Recent Advances in Diagnosing CardiacAbnormalities With an EKG During Exercise:A Review

Lesley E. Young, BVSc, DVA, DVC, Diplomate ECEIM, PhD, MRCVS

An EKG at rest provides only limited information in equine athletes, but now the technology to recordheart rhythm during exercise is affordable and practical for the veterinarian in the field. Author’saddress: Specialist Equine Cardiology Services, Moat End, Dunstall Green Road, Ousden, Newmar-ket, Suffolk, UK, CB8 8TZ; e-mail: lesleyeyoung@googlemail.com. © 2007 AAEP.

1. Introduction

Cardiac disease is a rare primary cause of poor per-formance in the equine athlete. On the rare occa-sion that performance is affected by cardiac disease,dysrhythmia is the most common underlying cause.1

The technique of taking a resting electrocardiogram(EKG) using a base-apex lead configuration is nowwell established in equine practice2; most equine prac-tices are equipped with both the expertise and thetechnology to perform the task. The method can bevaluable in confirming a diagnosis of sustained ar-rhythmias like atrial fibrillation or in diagnosing theorigin of premature beats. However, a resting EKGusually offers little to the client in establishing a prog-nosis for future use of their horse.

The limitations of a resting EKG arise largelybecause of the enormous cardiac reserve of thehorse,3 which means that performance-limiting car-diac disease or abnormal rhythms during exerciserarely manifest themselves at rest. This is com-pounded by the fact that alterations in resting car-diac rhythm are common in athletic horses becauseof normal high parasympathetic drive,4 yet some of

these rhythms can also occur as a result of cardiacpathology. As a result, the effect of a resting ar-rhythmia on performance or the effect of exercise onan arrhythmia can only be established if an EKGcan be taken during strenuous exercise when sym-pathetic tone and myocardial oxygen demand areincreased and parasympathetic influence is reduced.

In the past, exercising EKG examination, al-though well described in the equine literature, wasrestricted to the specialist cardiologist or the perfor-mance laboratory1; this was largely because of thevery high cost of radiotelemetric EKG equipment.However, the newest technology, based on palm-topand laptop computers or specialized battery-oper-ated devices, has become increasingly affordable forequine veterinarians in Europe. This equipment israpidly becoming standard for equine practitionersin the United Kingdom and is now available inNorth America. As a result, it is timely to reviewthe methodology and potential applications of thenew generation of EKG devices capable of producinghigh-quality exercising electrocardiographs in per-formance horses.

AAEP PROCEEDINGS � Vol. 53 � 2007 99

MEDICINE—EXERCISE

NOTES

This paper details the indications and diagnosticyield of this technique using data from an ambula-tory cardiology practice in the United Kingdom.

2. Materials and Methods

HorsesSeventy-three of a total of one hundred three horseswere referred for cardiovascular examination.These horses came from 22 equine and mixed prac-tices in the United Kingdom between January 2006and February 2007.

MethodologyThe use of limb leads for recording EKG traces is notappropriate during exercise. Wires and electrodesattached to the limbs are poorly tolerated, even atrest, and movement artifact during exercise rendersthem entirely useless. Crocodile clips to make elec-trical contact with the skin are also contraindicatedfor similar reasons. Therefore, limb leads have nopractical purpose in equids. The pattern of depo-larization of the equine ventricle precludes usingmultiple vectors to assess cardiac size and meanelectrical axis, so there is no point to using anythingother than a simple positive-negative lead system torecord cardiac rhythm. As a result, the base-apexlead in which single electrodes are placed above andbelow the heart is used for all equine EKG record-ings, because this lead system produces large com-plexes that are easy to identify.

The conventional base-apex lead placement wherethe positive electrode is placed on the sternum and thenegative electrode is placed on the right jugular furrowis modified slightly for exercising recordings; this re-duces movement artefact that is caused by neck flexionand extension during movement. This more verticalmodification of the familiar system still produces largeQRS deflections, but the atrial deflection (“P” wave) isslightly smaller in amplitude than that of a true base-apex configuration. Nevertheless, the “P” wave willstill be clearly visible during exercise.

Figure 1 shows the placement of four silver/silver-chloride adhesive electrodes that is suitable for re-cording an EKG during ridden exercise.

