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Re: [RC] Resting HR on young horses/Stroke Volume/Bradycardia - Beverley H. Kane, MD


I have been wondering about this thruout the RHR discussion.

In humans, and I assume in horses, the stroke volume is the amount of blood
that can be ejected/squeezed out from the left ventricle in a single
contraction ("stroke"--like the stroke of an engine piston). With increased
conditioning and fitness, the heart muscle enlarges. So in the
well-conditioned, enlarged heart, each contraction is stronger and the
stroke volume goes up. This has minimal, if any, direct effect on RHR.

The decreased heart rate--again, in humans--that comes from conditioning is
mainly from the decreased sensitivity of the pacemaker of the heart
(normally  the sinoatrial (SA) node in the right atrium) to the circulating
adrenalin of exercise. There are other HR determinants that have to do with
more efficient blood/oxygen extraction and utilization in lungs, heart
muscle, and skeletal muscle, and effects on the vagus nerve, which slows the
heart. So both the increased stroke volume and decreased RHR are a function
of the same conditioning factors.

What I don't get is how the -unconditioned- resting heart rate in horses is
any predictor of cardiovascular fitness for endurance. In humans, the
unconditioned resting heart rate is partly genetic, partly due to other
factors (age, foods, stress, etc). The more important thing to look at is
how the baseline, unconditioned HR decreases with conditioning.
  In humans, the recovery time to resting HR is a function of conditioning,
not of the absolute value of the RHR. Some humans with very low RHRs in the
unconditioned state become -dangerously- low in the conditioned state.
     Since the target/criterion HR at Rides is usu 60-64, any reasonably
fit horse should be able to recover to below that number. In a perfect
world, the Ride criterion should be something like "time to recover to 110%
of RHR," where RHR is in the absolutely calm, almost asleep horse. Since
this is somewhere between impractical and impossible, is it not true that
recovery criteria somewhat favor the horse with "genetically" low RHR **even
tho that RHR has nothing to do with the horse's fitness and fitness to
continue the ride** ???
  Summary point: in humans, unconditioned RHR and recoveries are unrelated.
  True or not in horses ???
Also in humans a slow RHR (bradycardia) in the unconditioned state can be a
sign of SA node or other heart disease. So given comparative human
physiology, I don't get why low RHR is so desirable in the purchase of an
unconditioned horse. I would even be concerned that such a bradycardia might
be a sign of a pacemaker or conduction (of electricity) disturbance.

Beverley...who has done >2,500 treadmill tests in humans of all levels of
fitness and with and without heart disease.

Beverley Kane, MD
Horsensei Equine-Assisted Learning & Therapy
Woodside, CA

On 9/10/06 3:02 PM, "Truman Prevatt" <tprevatt@xxxxxxxxxxxxxx> wrote:

 From all I have been able to find out - including input from some very
experienced vets one difference between humans and horses is
conditioning doesn't significantly impact the resting heart rate in
horses while it does in humans. The resting heart rate is closely
associated with "stroke volume" the higher the stroke volume the lower
the resting rate. It seems the stroke volume is genetically programmed
and fixed in horses.


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Re: [RC] Resting HR on young horses, Truman Prevatt