Mayooran Namasivayam, MBBS, PhD, FRACP, FACC, FASE
10 Jul,
20 tweets, 8 min read
Ok, it's #Tweetorial time. Today's topic: echo formulae. Specifically AVA and PISA. They oft cause angst and confusion among trainees so here's a quick primer on these #EchoFirst essentials...1/20...
2/20, Both rely on the Law of Conservation of Mass. Basically - amount of blood in = amount of blood out...
3/20, Aortic valve area and mitral regurgitant area (and subsequently volume) can be calculated using the "continuity equation" relying on this fundamental law of physics...
4/20, Let's do AVA first. Blood into LVOT must equal blood out Ao valve assuming no shunts in that location. There are 2 approaches to AVA. 1 uses peak velocities the other uses VTIs (velocity-time integrals). The key here is to think in units...
5/20, Using LVOT peak velocity (cm/s) x LVOT area (cm2) = volumetric flow (cm3/s=mL/s) = Ao peak velocity (cm/s) x AVA (cm2). LVOT peak velocity can be measured using V1 - pulsed wave Doppler. LVOT area is estimated using pi x (LVOTradius)^2...
6/20, This assumes circular LVOT (but we know it is really oval, so AVA is often underestimated (and hence AS severity overestimated) by this equation. Ao peak velocity (V2) is measured by continuous wave Doppler...
7/20, AVA can also be solved using VTI instead of velocity. VTI is velocity (cm/s) x time (s) = displacement (cm). Hence LVOT VTI x LVOTarea = volume (cm3 =mL = stroke volume) = Ao valve VTI x AVA. Again PW and CW Doppler are used for LVOT and AoV VTI, respectively...
8/20, Errors can come from mismeasurement of LVOT diameter (and hence radius), misalignment of Doppler beam, incorrect sampling location of pulsed Doppler (often taken too deep into LV), AF, among others...
9/20, The PISA formula is the same formula-the continuity equation- everyone forgets that because it seems more complex...but it isn't. It just uses the "proximal flow convergence" phenomenon to calculate flow on one side of the equation...
10/20, When blood regurgitates, e.g. in MR, the flow converges into a hemisphere on the atrial side, just before the orifice. The properties of this hemisphere can be used to calculate regurgitant volumetric flow rate...
11/20, The velocity of the blood at the edge of the hemisphere of flow convergence can be measured using the "aliasing velocity" where color Doppler signal changes from blue to orange...
12/20, In order get a nice PISA hemisphere and radius you need to move the baseline shift aliasing velocity on color Doppler from neutral toward the direction of the regurgitation. I.e. toward the orange side of color spectrum on TEE and toward the blue side on TTE...
13/20, You can then measure the PISA radius and read the newly adjusted aliasing velocity...
14/20, Now use the continuity equation again, but first understand how the atrial side of the equation is derived. Volumetric flow rate (cm3/s=mL/s) = surface area of flow convergence hemisphere (cm2) x blood velocity (cm/s) at surface of hemisphere...
15/20, The surface area of a hemisphere = (4 x pi x r^2) / 2 = 2 x pi x r^2. Here r = PISA radius. Blood velocity at surface = aliasing velocity...
16/20, On the other side of the equation you have the volumetric flow rate (cm3/s=mL/s) = effective area of mitral regurgitant jet at valve level ("effective regurgitant orifice area EROA" in cm2) x MR peak velocity (cm/s).
17/20, Putting it together, you have 2 x pi x (PISA radius ^ 2) x aliasing velocity = EROA x MR peak velocity. You then solve for EROA because everything else can be measured. Once you have EROA (cm2), if you multiply by MR VTI (cm) you can then get regurgitant volume (cm3 = mL)
18/20, Problems come from hemispheric assumption, eccentric jet, misalignment of Doppler, AF, 2D measurement (can do 3D PISA), among others...
19/20, So there you have it - quantitative assessment of AS and MR (and other forms of regurgitation) just comes down to the continuity equation. Remember these basic principles, read more to consolidate...and practice, practice, practice. Now, go forth and quantitate!
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@ACCinTouch @MGHHeartHealth @SVHSydney @VictorChangInst @SamuelB316 @Ph_Bertrand @TimChurchillMD @mghecho #CardioTwitter #echotwitter @iamritu @purviparwani @DocStrom @JudyHungMD @PPibarot @CapouladeR @ddefariayeh @pe_bartko @MarcDweck @hahn_rt 20/20
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