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Treatment of arrhythmias
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Summary
This talk will summarize which equine arrhythmias are physiologic and not concerning, which require evaluation of underlying systemic or heart disease but no therapy, which require close monitoring and which require therapy. A summary of therapies available will also be provided.
Clinical decisions after rhythm diagnosis are black white and grey
White
Vagally mediated arrhythmias: 2nd degree AV block and sinus blocks, pauses or arrests that go away with exercise or excitement and sinus arrhythmias can be considered benign. Sick sinus syndrome or 3rd degree AV block are the pathologic and black version of these physiologic arrhythmias and are very rare but very concerning as can make the horse collapse. They will need therapies similar to the used in small animals (large doses of corticosteroids or pacemakers).
Light grey
Rare or occasional SVPCs or unifocal VPCs do not have, in and of themselves, clinical consequences in the resting horse. They would rarely need antiarrhythmic therapy. Electrolyte abnormalities, systemic inflammation, systemic disease or underlying heart disease can predispose to these arrhythmias. So the arrhythmia may be a sign that something else is wrong with the horse that you may need to address. SVPCs are one of the triggers for AF but the risk is difficult to quantify. The increase in the risk is likely small and many horses with frequent APCs have successful athletic careers and stay in sinus rhythm for many years after diagnosis. VPCs are somewhat more concerning than SVPCs but do not panic when you identify VPCs in an ECG strip. It is often said that VPCs should be interpreted considering the ‘company that they keep’.
Meaning the rest of the evaluation of the horse, the heart and other ECG abnormalities are critical to make decision about how to manage horses with VPCs. Take home message: worry about why the arrhythmia is present and worry about the potential for worsening. For example: 1-You are treating a horse for acute colitis (or post-operative colic, pneumonia…) and you detect SVPCs. It would be advisable to evaluate and correct electrolytes, acid base status and inflammatory response. 2- You are auscultating a high level three day eventer prior to sedating it to float his teeth. You hear a loud left sided systolic murmur and some premature beats. It would be advisable to explain the owner that the loud murmur is likely the cause of mitral regurgitation. Recommend doing an echocardiogram and a holter and exercising ECG to evaluate the underlying heart disease, prognosis and risk for progression.
Grey: Atrial fibrillation (AF)
The most common clinically relevant arrhythmia in the horse. You definitely need to take this arrhythmia into account for the management of the horse but atrial fibrillation is seldom an emergency. The first step is to assess electrolytes, acid base status and a potential of an inflammatory response similar to the explained above for premature complexes. The next question to decide if the horse should be converted to normal sinus rhythm (cardioverted). There are many considerations that play a role in this decision like: the duration of the arrhythmia, the presence of underlying cardiac disease, the type of exercise the horse does, the potential complications of the cardioversion, the cost of the evaluation and treatment etc. The decision is multifactorial and would be topic for a full instructional video. These are some facts about AF to keep in mind: 1-Not all horses need to be converted to normal sinus rhythm. 2-The success rate for treatment is in general terms high, approximately 85-95%. 3-If you decide to treat the horse you have to main choices: antiarrhythmic drugs (most common choice is Quinidine) or transvenous electrical cardioversion. Both have pros and cons. 4-For any cardioversion attempt the main complication is sudden death. This is very rare but mention it. 5- Paroxysmal AF may occur in young racehorses. It will often spontaneously cardiovert within 48 hours. 6-Treatment may not be necessary in horses performing up to expectations if exercising ECG is acceptable (rate and rhythm) or in ‘pasture pets’. Horses that exercise at high intensities will not be able to do successfully (or allowed) while on AF. 7- Echocardiography is needed to assess the presence of underlying cardiac disease. 8- Echocardiogram and exercising ECG are mandatory if a horse is going to be exercised while on AF.9-Cardioversion is contraindicated if there is heart failure or severe underlying disease. 10- The recurrence rate varies between 25% and 100% depending on the situation.
Treatment with quinidine often successfully converts horses to normal sinus rhythm. If less than 24-48 hours duration no treatment is instituted and underlying metabolic or cardiac problems are assessed.
