Hi and welcome to this webinar on myth busting in equine cardiology. So the outline of this session is to discuss some of perhaps the commonly believed myths in equine cardiology relating to auscultation, significance of murmurs and significance of rhythm abnormalities. So I thought we'd start with the first one, which is there's no value in listening to an otherwise well horse at a vaccination.
And I think that we need to appreciate that the horse has got an amazing cardio respiratory reserve, which means there can be some pretty significant abnormalities going on with their heart, and that doesn't mean that there's gonna be any outward clinical signs to the client. Particularly if that horse is competing at a low level. And but the caveat to that being, it doesn't mean that there aren't problems, even if they haven't had demonstrable clinical signs that aren't a threat in terms of safety for ridden work.
So. I think I would urge people to listen to horses at vaccinations or other routine examinations in order to identify normal rate and rhythm and whether there are any cardiac murmurs. The next one is the belief that we can tell whether murmurs are pathological or physiological by auscultating after exercise.
And this was something that got proposed in the literature in the 90s and has sort of been perpetuated as we've gone by. And when we're thinking about murmurs, it's really being cognizant of what causes us to be able to hear that murmur. And it all surrounds turbulent blood flow and as we know, most murmurs in the horse are caused by valvular endocardiosis or degenerative changes in the valves.
Occasionally we hear them with . Infections around the valves or to do with ventricular septal defects, but fundamentally, when we're listening to when we're listening to murmurs, we're listening to turbulent blood flow and that's also true for heart sounds that we hear. Now whether that blood flow is turbulent or laminar is gonna depend a lot on physiologic states and it's gonna depend upon heart rate.
And what we now know with further data that we've got is physiologic murmurs. Can become louder or quieter after exercise, as can pathologic murmurs. The other thing is that for many of us that only do horses, we're in this luxury position that we're usually listening to hearts when that rate is very, very low compared to our small animal colleagues who, you know, might have to encounter cats with heart rates above, or, or small fairies with heart rates above 150 or higher.
And so actually it becomes much more difficult for us to be able to hear those murmurs when the heart rate increases. So be mindful that the most important thing on our physical exam when we're thinking about significance of murmurs is based upon the grade of that murmur at rest rather than what it sounds like after exercise. So then we move to the next point, which is quiet murmurs are unlikely to be a problem.
And in some regards that myth is true depending upon what the cause of the murmur is. So remember that the horse's heart is about the size of a, of a, a 10 litre black bucket, slightly smaller than that, but it's pretty large. And depending where the point of maximum intensity of that murmur is, is gonna depend on where we're listening with our stethoscope is gonna depend how loud that murmur becomes.
And the other point is, it also depends how much tissue. Is between us and that valve as to the loudness of the murmur. And I can remember a very obese cob that had a very small ventricular septal defect, which meant lots of turbulent blood flow.
And should have had a very, very loud cardiac murmur on both the left and right sides, and we could barely hear it because of his barrel shaped chest, but also how obese he was. So this is gonna really depend somewhat on environmental conditions, the shape of the horse, the body condition score, and also what the cause of that murmur is. So I've just touched upon ventricular septal defects and in fact they are the paradigm when we're thinking about about heart heart murmurs.
We expect ventricular septal defects to be really, really loud when they're small. Because if we've got a small hole in the interventricular septum, we're going to have more turbulent blood flow. Whereas the converse is true in this image here.
So this is a 5 year old pony that presented with a. Mild to moderate murmurs, so it was probably about a grade 3 on the left and the right hand sides. And one of the reasons this murmur was so quiet was because of how large that interventricular, septal defect is.
So that you can see here, we've got a measurement, it's probably about half of the diameter of the aorta, which is, which we can see down here. And we've also got an overriding aorta, this pony didn't have a tetralogy of fallow, but he had some of the, he had some tendencies that sort of pushed him towards that. Now this is a pony that within 6 months had gone into heart failure.
That his heart, you can see that, his heart was, not, not normal in terms of its, its dimensions. But that murmur really wasn't that loud. So it's just a caveat to say it isn't always, that isn't always the, the case.
