Definition

Sinus rhythm in which impulses arise from the sinoatrial node at slower-than-normal rate for an animal's signalment and activity.

Pathophysiology

• Can be an incidental finding in healthy animals or during sleep.
• May represent normal physiologic response to athletic training; may result from enhanced cardiac parasympathetic tone or decreased sympathetic tone as well as from intrinsic changes in the sinus node.
• Automaticity of the heterogeneous sinus node is a very complex phenomenon invoking the calcium and voltage clock mechanisms. More than 16 autonomically influenced currents with the I_f (funny) channel predominating and Ca⁺⁺ release from the the sarcoplasmic reticulum are critical in maintaining autonomic balance and changes in heart rate.
• May represent pathophysiologic response due to high vagal tone, change in blood pH, PCO₂, PO₂, or serum electrolyte disorders, hypothyroidism, increased intracranial pressure, toxins and certain drugs.
• May be a result of SSS.

Systems Affected

Cardiovascular-most instances benign arrhythmia and may be beneficial by producing a longer period of diastole and increased ventricular filling time; can be associated with syncope if due to abnormal reflex (neurocardiogenic) or intrinsic disease of sinus node.

Genetics

Female miniature schnauzer, West Highland white terrier, boxer, cocker spaniel, dachshund, and pug predisposed to SSS with possible underlying heritable component-may cause bradycardia.

Incidence/Prevalence

• Common in the dog, less common in cat.
• Interpretation of sinus node rate also depends on environment and type of patient. For example, a sinus rate can be as low as 20 beats/minute in a normal dog that is sleeping.

Signalment

Signs

Causes

Risk Factors

• Any situation or disease that may increase parasympathetic tone
• Oversedation
• Hypoventilation under anesthesia
• Breeds predisposed to SSS

Differential Diagnosis

• Persistent and marked SB should raise possibility of SSS.
• Clinical signs may mimic cerebral dysfunction.

CBC/Biochemistry/Urinalysis

• Hyperkalemia, hypercalcemia, hypocalcemia, or hypermagnesemia possible.
• CBC and serum chemistry profile may reveal changes associated with metabolic disease such as renal failure.

Other Laboratory Tests

• Serum T₄, free T₄ (FT₄), and TSH assay if hypothyroidism suspected.
• Measure trough serum digoxin concentration, if applicable, 8 hours after last dose or close to next dosing; normal therapeutic serum concentration should be 0.5–1.5 ng/mL.
• Toxicologic screen.

Diagnostic Procedures

• Provocative atropine response test to assess sinus node function-administer atropine 0.04 mg/kg IV, wait 10–15 minutes, then record ECG or administer same dose IM, wait 30 minutes, then record ECG; persistent sinus tachycardia at >140 bpm is expected response. Lower doses of atropine have increased tendency to cause initial accentuation of sinus bradycardia and first- or second-degree AV block because of centrally mediated increase in vagal tone.
• 24-hour Holter monitoring or ECG event recorder, an owner triggered device, useful if transient bradyarrhythmia is suspected cause for clinical signs.

Appropriate Health Care

• Many animals exhibit no clinical signs and require no treatment. In dogs without structural heart disease, heart rates as low as 40–50 bpm generally provide normal cardiac output at rest.
• Therapeutic approaches-vary markedly; depend on the mechanism for SB, the ventricular rate, and severity of clinical signs.
• Inpatient or outpatient management-depends on underlying cause and clinical status of patient.

Nursing Care

• Provide general supportive therapy including intravenous fluid therapy for hypothermic and hypovolemic patients.
• Discontinue any causative drug.
• Correct any serious electrolyte imbalance with appropriate fluid therapy.

Client Education

• Discuss importance of complying with daily medical management when treating underlying disease.
• Advise that persistent symptomatic bradycardia may necessitate
• permanent pacemaker implantation for reliable long-term management.

Surgical Considerations

• If progressive bradycardia occurs during anesthesia and is attributed to hypoventilation, immediately discontinue inhalation anesthetics and provide adequate ventilation; atropine is generally ineffective in this situation.
• If surgical manipulation triggering vagal reflexes (eye, vagus nerve, larynx) is anticipated, pretreatment with atropine (0.04 mg/kg IM, SC) or glycopyrrolate (5–10 µg/kg IM, SC) may prevent bradycardia.
• Severe bradycardia may precipitate cardiopulmonary arrest; identify the causative agent or condition for effective management.

