Re: Heart contractility

[Tivers@aol.com]

In a message dated 2/11/00 1:27:21 PM Pacific Standard Time, 
sbrown@wamedes.com writes:

Interesting questions. I only have a couple of answers.

<< How many of these issues are relevant with horses?  Do horses have problems
 with increased or decreased SVR?>

One of the problems with racehorses is delayed vasodilation due to a general 
lack of cardiovascular fitness ( a low-responsive nitric oxide system). So, 
you can run into a temporarily elevated SVR that causes lung bleeding. There 
is also the splenic contraction that takes place with the onset of high 
intensity exercise--can increase red cell population by has much as 100%. 

  >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. >

In the horse there was a concern that high heartrates would lead to this kind 
of problem--but it was found that muscular action speeded up ventricular 
refill to compensate.


> 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?>

I don't think any long term studies have been done in athletic horses. 
Instead, just today's heart size vs performance characteristics. 

>  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?<

Cariac output has been studied in relation to conditioning and fitness, but 
not in relation to heart size, that I know of.

>  Are tests done
 with x-rays, echos, etc.?>

Ultrasonography, for the most part.
 
 >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?>

Hmm, you'r going to force me to go back to the science to take a look. 

  >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?>

The spleen will completely empty with 24 seconds of near-maximal exercise. It 
will partially empty with 3-5 miles of 160 HR exercise.  
 
 >(Has heart surgery ever been done on horses...and, if so, what for?)>

Never heard of it being done.
 
 >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! ;-)>

Please! Don't do that! You've got me running already.
 
 Sue

ti
 
 sbrown@wamedes.com
 Tyee Farm
 Marysville, Wa.
  >>