equis - Articles
Heart: atrioventricular block – 2nd degree
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.
Presenting signs
- Usually none.
- Rarely syncope and weakness.
Acute presentation
- Asymptomatic.
Geographic incidence
- Worldwide.
Breed/Species predisposition
- Most common arrhythmia.
- Found in up to 50% of fit horses.
- Detected in over 40% of healthy horses during 24 h continuous electrocardiographic monitoring.
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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Prevention
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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.