Vetwatch · murmurs and evaluate the severity. Echocardiography is a form of ultrasonographic...
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24 ABSOLUTE HORSE AUGUST 2013 ABSOLUTE HORSE AUGUST 2013 25 hile many horse owners’ hearts will sink on hearing the news “your horse has a heart murmur”, this news is often not quite as bad as it first sounds. The heart is a very efficient organ and minor changes in heart function may have no impact at all on the horse’s quality of life or its exercise tolerance. Horses have amazing cardiovascular systems when compared to their human riders. Every element of the equine cardiovascular is designed for maximum efficiency. Like many features of equine anatomy and physiology, it is all part of their evolution as a flight species when the choice is fight or flight. The purpose of the cardiovascular is to deliver fuel, primarily oxygen to the muscles and other tissues so that they can first support the minute- by-minute processes which keeps the body alive and in the case of the muscles, oxygen is the fuel that can propel the horse at running speeds of up to 55mph, the maximum speed it is claimed an American Quarter horse can reach. Oxygen reaches the blood stream via the lungs. The red blood cell numbers and function determine the oxygen carrying capacity of the blood; equine athletes (hot- blooded types) will typically have a higher red blood cell count than the more sedentary breed (cold bloods). But horses have a natural ability to increase their red blood cell numbers when needed; the spleen acts as a reservoir of red blood cells and, under the influence of adrenaline and its related compound, nor-adrenaline, the capsule of the spleen can contract to inject large numbers of red blood cells into circulation. Humans do not have this capacity and some sportsmen have been tempted to increase their red blood cell counts illegally by storing their own blood during breaks between competitions.This is then re- injected by the athlete pre-race - so called blood doping.Horses do their own blood doping naturally whenever they exercise. Then we come to the pump, the heart. The horse’s heart is an amazing and massive organ. In Thoroughbreds it represents about 1% of its body weight. Heart size is related to its maximal output and there are several examples of superlative Thoroughbred athletes who have exceptionally large hearts. Eclipse, Phar Lap and Secretariat were all believed to have had unusually large hearts. The horse has a very wide range in its heart rate: most horses have a resting heart rate of around 30-40 beats per minute, at the trot this rises to around 100 but at maximal gallop the heart rate may be as high as 230 beats per minute. With each beat around 900-950mls of blood leaves the heart such that at its maximum, the equine heart is capable of pumping an amazing 250-280 litres of blood per minute. The heart comprises four chambers: there are two ventricles, one serving the right side and the other the left side of the circulation. These are the main pumping chambers; the left one pumps blood out to the body while the right ventricle pumps blood towards the lungs. When the ventricles relax, blood rushes in to fill them passively and the other two chambers, the atria serve as pump primers, making sure their respective ventricles are filled effectively. The main blood vessel that carries blood out of the left ventricle to the body is called the aorta while the vessel that carries blood from the right ventricles to the lungs is the pulmonary artery. The aorta connects to the arteries of the body that are then filled with blood that is rich in oxygen, the small blood vessels within the muscle and other tissues are the capillaries. These are thin-walled and thus allow oxygen to diffuse out of the blood stream towards the cells. Once depleted of oxygen, blood leaves the capillary beds to return towards the heart in the veins. Veins from the body return to the right side of the heart, from where blood can then go to the lungs to replenish the red blood cells stores of oxygen. Veins from the lungs carry blood that is rich in oxygen to the left side of the heart and from there, out to the body for fuel. What is a heart murmur? Heart valves sit between each atrium and its respective ventricle and between the ventricle and the major vessel it supplies. There are four heart valves and these are thin sheets of tissue that open and close to allow the heart to fill and then empty at the appropriate time. As the valves are opening and closing, heart sounds are generated. The term heart murmur simply refers to any extra sound that occurs in addition to the normal heart sounds. One of the most important features of horses’ hearts from a veterinary point of view is that heart murmurs commonly occur in horses with completely normal hearts. The heart is large and powerful and blood flowing through a heart with an entirely normal structure and function can generate a sound which is picked up with a stethoscope. We don’t often hear that process in our small animal patients – the parallel is that standing next to a large river, you will not be surprised to hear water flowing but next to a small stream, flow might not be audible. Vets use various terms to describe this form of heart murmur but “physiological” or “flow” murmur is often mentioned. The second commonest cause of heart murmurs is leaking, or regurgitation, in one or more of the heart valves. You might expect that heart valves would form a tight seal allowing blood to flow in one direction only. But echocardiographic imaging has revealed that actually, at rest, equine heart valves can often have small leaks. This is particularly so in racehorses and large scale studies have shown that these leaks and the murmurs they are associated with have no impact on racing performance. On the other hand, leaks can be due to disease in the heart valves. There are various possible disease processes that can affect the valves, but the most common is due to degeneration linked to the ageing process. Over many months and years, the heart valves