Artificial Heart Design - Heart Beat Needed?

[oogabooga]

When designing an artificial heart, why don't they just use an impeller design that doesn't mimic the cardiac cycle.

I believe current design will build pressure behind pneumatic valves which are actuated to mimic the cardiac cycle. Is it possible to just use an impeller to maintain blood flow and blood pressure without the use of pneumatic valves.

Is there a physiological reason why we would need the cardiac cycle assuming that you can maintain sufficient blood flow/pressure through an impeller only design? Also assuming of course that the impeller can adjust to current body needs/situation and can increase/decrease pressure just like the heart can beat faster as needed.

 

[PottedMeat]

I think they've been doing that for a while now:

http://en.wikipedia.org/wiki/Ventricular_assist_device

Continuous flow VADs normally use either centrifugal pumps or an axial flow pump. Both types have a central rotor containing permanent magnets. Controlled electric currents running through coils contained in the pump housing apply forces to the magnets, which in turn cause the rotors to spin. In the centrifugal pumps, the rotors are shaped to accelerate the blood circumferentially and thereby cause it to move toward the outer rim of the pump, whereas in the axial flow pumps the rotors are more or less cylindrical with blades that are helical, causing the blood to be accelerated in the direction of the rotor's axis.[5]

More recent work has concentrated on continuous flow pumps, which can be roughly categorized as either centrifugal pumps or axial flow impeller driven pumps. These pumps have the advantage of greater simplicity resulting in smaller size and greater reliability. These devices are referred to as second generation VADs. A side effect is that the user will not have a pulse, or that the pulse intensity will be seriously reduced, and will need to carry documentation saying that the lack of a pulse does not mean that they are dead.

 

[William Gaatjes]

I do not know if the modern designs do not have the issue, but i vaguely remember that the cells in the blood get damaged when using an impeller.

This i think would cause immune reactions one does not want.

But my information is old and may be outdated.

 

[CycloWizard]

I do not know if the modern designs do not have the issue, but i vaguely remember that the cells in the blood get damaged when using an impeller.

This i think would cause immune reactions one does not want.

But my information is old and may be outdated.

That could happen if the impeller weren't designed well. As long as it keeps the local strain rates relatively low, this won't be a problem. There has been a lot of work done on rheology (flow behavior) of blood to figure out how much is too much.

 

[FirNaTine]

I think they've been doing that for a while now:

http://en.wikipedia.org/wiki/Ventricular_assist_device

That whole lack of a pulse thing leaves an interesting conundrum for paramedics trying to treat one of these patients.

Typically one would do CPR on a person without a pulse, however it may be normal for that person. Additionally, there is some fear that doing chest compressions on a person with one of these implants could cause it to tear out of the heart (I don't know how easily this can happen, sorry.)

There are people walking around with these outside of the hospital. The local hospital that handles these brought a patient out for a continuing ed class I had a few years back. He did not have a palpable pulse, but his walking and talking (while not craving brains) proved its quite possible to tolerate continual non-pulsatile flow.

If you place a stethoscope in the right place, you can auscultate a whirring sound that would indicate the device is functioning.

They also have color coded cards with them, with emergency contact information. You can call a toll free number and immediately speak to a (nurse/nurse practitioner? can't exactly remember) who can advise you on what device they have and troubleshooting/emergency treatment.

 

[oogabooga]

Thanks for the responses - I had thought they were using valve systems to emulate the heart but apparently they are not. It's interesting to read how they handled the "not having a pulse" - Kind of how I imagine they would.

I would think I'd have a tattoo somewhere saying "I HAVE AN ARTIFICAL HEART AND DONT HAVE A PULSE - CALL THIS NUMBER" or something to that effect on my chest to make every effort people who might be helping me know about my situation.

 

[QuantumPion]

So instead of heart palpitations, you could have heart cavitation? 😱

 

[:emaN resU]

New VADs are generally axial or impeller. They're smaller and have less risk of stroke than older pneumatic VADs. And they are usually used for left ventricular support in patients who have a decent right ventricle, so because of this they do produce pulsatility due to pulsatile filling of the left ventricle. People have lived for years on non-pulsatile VADs without end organ damage.

Pneumatic VADs pump at a set rate or sense filling and run based on demand, whichever is faster. Axial/impeller VADs spin at a set RPM where they produce enough support while not unloading the ventricle enough to cavitate air or suck the interventricular septum into the VAD inflow cannula (which is placed through the apex). Each type of implantable VAD can fall back on a lower set RPM or rate if filling is inadequate to prevent cavitation.

 

[CycloWizard]

I would be interested to see what happens to the blood vessels in a person with a continuous flow VAD, since the vessels' mechanical properties adapt to the blood flow conditions. My first guess is that they would probably stiffen up due to lower average shear stresses, but I'm not sure if this would cause further problems down the line.

 

[Ninjahedge]

CW, i was thinking of something similar.

You can also use a slower rate if you have constant flow, so does this effect other processes?

Will you be more prone to calcification or cholesterol buildups with a slower constant velocity?

Does the constant flow work well in O2 transferrence in the lungs? (does it work better when they sit still for a second?)

Is it BETTER for situations like clots and strokes?

 

[Dreamland]

There was a thread on freerepublic about this a few weeks ago. Apparently, Dick Cheney has no pulse.

ok, found it:

http://www.freerepublic.com/focus/f-news/2624613/posts