Our endurance horses, due to many factors including their
rapid
respiration rates are alkalotic.
The mechanism of metabolic alkalosis in heavily sweating
endurance horses and horses given the diuretic furosemide
(Lasix) is believed to be due to reduction in the
extracellular fluid volume (normally 60% of body weight)
from loss of water, sodium and chloride (in sweat), without
a matching loss in bicarbonate. The compensatory response to
metabolic acidosis is a decrease in respiration. I agree
with the other responders: the horses are panting
(tachypnea) in an effort to dissipate heat from the
respiratory tract. Cows don't sweat well - they do this all
the time!
If respiration rate increases further,
such as in an inversion the alkalosis worsens and the
foundation is laid
for development of thumps (synchronous diaphragmatic
flutter) and
rhabdomyolysis (tying up).
Thumps (synchronous diaphragmatic flutter) results from a
combination of metabolic alkalosis, low blood calcium,
potassium, and magnesium. Metabolic alkalosis alters the
ratio of free to protein-bound calcium. Protein bound calcim
is not avaible for cellular function. These derangements
disrupt the normal functioning of the phrenic nerve that
controls the diaphragm - making it fire with every heart
beat.
THe horse is blowing of CO2 to bring up blood pH--but
you imply that endurance horses are alkalotic--how can this
be when the
major muscle product is LACTIC ACID, which is causing the
problem? If the
blood pH is low because of HLa, it only makes sense to me
that the horse
would blow of CO2 to raise his pH to counteract HLa.
Am I missing something?
Gwen: your thinking is logical, and actually, we accepted
for years that tied up horses have lactic acidosis and
therefore must be metabolically acidotic. This was said in a
vet journal decades ago, and no one challenged it. Finally
someone did a study (Journal of the American Vet Med Assoc,
1983) and found that tied-up horses are metabolically
alkalotic. Many studies since have documented this in
exhausted endurance horses. As far as blood lactic acid
goes, studies show horses with chronic exertional
rhabdomyolysis have LOWER blood lactic acid concentrations
in affected muscles than normal horses. The most recent
studies suggest that muscle damage occurs due to problems in
reglating calcium inside the muscle cell.
Sometimes when a heart rate goes erratic it is a deficiency
of calcium, but
I'm so expert. We are all still learning, and frankly, I
think we know more
than most fo the vets!!
This comment is not appropriate for this list. We can have
discussions and exchange information without insulting
anyone. No one knows everything, and veterinarians are
highly educated, but are expected to know many things about
many species. If you want veterinarians who are more
educated about distance horses, support your nearest
veterinary school's chapter of the American Association of
Equine Practitioners.
Trisha Dowling, DVM, MS, Diplomate ACVIM, ACVCP
Associate Professor, Veterinary Clinical Pharmacology
Western College of Veterinary Medicine