And this is another horse. This is an endurance horse that was competing over quite long distances. And this is a diastolic murmur secondary to aortic regurgitation.
And I hope you can see there's like a little flappy bit on his aortic valve here. And that flappy bit was where he'd likely got a tear in that aortic cusp. Now, er the.
Heart, this is the M mode here of the left ventricle. You can see that this left ventricle is very, very large and has got what we call this end diastolic thinning of the interventricular septum because that regurgitant jet was very, very big. And again, this horse had got a grade 3.
Out of 6, pandiastolic murmur, and I think this is where quiet murmurs become, it becomes relevant. So, you know, a quiet murmur, systolic murmur associated with the tricuspid valve is unlikely to be. Significant, but in fact, a, a diastolic murmur, possibly of any, any level might warrant further evaluation, especially if it's not something you're used to listening to.
And interestingly, this, this aortic, regurgitation case had, its murmur was loudest on the left hand side, still only a grade 3, in fact. But sometimes when the jet's pointing towards the interventricular septum, we're gonna hear that murmur loudest on the right hand side. And again, sometimes.
Sometimes heart sounds can be much more, much quieter on the right because the heart is further from the chest wall. So, I think my, my summary with quiet murmurs are, it's always worth considering that diastolic murmurs, especially in older animals, are significant and probably require further evaluation. And if you suspect a ventricular septal defect and that murmur's quiet, then that isn't always, a good thing.
And then we move to our next myth buster, which is almost the opposite of what we've just discussed, which is loud murmurs are always associated with increased risk. And I think before the advent of echocardiography and, and easy ways that we can do echocardiography in the field, this was often, advice that was given based on. Auscultation to say to people, your horse has got a really loud murmur.
It could be at increased risk of collapse or sudden death, and therefore, I would recommend that you don't ride it. And I think without additional monitoring that we have with echo and with ECG, it wasn't an unreasonable, it wasn't an unreasonable assumption. But now that we are able to appropriately assess and monitor these patients, we know that this isn't necessarily true.
And this comes back, this is a ventricular septal defect actually, it's in a thoroughbred. And this horse had got a grade 4 out of 6, hollow systolic murmur on the left associated with increased blood flow through the pulmonary artery. And you've got a grade 5 out of 6 pansystolic murmur on the right hand side, but hopefully you can see here, as indicated by this little rainbow or proximal flow convergence, this horse had got a very small ventricular septal defect, and when we compare it to his aorta, and it was about 20% of the diameter of his aorta.
So this BSD is very unlikely to be er to be clinically significant and we we followed this horse for a long, long time and he's he's in retirement now in his twenties and it's very unlikely that his er anything to do with his heart will be the reason that he er needs to be er euthanized or dies. So really important and you know we don't see VSD's in thoroughbreds very often. If you listen to this horse, you would assume that he's got very loud mitral and tricuspid murmurs.
And then this is a view, a left sided view of a jet of aortic regurgitation, so the aortic valve is sitting down here and that this is the, this is the jet here. And other than, in certain circumstances, often the severity of aortic regurgitation does correlate with, with the grades of murmur, not always, but, but often. And this is a horse that's got a really, really large jet of aortic regurgitation.
And in fact, he did manage, we did, and and this is actually his, this is actually his valve. This is another horse that's got some, significant structural changes on his aortic valve. And I saw this horse in 2016 and he was a 14 year old Dartmoor pony.
Had got a grade 4 out of 6 hollows, hollow diastolic murmur, and had got that big jet of aortic regurgitation that I've just shown you. He was being used, he was being ridden by a teenager who was hunting and eventing him. And his heart at this point was slightly enlarged, but it wasn't huge.
And in fact, we kept this horse, he, he's still in, in work 9 years later. His heart's sort of enlarged over 1 to 2 years and then it's sort of plateaued. So he doesn't, he doesn't.
Hunted event anymore, but that's more because of his orthopaedic disease than his heart. But he's still in full ridden work and we redid his echo and exercising ECG actually just a few months ago, and he was still absolutely OK for, for ridden work. So, you know, this is one of those cases in the past we probably would have said.