can gradually become a little thickened, particularly towards their edges and this affects the seal that they may and allows progressively more and more blood to leak, or regurgitate through the valve. This may all sound rather alarming but although the heart can fail, in fact most horses that have degenerative valvular regurgitation are not very severely affected. There is a steady increase in the prevalence of this form of murmur with increasing age such that they are found in around 3% of horses aged less than 7 years; 8% of horses aged 8–14 years, around 14% of horses aged 15– 23 years and about 15% of those older than 24 years. The good news is that degenerative valvular disease rarely affects the horse’s lifespan and a study of a group of over 1100 horses in south east England showed that horses with these murmurs were no more likely to die than those without them. Most of the horses with cardiac murmurs eventually succumbed to non-cardiac related illnesses. Diagnosis of heart murmurs So, the statistics are on the horse’s side, but how can a vet tell whether a heart murmur is significant or not? Armed with a stethoscope alone, the veterinary practitioner can often rule out serious heart disease in many horses. They do this by carefully listening to the heart murmur, assessing its position relative to the structures of the heart and considering its timing: specific forms of heart disease are associated with characteristic murmurs in terms of both timing and position. The loudness or grade of the murmur is also taken into account, with louder murmurs being more likely to be ROSSDALES EQUINE HOSPITAL & DIAGNOSTIC CENTRE Cotton End Road, Exning, Newmarket, Suffolk CB8 7NN. www.rossdales.com Vetwatch Vet Profile Name: Celia M. Marr Qualifications: BVMS, MVM, PhD, DEIM, DipECEIM, MRCVS Year of Qualification: 1985 Celia joined the team at Rossdales Equine Hospital and Diagnostic Centre in 2003. She is a Specialist in Equine Internal Medicine and works with both inpatients and outpatients with medical problems. She graduated from the Glasgow University Veterinary School in 1985, then remained in Glasgow to complete both Masters and PhD degrees. She then held a Fulbright Scholarship studying equine cardiology and internal medicine at the New Bolton Center, University of Pennsylvania. Before joining Rossdales, she held positions at the University of Cambridge Veterinary School, Valley Equine Hospital, Lambourn and the Royal Veterinary College. Her clinical and research interests are in Cardiovascular Medicine, Internal Medicine, Adult and Neonatal Intensive Care & Medical Imaging. She has published over 50 research papers and educational material relating to a range of medical disorders of the horse, concentrating on cardiovascular disease and diagnostic methods in medical disorders including editing a book on Cardiology of the Horse, the 2nd edition of which was published in 2010. Celia is an Honorary Professor of the University of Glasgow, Editor- in-Chief of Equine Veterinary Journal, Chairman of the European College of Equine Internal Medicine’s Advanced Training Advisory Committee, Deputy Chairman of the Veterinary Advisory Committee of the Horserace Betting Levy Board and Chairman of the HBLB’s Thoroughbred Research Consultation Group. W Rossdales Equine Hospital & Diagnostic Centre (All horse admissions) Cotton End Road, Exning, Newmarket, Suffolk CB8 7NN. Tel: 01638 577754 (Office hours); Tel: 01638 663150 (24 hours) Email: [email protected] Rossdales Equine Practice (Ambulatory Practice, Pharmacy and Accounts) Beaufort Cottage Stables, High Street, Newmarket, Suffolk CB8 8JS. Tel: 01638 663150 (24 hours) Email: [email protected] problematic and thus justify investigation. Echocardiography is the primary tool used to identify the cause of murmurs and evaluate the severity. Echocardiography is a form of ultrasonographic imaging that provides a two-dimensional image of the heart and its internal structures. This is coupled with a technique called Doppler echocardiography that allows the speed and direction of blood flowing through the heart to be documented. It is based on the Doppler shift, which is a physical principle whereby the frequency of sound is altered when it is reflected off a moving structure. It is the same principle used by CONTINUED OVER PAGE Matters of the heart: diagnosing and assessing heart murmurs Horses can be born with holes in their hearts: also known as a ventricular septal defect. The hole allows blood to flow between the chambers. If the hole is small, it may not affect the horse, but if it is large, as in this example, the horse’s performance will be seriously affected. Notice in this young horse, the healthy valves are thin sheets of tissue. The heart of a 30 year-old gelding which has developed thickening of the mitral valve due to degenerative valvular disease. This valve sits between the left ventricle, the main pumping chamber supplying the body and the left atrium. Because blood is flowing backwards (regurgitating) through the mitral valve for many years, the walls of the atrium have developed scars on their surface, the heart has enlarged and eventually heart failure developed. An echocardiogram that shows a moderately large leak at the aortic valve. The Doppler technique displays blood flowing in an abnormal direction in a mosaic of yellow and turquoise. The size of the leak gives an indication of its severity. This image was recorded from a 13 year-old riding horse that had a murmur but no other problems. An echocardiogram that shows a very small leak in the tricuspid valve, indicated by the arrow. The tricuspid valve sits between the right atrium (RA) and the right ventricle (RV). This leak was found in an advanced Eventer and it does not affect the horse’s performance at all. The image also shows how the echocardiography unit is able to “slice” through the heart and show the structures on the left ventricle (LVOT) and Aorta (AR). By Professor Celia M. Marr MRCVS
Transcript of Vetwatch · murmurs and evaluate the severity. Echocardiography is a form of ultrasonographic...
