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RE: [RC] Resting HR on young horses/Stroke Volume/Bradycardia - heidiI'm late coming into this discussion, and don't have the technical
answers to all of Beverley's questions, but will share a few
comments.
First of all, the horse has some physiological adaptations of the
cardiovascular system that are unique to equids, one being that it
starts out far superior to anything the human cardiovascular system can
do. In general, mammalian heart rates follow their body size (ie
mice have REALLY high heart rates, and elephants REALLY low ones) but
the horse is the one species that is clear off the graph, with a far
lower resting heart rate than you would predict, given its body
size.
In addition to that, most species can raise their RHR about 4x to
their maximal HR. The horse can raise his about 6-7x--again,
clear off the charts for other species. This is one of the
adaptations that makes him so uniquely suited to quick flight from
predators, and likewise (because the range between the two is so wide)
makes him uniquely suited to endurance pursuits.
While it is true that the RHR may lower somewhat with conditioning,
there is still quite a range in RHRs--and this does in part get back to
stroke volume. However, stroke volume is not the only aspect
involved in RHRs in the horse--there are also some neurological aspects
that cause some variation.
In general, although one might want to shy away from horses with RHRs
of, say, 50, there is a point of diminishing returns in selecting for
lower and lower RHRs in prospects--once one is looking at horses in the
30s to low 40s, one is likely looking at normal-to-advantageous
stroke volumes, and the key becomes the recovery rate. Even
unconditioned, good prospects tend to have phenomenal recovery rates,
and ones that will have to condition a lot more to achieve them will
show poorer ones. (Biggest problem is that ones with such
superior cardiovascular systems tempt you to ride beyond what their
skeletal systems are conditioned to take.)
I'm no cardiologist, but hope that thumbnail sketch helps.
Heidi
Truman,
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