This horse shouldn't be ridden anymore, he's got a really loud murmur. It is associated with his aortic valve, but we've, we've monitored him and you know, he will probably retire for reasons other than his cardiac disease. So loud murmurs don't always have to be career ending, but this could have been a different story.
This could have been a horse that had got an enlarged left ventricle because he's got a loud diastolic murmur secondary to aortic regurgitation, where he did have an ectopic ventricular beats, so to exercise or maybe runs a ventricular tachycardia, which would have put him at increased risk in terms of collapse and sudden death, and we wouldn't know that based on auscultation. And then the next myth is it's not important er to listen on the right hand side. And so the the.
Right sided murmurs associated with the tricuspid valve are, changes to that tricuspid valve and tricuspid regurgitation are less likely to impact on the horse than if you have got left sided murmurs. But remember, sometimes your aortic regurgitation will be loudest on that right hand side. So I thought I'd just give you an example here.
So this is a 3 year old Welsh Section B gelding who presented for bilateral systolic murmurs and polyuria polydipsia. So I'd sort of made my decision that this pony was gonna have likely gonna have a VSD that was gonna be unrelated to his polyuria polydipsia. Anyway, I listened to him and he got pansystolic murmurs on the left and right sides.
They were quite harsh on the left. I did start to think, hm, it's got not necessarily got the normal characteristics I would expect of a, of a VSD. Anyway, I started to scan him.
And my first view I normally get is a four chambered view of the heart, and I looked at it and I was like, oh, that's very interesting because he has got a very, very large right heart, which would not be the chamber we would usually expect to be enlarged with a ventricular septal defect. And this is an M mode of his left and right ventricle, and you can see his right ventricular, sorry, his right ventricle's very big and he's got this sort of dipping of his interventricular septum, suggesting volume and maybe even pressure overload on that right heart. And both his mitral and tri.
Cusspid valves were very abnormal in appearance. Again, suggestive that this pony rather than having a VSD might have some dysplastic changes in his valves. So the fact that he's got a very big right heart could predispose him to develop pulmonary hypertension.
And therefore was something that we needed to monitor over a period of time. Whereas had he had loud murmurs associated with a small VSD that would have been much, much less important. And this was the jet that you've got, you can see he'd got very, very severe tricuspid regurgitation.
And then my next case was an elite national hunt horse. And this horse . I got asked to look at it following a cardioversion with quinidine sulphate that had been performed elsewhere.
And someone asked if I would just have a listen to see whether or not the horse was still in sinus rhythm or whether it was fibrillating again. So I listened on the left hand side and the horse was in sinus rhythm. I couldn't hear, any cardiac murmurs, and then I went to the, Right hand side and this mare had got a grade 5 out of 6 pansystolic murmur that sounded to be like it was associated with her tricuspid valve.
And this is just a view of the right ventricular outflow tract showing she'd got very, very severe tricuspid regurgitation. And this was her heart, and this is a, a short axis view. This is her right ventricle at the top and her left ventricle at the bottom.
And you can see that she's got a pretty big right ventricle. So this was. Somewhat unfortunate because this animal had got an enlarged right heart, secondary to, tricuspid regurgitation, very severe tricuspid regurgitation.
And so it was likely that this valvular disease and the chamber size changes were gonna be an underlying factor in her having developed atrial fibrillation. And they also, of course, increased the likelihood that she. She was going to refibrillate again, particularly when she went back into training and her heart started to, get bigger with, with, in, in that way that it does when you, when you're training, like it would for a marathon.
And this horse never raced again, because I assume she did refibrillate. And it's just worth, noting that probably had you listened on the right and got an echo done, you wouldn't have cardioverted this horse. You would have saved the money and said, you know, probably this is, this is not gonna, not gonna do.
So then the next myth is murmurs can be a cause of poor performance. And the short answer to this is, this is not correct. And the reason for that is that unless your horse has got an increased resting heart rate and is in heart failure, we are not gonna see impacts from murmurs on forward flow.
