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(Brevibloc), a rapid-acting beta-blocker, may also be helpful in managing SVT acutely. For oral maintenance therapy or nonurgent control of SVT, treatment recommendations include oral diltiazem, atenolol (Tenormin), or sotalol. Sotalol may be an excellent antiarrhythmic drug for a dog or cat that has both ventricular and supraventricular arrhythmias. Bradyarrhythmias are identified with cardiac auscultation because of either a regularly slow heart rate or intermittent periods of sinus arrest. Bradycardia is generally defined as a heart rate of less than 60 beats per minute in dogs and less than 140 beats per minute in cats. An ECG is necessary to further characterize the bradycardia into sinus bradycardia, a sinus node dysfunction, atrial standstill, or high-grade AV block, and either high-grade, second-degree AV block or complete AV block. Most animals with clinically significant bradyarrhythmias present for syncope or weakness.Sinus bradycardia is usually a result of high vagal tone caused by gastrointestinal, respiratory or central nervous system disease, or secondary to medical therapy that may cause a decreased in heart rate, such as the administration of sedative, analgesic, or negative chronotropic cardiac drugs. Overdose or an overzealous response to negative chronotropic drugs, such as a beta blockers, calcium channel blockers, or digoxin, can cause pathologic bradycardias. Sepsis and low body temperature in cats can also produce sinus bradycardia. Treatment usually is directed at the underlying cause or disease. When sinus bradycardia occurs with excessively long pauses (several seconds), sinus node dysfunction or sick sinus syndrome (SSS) is suspected (Fig. 6A). SSS is an idiopathic dysfunction of the sinus node that most commonly affects miniature schnauzers and West Highland white terriers. SSS can manifest as mostly a regular sinus bradycardia with long pauses (several seconds) or as a bradycardia-tachycardia syndrome, alternating long pauses with an SVT. Dogs with bradytachycardia do not respond to medical management and will require a pacemaker to improve clinical signs. Some dogs with SSS that do not have the bradytachycardia may improve with medical management with either positive chronotropic drugs, such as a bronchodilator, or an anticholinergic agent, such as terbutaline, theophylline, or propantheline. Most dogs with significant sinus node dysfunction disease will eventually require a pacemaker to resolve their syncope. Asymptomatic dogs with SSS do not require any treatment. 6(A) ECG (25 mm/s) from a syncopal miniature schnauzer with SSS. Note a predominant respiratory sinus arrhythmia with a couple of periods of sinus arrest (>2 normal R-R intervals). The markedly prolong period of sinus arrest is terminated with an escape complex. (B) ECG (25 mm/s) from a syncopal dog with complete AV block with a ventricular escape rate of approximately 40 per minute. Note complete dissociation between the P and QRS complexes. View Large ImageFigure ViewerDownload Hi-res imageDownload (PPT) Complete or third-degree and high-grade second-degree AV block is a common pathologic bradyarrhythmia identified in dogs (see Fig. 6B). Most dogs with high-grade AV block have no to little structural heart disease.71, 72 However, occasionally endocarditis, DCM, or neoplasia is identified echocardiographically. Many cats with complete AV block have underlying myocardial disease.73 Most dogs present for syncope, collapse, weakness, and lethargy. Occasionally some dogs with high-grade AV block may be asymptomatic and are identified on a routine examination.

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