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Heart: second degree atrioventricular block
Synonym(s): Partial AV block
Introduction
- Cause: slowed conduction throughout the atrioventricular node:
- Can be due to excessive vagal tone, metabolic disorders, cardiomyopathies or idiopathic.
- Can be found in clinically normal dogs - may be benign in some cases.
- Signs: may be incidental finding; if advanced, there can be weakness or syncope.
- Diagnosis: signs, missed beats may be heard at auscultation, electrocardiography.
- Treatment: may respond temporarily to atropine if vagally mediated; may not require therapy; permanent, advanced and symptomatic block should be treated with a cardiac pacemaker.
- Prognosis: very variable, from good to sudden death through asystole.
Presenting signs
- Weakness.
- Exercise.
- Syncope.
- Asymptomatic.
- Congestive heart failure.
- Sudden death.
Age predisposition
- Prevalence unknown; may be common in symptomless young animals - especially if they have marked sinus arrhythmia.
- Older dogs → AV nodal fibrosis.
Cost considerations
- Pacemaker implantation Heart: pacemaker.
- Vagolytic or sympathomimetic drugs.
- Often requires no therapy.
Special risks
- Sudden death, especially if advanced block. (Mobitz Type II second degree block may precede third degree block Heart: 3rd degree atrioventricular block.)
Pathogenesis
Etiology
- Congenital: reported in pugs Pug → bundle of His stenosis.
- Iatrogenic: cardiac glycosides (digitalis Digoxin ); alpha-2 adrenoceptor stimulant sedatives (xylazine Xylazine ).
- Class IA anti-arrhythmic agents, eg quinidine Quinidine , procainamide Procainamide ; calcium antagonists Diltiazem , eg verapamil Verapamil.
- Functional: Mobitz Type I may occur with increased vagal tone, especially in brachycephalic dogs, in association with sinus arrhythmia. Occasionally with CNS diseases, eg head trauma Brain: trauma , brain tumor Brain: neoplasia , encephalitis Encephalitis.
- Myocardial disease: Mobitz Type II may occur with myocardial diseases such as myocarditis Heart: myocarditis , infarction or with infiltrative disorders such as neoplasia or fibrosis.
- Degenerative disease: degeneration of the AV node and/or bundles of His may occur with age.
- Metabolic disease: occasionally associated with hyperkalemia.
Predisposing factors
General- Brachycephalic breeds can show Mobitz Type I block as a variant of sinus arrhythmia.
Specific
- Respiratory diseases, eg airway obstruction, and CNS disease, eg brain tumor, can cause AV block, particularly Mobitz Type I.
Pathophysiology
- Mobitz Type I: (Wenckebach: variable P-Q interval) is usually associated with a high vagal tone, which delays conduction through the AV node. This can be caused iatrogenically or be functional.
- Often reasonably benign if only occasional beat missed, and it may disappear at elevated heart rates.
- Mobitz Type II: (fixed P-Q interval) more often associated with more severe disease of the conduction pathways, disturbing the transmission of the impulse from atria to ventricles. Can be due to infiltrative or degenerative myocardial diseases.
- May progress to more advanced AV blocks such as third degree (complete) AV block. Multiple missed beats may be found and ventricular escape complexes may be recorded.
Timecourse
- Mobitz Type I disease may never progress to a more serious arrhythmia.
- Mobitz Type II disease may or may not be symptomatic, but may lead to collapse or sudden death, notably through the development of third degree (complete) AV block Heart: 3rd degree atrioventricular block.
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.
Other sources of information
- Lunney J & Ettinger S J (1995)Cardiac arrhythmias.In:Textbook of Veterinary Internal Medicine4th ed. Eds S J Ettinger & E C Feldman. Philadelphia: W B Saunders & Co.
- Smith F W K, Tilley L P & Miller M S (1994)Electrocardiography.In:Saunders Manual of Small Animal PracticeEds S J Birchard & R G Scherding. Philadelphia: W B Saunders & Co.
- Tilley L P (1992)Essentials of Canine and Feline Electrocardiography.3rd edn. Philadelphia: Lea and Febiger.
- Sisson D D (1989)Bradyarrhythmias and cardiac pacing.In:Current Veterinary Therapy X.Ed R W Kirk. Philadelphia: W B Saunders & Co.