Understanding pressure gradient in Aortic Stenosis

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PapaHappyStar

I wanted to know:
For an individual with a stenotic valve -- what is the physical effect of the pressure gradient across the valve.

The mean value of the pressure gradient during the pump phase is a measure of the resistance to blood flow through the valve -- if the pressure in the pump phase ( systole ) is normally 120mmHg ( the high value in your BP reading ) then the mean pressure in the left ventricle is higher by the mean pressure gradient across the valve.

So if there is a mean gradient of 35mmHg across the aortic valve ( moderate stenosis ) and a BP of 120/80 then the mean pressure in the LV is 120+35=155mmHg. This is a measure of elevated afterload.

To find out what this means physiologically -- I looked up some data of BP variation with exercise:

Hypertension. 2002;39:761 said:
ref: http://hyper.ahajournals.org/cgi/content/full/39/3/761
6FF1.gif

Figure 1. Percentile curves of SBP and DBP responses by HRR during ergometric tests in normotensive men. The curves were constructed by fitting a third-order polynomial model to the regression equations of SBP and DBP with HRR during submaximal workloads of 50, 75, and 100 W.
Click to expand...

So at 155 mmHg ( read off 155 on the vertical axis for a person on the 50th pecentile in BP range -- i.e. average) := Heart rate reserve is 35%
so if the HR(max) is 170 beats per min. and HR(rest)=70 bpm -- the effective heart rate at which the person is always "exercising" is (170 - 70) * 35% + 70 = 105 bpm

At severe AS ( pressure gradient 50mmHg and effective systolic pressure 170mmHg) the effective heart rate would be 130 bpm -- ( no wonder we feel slightly out of breath on small exertion -- at severe stenosis -- and the mild chest pain on exertion is probably due to a buildup of lactic acid in heart muscles )

Burair
 
Fascinating! I just wish I could honestly say that I follow all that. Hm. So. If you were to take my most recent echo.... aortic valve..... mean gradient 56 mmHg and peak gradient 86 mmHg with a resting BP of 122/80...... at what point while I'm exercising should I stop so that I don't pass out??!! That is, provided I can actually assess my heartrate accurately since it is so slushy (the doctors and nurses have trouble these days).

Your answer needs no disclaimer. I'm not pushing myself at all these days!! Just love watching that scientific/mathematical mind of yours at work!!:D

Marguerite
 
This is not string theory :)

I think its a pretty good approximation though to how things are being handled -- the heart is not a very complicated machine, thats the beauty of it.

If measured HR during exercise is at say 130 bpm with severe stenosis -- the effective heart rate reserve is down to 40 beats ( 130 'effective' rest, 170 max ). Have to extrapolate between these values now:

130 bpm is (130 - 70)/(170-70) * 100 % = 60 % of HR reserve

60 % of 40 is 24 ... 24 + 130 = 154bpm as a first approximation for the hypothetical person in my first post.

154 bpm is 80-90% of max HR -- the heart is working extremely hard at this point. It compensates by becoming hypertrophic ( muscular ), continued overload can lead to oxygen starvation and a heart attack, but you wont be able to sustain this sort of exertion without feeling extrmely exhausted.

This is very heuristic but it sort of works to make sense of the ACC guidelines ...
 
Burair,
Since you were curious about the physical effects of the pressure gradient, I'm hopeful that you will consider my situation and comment. I have been diagnosed with pericardial constriction; I have done a fair amount of reading, but I've found little information about attempting to exercise with the condition. What I've read says it's very difficult to do, but no restrictions are imposed (due to the difficulty, most people quit).

So far I have been able to walk briskly, on a level surface, for 50 minutes, 5 to 6 times a week. I'm tired when I finish a session, but I'm not really out of breath and my HR is about 110 with a blood pressure of 130/80. However, there are times during the day when I can go up and down a flight of stairs and I find myself noticeably short of breath and my pulse is racing. What's up with that? I understand that climbing a flight of stairs takes more energy than walking a level surface, but can it make that great of difference?

Since no one seems to have any data on exercising with constriction, I am using my own experience to develop a workout program. My goal is to live with the condition, without needing an extra surgery, until I have to undergo another valve replacement. The primary difficulty I have encountered is fluid retention if I overdo the exercise--either walking or swimming. When it occurs, I have to wait until I lose the excess fluid, or the symptoms of the condition really become pronounced.

Do you have any thoughts on what numbers I should be recording to measure the status of my heart? I have access to a pulse oximeter so my O2 and HR are easy to measure, but I haven't kept track of my bp. My August echo showed no problems with the new valve.
Thanks,
Mary
 
Mary said:
Burair,
Since you were curious about the physical effects of the pressure gradient, I'm hopeful that you will consider my situation and comment. I have been diagnosed with pericardial constriction; I have done a fair amount of reading, but I've found little information about attempting to exercise with the condition. What I've read says it's very difficult to do, but no restrictions are imposed (due to the difficulty, most people quit).

So far I have been able to walk briskly, on a level surface, for 50 minutes, 5 to 6 times a week. I'm tired when I finish a session, but I'm not really out of breath and my HR is about 110 with a blood pressure of 130/80. However, there are times during the day when I can go up and down a flight of stairs and I find myself noticeably short of breath and my pulse is racing. What's up with that? I understand that climbing a flight of stairs takes more energy than walking a level surface, but can it make that great of difference?

Since no one seems to have any data on exercising with constriction, I am using my own experience to develop a workout program. My goal is to live with the condition, without needing an extra surgery, until I have to undergo another valve replacement. The primary difficulty I have encountered is fluid retention if I overdo the exercise--either walking or swimming. When it occurs, I have to wait until I lose the excess fluid, or the symptoms of the condition really become pronounced.