Drug(s) Of Choice

• If patient is hypothyroid, supplement with l-thyroxine.
• For severe hypocalcemia (<6 mg/dL) administer 10% calcium gluconate (0.5–1.5 mL/kg IV) slowly over 15–30 minutes; monitor with ECG.
• For symptomatic drug-induced bradycardias, disorders causing excessive vagal tone, and initial management of bradycardia associated with SSS, administer atropine (0.04 mg/kg IV) or glycopyrrolate (5–10 µg/kg IV); anticholinergic therapy may be continued short-term using atropine (0.04 mg/kg IM, SC q6–8h) or glycopyrrolate (0.01 mg/kg IM, SC q6–8h). Consider propantheline bromide (0.25–0.5 mg/kg PO q8–12h) or hyoscyamine (3–6 µg/kg PO q8h), methylxanthine theophylline, an adenosine receptor antagonist (extended release formulation 10 mg/kg PO q12h, dogs; 12.5 mg PO q24h in the evening, cats), and/or terbutaline (0.14 mg/kg q8–12h PO, dogs; 0.625–1.25 mg/cat PO, cats) to manage symptomatic bradycardia associated with SA node disease.
• For temporary management of symptomatic persistent bradycardia until pacing can be accomplished, consider continuous IV infusion of isoproterenol (0.04–0.08 µg/kg/min IV. However, if temporary pacing is available this is initial procedure of choice.

Contraindications

• For hypothermia-induced bradycardia with a pulse, rewarming and supportive measures should be mainstay of treatment. Parasympatholytics generally not recommended.
• Parasympatholytic agents contraindicated for acidotic, hypercarbic patients under anesthesia (hypoventilation); bradycardia in this setting may protect the myocardium by decreasing oxygen consumption.

Precautions

• Close ECG monitoring recommended when administering calcium solutions for treatment of hypocalcemia; if QT interval shortening or bradycardia, stop administration temporarily.
• In patients with heart disease, a lower initial dose of l-thyroxine is advised to allow adaptation to higher metabolic rate.
• Administer atropine selectively; rapid IV administration may predispose to ventricular arrhythmias by altering autonomic balance.
• Caution when administering parasympatholytic agent to dogs with suspect SSS-could result in a tachycardia that overdrive suppress escape rhythms with potential consequence of asystole.

Alternative Drug(s)

• Bradycardia associated with structural heart disease is most reliably treated by permanent pacemaker implantation.
• Glycopyrrolate may have longer vagal blocking effect and cause less frequent ventricular ectopic beats than atropine.

Patient Monitoring

• Assess total T₄ 6 hours post pill.
• Addison's disease-assess electrolytes every 3–4 months after patient is stable.
• ECG check of pacemaker function and pacing rate is recommended during each follow-up examination.

Prevention/Avoidance

• Maintain normal PaO₂ under anesthesia with proper ventilation; monitor with pulse oximetry or blood gases.
• Avoid hypothermia intraoperatively.

Expected Course and Prognosis

• Signs, if present, should resolve with correction of causative metabolic or endocrine problem.
• Treatment of symptomatic SB with a permanent pacemaker generally offers a good prognosis for rhythm control.

Associated Conditions

• Sick sinus syndrome
• Heart block
• Sinus arrhythmia

Pregnancy/Fertility/Breeding

Post-parturient hypocalcemia usually develops 1–4 weeks postpartum, but can occur at term, prepartum, or late lactation.

See Also

• Digoxin Toxicity
• Eclampsia
• Hypercalcemia
• Hyperkalemia
• Hypermagnesemia
• Hypocalcemia
• Hypothermia
• Hypothyroidism
• Organophosphate and Carbamate Toxicosis
• Sick Sinus Syndrome

Abbreviations

• AV = atrioventricular
• ECG = electrocardiogram
• SA = sinoatrial
• SB = sinus bradycardia
• SSS = sick sinus syndrome
• T₄ = thyroxine
• T₃ = triiodothyronine
• TSH = thyroid stimulating hormone