LV Systolic Function
Ejection Fraction vs Stroke Volume
Ejection fraction is NOT stroke volume. It is heavily influenced by the end-diastolic volume (EDV): a high EDV will lower the ejection fraction but will simultaneously increase the stroke volume. Therefore, a low ejection fraction does NOT indicate cardiogenic shock. In fact, the average EF in the cardiogenic shock registry is 45%.
Ejection fraction is also NOT a measure of contractility. Contractility is myocardial shortening independent of preload and afterload. Ejection fraction, however, is very sensitive to changes in preload and afterload. Thus, it should not dictate the initiation of inotropic support.
Ejection fraction is a surrogate of cardiac efficiency with higher EF correlating with higher cardiac work efficiency. But maximal cardiac work is estimated to occur at an EF of 50%. Improving cardiac efficiency is important in improving long-term outcomes but unlikely to be a helpful short-term resuscitation goal. Patients in septic shock with a depressed EF actually have higher stroke volumes and better outcomes than those with preserved EF.
A resuscitationist should be much more interested in estimating stroke volume than EF.
Stroke Volume - Simpson's Biplane Disk Summation
Get a good A4 view and/or A2 view.
Zoom in on the LV and capture it in its maximal dimensions.
Freeze clip at the end diastole (right before Rw or after A wave of MV closing)
Trace the endocardial border including trabeculae and papillary muscles. Try to make LV length perpendicular to base width.
Freeze clip at end systole (Tw or before E wave of MV opening)
Trace endocardial border as in Step 2. The apex of the heart should be in the similar position to avoid foreshortening bias.
In the example above, LV EDV is estimated to be 132mL and LV ESV is estimated to be 47mL. Before going further, do the same thing in A2 view to check for agreement.
If there's not good agreement, don't fret; it's not uncommon even in experienced hands. But you should be more leery of the result.
Assuming there is good agreement, the EF is therefore (132-47)/132 = 64%. But more importantly the SV is 132-47 = 85mL. At a HR of 75 this estimates a CO of 85 * 75 = 6.4L. Knowing the MAP (80) and estimating the CVP (10), one can calculate the SVR = 80 * (80-10) / 6.4 = 875 (normal 800-1200). Unlike EF, SV is a key physiologic variable in understanding a patient's macrohemodynamics.
Good explanation of pitfalls to avoid: https://www.cardioserv.net/echo-lets-talk-lv-bi-plane-measurements/
Stroke Volume - LVOT Doppler
Calculating stroke volume by doppler is especially helpful when Simpson's is not possible. But it should be performed even if Simpson's is calculated so to provide an alternative method to assess for agreement. As is the case with all clinical data, never hang your hat on any single piece of information. However, multiple pieces pointed at the same conclusion merits credence.
Obtain PSL view, focus in on LVOT
Measure LVOTd in midsystole. Do it twice over different cardiac cycles. Measurement should be within 2mm else do it twice again.
Obtain A5 and/or A3 view
Pulse wave doppler over LVOT - close to but not within AV
Manually trace Velocity Time Integral of LVOT
Trace 3 and take the average
Stroke volume = cross-sectional area (CSA) * velocity time integral
In the example above, the LVOT VTI was traced out to be 17.5cm and the LVOTd was measured to be 2.3cm.
The LVOT radius is therefore 2.3/2 = 1.15cm. The LVOT cross sectional area is π r ² = 3.14 * 1.15 ² = 4.15 cm²
Stroke volume = LVOT VTI * LVOT CSA = 4.15 * 17.5 = 72 cm³ = 72 mL
Ejection Fraction - E Point Septal Separation
EPSS received a lot of hype in the point of care ultrasound world because its touted as easy to measure and has a good correlation to visual estimation of EF by cardiologists. Thereafter, it has been embraced and rapidly adopted in the ED and now ICUs to assess "cardiac function". Cardiologists and cardiac intensivists have not been so keen to jump on this bandwagon.
A few epistemological problems with this "shortcut":
- EPSS cannot be interpreted without assessment of valve function because both mitral stenosis and aortic regurg will spuriously affect its value.
- Abnormal EPSS is more-so driven by abnormal chamber size (EDV) than true contractility. So cardiomyopathies that affect regional wall segments (MI) or do not cause myocardial dilation (AS) can have normal EPSS despite profound cardiogenic shock. Plus, a high EDV actually buffers the stroke volume from a low EF making the assessment of cardiac output rather ambiguous.
- EF in and of itself is not that useful.
Medicine is a disciplined art/science which is antithetical to easy shortcuts. If you are to rely on EPSS, think about valvular function, the end-diastolic dimensions, and - most importantly - look at the entire clinical picture shrewdly.
Obtain PSL view
M mode through the tips of the mitral valve leaflets. M mode axis should be perpendicular to the long axis of the ventricle
Measure the distance from anteroseptal wall and anterior leaflet of the mitral valve during early systole (E wave)
Measure the distance between the inner walls of the LV (LVIDd) at the tip of the mitral leaflet. This will act as a surrogate of LVEDV if it cannot be calculated in A4.
Ejection Fraction - Fractional Shortening
Fractional shortening suffers from the same shortcomings as EPSS. Sometimes this is measured in the same M-mode tracing as EPSS but this adds another layer of error. The longitudinal shortening of the LV actually brings the anteroseptal wall and posterolateral walls that was measured during diastole further toward the apex during systole. Furthermore, if the longitudinal axis is not relatively parallel to the x-axis or if there is asymmetry to the thickness of the septal wall, fractional shortening will be less valid.
Hence, B-mode PSL method (bottom right) is most preferred.
Obtain PSS-basal view.
M-mode through the center of the LV
Ensure that you are not off axis by noting the circular shape of the LV.
M-mode PSL method
M-mode through PSL just past the distal tips of the mitral leaflet. Don't try to get catch the mitral leaflet as in EPSS because this would put you too far to the right and introduce significant bias (see right).
B-mode PSL method
Measure the distance between the inner walls of the LV (LVIDd) at the tip of the mitral leaflet. Scroll forward to end systole and measure LVIDs. Notice the movement toward the apex. And how M-mode would've caught the wrong walls in systole (red line).