bseymour@houligan.UUCP (Burch Seymour) (10/20/86)
I just read an article about the Jarvik-7 heart which reminded me of an
old question about artificial blood pumps. Does the human body require a
pulse? That is, could we not substitute a constant pressure pump instead
of the pulsating variety. I realize that the heart pulses, this gives us
the systolic/diastolic values of blood pressure, but does it have to?
It seems that it would be easier to build a constant pressure, fully
implantable pump.
Anyone out there know of any work/research along these lines. I think the
movie _Threshold_ used a pump of this type. In fact if memory serves, I think
the pump used in the movie was built by Dr Richard Jarvik. And yes, I know,
it's only a movie.
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"A nation that beats its swords into plowshares generally ends up
doing the plowing for one that has kept it swords." Anon
Burch Seymour -Gould C.S.D. at ....mcnc!rti-sel!gould!bseymour
-------------------------------------------------------------------------larry@kitty.UUCP (Larry Lippman) (10/22/86)
In article <116@houligan.UUCP>, bseymour@houligan.UUCP (Burch Seymour) writes: > I just read an article about the Jarvik-7 heart which reminded me of an > old question about artificial blood pumps. Does the human body require a > pulse? That is, could we not substitute a constant pressure pump instead > of the pulsating variety. I realize that the heart pulses, this gives us > the systolic/diastolic values of blood pressure, but does it have to? For comparatively short periods of time, steady blood flow is provided when a patient undergoes open-heart surgery and is placed on an extracorporeal circulation device (i.e., "heart-lung" machine). Today, most extracorporeal circulation machines use roller-pumps, which result in almost a steady-state flow. A common predecessor pump - known as the Sigmamotor pump which used a sequential series of "fingers" to milk blood through a section of tubing - also provided a relatively non-pulsatile flow rate. So the point is, for comparatively short periods of time, steady blood flow is used today. Over a "long" period of time, as would be encountered with an artificial heart, steady blood flow would present some problems. The human circulatory system is extremely elastic, and the arterial side (arteries and arterioles) are also extremely muscular. Such resultant elasticity presents what is referred to as "peripheral resistance". To force oxygenated blood into circulation at the ascending aorta AND MAINTAIN ADEQUATE PERIPHERAL CIRCULATION would require a steady-state blood flow whose resultant pressure would be in the same magnitude range as that of "normal" systolic pressure. Such steady-state flow would keep arterial and arteriole walls and capillary beds permanently distended, causing unnatural and excessive stress on their walls, eventually resulting in ruputure of smaller vessels. To maintain circulation at lower pressures (say around "normal" diastolic pressure) would result in insufficient peripheral circulation. To re-phrase the matter in an accurate, but somewhat oversimplified manner: the higher arterial pressure at the time of left ventricular contraction (i.e., systole) overcomes peripheral resistance and "opens" a door to allow peripheral blood flow to continue for a period of time at a LOWER PRESSURE THAN COULD ADEQUATELY OVERCOME PERIPHERAL RESISTANCE BY ITSELF. > It seems that it would be easier to build a constant pressure, fully > implantable pump. While it might be "easier" to build a "constant pressure, fully implantable pump", such pumps are simply not suitable for use in a long-term environment. Why? Because of something called "hemolysis", which in this case is the destruction of blood cells through the mechanical action of the pump. A roller pump, as used in an extracorporeal circulation machine, causes significant hemolysis as the pump tubing walls are pinched together by the rollers as the blood is "milked" through the tubing. So, no roller pump is suitable for long-term use in an artificial heart. Other types of pumps, such as centrifugal, gear, rotor, vane, etc. all cause massive hemolysis, and are NEVER even used for even short-term extracorporeal circulation. It just so happens, that a volume-displacement pulsatile pump appears to be the most feasible for use as an articial heart. Interesting, isn't it, that such pumps closely approximate the design and functionality of the natural heart? ==> Larry Lippman @ Recognition Research Corp., Clarence, New York ==> UUCP: {allegra|decvax|rocksanne|rocksvax|watmath}!sunybcs!kitty!larry ==> VOICE: 716/688-1231 {hplabs|ihnp4|seismo|utzoo}!/ ==> FAX: 716/741-9635 {G1,G2,G3} "Have you hugged your cat today?"