Re: Heart contractility

[Sue Brown <sbrown@wamedes.com>]

>Both muscle cell hypertrophy and hyperplasia should produce more power per 
>contraction. I've never paid attention to wall thickness (probably should 
>have), and never tracked a horse through conditoning with heart
measurements. 
>Just used it as a one-time tool to indicate performance potential.

Since my experience with heart function issues is with the human heart and
it's dynamics associated with surgery, heart bypass, and recovery, I can
only ask questions from that angle of education...and it gets me wondering
how many of the relative functions are comparable in the horse.  The human
body is tweaked during these situations with medication and circulatory
volume and, of course, repair to the coronary arteries, hypertrophied
ventricles, or to the valves...which is not a mode of treatment (except for
circulatory volume) applicable here for the horses.  

One of the factors considered with human heart issues is the SVR (systemic
vascular resistance) and it's relation to pre-load and after-load figures
and cardiac contractility.  Cardiac output is carefully monitored on heart
patients during and after surgery and the medications juggled to balance
the SVR and contractility.  Too much SVR (evidenced by increased blood
pressure and increased after-load (the blood left in the ventricle after
the contraction)) requires medication to decrease the SVR but not
necessarily increase the contractility.  Chronic hypertension often results
in ventricular hypertrophy having exercised so hard against the
resistance...not related to cardiac output and hypertrophic increase due to
increased oxygen demand.  A low SVR may require medication to increase the
SVR, increasing the cardiac output by increasing the contractility of the
heart, or by increasing the circulating blood volume.  Malfunctioning
valves also enter here in consideration to cardiac output and
cardiomyopathy.  Connected with this also is pulmonary hypertension.

How many of these issues are relevant with horses?  Do horses have problems
with increased or decreased SVR?  Valve problems?  Pulmonary hypertension?
Cardiomyopathy resulting in weakened contractility?  Are horses ever
monitored with arterial pressure lines or cardiac output (Swan Ganz)
catheters?  Have cardiac outputs been monitored in relation to heart rate?
In humans, increased heart rate results in decreased pre-load (less time
for the ventricles to fill before the next contraction) with subsequent
decrease in cardiac output...to both the lungs and the body.  I'm assuming
that the horse's heart responds the same way.  

Since cardiac hypertrophy is not necessarily a good thing, are tests done
on the horses to determine if increased heart size is due to ventricular
hypertrophy (usually resulting in thinning of the heart wall and a decrease
in effective contractility) or ventricular wall hypertrophy as a result of
increased SVR or improved cardio-respiratory conditioning?  Have
comparative studies *ever* been done to show evidence of heart size
increase with long term conditioning (ie pre-conditioning echo compared to
post-conditioning findings) and/or relative cardiac output?  Are tests done
with x-rays, echos, etc.?

Do horses have trouble with increased blood pressure in reaction to stress?
 The heart rates go up so I'm thinking that this may also be the case...not
"due" to the heart rate increase but due to the adrenalin kick.  Does the
adrenalin affect the SVR?  What happens to the BP when the spleen dumps the
stored blood volume?  Does this affect SVR, BP, CO, and HR?  Under what
circumstances does the spleen kick in? 

(Has heart surgery ever been done on horses...and, if so, what for?)

Then there are parts about the lungs, oxygen carrying capacity, and so on
that I'm trying to relate.  <g>  I could come up with a hundred more
questions! ;-)

Sue

sbrown@wamedes.com
Tyee Farm
Marysville, Wa.