Heart Cath vs Echo Results

[Michael]

I went in for a heart cath last Friday and received some good news. My aortic valve area was 1.5 sq cm instead of 1.2 sq cm calculated from my last echo. More importantly the pressure gradient was 15 mm Hg instead of the 38 mm Hg calculated in the echo.

When I talked to my Cardiologist about the accuracy of the echo measurements, he said he had been burned both ways on the pressure gradient.

I've been trying to find the techniques they use for the echo to calculate the valve area and the pressure gradient without much success. I think the echo techs actually take a picture of the valve opening and then outline the opening area and then let the computer calculate the area. But this all depends on the tech getting the shot perpendicular to the valve and if their shot is not perpendicular the area calculation will be on the low side. As for the pressure gradient, I'm guessing that the blood flow rate is determined from the doppler effect from the ultrasound and then using the valve and the flow rate they back out the pressure gradient. I'm guessing but it seems the valve area will be more accurate with the echo and obviously the pressure gradient is more nebulous.

Now the results of the heart cath are different. They actually have a pressure transducer so they can measure the actual pressure on both sides of the valve so the pressure gradient is accurate. But the valve area is backed out by some complicated equation involving your height and body weight (I was grilling the crew in the cath room. They said in the old days they used to have do the calculations by hand)

Anyway I getting a little long winded hear but does anyone know where I can find any information on how the valve area and pressure gradient are derived in both the echo and the heart cath?

 

[tobagotwo]

Here are some echocardiology references from the References Forum, in Ross's Must-Have References.

Atlas of Echocardiography
http://info.med.yale.edu/intmed/car...ents/index.html

Digital Echocardiography
http://www.echobyweb.com/htm_level1...examination.htm

You will find that the opening size can be wrong with the catheterization (angiogram) method as well. The proof of the pudding, as it were, winds up in the surgeon's hands. Both the echo and the cath were wrong in my case - however, the echo was closer. The interventionalist was extremely experienced and knowledgeable, but the catheter has its quirks as well.

Echocardiological computations are not for sissies. While probably not that difficult to learn in context, they are enigmatic to decipher from condensed texts.

Best wishes,

 

[Ross]

Thanks Bob, I was beginning to think no one ever used those links and wondering if it was worth having them there.

 

[PapaHappyStar]

I went in for a heart cath last Friday and received some good news. My aortic valve area was 1.5 sq cm instead of 1.2 sq cm calculated from my last echo. More importantly the pressure gradient was 15 mm Hg instead of the 38 mm Hg calculated in the echo.

When I talked to my Cardiologist about the accuracy of the echo measurements, he said he had been burned both ways on the pressure gradient.

I've been trying to find the techniques they use for the echo to calculate the valve area and the pressure gradient without much success. I think the echo techs actually take a picture of the valve opening and then outline the opening area and then let the computer calculate the area. But this all depends on the tech getting the shot perpendicular to the valve and if their shot is not perpendicular the area calculation will be on the low side. As for the pressure gradient, I'm guessing that the blood flow rate is determined from the doppler effect from the ultrasound and then using the valve and the flow rate they back out the pressure gradient. I'm guessing but it seems the valve area will be more accurate with the echo and obviously the pressure gradient is more nebulous.

Now the results of the heart cath are different. They actually have a pressure transducer so they can measure the actual pressure on both sides of the valve so the pressure gradient is accurate. But the valve area is backed out by some complicated equation involving your height and body weight (I was grilling the crew in the cath room. They said in the old days they used to have do the calculations by hand)

Anyway I getting a little long winded hear but does anyone know where I can find any information on how the valve area and pressure gradient are derived in both the echo and the heart cath?

Hi Michael,

Doppler echo measures the velocities in the outflow tract before the aortic valve and after -- the pressure gradient is calculated from these velocities ( its proportional to the square of the velcity difference )

The velocities though are used to directly determine the valve area using the continuity equation:

AVA = Area of the outflow tract * (flow)velocity before the valve / velocity after the valve

( from here: http://www.manbit.com/ERS/ERSbyAZ.asp?101 )

The Area of the outflow tract is determined by direct measurement with the 2-D echo. Both measurements ( diameter of the outflow tract and the doppler velocity measurments ) are going to be affected if the probe is not well positioned and focused

The cath uses a different formula ( Gorlin Formula ) which involves the cardiac output rate and the pressure gradient across the valve. The Gorlin formula in its original form gave a consistently low value for the valve area, it is usually corrected now.

( http://www.manbit.com/ERS/ERSbyAZ.asp?186 )

Both these methods can have significant deviation from the 'true' valve area, they are both dependent on the flow rate across the valve.

If you search for the "gorlin formula", "continuity equation" and "heart valve area" you can get many more references.

The cath is less dependent on operator error and is the more "direct" measurement, its questionable if it is more accurate

 

[tobagotwo]

Thanks, Burair. Beautifully illustrated, too.

Apparently, they're not much fun to learn in context, either...

Best wishes,

 

[Michael]

Bob and Burair.

Thanks for all the info and web sites. I haven't been able to sit down and really go thru them yet but I will. Burair, its been awhile since I took fluid mechanics in college but I do recognize the continuity equation. I guess I'm going to have dig my old college text and bush up on this stuff. Thanks again for the info.

Michael

 

[PapaHappyStar]

I haven't been able to sit down and really go thru them yet but I will. Burair, its been awhile since I took fluid mechanics in college but I do recognize the continuity equation.

Dont need to crack a text for that...

Fluid dynamics is not really my subject but the continuity equation is just a statement of fluid volume conservation. For incompressible fluids the volume going into a constricted opening is the same as the volume coming out, so area*velocity gives the volume of a cylinder of fluid passing through per unit time, this volume should be the same before and after the valve opening -- i.e area before * velocity before = area after * velocity after -- so if the area after is small then the velocity after must increase.

I think the incompressibility assumption needs correction proportional to the flow rate for compromised and calcified valves ( there is some squeezing going on probably because the size of the particles is comparable to the curvature in the valve )