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Heart: sinus bradycardia
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.
Presenting signs
- Exercise intolerance.
- Lethargy.
- Asymptomatic.
Acute presentation
- Collapse.
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:
- Alpha 2 adrenoreceptor agonist sedatives (romifidine Romifidine, xylazine Xylazine and detomidine Detomidine hydrochloride).
- 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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Prevention
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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.