Quinidine gluconate (IV) is indicated in fibrillation <2 weeks duration (quinidine sulfate also acceptable in these case). Quinidine sulfate (via NG tube) is indicated if fibrillation >2 weeks duration. If the arrhythmia has been present >4 months prognosis is slightly worse for successful chemical cardioversion ±80% vs. ±90% in less 4 months. If the arrhythmia has been present >4 months the recurrence rate is higher (60% vs. 25%) both in the absence of underlying heart disease). If more than one episode recurrence is almost certain regardless of type of therapy. Digoxin (0.011mg/kg) prior to quinidine therapy is used if rate is high or contractility poor or if cardioversion has not occurred after 24 hours of therapy. Usually quinidine sulfate is given at a dose of 22 mg/kg (±10g/horse) via NG tube q 2 hrs for 4 doses then q6 hrs until converted or signs of toxicity. The QRS duration should be checked prior to each dose. Continuous ECG monitoring and therapeutic drug monitoring (quinidine plasma concentration after 4th dose or toxic signs) are useful to guide therapy. If toxic or dangerous side effects are found discontinue therapy, administer NaHCO3. Idiosyncratic and side effects include: depression- all horses, paraphymosis- all males, anorexia, hypotension (weakness), other arrhythmias (SVT, VT, torsades, Vfib), congestive heart failure, GI signs (flatulence, colic, diarrhea), anaphylaxis, urticaria, laminitis, syncope and sudden death. Toxic effects (associated with high plasma concentrations) include prolongation QRS complex, severe swelling of nasal mucosa or neurologic signs: ataxia, convulsions.
Transvenous Electrical Cardioversion (TVEC) is performed after placing catheters with electrodes in the right atrium and left pulmonary artery. The catheters are connected to a defibrillator and shocks delivered under general anesthesia. Specialized equipment and expertise are needed. The success rate is up to 90-95%. There are no studies of recurrence rate this is likely the same than chemical cardioversion
Dark grey: Ventricular tachycardia (VT)
Ventricular tachycardia is a serious arrhythmia. Horses with ventricular tachycardia always need close monitoring, ideally with telemetry and often need antiarrhythmic therapy. However, not all horses with ventricular tachycardia need antiarrhythmic therapy. It is mandatory to assess electrolytes, acid base status and a potential of an inflammatory response similar to the explained above for premature complexes and atrial fibrillation. There are 5 classic scenarios in which antiarrhythmic treatment is indicated for horses with VT:1-Clinical signs of cardiovascular collapse (e.g. weakness, syncope…). This applies to all arrhythmias and in general terms to all medical problems. Look at the patient first 2- Heart rate >100 beats/min. The number is somewhat arbitrary but good place to start.3-Multifocal rhythms - Implies there is more widespread cardiac involvement. Also has prognostic implications. 4-R on T phenomena- This happens when the premature QRS is so early that ‘comes out’ of the previous T. Creates a very unstable electrical system that predisposes to more malignant arrhythmias (ventricular fibrillation) and sudden death. 5-Torsades de pointes: this is an emergency. Run and get the emergency drug box (or tell someone to do so) and put an IV catheter ASAP. Start pulling antiarrhythmics. If the horse is standing be aware that it may collapse at any time so have unnecessary or untrained people out of the stall and everybody else ready to get out of the way.
The most common drugs to treat ventricular arrhythmias in horses are lidocaine and magnesium sulfate. Magnesium sulfate is the drug of choice for Torsades. Magnesium sulfate should only be used in horses with low or normal magnesium. Hypermagnesemia is very rare unless it is iatrogenic in origin.
Antiarrhythmic drugs
Antiarrhythmic are used whenever rate and rhythm need controlling. Cardiologists often hesitate to use antiarrhythmics. The reason is that they alter automaticity, conduction velocity, action potential duration, refractory period, etc. and therefore they can be dangerous. Any antiarrhythmic drug can be proarrhythmic under certain conditions. As described above there are 5 general criteria for antiarrhythmic use VT can be treated using any antiarrhythmic that will stop the action potential by obliterating the upstroke or prolonging the refractory period (sodium channel blockers, potassium channel blockers). SVT can be treated in the same way as VT (but note that some AADs have different efficacy in the atrial vs. ventricular cardiomyocytes), or they can be treated by slowing conduction through the AV node (β-blockers, calcium channel blockers, digoxin). Antiarrhythmics potentially useful to treat ventricular ectopy are MgSO4, lidocaine, procainamide, amiodarone, propafenone, phenytoin, sotalol, propranolol, or quinidine. Supraventricular tachycardias can be treated using MgSO4, procainamide, propafenone, sotalol, diltiazem, digoxin, propranolol, or quinidine.
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