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Arrhythmias Katharyn Mitchell
COMMISSIONED REVIEW ARTICLE*
MURMURS AND ARRHYTHMIAS IN PERFORMANCE HORSES: WHEN DO WE WORRY ABOUT THESE? PART 2: ARRHYTHMIAS
Katharyn J Mitchell
Cornell University, NY, USA km424@cornell.edu
https://equisense.com/
PART 1 of this paper discussing the practical approach to murmurs in performance horses appeared in the September 2021 issue of EVP.
INTRODUCTION
Cardiac arrhythmias are common findings on physical examination of horses and the difficulty is deciding if you are hearing a normal so called ‘physiological’ arrhythmia such as a 2nd degree atrio-ventricular block (AVB) or a more pathological arrhythmias like atrial fibrillation (AF).1-6
Trying to decide if the arrhythmia is clinically relevant requires experience, interpretation of other historical and physical examination findings and the addition of further diagnostic investigations including echocardiography, ECG analysis (resting and at exercise) and in some cases stress testing under exercising conditions, when some arrhythmias become [most] apparent, and should be evaluated particularly closely.
RECOGNISING AND DESCRIBING ARRHYTHMIAS
When trying to identify the presence of an arrhythmias during a clinical examination, it is important to listen for a reasonable period, at least 30 to 60 sec. Arrhythmias that are paroxysmal or intermittent may be difficult to hear during a short period of auscultation while arrhythmias that occur frequently or are sustained are easier to identify during a clinical examination. The important features used to describe arrhythmias are listed in Table 3 and features of the most diagnosed arrhythmias and their consequences are listed in Table 4.
Table 3. Points to focus on when auscultating to detect arrhythmias
Rate
Is the background heart rate appropriate? Tachycardic (>44 bpm) Bradycardic (<28 bpm) Normal rate (28-44 bpm)
Rhythm
Is the rhythm regular or irregular? If irregular, is there a pattern (regularly irregular) or un-patterned (irregularly irregular)?
Irregular rhythms
Are there beats that are faster (premature) or are there pauses? Are there faster beats followed by pauses? How long are the pauses? Double normal interval? Shorter? Longer? Does the arrhythmia appear to go away at higher or lower heart rates? Do you hear the arrhythmia after exercise?
Table 4. Commonly diagnosed arrhythmias in horses and their hemodynamic consequences.13
Arrhythmia Auscultation Consequence Comments
2nd degree atrio-ventricular (AV) block
Typically heard at low heart rates.
Most often a regularly irregular rhythm (3-4 beats then a pause, twice the normal interval, repeating).
Disappears at higher heart rates (stress, excitement, exercise) but can be heard during the recovery period as the heart rate decelerates. Considered a normal finding in many horses.
If more than three beats are blocked in a row, or every 2nd beat is blocked for long periods, this is considered abnormal and warrants further investigation. Commonly heard after sedation with alpha two agonists. None in normal horses where the 2nd degree AV block disappears at higher heart rates.
If pathological ‘highgrade’ 2nd degree AV block develops and the arrhythmia persists at exercise, then exercise intolerance can be reported.
Atrial fibrillation (AF)
Irregularly irregular rhythm (combination of faster beats, pauses without any clear pattern). Often at rest the heart rate can be normal when averaged over a 3060sec period. Higher than normal heart rates are possible with stress or exercise and the irregularity maybe more difficult to hear. Loss of atrial booster pump function causes a reduction in cardiac output and results in reduced athletic capacity and poor performance.
Inappropriately high heart rates (ventricular response rate) during stress or exercise can result in decreased filling time and poor cardiac output.
Wide complex QRS tachycardia can develop during stress or exercise and this can predispose to more malignant arrhythmias that occasionally result in collapse or sudden death. AF is the most common pathological arrhythmia detected in horses.
It can occur as paroxysmal AF (spontaneous conversion after a short period of time) or can become sustained and require treatment to return to a normal sinus rhythm (NSR).
It is important to distinguish AF that occurs secondary to underlying structural disease (e.g. atrial enlargement) from that occurring spontaneously in a relatively normal appearing heart, so called ‘lone AF’. Reduced athletic capacity and poor performance is commonly reported at higher exercise intensities.
Some horses develop epistaxis if AF occurs during exercise.
If AF occurs secondary to cardiac enlargement, decompensation and congestive heart failure can develop (jugular vein distension/ pulsation, ventral and limb oedema, ascites, pulmonary oedema, respiratory distress).
