I have been enjoying the discussions from the wings for a while now but
since Truman Prevatt hinted that I am shy regarding the work I have
been doing, I decided to prove him wrong and offer some of what I have
been studying with the rest of you.
My background is that I have a Ph.D. in Pharmacology, specializing in
renal and cardiovascular,and have been
teaching Medical school for 8 years now. My husband, Carlos, and I got
involved with endurance 2 years ago; He rides and I crew. We have 2 boys
Daniel, 9, who also rides with his father, and Scott, 5 who helps Mom.
Our most recent accomplishment where we "field tested" some of my
knowledge was the Old Dominion, where Carlos and Cody were 1 of the 10 to
finish the race. Cody's recovery rates at all the vet checks were 2
minutes or less, with no need to sponge him for cooling, even though the
temperatures were in the 90's and humidity was the same. Much of the
success is due to the fact that Cody is a remarkable horse, and we like
to think that our planning and program helped with the rest.
Just a little background on electrolytes to begin with. The major
electrolytes that are of importance in the endurance situation are sodium
(Na+), potassium (K+), calcium (Ca2+), chloride (Cl-) and magnesium (Mg2+).
Of all of these, K+ is the most crucial to maintain in order to avoid
most of the metabolic problems, such as "thumps" and cramping and "tying-up".
Without going in to a complete lecture, the body is extremely efficient
at reabsorbing Na+ in the kidneys in order to reabsorb water during
periods of dehydration. This Na+ reabsorption and water reabsorption is
done at the expense of K+ losses. In other words, when the horse starts
to become dehydrated his kidneys will filter the blood and reabsorb
almost 99% of the Na+ in it. Whenever Na+ is reabsorbed back into the
bloodstream water will follow so that the urine output is decreased and
the animal conserves water. The downfall is that as the kidneys reabsorb
this Na+ back into the blood, they are eliminating K+. Therefore the
urine contains a lot of K+ and little Na+.
This K+ loss in the urine is compounded by extreme K+ losses in the
sweat. K+ may be concentrated in horse sweat 10-20x greater than in the
blood.
Other factors causing K+ depletion in these horses are alkalosis, which
results from the horses breathing rapidly, epinephrine (adrenaline -
which is really flowing at the beginning of the race!) and insulin
release, which occurs most when the animals are fed sugary concoctions.
So what's the big deal about K+? K+ is the electrolyte in the body that
is responsible for setting the "resting membrane potential" of all
electrically active tissues; which includes the heart, the entire nervous
system, and the response of muscle tissue to contract. The "resting
membrane potential" is going to determine
1) How sensitive the tissue is to electrical stimulation.
2) Whether that tissue will react, or whether it will relax.
3) How fast the tissue will react.
4) Whether the tissue can react a second time, or does it
becomes refractory.
For those who don't care about all the background, the bottom line is
that K+ depletion will lead to:
1) Vasoconstriction and reduced blood flow to the muscles.
This will result in cramping, tying-up and the breakdown
of muscle tissue. When muscle tissue breaks down (known as
rhabdomyolysis) it releases a protein called myoblobin.
Myoglobin is toxic to the kidney - resulting in damage and
reduced ability of the kidneys to compensate for further
dehydration and reduced ability to rehydrate after the race.
2) Increased sensitivity of the nerve that controls the diaphragm
and therefore respiration rate. When this nerve (the phrenic
nerve) becomes sensitive it begins to respond to the
electrical impulses coming from the pacemaker of the heart and
will fire and cause the diaphragm to contract in synchrony with
every heartbeat. This is "thumps" - or medically known as
synchronized diaphragmatic flutter.
Treatment of thumps usually entails the administration of electrolyte
solutions containing extra calcium. The calcium itself will suppress
electrical activity, therefore it will decrease the sensitivity of the
nerve to the diaphragm so that it does not respond to the heartbeat.
However, calcium alone does not treat the underlying cause of the
problem, which is potassium depletion (compounded by alkalosis). Thumps
can be avoided by maintaining K+ to begin with.
So the question of the day is "How much K+ is needed to avoid these
problems?" Obviously there is much research aimed at just this
question. The point is, that a 2 oz. dose of standard electrolytes is
nowhere near sufficient to make up for what the horses are losing. In
addition, some calcium is essential to balance what is happening
electrically in the animals.
The electrolyte formula that I used for our horse at the Old Dominion was
4 oz of the powdered Equilytes (which would be 1420 mg of K+) plus an
additional teaspoon of KCl (which is an additional 2240 mg of K+) mixed
in unsweetened applesauce. We administer this every 10-15 miles, which
often means between vetchecks. In addition, early in the race I add 600
mg of Ca2+ with Vitamin D to the electrolyte mixture.
A few words about calcium. No matter what form it is in, the absorption
of calcium out of the gut into the bloodstream is absolutely dependent on
vitamin D. In horses fed routine diets of alfalfa which contains high
amounts of calcium and phosphorous, these animals have shut down the
production of the hormone that is responsible for signalling the
manufacture of vitamin D. Therefore they do not absorb calcium when it
is presented to them orally in electrolyte solutions. The calcium then
remains in the gut and will suppress electrical and muscle activity in
the gut and can lead to suppression of gut motility, colic and
impactions. Even if the horses do not receive alfalfa in their diets,
too much orally administered calcium may reduce gut motility. If gut
sounds are not normal, I do not add extra calcium to the electrolyte
solution. However, I always give calcium with VITAMIN D.
Now as you can all see, I am never short of words so I would like to
dispel the myth that I am "shy". I hope this will help some of you
understand the overall importance and "why" we give electrolytes. Let me
finish by saying that I have not had the opportunity to do the necessary
blood tests on a large number of horses to absolutely document our
results. I do know that under the most extreme conditions our horse had
unbelievable recoveries (1-2 minutes) and never showed any indications of
cramping or metabolic problems. His pulse after trotting him out for the
CRI was many times lower than the initial reading. This is a young 6 year
old gelding who has less than 500 total miles.
I would love to hear anybody else's opinions or answer any questions you
might have. You know where to find me!
Lynn Crespo
Associate Professor and Chair
Department of Pharmacology
Nova-Southeastern University
My home is in Davie, FL!
L. M. Crespo
lynn@alpha.acast.nova.edu