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Heart: sinus block
Introduction
- Cause: usually high vagal tone. Rarely, sinus node dysfunction (sick sinus syndrome).
- Signs: usually none. Rarely, exercise intolerance, syncope.
- Diagnosis: electrocardiography.
- Treatment: treatment of primary cause and if necessary short-term use of atropine or glycopyrrolate; use of an artificial pacemaker has been reported although this is rarely indicated.
- Prognosis: usually good. Guarded if associated with clinical signs.
Presenting signs
- Usually asymptomatic.
Pathogenesis
Etiology
Irritation or stimulation of the vagus nerve
- Commonly alpha-2 adrenoreceptor agonist agents (romifidine Romifidine, xylazine Xylazine, detomidine Detomidine hydrochloride).
- Normal homeostatic mechanism to reduce arterial blood pressure.
- More rarely surgical manipulation.
- Thoracic neoplasia Respiratory: neoplasia.
- Cervical neoplasia.
- Electrolyte imbalance.
- Pathologic conditions of the atria, including fibrosis, inflammation and cardiomyopathy Heart: cardiomyopathy (rare).
Pathophysiology
- Sinus block is caused by the failure of impulses to leave the sinoatrial node due to a depression in conductivity in the node. This is most commonly attributable to high vagal tone and less commonly fibrosis.
- Sinus block cannot usually be differentiated from sinus arrest clinically and on ECG.
- Sinus arrest is the failure of impulse formation within the sinoatrial node due to a depression in automaticity in the node. This is most commonly attributable to high vagal tone and less commonly a degenerative process (fibrosis).
- Theoretically, the duration of the pause during sinus block is a multiple of the baseline underlying rhythm (for example a 4-second pause is 2 times the baseline interval when the heart rate is 30 beats per minute). In the presence of sinus arrest, there is no relationship between the duration of the pause and the baseline heart rate.
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
- Gasthuys F et al (1990) A preliminary study on the effects of atropine sulfate on bradycardia and heart blocks during romifidine sedation in the horse. Vet Res Commun 14 (6), 489-502 PubMed.
- McGuirk S M et al (1985) Diagnosis and treatment of cardiac arrhythmias. Vet Clin North Am Eq Pract 1 (2), 353-370 PubMed.