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For any reason, a human heart is replaced with a pump that instead of pulses provides a continuous stream of blood. It varies the pressure, as a heart does, as needed.

Ignore such technical issues as energy, immune system rejecting strange body, etc...

Is that viable/compatible with life?

What problems derived from this continuous blood-flow will arise?

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    $\begingroup$ That exists for real. $\endgroup$ – JDługosz Aug 26 '16 at 10:06
  • $\begingroup$ No problem here, no obvious problems. Why should it in first place. It works that way, but not to have to work that way. $\endgroup$ – MolbOrg Aug 26 '16 at 10:08
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    $\begingroup$ Dick Cheney has no pulse. $\endgroup$ – Chloe Aug 26 '16 at 17:31
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    $\begingroup$ One of the problems is that an encounter with a first responder can lead to unnecessary CPR when they can't find a pulse. Note: for CPR to be effective they press hard enough to break your ribs. $\endgroup$ – candied_orange Aug 27 '16 at 15:14
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    $\begingroup$ I wonder if there could be subtle psychological effects from the lack of "pulsing" - probably not much of a stretch. Sometimes people can feel their own heartbeat or even hear it. Could this subtly effect things like falling asleep (or sleeping), relaxation, etc.? $\endgroup$ – Steve Aug 28 '16 at 14:00
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That’s a real thing.

The pump console usually comprises several rotating motor-driven pumps that peristaltically "massage" tubing. This action gently propels the blood through the tubing.

Or

The pumps used in VADs can be divided into two main categories – pulsatile pumps, that mimic the natural pulsing action of the heart, and continuous flow pumps.

Start reading there to learn more about them.

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    $\begingroup$ @JohnFeltz Not really. Veins are flexible - try pushing a fluid through flexible piping upwards, and you'll see why it doesn't work :) It's kind of like trying to push a string from the bottom. Of course, the question then changes to "could we live without flexible veins if we had a continous stream of blood instead of pulses?" :D $\endgroup$ – Luaan Aug 26 '16 at 12:14
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    $\begingroup$ I think he’s on to something: a check valve would never be engaged if tge flow never stopped. $\endgroup$ – JDługosz Aug 26 '16 at 12:50
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    $\begingroup$ @JDługosz Not necessarily. Our blood pressure NOW doesn't go to zero between heartbeats, it varies between 120 and 80 mmHg on the arterial side and 5-15 mmHg on the venous side. A continuous pressure on the arterial side will still lead to a very low venous side pressure since red blood cells have to go single file through terminal capillaries. Plus the flow would still have to be variable to deal with increased O2 demands when fight or flight responses are activated (or just going from laying down to standing). $\endgroup$ – Jason K Aug 26 '16 at 13:27
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    $\begingroup$ To extend on what @JasonK said, there is a process where contraction of skeletal muscle in the legs help move blood back to the heart. There is enough pressure placed on the veins that squeezes blood out. The one-way 'check valves' mean it can only go in one direction. Effectively, your skeletal muscles work like indirect mini-hearts throughout your body. $\endgroup$ – BrownRedHawk Aug 26 '16 at 18:58
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    $\begingroup$ Jason & Hawk are right. When the area surrounding a checked-valved vein increases in pressure, it causes a local pressure increase sufficient to engage the check valves and provide a net return of blood. (Consider the delta pressure due entirely to gravity vs. the tidal pressure of the heart.) $\endgroup$ – The Nate Feb 19 '17 at 23:27
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It is possible, indeed it is already done today, it is called a Extracorporeal membrane oxygenation.

But, I can't find any problem related to the continuous blood flow nor its benefits.