Premature beats (atrial or ventricular in origin)
A beat that comes earlier than expected. Often followed by a pause before the normal rhythm resumes. The premature beat can sound louder or softer than the normal beats.
It is very difficult to determine the origin of the beat on auscultation. Atrial premature complexes (APC): single APCs are usually of very little hemodynamic consequence, even when occurring at peak exercise, however they act as triggers for the development of AF. Multiple APCs at peak exercise could influence performance.
Ventricular premature complexes (VPC): single VPCs are usually of little hemodynamic consequence at rest. If occurring frequently or in multiples at peak exercise an effect on performance could be appreciated. A VPC could act as a trigger for more malignant ventricular arrhythmias (ventricular tachycardia, R-on-T phenomenon, torsade de pointes etc) that could result in collapse or sudden death. An ECG is required to try and determine the origin of a premature beat.
Premature beats occur infrequently at rest in healthy horses (APCs approx. 1-3/hr, VPCs approx. 1/3hrs) therefore auscultation of premature beats during a physical examination is considered clinically relevant. If the arrhythmia burden is high enough during peak exercise, a limitation on performance is possible.
Premature beats are considered triggers for more complex arrhythmias.
Ventricular arrhythmias pose the biggest concern over safety, particularly when combined with underlying structural heart disease (e.g. as seen in a horse with AR).
Clinical signs
Ventricular tachycardia (VT)
Tachyarrhythmia.
Slow VT (also called accelerated idioventricular rhythm or AIVR) can occur at rates of 50-100 bpm. VT occurs at rates above 100 bpm.
Intensity of beats can fluctuate during auscultation.
Pulse deficits can occur. At higher heart rates, jugular pulsation can be observed. The consequences of AIVR or VT depend on a) the rate and b) underlying cardiac or systemic disease c) degree of cardiac dysfunction.
Typically, clinical signs of poor perfusion occur at rates over 80-100 bpm.
Rapid, multiform (polymorphic) VT, R-on-T phenomenon, or torsade de pointes can rapidly deteriorate into a life-threatening malignant rhythm like ventricular flutter or fibrillation. Both AIVR and VT can occur secondary to many systemic diseases or can occur spontaneously. Treatment is indicated for horses that show clinical signs of poor perfusion, rapid heart rate, multiform QRS complexes or show the presence of R-on-T phenomenon or torsade de pointes. Although occasionally an incidental finding, most horses with AIVR or VT show clinical signs including lethargy, inappetence, colic signs, tachypnoea, jugular distension and/ or pulsation or ventral oedema. Severely affected horses may show weakness, ataxia, or collapse.
ECHOCARDIOGRAPHIC ASSESSMENT AND ECG ANALYSIS
While the clinical examination is critical to alert the practitioner to a problem, it is the additional diagnostic evaluation (echocardiography and ECG analysis) that will provide information about the severity, hemodynamic relevance, and likely rate of progression of any cardiac disease present.
A standardised approach to echocardiography and ECG analysis14; 15 is recommended, to assess all the cardiac structures fully. The specific indications for echocardiography and ECG analysis are listed in Table 5.16 Still images and cineloops should be stored and standardised measurements performed during echocardiography, which will allow comparison of structures over time, particularly important when evaluating the rate of disease progression. Longer duration ECG recordings (often overnight) will be required to determine the importance of intermittent or paroxysmal arrhythmias. Exercising ECGs are essential when assessing the hemodynamic relevance of an arrhythmia and for the risk assessment when determining the safety of a horse during exercise.
Table 5. Clinical findings being indications for echocardiography ± ECG analysis (rest and exercise)16 .
Previously diagnosed ‘functional’ murmur that is louder on serial examinations
Grade 3-6/6 left sided systolic murmur compatible with mitral regurgitation
Grade 3-6/6 left sided diastolic murmur compatible with aortic regurgitation (Grade 2/6 diastolic murmurs when combined with a hyperkinetic pulse)
Grade 4-6/6 right sided systolic murmur compatible with tricuspid regurgitation (in a non-athletic breed, a grade 3/6 should also be investigated)
Any suspected ventricular septal defect or other congenital heart disease
Continuous or combined systolic-diastolic murmurs
Any irregularly irregular arrhythmia
When premature beats are auscultated
When an arrhythmia doesn’t disappear with exercise
In a horse with unexplained tachycardia or bradycardia
In a horse with a history or clinical signs of weakness or collapse
These data on cardiac chamber size, systolic and diastolic function, valvular and chamber morphology, and the presence of any concurrent arrhythmias, assist in the evaluation of identified cardiac abnormalities as a likely cause (or future cause) of poor performance and for risk assessment when it comes to the question about safety.
