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Cardiac dysrhythmias in horses Celia M Marr [email protected] Control of heart rate at rest • Equine resting heart rate is low: 25 - 42 bpm • Horses have very little or no sympathetic tone at rest • Parasympathetic tone is high • The vagus innervates both the sinoatrial and the atrioventricular nodes • The horse can reduce its heart rate by delaying conduction at the atrioventricular node 3 © cmarr 2013 SECOND DEGREE ATRIOVENTRICULAR BLOCK • • • First: • closure of the atrioventricular valves • opening of the semilunar valves Second: • closure of the semilunar valves • opening of the atrioventricular valves Fourth: • atrial systole Le-lub-dub 4 Le-lub-dub © cmarr 2013 Le Le-lub-dub ECG Interpretation 5 SECOND DEGREE ATRIOVENTRICULAR BLOCK P QRST P QRST P P P P Sinus rhythm restored during exercise © cmarr, 2013 6 QRST P QRST Pathogenesis of atrial fibrillation RE-ENTRY Requires an area of Unidirectional block UNIFORM REFRACTORY PERIODS CRITICAL ATRIAL MASS © cmarr, 2013 VARIABLE REFRACTORY PERIODS Pathogenesis of atrial fibrillation Normal heart: high vagal tone maintains low heart rate Intermittent conduction through atrioventricular node Irregular R-R interval with f waves © cmarr, 2013 8 Significant underlying heart disease: increased sympathetic tone: high ventricular rate Pathogenesis of atrial fibrillation CRITICAL ATRIAL MASS VARIABLE REFRACTORY PERIODS • High vagal tone • normal horses • Myocardial pathology • Electrolyte disturbances disease • Normal horses • Atrial enlargement • Mitral or tricuspid regurgitation • Congenital heart disease 9 © cmarr 2013 ATRIAL FIBRILLATION: PRESENTING SIGNS • Atria contribution to ventricular filling = 15% • With no other cardiac disease, AF will only cause exercise intolerance if in vigorous exercise • Racehorses, Eventers, some hunters • Can be an incidental finding • Breeding stock, hacks, some show jumpers 10 © cmarr 2013 ATRIAL FIBRILLATION • Exercise intolerance/poor performance • Reluctance to participate in exercise • Irregularly-irregular cardiac rhythm • intermittent long pauses interspersed with rapid runs • no fourth heart sound • loud third heart sound • Variable pulse quality • Variable intensity of heart sounds • Exercise-induced pulmonary haemorrhage 11 © cmarr 2013 PAROXYSMAL ATRIAL FIBRILLATION Ohmura H et al, JAVMA, 2003, 223, 84-88 • gastrointestinal disease (ventricular arrhythmias more common) • exercise-induced • minimum frequency = 0.03% (no. of episodes/no.of starts) minimum prevalence = 0.29%(no. of horses with AF/no. of horses) • prevalence in slow/non-finishing horses = 1.39% • recurrence rate = 6.1% (10 year study, 115 horses, 108 single episode, 6 two episodes, 1 three episodes • 92% resolved spontaneously within 24 hours without treatment, • 12 2 with 15-20g QS at 24 hours, 2 spontaneously within 48 hours, 4 with 15-20g QS within 48-120 hours © cmarr 2013 Sustained Atrial Fibrillation Diagnostic Approach Goal • identify and characterise any underlying cardiac disease • clinical exam • echocardiography • clinical pathology 13 © cmarr 2013 • Atrial fibrillation is an effect and not a cause of congestive heart failure 14 © cmarr 2013 Clinical Findings that might indicate underlying heart disease: • tachycardia • heart rate > 50 bpm • cardiac murmurs • particularly murmurs of AV regurgitation • venous distension and pulsation • peripheral oedema 15 © cmarr 2013 CLINICAL DECISION-MAKING Incidental Finding Athlete with exercise intolerance Heart failure Cardioversion Palliative furosemide digoxin ACE inhibitors No treatment 16 © cmarr 2013 Quinidine • Class 1A sodium channel blocker • Prolongs the effective refractory period • Decreases myocardial conduction velocity & prolongs QRS and QT intervals • • • • 17 Vagolytic Alpha-adrenergic antagonist Proarrhythmic Gastrointestinal ulceration © cmarr 2013 18 © cmarr 2013 Quinidine Sulphate 6 4 2 0 0 2 4 6 8 10 12 14 16 18 20 22 • Administer by nasogastric tube until either normal sinus rhythm restored or serious side-effects occur © cmarr 2013 Extra-cardiac side effects • • • • • • • respiratory stridor penile protrusion neurological deficits flatulence diarrhoea colic tend to be seen in horses that require prolonged treatment • GI effects are the main cause of treatment failure 20 © cmarr 2013 Cardiovascular side-effects • hypotension • rapid supraventricular tachycardia • ventricular arrhythmias • both potentially fatal arrhythmias • independent of plasma concentrations • monitor the ECG continuously 21 © cmarr 2013 Hypotension ALPHA ADRENERGIC ANTAGONIST 22 • • keep the horse calm • iv fluids (or phenylephrine) may be necessary do not allow any