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    $\begingroup$ Well, uninformed medics declaring you dead when they don't feel a pulse could be a downside haha $\endgroup$ – Martijn Aug 26 '16 at 14:35
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    $\begingroup$ @渡し守シャロン I doubt it. When you check your pulse, you don't feel any flow, you just feel the pressure increasing and decreasing. $\endgroup$ – Walt Aug 26 '16 at 16:57
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    $\begingroup$ @Amadeus9 I think trained paramedics / EMS respondents do look for and feel a flow, but I don't know for sure. I just remember my paramedic buddy describing some condition he always looks for where the flow feels ... "wooshy" ... and I don't remember the word for it or any more details about that story, or even the context he was describing it in. But yeah it would probably take them by surprise if they didn't find a pulse regardless since it'd be so unusual, and wouldn't make the monitoring equipment in the ambulance happy either. $\endgroup$ – Jason C Aug 27 '16 at 1:11
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    $\begingroup$ Nobody relies on just one life sign. They will check various things in addition to pulse (e.g. breathing, pupil dilation, blood oxygen levels) before declaring you dead. $\endgroup$ – JBentley Aug 27 '16 at 16:09
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    $\begingroup$ "I don't want to go on the cart..." youtube.com/watch?v=HLgQMtquS6Y $\endgroup$ – A. I. Breveleri Aug 28 '16 at 0:26
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From what I have read in medical magazines, the exact answer is still subject to research. There are "artifical hearts" that do exactly that (inside or externally, both exist) but some have the suspicion that this kind is suboptimal for prolonged use.

There are other studies that show that stem cells (and probably also other cells) react and differentiate differently whether they are under a constant or a (rather) quickly varying pressure.

The details are not very well understood, but at least it is understood that if there are any risks with a continuous flow pump involved, their statistic relevance is likely lower than any other issues the patients have, so research is done in that field as many people believe those are more reliable.

It should be noted that everything that is involved here is about patients with (possibly terminal) illnesses and that the artificial hearts are often used to fill the gap until a transplant becomes available. In the context of worldbuilding one might want to consider that doing this on a big scale for healthy people might have a much worse outcome, given current understanding of the human body.

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    $\begingroup$ One more thing to note is that even if there are issues with continuous flow in humans, they might not exhibit in creatures that were designed/evolved to cope with continuous flow naturally. $\endgroup$ – John Dvorak Aug 26 '16 at 20:01
  • $\begingroup$ @JanDvorak: true that, though I have a hard time imagining an evolutionary path to there. I think the problems will in any case be minor enough to be able to solve them when wanting to go full scale all humans. I would have more problems with a motivation on why you want to do it thought.... $\endgroup$ – PlasmaHH Aug 26 '16 at 20:06
  • $\begingroup$ Motivation to replace hearts with artificial pumps in healthy humans? The only reasons I can think of are prevention and cyborgification - once you have solved the issue of rupturing blood vessels (say, by limiting the flow to the less augumentable parts), you might want to go up with the blood pressure to ensure speedier delivery for your super-muscles. And, once you have an artificial heart, it might as well be continuous flow - familiar tech, easier on veins, possibly more efficient (just stick a fan inside a pipe). $\endgroup$ – John Dvorak Aug 26 '16 at 20:14
  • $\begingroup$ I know that a pig's heart valve will disintegrate unless subjected to pulsitile flow. Presumably, this applies to other circulatory organs and structures as well as other species. Dr. Warnock is a fellow that I know did that research (at MIT, IIRC), but I don't currently have a link to it. If you actually care I could dig around a bit to find the paper. $\endgroup$ – The Nate Feb 19 '17 at 23:32
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I'm not aware of any negative physiological effects of maintaining a steady blood pressure, PROVIDED the pressure is around the regular average a human has today. So, for example, if a normal human has an arterial pressure between 120 mmHg and 80mmHg, their average pressure is 100mmHg. So you couldn't run your synthetic heart at 120mmHg all the time, as that would start to induce the negative changes we associate with hypertension. The higher the continuous pressure, the more negative the effects (typically increased risk of stroke, decreasing kidney function, etc). Of course these devices have lots of other complications, but we'll ignore those (the need for anti-coagulation, etc).

But you have to be able to vary the pressure to some extent because otherwise in a fight or flight response (gotta run!!!) you won't be able to increase blood flow to deliver more oxygen and remove lactic acid. Plus it helps to be able to vary blood pressure when going from laying down to standing up (problems doing this is called orthostatic hypotension). So a continuous pressure heart, inherently, isn't a problem, but one that is LOCKED TO A SPECIFIC PRESSURE is.