SUMMARY
Arrhythmias are a common finding in many normal horses, but it is important to recognise those arrhythmias that can result in performance limitation or predispose to an adverse event during exercise.1-3;8 The combination of a murmur and development of a pathological arrhythmia like atrial fibrillation certainly might be very relevant when it comes to performance limitation and should be considered when monitoring for disease progression.
Far less commonly, a rapidly progressing, severe valvular regurgitation can become hemodynamically relevant, resulting in reduced exercise capacity, poor performance and decrease life expectancy.5; 18; 19
Most (but not all) congenital defects are identified earlier in life, and depending on the severity of the defect, some horses can perform and have a normal life expectancy, while others will develop severe clinical signs and present with exercise intolerance or signs of congestive heart failure.10
As practitioners, the key is to recognise cardiac abnormalities on a thorough clinical examination, investigate those meeting the criteria for additional diagnostic evaluation and then closely monitor the horse for any change in performance.
References
Durando, M.M. Cardiovascular Causes of Poor Performance and Exercise Intolerance and Assessment of Safety in the Equine Athlete. Vet Clin North Am Equine 2019;35:175-190.
Martin, B.B., Jr., Reef, V.B., Parente, E.J. et al. Causes of poor performance of horses during training, racing, or showing: 348 cases (1992-1996). J Am Vet Med Assoc 2000;216:554-558.
de Solis, C.N., Althaus, F., Basieux, N. et al. Sudden death in sport and riding horses during and immediately after exercise: A case series. Equine Vet J 2018;50:644-648
Marr, C.M. Equine Acquired Valvular Disease. Vet Clin North Am Equine 2019;35:119-137.
Leroux, A.A., Detilleux, J., Sandersen, C.F., et al. Prevalence and Risk Factors for Cardiac Diseases in a Hospital-Based Population of 3,434 Horses (1994-2011). J Vet Inten Med 2013;27:1563-1570
Patteson, M.W. and Cripps, P.J. A survey of cardiac auscultatory findings in horses. Equine Vet J 1993;25:409-415.
Keen, J.A. Examination of Horses with Cardiac Disease. Vet Clin North Am Equine 2019;35:23-42. Buhl, R., Carstensen, H., Hesselkilde, E.Z., et al. Effect of induced chronic atrial fibrillation on exercise performance in Standardbred trotters. J Vet Intern Med 2018; 32:1410-1419.
Boegli, J., Schwarzwald, CC., Mitchell KJ. Diagnostic Value of Non-invasive Pulse Pressure Measurements in Warmblood horses with Aortic Regurgitation. J Vet Intern Med 2019;33:1446-1455.
Scansen, B.A. Equine Congenital Heart Disease. Vet Clin North Am Equine 2019;35:103-117.
Reef, V.B. Evaluation of ventricular septal defects in horses using two-dimensional and Doppler echocardiography. Equine Vet J 1995;27:86-95.
Decloedt, A. Pericardial Disease, Myocardial Disease, and Great Vessel Abnormalities in Horses. Vet Clin North Am Equine 2019;35:139-157.
Mitchell, K.J. ECG interpretation in Equine Practice. CABI International, Oxford, UK. First edition. 2020
Schwarzwald, C.C. Equine Echocardiography. Vet Clin North Am Equine 2019;35:43-64.
Mitchell, K.J. Equine Electrocardiography. Vet Clin North Am Equine 2019;35:65-83.
Reef, V.B., Bonagura, J., Buhl, R., et al. Recommendations for management of equine athletes with cardiovascular abnormalities. J Vet Intern Med 2014;28:749-761.
Trachsel, D.S., Schwarzwald, C.C., Bitschnau, C., et al. Atrial natriuretic peptide and cardiac troponin I concentrations in healthy Warmblood horses and in Warmblood horses with mitral regurgitation at rest and after exercise. J Vet Cardiol 2013;15:105-121.
Reef, V.B., Bain, F.T. and Spencer, P.A. Severe mitral regurgitation in horses: clinical, echocardiographic and pathological findings. Equine Vet J 1998;30:18-27.
Reef, V.B. and Spencer, P. Echocardiographic evaluation of equine aortic insufficiency. Am J Vet Res 1987;48:904-909.
AAEP INFECTIOUS DISEASE GUIDELINES
AAEP guidelines are created to provide instruction about horse health topics for the practitioner and the equine industry. A full listing is available HERE. Below is access to downloadable PDF guidelines for those diseases pertinent to Aotearoa NZ.