form of exercise during treatment © cmarr 2013 Ventricular Arrhythmias PROARRHYTHMIC EFFECT • Magnesium sulphate • Propanolol • Lignocaine • NOT Procainamide (same class as quinidine) 23 © cmarr 2013 Atrial fibrillation: Pre-treatment Supraventricular tachycardia: ventricular rate around 200 Supraventricular tachycardia with variable conduction following digoxin Normal sinus rhythm 24 © cmarr 2013 RAPID SUPRAVENTRICULAR TACHYCARDIA VAGOLYTIC EFFECT • Emergency treatment required if ventricular rate > 120 /min and rising • • • 25 Digoxin Propanolol Diltiazem © cmarr 2013 Can we predict which horses will develop major problems during quinidine treatment? IN ADVANCE • Tachyarrhythmias • not really - appears to be an individual horse, idiosyncratic response • GI and other extra-cardiac side effects • prolonged treatment duration, usually horses with under-lying heart disease and/or longstanding AF 26 © cmarr 2013 Can we predict which horses will develop major problems during treatment? DURING TREATMENT • Prolonged QRS duration • Ventricular premature depolarisation • Long QT and ST segment changes 27 © cmarr 2013 How many horses develop major problems during quinidine treatment? • Depends on case type • dose and duration of treatment are influenced by AF duration and underlying heart disease • Variable population including older animals around 40% have side-effects • Young, racing STD/TBs have problems less commonly • Life threatening arrhythmias: 4 - 8% • Diarrhoea: 15 - 25 % 28 © cmarr 2013 Electrical Cardioversion RA RPA LA LPA Frye et al, 2002, J. Am Vet Med Assoc, 2002; 220: 1039-1045. McGurrin MK, Physick-Sheard PW, Kenney DG.J Vet Intern Med 2008;22:609-615. McGurrin MK, Physick-Sheard PW, Kenney DG, et al. J Vet Intern Med 2005;19:695-702. McGurrin MK, Physick-Sheard PW, Kenney DG, et al. J Vet Intern Med 2003;17:715-718. De Clercq D, van Loon G, Schauvliege S, et al. Vet J 2008;177:198-204. 29 © cmarr 2013 McGurrin, KJ, Physick-Sheard, PW, Kenney, DG (2005) How to perform transvenous electrical cardioversion in horses with atrial fibrillation J. Vet. Cardiol. 7, 109-119 RPA LPA 30 © cmarr 2013 31 © cmarr 2013 Side-effects: fatal arrhythmias • Ventricular fibrillation • Third degree AV block 32 © cmarr 2013 Atrial Fibrillation and poor performance Onset within last 48 hours Monitor 33 Onset >48 hours, < 3 - 4 months normal echocardiogram Quinidine sulphate © cmarr 2013 Uncertain or prolonged duration, mild heart disease Quinidine failure 1. DC conversion, 2. alternative anti-dysrhythmics Long-term Prognosis Depends on • degree of underlying cardiac disease • duration prior to treatment • less than 3 months • recurrence rate 15% • greater than three months • recurrence rate 60% • higher prevalence of side-effects associated with prolonged duration of treatment 34 © cmarr 2013 Does sustained atrial fibrillation cause collapse or death during exercise? 35 • Typically, horses with sustained atrial fibrillation do NOT collapse • Sudden onset (paroxysmal) AF can be associated with collapse or severe distress • Some horses with sustained AF have exerciseinduced ventricular tachycardia or inappropriately high ventricular rates • Some horses will have EIPH at relatively low intensity exercise © cmarr 2013 ORIGIN OF PREMATURE DEPOLARISATIONS Base-apex lead Sinus: bifid P negative QRS Supraventricular: premature abnormal P with normal QRS-T Ventricular No P premature abnormal QRS-T © cmarr, 2013 36 PATHOGENESIS OF TACHYARRHYTHMIAS Primary Myocardial disease • Viral • • Bacterial • Toxic • • Nutritional • • Neoplasia • Idiopathic Parasitic Cardiomyopathy Secondary to other factors • Hypoxia • Endotoxaemia • Electrolyte disturbances • Acid-base disturbances • Catecholamine-induced • Vagally-induced Immune-mediated © cmarr 2013 • Hypocalcaemia, hypomagnesaemia, hypokalaemia and metabolic acidosis © cmarr 2013 Clinical management of tachydysrhythmias • High index of suspicion required • Judge arrhythmias by the company they keep • Aim treatment at underlying cause • Reserve specific anti-arrhythmic therapy for life-saving situations 39 © cmarr 2013 Summary • Many normal horses have bradydysrhythmias at rest that are of no clinical significance • Atrial fibrillation is the most important clinically significant arrhythmia • Atrial fibrillation requires treatment if it is affecting the horse's role as a performance animal • Other tachydysrhythmias are seen occasionally, usually secondary to physiological/pathophysiological states associated with exercise or systemic illness 40 © cmarr 2013 DVD includes 70 cases contributed by 17 authors 42 © cmarr 2013 If you buy this book also buy a stethoscope speaker pad: www.blaufuss.org