Re: [RC] Resting HR on young horses/Stroke Volume/Bradycardia - Beverley H. Kane, MDTruman, I have been wondering about this thruout the RHR discussion. In humans, and I assume in horses, the stroke volume is the amount of blood that can be ejected/squeezed out from the left ventricle in a single contraction ("stroke"--like the stroke of an engine piston). With increased conditioning and fitness, the heart muscle enlarges. So in the well-conditioned, enlarged heart, each contraction is stronger and the stroke volume goes up. This has minimal, if any, direct effect on RHR. The decreased heart rate--again, in humans--that comes from conditioning is mainly from the decreased sensitivity of the pacemaker of the heart (normally the sinoatrial (SA) node in the right atrium) to the circulating adrenalin of exercise. There are other HR determinants that have to do with more efficient blood/oxygen extraction and utilization in lungs, heart muscle, and skeletal muscle, and effects on the vagus nerve, which slows the heart. So both the increased stroke volume and decreased RHR are a function of the same conditioning factors. What I don't get is how the -unconditioned- resting heart rate in horses is any predictor of cardiovascular fitness for endurance. In humans, the unconditioned resting heart rate is partly genetic, partly due to other factors (age, foods, stress, etc). The more important thing to look at is how the baseline, unconditioned HR decreases with conditioning. In humans, the recovery time to resting HR is a function of conditioning, not of the absolute value of the RHR. Some humans with very low RHRs in the unconditioned state become -dangerously- low in the conditioned state. Since the target/criterion HR at Rides is usu 60-64, any reasonably fit horse should be able to recover to below that number. In a perfect world, the Ride criterion should be something like "time to recover to 110% of RHR," where RHR is in the absolutely calm, almost asleep horse. Since this is somewhere between impractical and impossible, is it not true that recovery criteria somewhat favor the horse with "genetically" low RHR **even tho that RHR has nothing to do with the horse's fitness and fitness to continue the ride** ??? Summary point: in humans, unconditioned RHR and recoveries are unrelated. True or not in horses ??? Also in humans a slow RHR (bradycardia) in the unconditioned state can be a sign of SA node or other heart disease. So given comparative human physiology, I don't get why low RHR is so desirable in the purchase of an unconditioned horse. I would even be concerned that such a bradycardia might be a sign of a pacemaker or conduction (of electricity) disturbance. Beverley...who has done >2,500 treadmill tests in humans of all levels of fitness and with and without heart disease. _____________________________________ Beverley Kane, MD Horsensei Equine-Assisted Learning & Therapy Woodside, CA http://www.horsensei.com On 9/10/06 3:02 PM, "Truman Prevatt" <tprevatt@xxxxxxxxxxxxxx> wrote: From all I have been able to find out - including input from some very experienced vets one difference between humans and horses is conditioning doesn't significantly impact the resting heart rate in horses while it does in humans. The resting heart rate is closely associated with "stroke volume" the higher the stroke volume the lower the resting rate. It seems the stroke volume is genetically programmed and fixed in horses. =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Ridecamp is a service of Endurance Net, http://www.endurance.net. Information, Policy, Disclaimer: http://www.endurance.net/Ridecamp Subscribe/Unsubscribe http://www.endurance.net/ridecamp/logon.asp Ride Long and Ride Safe!! =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
|