Re: RC: Heart contractility

[Tivers@aol.com]

In a message dated 2/10/00 6:26:37 PM Pacific Standard Time, Petdoc6@aol.com 
writes:

<< In the heart's response to training, wouldn't the strength of each 
myocardial 
 fiber remain the same?   Would there be an additive effect due to increased 
 muscle thickness?  A larger chamber would result in an increased stroke 
 volume while the smaller cavity due to hypertrophy of chamber walls would  
 result in a smaller stroke volume but a higher pressure at the end of 
systole?
 
 Carol Wingate >>


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. Stakes 
level racehorses always have larger hearts than average, and most racehorses 
are not cardiovascularly fit at all. Heart size also tracks with body weight, 
though--so bigger horses have bigger hearts, automatically. It's the medium 
sized horse with the big heart that really is the best flat racing prospect. 

Left vetricle diameter tracks inversely with contractile fraction (what 
percentage of the  ventricle closes with contraction) at rest. Don't know 
what it does at work. Fred Fregin still likes the QRS duration method of 
heart scoring--says it tracks very well with performance. Wouldn't an ECG 
show a bigger contractile spike for a larger muscle mass--like an 
electromyogram does?

ti