The caveat to that is, although the murmurs are not likely to be the cause of poor performance. Chamber enlargement and rhythm abnormalities can certainly be. And so horses that have got mal or as I've just shown you, severe tricuspid regurgitation are gonna be at increased risk of developing atrial fibrillation and horses that have got left ventricular enlargement, which again we can see with mitral regurgitation or with aortic regurgitation.
Are gonna be at risk of ventricular ectopic beats. Now, the odd aberrant beat, either ventricular or superventricular, probably isn't gonna be a cause of poor performance, but if we have atrial fibrillation, where at canter and gallop, we're not getting that extra contraction, extra blood moving from the atria into the ventricles, that can certainly impact on poor performance. As can high ventricular rates with lots of ventricular premature depolarization or ventricular tachycardia, which are gonna affect ventricular filling and then forward flow.
So it's not true, it's true to say murmurs on their own are not gonna be a cause of poor performance, assuming the horse isn't in heart failure, but they can can definitely result in. Sometimes a transient er dysrhythmias, at exercise which certainly are can be performance limiting. So this is an example here.
This was a a 12 year old event horse, she was actually out hunting and pulled up at the end of the, of the meet with epistaxis. When she was examined about 2 hours later, this was the underlying rhythm that we'd got. So, she's got an irregular, irregular rhythm.
She's got F waves. There's no P waves. And she's got underlying, atrial fibrillation.
No, most horses with atrial fibrillation, she probably can't go hunting or eventing with this underlying rhythm abnormality, but most of the time that, that will be performance limiting, but it's not gonna have an impact on safety. So this horse, because of her job, needed to be cardioverted, so she underwent DC cardioversion and has never refibrillated over the last 5 years that we have followed her. The next myth is sedating horses with cardiac murmurs poses a significant risk.
And this is a myth. So if the horse has got a, a normal resting heart rate and a normal rhythm. Sedating those horses is er gonna be have minimal risk for the procedures that you want to do.
Now it does somewhat depend what procedure you want to do as to whether you want to do that before or after there's evaluation of that murmur depending on what the horse's situation is and what it does for a living. Now if that horse has also got an underlying rhythm abnormality, it could increase that risk of collapse, but it is very, very rare. So most of the time when you're listening to horses, as long as they've got a normal resting heart rate, sedation isn't gonna cause a problem.
And that's not never, but it is it's very, very unusual that it will, it will be an issue. And then I think the next myth is significant cardiac disease is rare. And I am, I'm often, think about this in terms of, you know, my caseload, which is predominantly hunters, sports horses.
We don't see as many ponies, and it falls into the same category is, well. Maybe ponies don't get as much cardiac disease as horses do. Now that could be true, or it could just be they're the group of animals that we don't ever evaluate.
And I think it depends on what your definition of significant is. So we know that many horses can. Continue their careers with loud cardiac murmurs, and some of them can continue their career with, intermittent or persistent rhythm changes depending on what they do for a living.
But I think that, We probably underestimate how significant cardiac disease is because many times we'll, we are, we either don't evaluate them or we haven't identified those abnormalities. Some of them are not there all the time. Especially with things like paroxysmal atrial fibrillation and.
Actually with things like aortic regurgitation and maybe sometimes mitral regurgitation, if we haven't done further diagnostic tests, we don't know what that animal's rhythm is at exercise. What we do know is that the number of horses. In general practise that .
Die during exercise, we've, we all come across them, every year there will be reports in our caseload that this has, this has occurred, but we don't always know why, why that is. So this is a case I saw not that long ago. This was a six year old homebred warm blood mare, and she had.
The owners had known that she'd had bilateral murmurs for a long time, but that she'd had a normal heart rate. Anyway, she presented because she'd got a cough, and the cough got evaluated very appropriately. And then she develops some edoema in front of her it's sort of inner pectoral region, which is a very classic site for edoema in horses when they go into failure.
So I got to examine her, the resting tachycardia developed around or just after the time that the ventral edoema appeared. And at this point the horse had got a resting tachycardia of around 80 beats per minute, and she'd got bilateral loud systolic murmurs. So this left hand view is a view of her.
