ISSN 2398-2977      

Heart: sinus bradycardia

pequis

Introduction

  • An abnormally slow (<24 bpm) but regular heart rate.
  • Cause: variable.
  • Signs: depend on inciting cause; may show poor athletic performance if unable to elevate heart rate appropriately at exercise. Can also cause lethargy.
  • Occasionally highly conditioned and very relaxed horses will have heart rates <24 bpm, but this will be rapidly reversible with exercise or excitement; such low heart rates are common with intravenous sedation.
  • Diagnosis: auscultation, ECG.
  • Treatment: indicated in profound irreversible bradycardia.
  • Prognosis: guarded to poor in symptomatic animals; excellent if bradycardia is physiological or drug-induced.

Pathogenesis

Etiology

Physiological elevation of vagal tone

  • Eyeball pressure, eg in ocular surgery.
  • Elevated intracranial pressure, eg neoplasia, trauma.
  • Hypothermia.
  • Altered carotid sinus pressure.

Pathologic

  • Systemic disease with cardiac toxicity.
  • Electrolyte imbalance.
  • Sick sinus syndrome.

Central nervous system lesions

  • Drug associated:
  • Calcium channel blockers, beta-blockers.
  • Digoxin Digoxin.
  • Anesthetic agents: sinus bradycardia can be a warning of impending cardiac arrest during anesthesia.

Pathophysiology

  • Usually secondary to high vagal tone causing a slowing of sinus node pacemaker cells.
  • Rarely sinus node dysfunction secondary to sick sinus syndrome, which in part is a degenerative process of the sinus node with fibrosis.

Diagnosis

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Treatment

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Outcomes

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Keen J A (2020) Pathological bradyarrhythmia in horses. Vet J, 259-260 PubMed.
  • Pimenta E L M et al (2011) Comparative study between atropine and hyoscine-N-butylbromide for reversal of detomidine induced bradycardia in horses. Equine Vet J 43 (3), 332-340 PubMed.

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