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tachyarrhythmias include DCM, infectious endocarditis, myocarditis, severe congenital heart disease, advanced congestive HF secondary to DCM or MVD, and cardiac neoplasia. Traumatic myocarditis resulting from blunt chest trauma, possibly related to myocardial bruising or ischemia-reperfusion injury, may also result in ventricular arrhythmias. Establishing a diagnosis of VT is made from the interpretation of the ECG. Ventricular premature beats and ventricular tachycardia have wide and bizarre QRS complexes with no associated P waves. Other findings suggestive of VT include atrioventricular (AV) dissociation and fusion beats. The QRS axis or polarity of a ventricular arrhythmia is usually negative in lead II but could be positive, particularly in VT commonly seen in boxer dogs with ARVC 67 ECG (25 mm/s) from a syncopal Boxer dog with ARVC showing a predominant rhythm of ventricular tachycardia. Note the wide QRS tachycardia that is positive in lead II, typical for boxer ARVC. (B) ECG (25 mm/s) from a dog with congestive HF and atrial fibrillation. Note the narrow QRS irregular tachyarrhythmia with lack of P waves and varying QRS amplitude. The need and urgency of treatment of VT depends on the hemodynamic status of patients, the severity of the arrhythmia, and underlying myocardial dysfunction. Some ventricular arrhythmias may not require specific antiarrhythmia therapy and would benefit from supportive maneuvers, such as supplemental oxygen, adequate fluid/electrolyte therapy, and treating the underlying condition, such as HF or sepsis. Ventricular arrhythmias are considered more hemodynamically significant and life threatening if they occur at a fast rate (>150/min in dogs or >250/min in cats) or in sustained runs (>30 seconds), if they are multiform, or are very premature (R-on-T phenomenon). Acute management of ventricular arrhythmias in cats with injectable antiarrhythmic drugs is problematic because of the risk of adverse effects. The author often suppresses symptomatic feline VT with compounded oral sotalol (Betapace) For acute management of severe and symptomatic ventricular arrhythmias in the dog, treatment recommendations include lidocaine (Xylocaine) as a 2 mg/kg IV bolus, which is often repeated to effect up to a total of 8 mg/kg or until an adverse effect is observed, such as nausea or transient neurologic tremors or seizure, waiting 3 to 5 minutes in between each bolus to evaluate efficacy. If the bolus injections are successful in controlling the VT, then a CRI of lidocaine is initiated. Because the half-life of lidocaine in dogs is approximately 60 minutes, the CRI may take 5 to 7 hours to reach a steady state; therefore, intermittent, smaller 1-mg/kg boluses may be necessary in the interim. If lidocaine is unsuccessful, ensure adequate serum potassium and magnesium concentrations, then try administering procainamide (Procan). IV procainamide boluses are given slowly over 3 to 5 minutes because of the possible development of hypotension in 2- to 8-mg/kg increments to effect, up to a total of 16 mg/kg. If the boluses are successful in controlling the arrhythmia, then a CRI of procainamide should be initiated. Alternatively, if a CRI is not feasible, IM procainamide (7–10 mg/kg) every 6 to 8 hours could be used. For oral maintenance therapy or nonurgent control of a ventricular arrhythmia, treatment recommendations in dogs include the use of sotalol (a beta-adrenergic blocker + potassium channel blocker or Vaughn Williams type II and III antiarrhythmic, respectively) or mexiletine (Mexitil) (a sodium channel blocker or Vaughn Williams type IB antiarrhythmic) or a combination of both. Sotalol is often started shortly after lidocaine if chronic oral suppression of the arrhythmia is anticipated, as in a syncopal ARVC boxer. For medically refractory, incessant VT associated with the loss of consciousness or hypotension, other antiarrhythmics, such as amiodarone (Pacerone), or electrical cardioversion may be considered.68, 69 Electrical cardioversion of VT is rarely performed in veterinary medicine but not uncommonly performed in human emergency medicine. The management of SVT is importantly different than a ventricular arrhythmia. SVTs are usually narrow QRS complex tachycardias that include atrial fibrillation, atrial flutter, multifocal atrial tachycardia, reentrant accessory pathway, or reentrant AV nodal tachycardias. The QRS is typically positive in lead II. SVTs can be rarely aberrantly conducted resulting in a wide-complex tachycardia, which can be confused for a VT. Aside from atrial fibrillation and multifocal atrial tachycardia, most other causes of SVTs have regular QRS intervals. Atrial fibrillation is typically a sustained tachyarrhythmia, whereas regular SVTs and multifocal atrial tachycardias are typically intermittent. The ECG diagnosis of atrial fibrillation includes the lack of P waves; presence of baseline undulation waves (f waves); and a fast, irregular rhythm, sometimes with varying QRS amplitudes (see Fig. 5B). In the emergency setting, most dogs and cats presenting with atrial fibrillation are in congestive HF and are typically managed with a combination of digoxin and diltiazem.70 The combination of these two drugs is superior to either drug alone at reducing the ventricular response rate of atrial fibrillation, which is the goal of therapy. In most cases, these drugs are given orally but can be given initially IV for more rapid onset of action. Treatment of other supraventricular arrhythmias depends on the frequency of the rhythm disturbance and the presence of any underlying myocardial dysfunction. If the intermittent SVT is frequent, occurring in long runs (several minutes), or is causing symptoms, treatment is recommended. It is important to emphasize that chronic, sustained SVT may result in a tachycardia-induced cardiomyopathy and congestive HF. A vagal maneuver (eg, ocular or carotid sinus massage) may be successful in transiently breaking an SVT. However, medical treatment will usually be necessary to chronically control the SVT. For peracute management of sustained, symptomatic SVT, IV diltiazem is usually successful in both dogs and cats. IV esmolol