Of the four chambers of her heart, 2 atria and 2 ventricles, and hopefully you can see that this left atrium is very, very, very large, as is her left ventricle, and it's working pretty hard. But also on this short axis view, this is her mitral valve, you can actually see really thickened edges that she's got to this mitral valve, but she's also got an enlarged left ventricle with like bowing of her interventricular septum. So she's got to 6 years old and she's sort of coped.
This mare, if you were gonna take a, a punt, would say she had probably again got tricuspid and mitral valve dysplasia, and she's managed to get all the way to 6 before she's gone into heart failure. And she's done exactly what most horses do when they go into heart failure, is that she's got biventricular failure, so she's got abnormalities with both her left and right ventricles. And without a valve replacement, there isn't going to be very much unfortunately that we can, we can do for this horse.
We might be able to palliatively manage her with, with drugs, but she was destined to be an event horse. So, a decision was made at this point that we would euthanize her. And this is just showing you how abnormal.
Play that again, sorry. How abnormal this tricuspid valve is, this is the right side of her heart, but she's got abnormalities just in here of how that tricuspid valve moves. And then the next case is a 17 year old Irish draught mare that is is purchased to be a hunter.
The client had tried the horse, loved her, had taken her hunting on several occasions, and there was no evidence of poor performance or epistaxis. But when the referring vet came to examine her, she'd got an irregularly irregular rhythm with a resting heart rate of 32 beats per minute. And she'd also got a grade 3 out of 6 hollow systolic murmur on the left, consistent with mitral regurgitation, and she'd got a 2 out of 6 hollow systolic murmur on the right, consistent with tricuspid regurgitation.
So these are her measurements that we've got, we've got down here and you can see her left atrial diameter's quite big. I don't know if I've got her ECG but she had got an ECG diagnosis of atrial fibrillation. And she's got a big jet on her tricuspid valve and she had got a big jet here on her, on her mitral valve.
So this is a real quandary because you know, she's going to be a hunter. We've done, we've done an echo, she does have an enlarged left atrium, but she has got a normal sized just about left ventricle, remembering these normal ranges that I've got here are largely for thoroughbreds and she was quite a big, she was quite a big hunter. Her resting ECG diagnosis was atrial fibrillation.
When we did an exercising ECG her heart rate increased more than a normal horse's heart rate would, but it was still, it was still largely er within normal limits, it didn't get too high, she didn't have any bad rhythms at exercise. So she's a really tricky case because we normally would say we wouldn't recommend horses hunt when they've got atrial fibrillation, they need that atrial contraction to support cardiac output. But this horse was showing no signs of, showing no signs of er, poor performance and the client wouldn't have known had, had it not been for the referring vet, having listened to her prior to doing a, a dental.
So This was a a discussion to have, do we know that this horse is at increased risk? No, does she have significant cardiac disease? Yes.
And so in fact the owner has decided to continue hunting her. We will check her again before the next hunt season and see how, how her cardiac disease has progressed, whether she's still got a relatively OK rhythm, it's obviously not normal as she's got atrial fibrillation. And we can then monitor and see and see, you know, how, how long we, we go.
And we've talked about, you know, what the relative risks are in terms of this, but this horse has got significant cardiac disease and you would have had no idea, just from looking at it or by the sound of it, even riding her. So they're my, they're my murmur. I suppose they're, I think they're my, my murmur myths.
So I'm now gonna move on to have a think about some of the rhythm abnormalities. So one of the things that is quite commonly said is that it's easy to distinguish between 2nd degree AV block and atrial fibrillation, especially after exercise. And I truly believe that this is very much a myth.
So. Most of the time when we auscultate horses at rest and we've got second degree AV block, we can hear S4, S1 and S2. So we'll get bum bum, barum bum.
But I bum Oh And so we can hear that S4 in the we can hear S4 if you like in the pause where we get the block. Now, not every horse can we hear an S4, we can probably hear in about 85% of horses. And as many of us know, sometimes we can hear all four heart sounds in the horse, or we can hear 3, S1, S2, and S3.
So remember that S4 is closely related to S1, comes first and is associated with the P wave or atrial contraction. And then S1. Turbulent flow associated with closure of the AV valves, S2 closure of the semi-unar valves, and then S3 associated with passive ventricular filling.
