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Old 10-19-2015, 01:16 PM
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Default Does a pacemaker change heart sounds?

I got a question about this once, and found it in my stash. There's what I said.

Heart sounds are made by liquids going through structures, like plumbing. Pacers work with electrical conduction, like wiring. The plumbing isn't going to change whether there's a pacer driving the conduction system or not.

Let's go back to the beginning.

You've got the SA node that normally sends out an impulse that makes the atria contract in a nice organized fashion, wringing blood through the mitral valve (in the left heart) and the tricuspid valve (in the right heart).
Then the impulse gathers together at the AV node and goes down the conduction system in the ventricles to make them contract in a nice organized fashion to wring the blood out the aortic valve (left) and pulmonic valve (right). Pacemakers have sensors in them in the atria and the ventricles, and the little things think like this:
Atrial circuit: "OK, do I see an atrial impulse in the specified frequency? Yes? Great, I do nothing, but I do tell the ventricular circuit. No? OK! Must be a problem with the SA node! I will stimulate the atria! I tell the ventricular circuit I have done this! OK! My work is done until it's been too long since the last atrial contraction, so then I do it again!"
Ventricular circuit: "Do I see an impulse coming down from the AV node to the ventricles? Yes? OK, I do nothing. No? Well, did the atrial circuit sense one or send one? Yes? OMG, must be an AV block! Then I must send one through the ventricles at a specified interval after that happened."
This results in hemodynamics, pushing blood around, which results in heart sounds.

It's helpful if you can step back first and think of what the anatomy of the circulatory system is supposed to accomplish. It's supposed to move a fluid around in a bunch of blood vessels, pumped out at high pressure from the left side of the heart, returned to the heart by passive squeezing in the veins and kept from sloshing backwards by valves in the vessels. Then the right side of the heart is supposed to push it through the lungs (at a lower pressure, because it only has to perfuse the lungs right next door, not all the way down to the toes like the arterial system) to do the gas-exchange thing. Then the fluid goes back to the left side of the heart and out to the body again.

Let's look at the blood flow in a linear fashion-- it's a much easier way to visualize what's going on. I give these in color so you can see the blue of venous, the red of arterial. Draw it on a piece of paper so you can fool with it.

Body > Veins > Vena Cava > Right Atrium > tricuspid valve > Right Ventricle > pulmonic valve > Pulmonary Artery > LUNGS >Pulmonary Vein > Left Atrium > mitral valve > Left ventricle > aortic valve > Arteries > Body

If you think about what's going on at each valve in systole and diastole AND you remember to listen in the right place for each valve, you can figure out whether you have a systolic or diastolic murmur and which valve it's coming from.

Draw a picture of the 4 chambers of the heart, following the nice one Meli posted. Yes, it's good to do the drawing yourself, because it makes different part of your brain save the information. . Include the mitral valve between the a & v on the left side, and the aortic valve at the outflow to the ventricle into the aorta. Have your linear blood flow line handy; they are complementary.

What goes on at the mitral valve? In diastole, it's wide open, so blood can go from the atrium to the ventricle (a quiet thing). In systole, it slams shut (the S1). You hear the mitral valve best at.... the apex, right. SO... if you have a mitral stenosis, things'll be noisiest there in diastole, because the blood will be going thru a too-small hole into the ventricle. If you have a mitral regurgitation, thereíll be a blurry noise there at systole, as blood goes backwards through it when it should be slamming shut. Think about this and don't go on until it is clear to you.

What goes on at the aortic valve? In systole, it's open so blood can get out to the aorta (quiet). In diastole it's slammin' the door, so blood can't go backwards (thatís the S2). So if you have a stenosis there, when and where do you hear the blurry noise?Right, in systole, and just downstream of the valve in the arch, as the ventricle tries to force blood thru a tight (stenotic) opening. OK, so if you have a regurgitation there, when do you hear it? Bingo, in diastole, because the partly-ajar valve is unable to slam shut and blood comes backwards thru it, making a racket.

Now, you will notice that these things happen whether or not the rate is regular; if there is a P wave, there is blood being pumped efficiently (more or less) into the ventricle. Even if there isn't a P wave (atrial fib), you will notice that the two heart sounds S1 and S2 are dependent on the ventricles doing their thing, no matter how regularly. If they aren't, then your patient is dead and has no heart sounds, or else he is in VF and there's too much racket what with CPR and defibrillating going on to hear anything, not that anyone would be listening to heart sounds with all that going on anyway.

So, back to your pacemaker question. Since an artificial pacer is designed to drive the electric system, this results in atrial contractions and ventricular contractions, with luck in a nice synchrony to duplicate normal conduction. Since the S1 and S2 heart sounds are valvular, driven by blood flow, the only influence the pacer impulses have is to be sure there is some flow, and that's what makes the heart sounds.

S3 and S4 are made by the blood banging around inside the ventricle itself, turbulence in there, NOT having anything to do with the S1/S2 valvular sounds. They have to do with the general health (flabby? stiff) of the ventricular wall itself, an indication of the condition of the muscle (failing? too thick? dead spots? coronary artery disease? aneurysm? contusion? organ rejection / inflammation post transplant?) and this will not be influenced by a pacer either. Alas, a pacer will not head off or change the need for a VAD or transplant if the basic protoplasm is crappy for any of the above reasons, including progression of any of the above reasons. All it does is turn on the electricity. If the lights don't come on, you need to get a flashlight or change the bulbs.

I hope this helps you put it all together. Ask again if not.
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