Do you have any thoughts on what numbers I should be recording to measure the status of my heart? I have access to a pulse oximeter so my O2 and HR are easy to measure, but I haven't kept track of my bp. My August echo showed no problems with the new valve.
Thanks,
Mary
Click to expand...

If pericardial constriction restricts the expansion of the heart then I would be interested to know the ejection fraction from the left ventricle, the pulse rate and the cardac output. If you want to continue to do exercise and you have a cardiologist you trust then you can ask about a ( possibly dobutamine ) stress echo. This will measure the response of the pumping ability of the heart to increasing levels of exertional stress.

I am not sure why ( or even exactly if ) the body retains fluid as a result of chronic heart overload ( sort of like CHF ), the reasons for this would be interesting to find out.

Burair
 
As measured by the last echo, the left ventricle had an ejection fraction of 60%, but how is the cardiac output determined?
 
The difference between climbing stairs and walking ( even quite briskly ) is the difference in power output ( energy used in equivalent time intervals ) -- both sliding friction and the gravitational force ( weight ) increase proportionally with mass but the constant of proportionality is ( say ) about an order of magnitude ( factor of 10 ) different so you would have to walk around 10 times faster than you climb in order to expend the same amount of energy in equal time.

If you climb 3 meters in 10 seconds by stair you would be going at 3 meters every second walking ~ 10 km every hr ( 10 min for each mile ) which is a trot.
 
Mary said:
As measured by the last echo, the left ventricle had an ejection fraction of 60%, but how is the cardiac output determined?
Click to expand...

volume of left ventricle * EF * pulse rate

They can also do it by measuring the mean cross-sectional area of the aortic root and the mean velocity by doppler. The number should be on your echo report ( in l/min or in ml/sec ). I might not be able to tell you much from it though... not enough experience to tell what ranges are normal ... but these would be the sort of things I would be looking at I guess if I was to understand how I was progressing given pericardial constriction.
 
Thanks

Thanks

Hi Burair,
Just wanted to thank you for sharing this info - I am still trying to make sense of it all. I am going to ask for a report of my echo from my cardio and see where I might fit in. Assuming I can understand it all :D
Thanks again for the great info - very intersting.
Ashley
 
Ashley said:
Hi Burair,
Just wanted to thank you for sharing this info - I am still trying to make sense of it all. I am going to ask for a report of my echo from my cardio and see where I might fit in. Assuming I can understand it all :D
Thanks again for the great info - very intersting.
Ashley
Click to expand...

You are welcome. Let us know what you find from your echo.
Regards,
Burair
 
Hi Burair--
what you're saying makes sense from what I can remember of my TEE in 11/04 and my 3D echo in 2/2005 where they estimated my valve area @ 0.6 cm2. I remember them measuring the gradient in the TEE (where my valve area was measured as 0.8cm2 and saying I was borderline for the "you need surgery measurement". However, on my echo done at CCF for my "big workup", my gradient had essentially doubled, and my valve area had decreased 25% to 0.6. During that time I can think of at least 2 events where climbing stairs made me extremely short of breath, as well as 1 event where I definitely had some chest pain. Since my valve replacement that has not been an issue, although my area with my mechanical is only something like 1.8 cm2 (I had a small root and they could only get a 21mm valve in me.
 
Thanks Cort, I dont watch that show -- didnt know it existed, will try and remember to turn the TV on then next week ...

Carolyn, it does make sense -- cant vouch for numerical accuracy but this sort of understanding can lead to pretty good explanations/predictions for what can happen with untreated AS ( also AR which produces an increase in afterload as well ).

Chronic severe AS will lead to muscular changes in the heart ( hypertrophy ) this means there is more mass to oxygenate but the blood supply is affected due to the AS, eventually causing either rhythm problems or serious angina.

The effect of the AS is to reduce the reserve capacity of the heart.
Also :
-- Hypertension
-- Lack of cardiac conditioning ( high resting pulse )
-- Bad lungs ( due to for e.g. smoking )
have the same effect and exacerbate the AS.

Sudden death in athletes can also be understood:
AS affects the heart by making it work against an increased load, causing an oxygen debt in the heart earlier at high levels of activity. But physical signs of fatigue and oxygen debt from the ( rest of the ) body come later since its not working as hard -- so athletic individuals with high pain thresholds ( and adrenaline levels ) might ignore ( or not even be aware of ) the sensations from their chest, in extreme cases this leads to a heart attack.

Burair
 
You're welcome, Burair! Hope you remember ... it is, imho, an AWESOME show :).
 
PapaHappyStar said:
volume of left ventricle * EF * pulse rate

They can also do it by measuring the mean volume of the aortic root and the mean velocity by doppler ( and pulse rate ). The number should be on your echo report ( in l/min or in ml/sec ). I might not be able to tell you much from it though... not enough experience to tell what ranges are normal ... but these would be the sort of things I would be looking at I guess if I was to understand how I was progressing given pericardial constriction.
Click to expand...

I checked my echo, and apparently my new cardiologist, who read it, doesn't subscribe to the "let's post all the values" school of thought, because he's left many measurements off. He was so gaga over my "echodense pericardium" and my "impressive diastolic septal knock" that I guess he wasn't concerned with much else.
Hopefully the echo tech that I really like will do my next scheduled one, and I will ask him what the values are.
Thanks,
Mary
 
I've been having problems with my pressure gradient. My aortic valve was replaced four years ago with a tissue valve. The first few echos afterwards were fine. However now I am having echos done every 6 months as they are not happy with the results. They say my valve is opening and closing as it should but my pressure gradient is too high, which seems to be a contradiction. My mean gradient is around 26 but my peak gradient is 60. They seem to be assessing me on my peak gradient not the mean. I'm very confused.
 
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