24 ABSOLUTE HORSE AUGUST 2013 ABSOLUTE HORSE AUGUST 2013 25
hile many horseowners’ hearts willsink on hearing the
news “your horse has a heartmurmur”, this news is often notquite as bad as it first sounds.The heart is a very efficientorgan and minor changes inheart function may have noimpact at all on the horse’squality of life or its exercisetolerance.Horses have amazingcardiovascular systems whencompared to their human riders.Every element of the equinecardiovascular is designed formaximum efficiency. Like manyfeatures of equine anatomy andphysiology, it is all part of theirevolution as a flight species whenthe choice is fight or flight. Thepurpose of the cardiovascular is todeliver fuel, primarily oxygen to themuscles and other tissues so thatthey can first support the minute-by-minute processes which keepsthe body alive and in the case ofthe muscles, oxygen is the fuel thatcan propel the horse at runningspeeds of up to 55mph, themaximum speed it is claimed anAmerican Quarter horse can reach. Oxygen reaches the blood streamvia the lungs. The red blood cellnumbers and function determinethe oxygen carrying capacity of theblood; equine athletes (hot-blooded types) will typically have ahigher red blood cell count thanthe more sedentary breed (coldbloods). But horses have a naturalability to increase their red bloodcell numbers when needed; thespleen acts as a reservoir of redblood cells and, under theinfluence of adrenaline and itsrelated compound, nor-adrenaline,the capsule of the spleen cancontract to inject large numbers ofred blood cells into circulation.Humans do not have this capacityand some sportsmen have beentempted to increase their red bloodcell counts illegally by storing theirown blood during breaks betweencompetitions.This is then re-
injected by the athlete pre-race -so called blood doping.Horses dotheir own blood doping naturallywhenever they exercise. Then we come to the pump, theheart. The horse’s heart is anamazing and massive organ. InThoroughbreds it representsabout 1% of its body weight.Heart size is related to its maximaloutput and there are severalexamples of superlativeThoroughbred athletes who haveexceptionally large hearts. Eclipse,Phar Lap and Secretariat were allbelieved to have had unusuallylarge hearts. The horse has a very wide range inits heart rate: most horses have aresting heart rate of around 30-40beats per minute, at the trot thisrises to around 100 but atmaximal gallop the heart rate maybe as high as 230 beats perminute. With each beat around900-950mls of blood leaves theheart such that at its maximum,the equine heart is capable ofpumping an amazing 250-280litres of blood per minute.
The heart comprises fourchambers: there are twoventricles, one serving the rightside and the other the left side ofthe circulation. These are themain pumping chambers; the leftone pumps blood out to the bodywhile the right ventricle pumpsblood towards the lungs. Whenthe ventricles relax, blood rushesin to fill them passively and theother two chambers, the atriaserve as pump primers, makingsure their respective ventricles arefilled effectively. The main blood vessel that carriesblood out of the left ventricle tothe body is called the aorta whilethe vessel that carries blood fromthe right ventricles to the lungs isthe pulmonary artery. The aortaconnects to the arteries of thebody that are then filled withblood that is rich in oxygen, thesmall blood vessels within themuscle and other tissues are thecapillaries. These are thin-walledand thus allow oxygen to diffuseout of the blood stream towardsthe cells. Once depleted of
oxygen, blood leaves the capillarybeds to return towards the heartin the veins. Veins from the bodyreturn to the right side of theheart, from where blood can thengo to the lungs to replenish thered blood cells stores of oxygen.Veins from the lungs carry bloodthat is rich in oxygen to the leftside of the heart and from there,out to the body for fuel.