For four-lead systems, the positive left-arm elec-trode (usually green) is positioned near to the leftcardiac apex, and the negative right-arm electrode(usually red) is placed on the left shoulder area. Thethird Einthoven lead (usually yellow) is also placedcaudal to the cardiac apex. This allows two identicaltracings to be obtained from lead 1 and lead 2 in theEinthoven three-lead system; as a result, a “spare” isproduced in case one of the ventral leads is displaced.The trace in lead 3 that is recorded between the leftarm and left leg will be of no diagnostic value becauseof the close apposition of these electrodes at the cardiacapex. However, a rhythm trace only is required froma horse, so this is not a serious limitation. The fourthearth electrode (usually black), when present, is thenattached on the shoulder close to the negative, right-arm electrode.

The precise positioning of the electrodes is unimpor-tant provided that the positive is below and slightlycaudal to the heart and the negative is above andslightly cranial to it. Ideally, the electrodes shouldremain visible to the rider and/or the examiner, sothey can be reattached easily if they become dislodged.A position where they are least likely to be affected,such as by the saddle or girth or by the rider’s hands orlegs, is obviously ideal. The recording device can beattached to the metal “D” ring just below the pommelof most saddles. For harness racing, the electrodesmust be similarly positioned away from any movingharness straps. The precise location of the ventralelectrodes and the device must often be varied slightlydepending on the style of tack or the rider’s legposition.

EKG Equipment

Traditionally, radiotelelemetric or digital Holtera re-cording systems were used to obtain exercisingtraces. The radiotelelemetric units use a local

Fig. 1. Configuration of electrodes to record an EKG during ridden exercise.

100 2007 � Vol. 53 � AAEP PROCEEDINGS

MEDICINE—EXERCISE

transmitter carried by the horse that continuouslyradio transmits the EKG signal to a local recorder.These units are expensive and also require that therecorder remain within at least 250 m of the exer-cising horse. This can create practical problems formany equine athletes, unless the horse regularlyexercises with a scurry or there is good vehicularaccess to the exercise grounds.

Digital Holter monitors are also effective, but theycan be expensive unless they can also be used forother applications such as 24-h EKG recordings in asmall-animal environment. These devices requirespecialized software for reading and interpretationof the stored EKG data, although data can be ac-quired for days at a time and is especially valuablefor investigating horses with sporadic collapse.Commercial companies or individuals will readthese recordings on a per-case basis, which reducesthe practice’s capital outlay; however, these provid-ers are generally more familiar with small-animaltraces.

The newest technology used to obtain these datais based on personal computers and a digital datalogger.b These units are the most widely appli-cable and affordable for equine practitioners in afield environment. For general practitioners whomight be unfamiliar with interpretation of EKGrecordings during exercise, the digital format ofthe data allows them to be emailed easily for spe-cialists for review.

3. Results

There are many indications for exercising EKG ex-aminations in the equine practice based on the U.K.experience.

From the 103 cases referred for cardiovascularexamination, 73 were deemed to require an exer-cising EKG examination. The type of exercisevaried depending on the function and fitness of thehorse, but in general, every attempt was made toreplicate or slightly exceed the intensity of thehorse’s normal activity. In this series, 52 horseswere exercised under saddle, and 21 were workedon the lunge.

The horses not subjected to exercising EKG ex-amination were horses with clear physical signs ofcardiac failure at rest (tachycardia, tachypnea, ordependent edema; six horses), horses with bacte-rial endocarditis (three horses), horses with low-grade regurgitant murmurs or functionalmurmurs for whom procedure was not deemednecessary (fifteen horses), horses that were notused for ridden work (two unbroken youngstersand one retired horse), and horses affected byconcurrent lameness or other problems that pre-cluded fast exercise (three horses).

In summary, the distribution of cases and condi-tions in which the technique was used and theirdiagnostic yields were as follows:

● There were twelve Thoroughbred horseswith disappointing training or race perfor-mance. The procedure was performed toelucidate whether or not exercise-induced ar-rhythmias such as paroxysmal atrial fibril-lation, a condition with relatively highprevalence in racing Thoroughbreds, waspresent.5 No case was positive for paroxys-mal atrial fibrillation nor were any atrialpremature beats, which are potential trig-gers for the condition, identified in this groupof horses. However, two horses had multi-ple ventricular premature beats during exer-cise and recovery, and four horses had one ormore ventricular premature depolarizationsduring recovery. The latter group was con-sidered to be “normal,”6 and they underwentfurther investigations of their upper airwayfunction. The abnormalities in the formergroup were considered to be potentially sig-nificant, and these horses underwent furthercardiac investigations. An inappropriatelyhigh heart-rate response to exercise occurredin one horse.

