Equis ISSN 2398-2977

Heart: atrioventricular block - 2nd degree

Contributor(s): Karen Blissit, Karen Blissitt, Christopher Brown, Wendy Furness, Lesley Young, Nicola Menzies-Gow

Introduction

  • Cause: impulse of supraventricular origin (from sinus node or an atrial ectopic beat) is blocked at the AV node and does not progress to activate the ventricles.
  • Significance: usually non-pathologic.
  • Signs: dropped beat.
  • Diagnosis: ECG and/or auscultation.
  • Treatment: usually condition is physiological and no treatment is required, very rarely an artificial pacemaker, or sympathomimetic agents may be indicated in advanced (pathological) 2nd  degree atrioventricular block (AVB).
  • Prognosis: generally excellent due to physiological nature of the condition, but conversely prognosis is guarded to poor if arrhythmia is not abolished by exercise and horse is symptomatic.

Pathogenesis

Etiology

  • Usually a normal homeostatic mechanism to regulate blood pressure. Mediated by the vagus.
  • During the conducted beats, blood pressure gradually rises. This stimulates baroreceptors which act via the vagus to slow or block conduction at the atrioventricular and sinoatrial node.
  • In advanced (pathological) second degree AVB:
    • Occasionally structural conduction system disease, eg congenital, degenerative, inflammation or neoplasia.
    • Drug induced, eg α2 agonists (detomidine Detomidine hydrochloride, romifidine Romifidine and xylazine Xylazine).
    • Electrolyte imbalances.
    • Digitalis toxicity.

Pathophysiology

  • Usually a normal homeostatic mechanism to regulate blood pressure. Mediated by the vagus.
  • Rarely pathological.

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.
  • Matsui K et al (1985) Second-degree atrioventricular block observed in a Thoroughbred foal on 2.5 months of age. Nippon Juigaku Zasshi 47 (1), 175-178 PubMed.
  • Smetzer D L et al (1969) Second-degree atrioventricular block in the horse. Am J Vet Res 30 (6), 933-946 PubMed.
  • Smetzer D L et al (1969) High-grade second-degree atrioventricular block in a horse. Am J Vet Res 30 (3), 337-343 PubMed.

Other sources of information

  • Marr C M & Bowen I M (2010) Eds. Cardiology of the Horse. 2nd edn. Saunders Ltd, UK. ISBN: 9780702028175.


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