What is a heart murmur?Heart valves sit between eachatrium and its respective ventricleand between the ventricle and themajor vessel it supplies. There arefour heart valves and these arethin sheets of tissue that openand close to allow the heart to filland then empty at theappropriate time. As the valves areopening and closing, heart sounds
are generated. The term heartmurmur simply refers to any extrasound that occurs in addition tothe normal heart sounds. One of the most importantfeatures of horses’ hearts from aveterinary point of view is thatheart murmurs commonly occurin horses with completely normalhearts. The heart is large andpowerful and blood flowingthrough a heart with an entirelynormal structure and function cangenerate a sound which is pickedup with a stethoscope. We don’toften hear that process in oursmall animal patients – theparallel is that standing next to alarge river, you will not besurprised to hear water flowingbut next to a small stream, flowmight not be audible. Vets usevarious terms to describe thisform of heart murmur but“physiological” or “flow” murmuris often mentioned.The second commonest cause ofheart murmurs is leaking, orregurgitation, in one or more ofthe heart valves. You might expectthat heart valves would form atight seal allowing blood to flowin one direction only. Butechocardiographic imaging hasrevealed that actually, at rest,
equine heart valves can oftenhave small leaks. This isparticularly so in racehorses andlarge scale studies have shownthat these leaks and the murmursthey are associated with have noimpact on racing performance. On the other hand, leaks can bedue to disease in the heart valves.There are various possible diseaseprocesses that can affect thevalves, but the most common isdue to degeneration linked to theageing process. Over manymonths and years, the heartvalves can gradually become alittle thickened, particularlytowards their edges and thisaffects the seal that they may andallows progressively more andmore blood to leak, or regurgitatethrough the valve. This may all sound ratheralarming but although the heartcan fail, in fact most horses thathave degenerative valvularregurgitation are not very severelyaffected. There is a steady increasein the prevalence of this form ofmurmur with increasing age suchthat they are found in around 3%of horses aged less than 7 years;8% of horses aged 8–14 years,around 14% of horses aged 15–23 years and about 15% of those
older than 24 years. The goodnews is that degenerative valvulardisease rarely affects the horse’slifespan and a study of a group ofover 1100 horses in south eastEngland showed that horses withthese murmurs were no morelikely to die than those withoutthem. Most of the horses withcardiac murmurs eventuallysuccumbed to non-cardiac relatedillnesses.
Diagnosis of heartmurmursSo, the statistics are on the horse’sside, but how can a vet tellwhether a heart murmur issignificant or not? Armed with astethoscope alone, the veterinarypractitioner can often rule outserious heart disease in manyhorses. They do this by carefullylistening to the heart murmur,assessing its position relative tothe structures of the heart andconsidering its timing: specificforms of heart disease areassociated with characteristicmurmurs in terms of both timingand position. The loudness orgrade of the murmur is also takeninto account, with loudermurmurs being more likely to be
ROSSDALES EQUINE HOSPITAL & DIAGNOSTIC CENTRECotton End Road, Exning, Newmarket, Suffolk CB8 7NN. www.rossdales.comVetwatch Vet Profile
Name: Celia M. MarrQualifications: BVMS, MVM,PhD, DEIM, DipECEIM, MRCVSYear of Qualification: 1985
Celia joined the team at RossdalesEquine Hospital and DiagnosticCentre in 2003. She is a Specialistin Equine Internal Medicine andworks with both inpatients andoutpatients with medicalproblems.She graduated from the GlasgowUniversity Veterinary School in1985, then remained in Glasgowto complete both Masters and PhDdegrees. She then held a FulbrightScholarship studying equinecardiology and internal medicineat the New Bolton Center,University of Pennsylvania. Beforejoining Rossdales, she heldpositions at the University ofCambridge Veterinary School,Valley Equine Hospital, Lambournand the Royal Veterinary College.Her clinical and research interestsare in Cardiovascular Medicine,Internal Medicine, Adult andNeonatal Intensive Care & MedicalImaging. She has published over50 research papers andeducational material relating to arange of medical disorders of thehorse, concentrating oncardiovascular disease anddiagnostic methods in medicaldisorders including editing a bookon Cardiology of the Horse, the2nd edition of which waspublished in 2010.Celia is an Honorary Professor ofthe University of Glasgow, Editor-in-Chief of Equine VeterinaryJournal, Chairman of the EuropeanCollege of Equine InternalMedicine’s Advanced TrainingAdvisory Committee, DeputyChairman of the VeterinaryAdvisory Committee of theHorserace Betting Levy Board andChairman of the HBLB’sThoroughbred ResearchConsultation Group.
