Irregular Heartbeat in a Three-Year-Old

I have a 3-year-old Holsteiner mare, bought at auction, that I had vetted yesterday as if it were a routine PPE. Her results were excellent in all but one category. Her resting heartbeat was completely normal, but after strenuous exercise became irregular.

The research I’ve done suggests that this is a very rare situation, so rare that I would like to repeat the test in a couple of weeks. And for another reason too.

Four days before the vetting my partner trailered the mare out to have her teeth done for the first time That process was mismanaged in my opinion–the mare became so frightened that the dentist administered four times the ordinary dose of sedative to get the job done.

Any thoughts on the possibility that lingering sedative in her system could account for the irregular heartbeat after excercise four days later?

No idea on the sedative but I would recommend taking her to a cardio specialist. If you post your general location I’m sure COTH can help you find one.
From my experience, a lot of general vets tend to be not super well versed in determining heart issues. Sometimes irregular heart rate in a horse is not a problem at all, and exercise will help fix the issue. And other times it’s much more serious. Until a specialist can run all the tests, you really don’t know.

However, I will say, make sure your horse knows how to lunge before you call the specialist. They will need to lunge them.


Totally agree. I have seen a few horses “fail” vettings over heart issues that were not issues when checked by a specialist.

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The benign irregular heart rhythms I’m familiar with usually go away with exercise. Not get worse. And I can’t think of any reason the sedation given four days ago would still be having any effect at all on the horse.

I would get a second opinion of some sort.

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Yeah, one of mine actually has that. I can’t ever remember the name.

However, there’s a different issue that happens both during and not during exercise. They usually tell the owners to do a lot of low level cardio with the horse and avoid high heat situations. It prevents the horse from being a 3 day eventer or racing, but they are usually 100% fine for ‘lower’ level work like hunters, jumpers, dressage, etc. I’m hoping an actual vet can chime in here, and not an armchair vet such as me :smile:

Atrial Fibrilation? (Uh, that might be the armchair vet spelling, not the textbook one… we’ll say A-fib). I had one with that, which was described to me as an “Irregularly Irregular Heartbeat”. As in, no specific pattern to the deviations from your normal lub-dub. I was told it “could” lead to exercise intolerance. [As it were, I didn’t have enough exercise tolerance myself to see if I could exhaust that horse :rofl:]

To the OP, I agree with getting a recheck and likely a specialist. Also get yourself a stethoscope (and someone to show you how to use it, if needed) and start listening in to her heart regularly yourself, so you can determine if there is any pattern or commonality to when her heartbeat is normal vs. not.

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Probably that! It’s been years since I took classes on this stuff. I’m one of those people who knows just enough to be dangerous but not enough to be entirely helpful :joy:

A-fib is not ‘benign’ and can be associated with underlying serious conditions or even something relatively simple to fix, like magnesium deficiency.

If the horse is in A-fib, she definitely needs an ECG. And to be evaluated for conversion. Especially if you ever want to sell her.


I owned a TB some years ago that was diagnosed at 17 with A-fib and a murmur. No outward symptoms, was working/showing 4th level dressage at the time. Took him to Ohio State, where he had a full cardiac workup. They attempted to convert him to normal rhythm with a drug called quinidine; it was not successful. Ultimate diagnosis was mild congestive heart failure, ok to work but watch for intolerance of exercise, lethargy, weight loss, depression. He was fine to ride/work until 22 when cervical arthritis got the better of him. Heart measurements at that time showed only modest change. He was 25 when the heart caught up with him.
With such a young horse I’d say get all the tests/measurements you can to understand where in the heart and what the problem is. Go to best clinic/teaching hospital within your reasonable travel distance. One that has cardiac specialists for sure.
Good luck with this…


I’m going to be most unhelpful as I have only tangential experience and the details below are sketchy at best and could be inaccurate.

I have a friend with a horse that has heart issues. I think A-Fib but can’t be 100% sure. I know they did a few ECG’s, including a baseline - intervention - re-check (that did not show any improvement). Irregularity was identified around age 3 during a PPE (friend bred horse) – sale was going to go through if intervention ‘fixed’ the problem but it did not.

She ended up keeping him and deems him ‘unsellable’. He’s now 7ish and doing mid-level dressage (competing very competitively at 2nd level and schooling 4th level movements). He has physical aptitude for GP but unclear if he has the mental fortitude for the top levels. She’s content to keep him and see how far he goes and just enjoy him as he’s also a great hack and ‘play around doing everything’ type horse. Because of his heart condition, she doesn’t push him too hard in sessions and monitors his breath closely.

I know she’s been warned that he could just up and die underneath her at any time, which is why she deems him unsellable. If a rider was injured in a sudden death she would never forgive herself, although she’s comfortable taking the risk personally.

I’d seek out a cardiac specialist to fully understand what you’re dealing with.


I didn’t mean to imply with my earlier reply that A-fib is benign! But looking back at the order of previous posts and my attempt at a being brief (not a strong suit of mine) I can see how it looks that way. I was really just trying to put the possible “name to the face” for the “different issue” @StormyDay mentioned.


Hmmmmm. I would also get a second opinion. And, i’d find out exactly what was irregular about the heartbeat, what the vet was finding and how it was recorded. At the very least, consult with equine cardiologist at your state’s vet school. You can likely make an appt and send them records,

My Westfalen would occasionally drop beats when he was a late 3 year old. This was caught by the internist that scoped him for ulcers. The internist stated that she had seen that in some young warmbloods and they usually grew out of it. My horse was an absolute stress case all the time, which likely contributed to the condition. It didn’t slow him down at all and the vet who became his regular vet never mentioned it.

Thank you for all these responses. Over all, they’re hopeful. Yesterday we repeated the original test, using a stethoscope to listen to the mare’s heart at rest and then after lunging. We did this three times. The first time my barn mate, who’s a nurse, thought she heard a tiny hitch at the beginning of the post-exercise heartbeat. Post-exercise the second and third times the heartbeat sounded normal.

My co-owner and I are going to take her to the vet school at the state U. An ECG should give us reliable information. It’s good to know that some young warmbloods may grow out of slight heart irregularities, but we’ll see. I’ll report back.