Now, when we've got atrial fibrillation, we are never gonna be able to auscultate S4 because they don't have any atrial contraction. But they do normally have a really loud S1 and they often will have that audible S3. So their rhythm will be bom bom boom, bom bom bom, bom bom bom.
But obviously it's not gonna be regular. Now horses that develop chronic atrial fibrillation will become more regular over time, so that's a caveat for when we're listening at when we're listening at rest. Now the challenge after exercise is the R2 hour intervals get closer together and it comes back to what I said at the beginning.
We aren't all necessarily that practised at listening to cat hearts with those much higher rates. But your rhythm at sorry, your rhythm when the heart rate increases is even when it's irregular at rest, is always gonna sound much, much more regular after exercise. So.
So just be, be mindful of that. The other thing that I wanted to discuss here about atrial fibrillation is always remember that because, er, because these horses can end up with these sort of sympathetic drives, if you go and see a horse that's colicing, that's also got atrial fibrillation. He could have a heart rate of 60, 70 or 80 with a spasmodic colic or with a a displacement.
So just always be mindful that that heart rate in those horses with AF is not always reflective of severity of hypovolemia and severe disease that we would see in a normal horse that is colicing. So then the next myth or not is that cardiac dysrhythmias are a common cause of poor performance. And this is a really tricky one because this was a really .
There have been a few really nice studies that have looked at rhythm abnormalities in normal horses and rhythm abnormalities in horses that are poorly performing. So this was a study, it's quite old now, but it came out of, Newmarket looking at numbers of cardiac dysrhythmias during and after treadmill exercise that has its own challenges. In thoroughbred racehorses with poor performance, so they had 88 horses that were poorly performing.
Lots of them had got upper airway abnormalities. No diagnosis got made in 27 cases. And what they found in this group of horses was that they had one premature beat, primarily in the early part of the recovery phase, in 63% of horses.
Now remember, that early part of the recovery phase is not peak exercise, it's not when they're, they're like, we're likely to be looking for poor performance. And when we look at the prevalence of those different groups, we've got 26% had ventricular beats, 19% superventricular beats, and 17% had both superventricular and ventricular beats. And then during the exercise phase of these group of poorly performing horses, 5% had VPCs, 10% had supraventricular, and 2.3% had just got severe.
They classified them as severe abnormalities. So just keep those numbers in mind. This was another study that looked at er causes of poor performance.
This was a bigger group, this is a slightly older paper that came out of the New Bolton Centre. And it was retrospective. Again, horses were put on a high speed treadmill and evaluated for poor performance.
They got a definitive diagnosis in 74% of horses, lots of them with upper airway disease, and then nearly 10% were classified as having clinically important cardiac dysrhythmias alone. And then there was this group of 6% of them, so 22, that had got both dynamic airway obstruction and cardiac arrhythmias. And then 20 had got poor cardiac fractional shortening immediately after exercise.
And again, this group of horses that had the significant cardiac dysrhythmias, 10% had VPCs, 3% had ventricular tachycardia, and about 3% had superventricular, premature beats. Again, remember those numbers. I think where you've got ventricular tachycardia at exercise, I think we can be pretty sure that could well be a cause of poor performance.
And then I wanted to bring you to this paper, this was also done in Newmarket. And this was looking at cardiac dysrhythmias during submaximal and maximal exercise when horses were being trained in Newmarket on the gallops. And what they found in this group was that dysrhythmias occurred at all stages of exercise, these are.
Two and 3 year old racehorses, very prevalent in warm up and warm down phases, and they found VPCs in 24%, in, in at least one of those phases. So we, we always have it as rest, warm up, peak exercise, and then we usually have two phases, two recovery phases. 4% had VPCs in two phases of that exercise cycle, 1% in 4 out of 5, and 3% had them at peak exercise.
And with SVPDs, about 18% in ways, 4% in two phases, and 1% in 3 out of 5 phases. So overall, of this group of horses, of which there were nearly 100, 45% had one premature depolarization and 8% had both SVPDs and VPDs. So let's just put all of that together.
