Why give humans pig hearts?

03 December 2019

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Question

Why give humans pig hearts? Why are we focused on pigs?

Answer

Cardiologist James Rudd answers...

James - Well, it's really all down to the fact that pig hearts are very similar to the size of human hearts. And let's talk about heart valves for a little bit. There are four heart valves in everybody's heart. And their job is to make sure that the blood flows the right way through the heart. And the valves can become diseased and damaged and they can either be too narrowed down so the blood doesn't flow properly, or they can become leaky. So the blood flows back where it shouldn't be. And so what happens is we can use pig heart valves which are specially treated. It takes about four weeks to actually prepare a pig heart valve to go into a patient.

Chris- So we would actually take the native valve from the pig heart?

James-Indeed, native valve comes out of the pig heart. It's treated with glutaraldehyde which completely sterilizes the valve, removes some of the immunological triggers that might cause people to reject it, and it's then placed in a kind of a strut which is suitable for sewing into a patient.

Chris- And do they substitute like for like, so the valve that you were talking about, the aortic valve earlier that you're interested in with that furring up and becoming narrowed in patients and you can spot that with your radioactive toothpaste, would it be a pig aortic valve in place of a human valve? Would it be a pig mitral valve in place of a human mitral valve? Would you go anatomically like for like?

James- Yes, you do indeed. And as I say, it really boils down to the fact that pigs and humans have roughly the same size heart.

Giles- Where are we with actually genetically modifying pigs or the heart so that we actually remove the immunological problems?

James- So in humans we're nowhere near, but there have been some experiments done where pigs have been modified and then their hearts have been transplanted to other species, which I think chimpanzees and the chimpanzees have stayed alive for about a year. So I think we're on the road to that. There are clearly perhaps ethical concerns that we need to think about before we take that last jump of xenotransplantation. But pig valves have certainly served as well for the last 40 years.

Chris- I spoke to Professor George Church from Harvard, who's a pioneer of a lot of this genomic technology earlier this year. And one of the things that he did was to work out where all of these viruses that loiter in the genome of all animals actually, but where they all are in pigs. Because one worry is when you take a heart from a pig and put it into a person, because they'd be immunosuppressed as we were discussing earlier, if any of these viruses reawaken, they would then be growing in the face of a, of a new body without any kind of immune response to tackle them and that could have all kinds of consequences. So he's gone through and found where in the genome these things are lurking and they've removed them. And this is obviously a step towards making these things safer. But as you say, we're not quite there yet with the whole question of how to stop the immune system still regarding this as pig, not human.

James- Yeah indeed. And with the heart valves, as I say, we actually blast the valve tissue itself with very strong detergent under high pressure and high temperature to kill any kind of immune signal.

Chris- Does it regrow new cells? Human cells of the recipient over that tissue then over time, so it's like road resurfacing, but then you grow a layer of your own cells. So over time you don't see pig tissue, you see human tissue.

James- Exactly. Yeah. It takes about three or four months to do that completely. So patients need to be on anticoagulant drugs for the first few months, but afterwards they can stop that. Once the valve has been endothelialised, we call it.

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