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Re: Heartrate Threshold

Susan Evans Garlinghouse wrote:

> I think I read somewhere
> that hummingbirds have a normal HR of around 2400 bpms, although their
> heart is a little different.  So I'm not so sure a horse's heart
> wouldn't be capable of re-filling and pumping out a fresh load of blood
> at 240 bpms---after all, it has been documented on more than one
> occasion.

His Susan,

I think there are some physical limits here:
- cardiac output (=stroke volume x heartrate). I'm not sure if any animal has such a
high cardiac output max. as horses. OK some animals have higher HRmax. but smaller
hearts also.
- arterial pressure and blood flow speed at HRmax. There must be a limit somewhere,
because theses structures are not made of carbon steel. I heard about arterial
ripp-offs and other awfull stress failures which happens in racing horses but very
seldom in humans.
- refill time of the heart itself. I remember reading an article of Engelhard. He
said that if HR goes up, the length of working phase (pressing blood) stays constant
at one point, because there is no way to save time anymore. However, the refill
phase (which is also the relaxing phase of the heart muscle itself) is shortened
more and more. Up to some 1/1000 sec. where the time is too short to refill the
heart again. I think this is what Tom mentioned as fibrillation.

Thats also interesting with the HRmax. of ponies (maybe small Arabians also). I
suggest they should be higher (?)


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