Humans vary their blood pressure through adjusting heart rate AND changing the muscular tension on arterial side blood vessels. Venous blood vessels have little to no muscular walls so they can only dilate to compensate for increased pressure or respond to the muscular action of the tissues they are embedded in. If you remove the natural heart, the replacement will have to either be manually controlled, somehow interpret the brains call for increased heart rate by increasing the flow rate and elevating blood pressure (or sense dropping oxygen levels, rising lactic acid, or whatever), or the person would be forced to regulate their activity very carefully lest they deplete oxygen and cramp up due to lactic acid build up. Here is a good technical article discussing various total heart and left ventricular assist devices and exercise tolerance. To my knowledge, all artificial heart type devices meant for prolonged use (i.e. getting out of bed and walking around) replicate or assist a beating heart. ECMO and the like are for sedated/bed-ridden patients, so exercise tolerance is not an issue.

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  • $\begingroup$ I'm sure there's an app for that.... $\endgroup$ – Criggie Aug 26 '16 at 22:02
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These already exist. In 2010, Dick Cheney received exactly this sort of thing; it wasn't a fully artificial heart, but it took over for his failing left ventricle, and for over a year he lived with no pulse at all. The comedians went nuts over this, but the truth is somewhat more boring: his life during this time was fairly typical for someone with severe heart trouble. After about 15 months he got a heart transplant, so now he has a pulse again.

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    $\begingroup$ While normally these devices assist the heart, which is still there and might give some weak beat, there was at least once case where the doctors removed the patient's heart altogether (due to cancer) and replaced it completely with two pumps. The case I heard about the patient died after over half year from infection (the devices still need external batteries and the leads through skin increase the risk), but appeared to be recovering fine before getting that infection. $\endgroup$ – Jan Hudec Aug 26 '16 at 14:14
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It is entirely possible and the subjects are still able to live a very healthy, normal life. Some, like Andrew Jones in the link, even reach extraordinary levels such as bodybuilding while still being without a pulse.

It is important to note that this is usually temporary while the subjects wait on the donor list for a new heart.

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Long-term continuous blood flow does cause physiological problems, but the full scope of the problems it causes is still the subject of ongoing research.

Pulsatile blood flow causes a phenomenon called endothelial shear stress (ESS). I still remember a set of electron micrograph slides from medical school a few years ago comparing arterial endothelium - that's the innermost layer of arteries - which have been subjected to both continuous and pulsatile blood flow. I no longer remember the experimental details, but there is a clear difference between the two. The surface of the endothelium subjected to pulsatile flow looks smooth and healthy, the other one looks positively jagged.

This jagginess causes all sorts of problems, especially the deposition of artheroscletoric plaques. Here is a 2007 paper from the Journal of the American College of Cardiology that describes the phenomenon in detail.

In the context of worldbuilding, while it certainly will not kill the subject immediately, there may be other long-term effects that we don't know about. If the continuous blood flow is the side-effect of a life-saving operation, like a heart transplant, I'd say that's not too bad :)

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  • $\begingroup$ Welcome to Worldbuilding and SO in general! $\endgroup$ – JDługosz Aug 28 '16 at 8:51
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    $\begingroup$ This is wonderfully interesting. Quite counter-intuitive. It looks like ESS is needed to keep the endothelium smooth and healthy. A good start to Worldbuilding SE. $\endgroup$ – a4android Aug 28 '16 at 9:02
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Taking an intuitive/common-sense perspective, with a little everyday science thrown in...

The goal of the device is to match the mean flow rate (litres per minute) of a natural heart. Continuous flow will result in a certain uniform fluid velocity through the arteries, veins, vessels, and capillaries; pulsed flow will produce time-varying velocities, peaking much higher than the mean to make up for reduced velocities occurring between the pulses. Blood is a suspension; when not in sufficient motion, things in a suspension will begin to settle out. The higher peak velocities could be beneficial in preventing things normally suspended in the blood from settling out on the vessel walls, or "scouring" things which have begun to settle. The uniform velocity produced by a continuous flow device might not be high enough to accomplish this.

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