So we've got the first study I talked about in Newmarket on the treadmill. We're really, when we're thinking about poor performance, most interested in what happens at peak exercise, so about 5% with VPDs, 10% SVPDs, 2% classified as severe. The Martin study, which was the big group of, again, predominantly these are gonna be racehorses, out of the new Bolton centre in Pennsylvania, 10% VPCs, 2.5% VTAC, 2.6% SVPDs.
And then the new market study in normal horses. 3% VPCs and then I did a small study as a student project where we took 30 normal horses, they were predominantly low level sports horses, and we found the same, about 3% of VPCs and about 16% of SVPDs at peak exercise. So, we have to be super careful when we're thinking about rhythm abnormalities of we know we see them in.
Normal horses. So how many is too many, and how many are likely to cause us a problem in terms of either poor performance or where VPCs are concerned, safety for ridden work. And we, we don't have the answer to that, but I think it's really understanding the odd premature beat is unlikely to have a significant impact on performance.
Now whether it has an impact on safety is a very different question. And we do need more data to be able to know more about that. And then the next er question is, horses rarely die during exercise due to cardiac disease.
And I think that as we talked about earlier, many of us can think of a horse, probably in the last year that we've had a client or we've had someone we know say, oh I was out doing a, doing a ride, or I was out hunting and the horse next to me dropped dead. So I think for a very, very long time we'd got this idea that horses probably died during exercise due to rupture of great vessels. And then this was a really nice study as part of Catlile's PhD.
She's got multiple papers that . That made up her PhD looking at reasons for exercise related sudden death in thoroughbreds. Now that maybe we're seeing something quite different when we're considering our our sports horses or our general purpose riding horses, but at least we've got some data on nearly 300 horses that looked at why these horses had died.
They were collected from all around the world, so 6 racing jurisdictions, and sudden death was defined as acute collapse in death in apparently healthy horses during or within 1 hour after exercise. And what they found was these horses all went to boarded pathologists in order to make, to try and come up with a diagnosis. So the pathologist recorded a definitive cause of death in 53% of cases.
Those definitive causes included cardiac failure, apparent pulmonary failure, pulmonary haemorrhage, haemorrhage associated with pelvic fractures or idiopathic blood vessel rupture and spinal cord injury. And they had a presumptive cause of death made in 25% of cases, but death remained unexplained in 22%. So we have got some that are likely to be because of vessel rupture, we've probably got some where they get cardiac failure.
It's always very difficult because every horse. That dies, especially dies suddenly, will often develop those underlying pathologic changes consistent with cardiac failure, they develop pulmonary edoema, they develop other things. And but it it that group of deaths remains unexplained is what's really interesting.
So that was 22% of cases. If they couldn't find. If they couldn't find a, excuse me.
Structural cause. Then it was very likely that that cause of death was functional, and that most likely cause of functional cause of death is going to be some form of cardiac dysrhythmia. So I think when I remembered going through this data in detail, you looked at it and there was probably about 40 or 45% of these cases that died suddenly because of some form of cardiac disease.
So I think that's just worth bearing in mind, and these are horses that are unlikely to have had rhythm abnormalities at rest, which is why really making sure that if possible, if we do have reports of these cases or that we have got. Likely significant murmurs at rest, so greater than grade 3 murmurs on the left and right sides in sports horses and greater than grade 3 or greater than grade 4 for tricuspid murmurs, in race horses, that we should try and do a bit more evaluation because there, there is some risk that these horses could die. Then the last .
Well, last but one actually, I think I've got 12. The last myth is atrial fibrillation is a significant risk factor for sudden death. And this would certainly, this has certainly been proposed by cardiologists, often based on a few anecdotal cases.
I would say we see a lot of atrial fibrillation cases in our population. Some of which we cardiovert, some of which we have evaluated and they continue doing written work, and actually those risk factors for them developing sudden death are rare in our hands. Now this was sort of questioned by, this was a study that came out of the University of Ghent, in over 10 years ago now, where they brought a group of horses to.
