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[RC] Heart Rates and Fitness--long - Bruce Weary

In light of recent ideas put forth about heart rates and fitness, I would like to make a few comments. Now, I'm no cardiac expert, but I did have my heart broken in 4th grade. By Barb Remigio, and, no, I don't want to talk about it.
Beverly was wondering about the usefulness of a low resting heart rate in an unconditioned endurance candidate. Heart rate in people is determined by, among other things, the strength of contraction, stroke volume, elasticity of peripheral vessels and the health, age and physical condition of the person.
In people, exercise affects all of these things (except age) and has additional benefits, as well. There is increased vascular supply to the periphery and the heart itself. Sweating efficiency and heat tolerance may increase. There are some other interesting effects that occur in the nervous system, as well, that are worth noting. The Vagus nerve increases it's output in fit individuals, and this increased "vagal tone" actually slows the heart rate, and stimulates gut function. In tests involving fit individuals vs. less fit individuals, the fitter people tested had higher vagal tone, and thus lower resting rates. These fitter individuals also have less sensitivity of the baroreceptors (pressure receptors) in the upper aorta, and this makes the heart less reactive to blood pressure, so the heart rate is less during exercise in conditioned individuals. This vagal tone is also what's largely responsible for our lowered heart rates during sleep. Low blood pressure is rare, and not directly a function of bradycardia. It is usually from the effects of myocardial infarct, blockage, or the effects of medications, (beta blockers, digoxin) dehydration, blood loss or other medical condition. The term "pathological bradycardia" applies to these situations. Fainting from "physiological bradycardia" (slow heart rate from fitness) is rare enough that other causes should be suspected first.
So, since the horse is a mammal, too, it's reasonable to assume that many of these principles apply to the horse, as well. The lower resting heart rate in an unfit horse may mean that he is able to meet his oxygen demands more easily from the genetic get-go, and exercise will only increase his advantage. His "fit" rate may or may not change, (it frequently does in people) but he will presumably be more efficient at distributing blood and retiring his oxygen debt with fewer contractions, and probably meet ride criteria that much quicker. Horses do vary, though.
As far as a horse recovering to a "pathologically low" heart rate, this is unlikely, unless the horse's sympathetic nervous system is completely inoperable. There are other criteria that will keep the healthy horse's heart rate at a sufficient level until he stablizes, such as oxygen debt, internal heat, dehydration, electrolyte loss, fatigue, pain, and other factors that stimulate the sympathetics, which tell the heart rate to stay up. When the sympathetic influence decreases, the parasympathetic system slows the heart rate. There are a series of "checks and balances" that drive all this. Horses that crash may have criteria that overwhelm these mechanisms and the horse is in trouble before he can right himself, and thus he needs veterinary attention. So, good recoveries are indeed, all other things being equal, a good indicator of fitness and exercise tolerance. Every sanctioned endurance in the country has, as its core recovery criteria, an acceptable heart rate. Good or bad, finishing criteria doesn't demand that normal gut sounds, capillary refill, jugular refill, skin tenting or anal tone be present for a completion, as important as they are. This might be something worth looking at. And, Dr. Ridgeway's CRI is based entirely on the heart rate recovery, due to all the other physiological factors acting on the heart rate, which helps give an overall view of how the horse is tolerating his work, not just how he's retiring his oxygen debt.
As far as Barb Remigio is concerned, I sent her love and moved on to 5th grade. I was 16, and it was time to get on with my life. Dr Q, world renowned heart throb.


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