W
Rossdales Equine Hospital & Diagnostic Centre(All horse admissions)
Cotton End Road, Exning, Newmarket, Suffolk CB8 7NN. Tel: 01638 577754 (Office hours); Tel: 01638 663150 (24 hours)Email: [email protected]
Rossdales Equine Practice(Ambulatory Practice, Pharmacy and Accounts)
Beaufort Cottage Stables, High Street, Newmarket, Suffolk CB8 8JS. Tel: 01638 663150 (24 hours) Email: [email protected]
problematic and thus justifyinvestigation.Echocardiography is the primarytool used to identify the cause ofmurmurs and evaluate theseverity. Echocardiography is aform of ultrasonographic imagingthat provides a two-dimensionalimage of the heart and its internalstructures. This is coupled with atechnique called Dopplerechocardiography that allows thespeed and direction of bloodflowing through the heart to bedocumented. It is based on theDoppler shift, which is a physicalprinciple whereby the frequency ofsound is altered when it isreflected off a moving structure. Itis the same principle used by
CONTINUED OVER PAGE
Matters of the heart: diagnosing and assessing heart murmurs
Horses can be born with holes in their hearts: also known as a ventricular septal defect. The hole allows blood to flowbetween the chambers. If the hole is small, it may not affect the horse, but if it is large, as in this example, the horse’sperformance will be seriously affected. Notice in this young horse, the healthy valves are thin sheets of tissue.
The heart of a 30 year-old gelding which has developed thickening of themitral valve due to degenerative valvular disease. This valve sits betweenthe left ventricle, the main pumping chamber supplying the body and theleft atrium. Because blood is flowing backwards (regurgitating) through themitral valve for many years, the walls of the atrium have developed scars ontheir surface, the heart has enlarged and eventually heart failure developed.
An echocardiogram that shows a moderately large leak at the aortic valve.The Doppler technique displays blood flowing in an abnormal direction in amosaic of yellow and turquoise. The size of the leak gives an indication of itsseverity. This image was recorded from a 13 year-old riding horse that had amurmur but no other problems.
An echocardiogram that shows a very small leak in the tricuspid valve,indicated by the arrow. The tricuspid valve sits between the right atrium(RA) and the right ventricle (RV). This leak was found in an advancedEventer and it does not affect the horse’s performance at all. The image alsoshows how the echocardiography unit is able to “slice” through the heartand show the structures on the left ventricle (LVOT) and Aorta (AR).
By Professor Celia M. Marr MRCVS
26 ABSOLUTE HORSE AUGUST 2013
Vetwatch
CONTINUED FROM PREVIOUS PAGE
Police speed traps.In a horse with valvular regurgitation,echocardiography will demonstrate howthickened the valve has become and howlarge the leak is. The heart size will also betaken into account because just like anyother muscle, the heart will tend to getlarger if it is working harder. Up to a point,this can be a good thing, but if the heartbecomes excessively large, it will begin toloose its efficiency.Valvular regurgitation is not the only causeof murmurs. Horses can be born with holesin their heart, just as children can. Withsmall holes, the horse can be unaffected sothat the hole is only picked up when thehorse or pony has already started use as ariding animal. Occasionally foals are born with majorstructural deformations that sadly willleave them unlikely to survive. When thereis serious heart disease, the horse, pony orfoal may be showing signs of lethargy,particularly with exercise, it may becomebreathless and sometimes, because thecirculation is failing, the veins may become
distended and fluid swellings may developaround the sheath and lower aspect of theabdomen. Fortunately, it is a very uncommon forhorses to develop severe heart disease, butif a horse owner does become worried thattheir horse might have heart disease, thefirst step is to have a vet listen to the heartso that they can either reassure you that allis well, or organise further investigations ifappropriate.
An echocardiogramthat shows bloodflowing through ahole in the heart, aventricular septaldefect.
This middle-aged gelding has developedswelling of his sheath, distension of the veinsand a fluidy, oedematous swelling along thelower aspect of his abdomen. He has heartfailure due to severe valvular regurgitationaffecting all four heart valves.
This Welsh pony foal wasborn with multiplecongenital cardiac defects.It is depressed and poorlygrown with a distendedabdomen due to fluidaccumulation.