● There were nineteen horses with diastolicmurmurs of aortic valve regurgitation. Theprocedure was performed alongside echocar-diography to ensure that ventricular prema-ture beats were not present and that theanimals were not at an increased likelihood ofsudden death.7 A normal EKG was obtainedfrom fourteen of nineteen horses. Ventricularpremature beats during appropriate riddenwork were detected in five cases, all of whichwere then immediately retired. Advanced aor-tic valve regurgitation results in left ventriculardilation and increases cardiac work and after-load, which directly increases myocardial oxy-gen demand. Simultaneously, diastolic aorticpressure progressively decreases as valve dys-function progresses, reducing coronary perfu-sion and myocardial oxygen delivery.Myocardial oxygen demand is further in-creased during exercise, and in advanced casesof aortic valve regurgitation, ventricular isch-emia can lead to ventricular ectopic activityand increase the risk of sudden death or col-lapse during exercise (Fig. 2). These changesare often present before the onset of clinicalsigns of heart failure, and as a result, regularexercising EKGs are mandatory in this groupof patients if they continue to be ridden.8

● There were two horses assessed for the effectof exercise on an abnormal rhythm at rest tobetter determine the prognosis and thehorse’s suitability for ridden work. Onehorse was affected by atrial tachycardia withsecond-degree atrioventricular (AV) block,and another horse had multiple atrial pre-mature beats at rest (Fig. 3). Both wereasymptomatic, and in both cases, cardiac

AAEP PROCEEDINGS � Vol. 53 � 2007 101

MEDICINE—EXERCISE

rhythm normalized during exercise with anincrease in their sinus-node firing rates.

● Ten horses with sustained atrial fibrillationwere assessed. Conversion of atrial fibrilla-tion to normal sinus rhythm is not always in-dicated in pleasure horses. However, beforemaking this decision, it is important to ensurethat their heart-rate response is reasonableand that they do not suffer from uncontrolledsupraventricular tachycardia during exercise.This is crucial to determine if a horse withatrial fibrillation is safe to be ridden withouttreatment or if treatment has failed. In thisgroup, only one horse failed to maintain ac-ceptable heart rates during its usual levels ofwork (Fig. 4).

● Thirty-two horses affected by murmurs of mi-tral- and tricuspid-valve regurgitation wereassessed to ensure that these horses’ heart-rate responses to exercise were normal andthat no abnormal rhythms developed duringexercise. Twenty-two of these horses were ex-

amined after their murmur was detected at apre-purchase or insurance examination. Nosignificant EKG abnormalities were detectedin this group of animals.

4. Conclusion

Given that the horse is endowed with a large cardiacreserve, evaluation of the equine cardiovascular sys-tem and EKG at rest provides only limited informa-tion. An EKG during exercise, however, is anintegral tool in the clinical evaluation of horses pre-sented for episodes of exercise-associated collapse,decreased exercise tolerance, poor athletic perfor-mance, or cardiac murmurs. Recent technologicaladvances now allow this technique to be easily per-formed in the equine practice, and the new devicesare ideally suited to performing resting EKG andlong-term monitoring. Additionally, digital storageof the acquired EKG data allows the traces to beeasily transferred to a specialist cardiologist for in-terpretation, if required.

Fig. 2. The example shown here was taken from a 16-yr-old Thoroughbred during fast work on an all-weather gallop (10 mm/mV; 25mm/s). There had been no change in the clinical characteristics of his grade 4 of 6 diastolic murmur over 3 yr. His resting heart ratewas 32 beats/min with regular second-degree atrioventricular block. The trace shows a triplet of ventricular ectopic beats. Thistriplet occurred at a heart rate of 168 beats/min during hack canter. Multiple episodes of ventricular and junctional ectopic beatsoccurred during this exercise session; however, the horse seemed clinically normal and had no history of poor performance. Becausethis rhythm is a trigger for ventricular fibrillation, the horse was immediately retired from ridden work.

