition, because of the relationship of velocity and pressure described in the Bernoulli equation, the rate of increase in velocity of the flow across the MV can estimate the change in pressure over time (dP/dt), which is an indicator of systolic function (Figure 4-14). A low dP/dt (<600 mm Hg/s) is an indicator of a poor prognostic, and may be helpful in determining management.
Key Point
• Although most modern echocardiography machines will automatically estimate a dP/dt, to manually estimate dP/dt on echocardiography, measure the time difference between when the mitral regurgitant jet is 1 m/s and 3 m/s. Because the change in pressure will be 32 mm Hg (36 mm Hg at 3 m/s − 4 mm Hg at 1 m/s), dP/dt = 32/Δt (mm Hg/s).
Step 5: Integration
• While 2D, color Doppler, and spectral Doppler all are valuable independently, comprehensive quantitative assessment of MR severity requires a combination of all three.
• Flow-convergence analysis uses both color and spectral Doppler to estimate effective regurgitant orifice (ERO) area and regurgitant volume (RVo) by the proximal isovelocity surface area (PISA) method. The surface area of the jet approaching the MV is measured on color Doppler and the velocity and velocity-time integral (VTI) are measured on spectral Doppler (see Chapter 2 for further description of PISA). ERO above 40 mm2 and RVo over 60 mL are considered severe mitral regurgitation.
• RVo can also be estimated by subtracting the SV across the aortic valve during systole from the volume across the MV during diastole. Stroke volume is estimated by measuring the diameter of the LV outflow tract and the VTI in the left ventricular outflow tract (LVOT). Diastolic flow across the MV is similarly estimated (assuming a circular cross section and obtaining 2D and spectral measurements at the level of the annulus).
• RVo can be estimated by subtracting SV from total change in volume of the LV cavity. SV is estimated as described above. LV volumes are typically estimated in 2D using Simpson’s method.
• 3D echocardiography provides the promise to avoid some of the inaccuracies resulting from the geometric assumptions necessary for 2D quantitative analysis. 3D has been shown to provide more accurate assessments of LV volumes. LVOT and MV annular area assessment is likely improved with 3D. In addition, important advances are being developed to better render color Doppler 3D images of regurgitation.
Pulmonary Artery Systolic Pressure
Step 1: Estimating the RV-to-RA Pressure Gradient
• The peak velocity of the tricuspid regurgitant Doppler envelope is measured in multiple views. The highest value from a regurgitant jet is used to estimate the systolic pressure gradient across the tricuspid valve with the use of the simplified Bernoulli equation (P = 4v2) (Figure 4-15).
Key Points
• While most patients have a sufficient tricuspid regurgitant jet for a reasonable measurement, a significant percentage do not. Care should be taken in using only those jets with a reasonable certainty that they represent the true peak velocity of blood across the tricuspid valve. Reporting that an estimation is not able to be accurately made is preferable to reporting a potentially inaccurate estimation.
• If a reasonable jet is obtained in multiple views, the jet with the highest velocity should be used (e.g., not an average of multiple views). The view with the jet with the highest velocity likely is the closest to parallel to the true jet. However, if within one view multiple different velocities are obtained (e.g., atrial fibrillation), an average of 3 to 5 beats is acceptable.
Step 2: Estimating the RA Pressure
• Right atrial pressure is estimated, usually based on evaluation of inferior vena cava size. A normal-sized inferior vena cava (<2.0 cm) and normal respiratory variation of this size with inspiration (e.g., “sniffing”) (>50% decrease) signifies normal right atrial pressure (<10 mm Hg). Mildly increased size (2.0 to 2.5 cm) or decreased respiratory variation signifies mildly elevated right atrial pressure (10 to 15 mm Hg). A dilated inferior vena cava (>2.5 cm) and decreased respiratory variation signify elevated right atrial pressure (>15 mm Hg).
Step 3: Estimating PASP
• Assuming no significant pulmonary valvular stenosis, the PASP is estimated to be the sum of the estimated right atrial pressure and the estimated pressure gradient across the tricuspid valve. Often the values are reported in 5-mm Hg increments, as smaller increments suggest a higher precision than justified. Generally, values under 40 mm Hg are considered normal.
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