A university teaching hospital, and they had found that they had these, what they've classified as ventricular premature depolarization in 80% of horses, 16% of rest, 70% during exercise, 2% during recovery. And I remembered reading this thinking I see quite a lot of warmbloods in my population and I would not see this number of abnormalities. Now as part of their work up, some of these, these horses were er exposed to other animals and other smells that perhaps they wouldn't normally be.
And this was an editorial that got written by Professor Physic Sheard, who's probably the best equine electrophysiologists that there are, and he sort of questioned the methodology behind this. So, the, the comments that were made were. These cases, usually horses with atrial fibrillation that show signs of weakness, incoordination or collapse have usually got other conditions as well as having atrial fibrillation.
There have been concerns in the racing press about safety of horses diagnosed with atrial fib, but no evidence presented of those practical impacts. And that most people who would see a horse with lone atrial fibrillation that haven't got any cardiac murmurs would give a pretty good prognosis without raising major safety issues. Now, what we don't know is there are a small number of horses that do have what I like to call idioventricular rhythms that exercise that we're not going to, that we're not gonna be able to pick up based on simply on, resting auscultation.
Professor physic Sheard worked at Guelph where they saw lots of standardbreds with atrial fibrillation and felt that the, . That one of the reasons that you might see these, we'll call them aberrant beats, is to do, do with changes in sympathetic response. And that they felt that this study that came out of Ghent was probably a reflection of autonomic instability, rather than the atrial fibrillation per se.
And that, that excitement was. The reason why we saw these aberrant beats rather than necessarily the atrial fibrillation per se. And his interpretation was that he wouldn't recommend any change in management based on this paper, but that we probably need more data collection, which to my knowledge we haven't published that yet.
And really thinking about these groups of horses and the risks that they pose. He personally, I think that probably we should be doing exercising ECGs. In horses diagnosed with atrial fibrillation, we might only need to do it once, but we probably should at least look because we do want to not miss that pretty small group in my experience of horses that do have these different rhythm abnormalities, but really interpret those that paper with care.
So I'm gonna come back to Ny Megan, who I told you about earlier. This was her resting ECG she presented literally very very quickly after she had er. Developed epistaxis out hunting.
She'd got no murmurs on er on her cardiac auscultation and she had a normal echocardiogram. And this was her resting ECG, she's got atrial fib, she'd say regularly irregular rhythm. So we cancelled her on the lunge and this was what her rhythm did on the lunge.
So you can see for lunging exercise, her heart rate went up to about 190, which is pretty high. But what was more concerning for me was this sort of RMT phenomenon that we've got. And whether, wherever that originates from, whether it's from the ventricle or whether it's from somewhere else, when you've got a rate as fast as this, it's going to have an impact on cardiac output.
And if I show you a slightly trace over a slightly longer period of time, you can see that this was quite a frequent finding that we had got here. So it's just putting that into perspective. Now I've probably only seen a handful of horses that do this at exercise.
So it would be, I think I looked through our, our data records and I've probably found less than 3% of our horses with atrial fibrillation. That had these abnormal rhythm abnormalities to exercise, but I wouldn't have been able to predict which horses that was, without doing an exercising ECG. So, my summary is that horses do have a huge cardiac reserve, and we do know cardiac failure's really rare compared to what would happen in people or small animals.
And usually, when it occurs, it's secondary to mitral regurgitation. I think we need to be mindful that what we find on auscultation can really help us guide the need for further evaluation, but they don't necessarily tell us about severity or the likely safety risk. My personal views are murmurs greater than grade 3 should be, we should recommend further evaluation, and if we hear a diastolic murmur, then we probably should, we should recommend evaluation for those as well.
It certainly doesn't have to be career ending, and my normal figures I state to clients are 90% of horses can safely continue competing with appropriate monitoring. Now safely is always a strong word because riding horses is a dangerous sport and I never make any promises that I can always guarantee. That are, I'm gonna be able to identify whether a horse is gonna collapse at exercise, but I think we can certainly help clients with what that relative risk might be.
And we really need to understand there are still a lot of knowledge gaps. And with that, I will thank you for your attention, and if you do have any questions, I'm always happy to answer them via email.