Fig. 3. (A) This resting EKG (25 mm/s; 10 mm/mV) was obtained from a 13-yr-old cob after an abnormal rhythm was noted duringauscultation before his annual vaccination. The horse was otherwise asymptomatic. There are multiple atrial premature beats(arrowed) present at rest. (B) The second trace comes from the same horse during lunging exercise (heart rate � 169 beats/min).The rhythm is now entirely regular, because the sinus-node rate has now overridden the ectopic focus. The ectopic beats onlyreturned 15 min after the horse returned to his stable and his rate fell to �60 beats/min. Exercising EKG in this case allowed afavorable prognosis to be given to the owner, because the rhythm did not deteriorate during exercise and the horse remained in work.

102 2007 � Vol. 53 � AAEP PROCEEDINGS

MEDICINE—EXERCISE

References and Footnotes1. Birks EK, Durando MM, Martin BBJ. Clinical exercise test-

ing: evaluation of the poor performing athlete. In:Hinchcliff KW, Kaneps AJ, Goer RJ, eds. Equine sportsmedicine and surgery: basic and clinical sciences of theequine athlete. Edinburgh: W.B. Saunders Co., 2003;9–19.

2. Durando MM, Young LE. Cardiovascular examination anddiagnostic techniques. In: Robinson NE, ed. Current ther-apy in equine medicine; 571–584.

3. Poole DC, Erikson HH. Heart and vessels: function duringexercise and response to training. In: Hinchcliff KW, KanepsAJ, Goer RJ, eds. Equine sports medicine and surgery. Oxford,UK: Elsevier Science Limited, 2004;699–727.

4. Hamlin RL, Klepinger WL, Gilpin KW, et al. Autonomiccontrol of heart rate in the horse. Am J Physiol 1972;222:976–978.

5. Ohmura H, Hiraga A, Takahashi T, et al. Risk factors for

atrial fibrillation during racing in slow-finishing horses. J AmVet Med Assoc 2003;223:84–88.

6. Martin BB Jr, Reef VB, Parente EJ, et al. Causes of poorperformance of horses during training, racing, or showing:348 cases (1992–1996). J Am Vet Med Assoc 2000;216:554–558.

7. Horn J. Studies on aortic regurgitation in horses. Disser-tation. Royal Veterinary College, London, UK, 2001.

8. Young LE. Diseases of the heart and vessels. In:Hinchcliff KW, Kaneps AJ, Geor RJ, eds. Equine sportsmedicine and surgery: basic and clinical sciences of theequine athlete. Edinburgh: W.B. Saunders Co.,2003;728 –769.

aLifecard CF, Del Mar Reynolds Medical, Irvine, CA 92618.bTelevet 100, Kruuse, DK-5290, Marslev, Denmark.cJørgen KRUUSE A/S, Marslev Byvej 35, DK-5290, Marslev,

Denmark.

Fig. 4. This trace was taken from a 6-yr-old Warmblood gelding used for dressage. An irregularly irregular rhythm wasdiagnosed when the horse was presented for a pre-purchase examination. There was no history of performance problems sincethe gelding had been imported from Holland 1 yr previously. Resting heart rate was 38 beats/min, and resting EKG confirmedthat the irregularly irregular rhythm was caused by atrial fibrillation. The trace, taken during sustained canter, shows that thegelding develops a rapid supraventricular rhythm with a rate of almost 400 beats/min that is sustained for 1.4 s. The complexesare wide and bizarre. This is probably caused by aberrant conduction of the rapid supraventricular rhythm and not aventricular problem. Regardless of the rhythm diagnosis, ventricular filling and myocardial oxygenation would be severelycompromised if this rhythm and rate were sustained for any length of time. Therefore, the horse is not safe for his rider, unlesshis rhythm can be treated, despite the fact that the abnormal rhythm was not adversely affecting his performance. In this case,conversion to normal sinus rhythm was achieved with quinidine sulphatec orally, and the repeat EKG taken during similarstrenuous exercise 18 wk later was entirely normal.

AAEP PROCEEDINGS � Vol. 53 � 2007 103

MEDICINE—EXERCISE