Felis ISSN 2398-2950
Heart: dysrhythmia
Synonym(s): Arrhythmia, Heart block, Ectopic beat
Contributor(s): Serena Brownlie, Phil Fox, Philip K Nicholls, Penny Watson
Introduction
- Definition: abnormalities of rate and rhythm of the heartbeat.
- Cause: primary cardiac disease - muscular or conduction abnormalities; systemic disease; trauma.
- Signs: many are asymptomatic; syncope, sudden death, congestive heart failure.
- Diagnosis: auscultation; electrocardiography (ECG).
- Treatment: anti-arrhythmic drugs depending on type of arrhythmia; correction of metabolic factors.
- Prognosis: depends on cause and response to therapy.
Follow the diagnostic trees forCommon Cardiac Arrhythmias Common cardiac arrhythmias and Bradyarrhythmia & Tachyarrhythmia Bradyarrhythmia & Tachyarrhythmia.
Pathogenesis
Predisposing factors
General
- Toxemia Shock: septic.
- Hypoxia.
- Myocardial wall abnormality.
- Drugs.
- Neurological abnormality, ie 'brain-heart syndrome'.
- Systemic/metabolic diseases.
- Cardiomyopathies.
Specific
Bradycardia
AV block in cats is always abnormal and needs investigation.
Tachycardia
- Hyperthyroidism Hyperthyroidism.
- Myocardial disease.
- Drugs:
- Atropine (parasympatholytics).
- Anti-arrhythmics.
- Anesthetic agents.
- Cardiac glycoside toxicity (can → VPCs).
- Electrolyte imbalance:
- Hypomagnesemia.
- Acidosis.
- Nervousness/adrenaline.
- Fever.
- Pain.
Pathophysiology
- Conduction abnormalities or development of ectopic foci → trigger dysrhythmias.
- Depolarization of pacemaker cells in sino-atrial node (dictates intrinsic heart rate):
- Increased by sympathetic stimulation (excitement, fear and pain).
- Decreased by parasympathetic stimulation.
- Altered by drugs, hormone levels, electrolytes.
- Reflected by damage to conduction tissues → AV or branch bundle block.
- Ectopia:
- Myocardial disease (myocarditis, fibrosis or hypoxia), electrolyte imbalances, endotoxins or toxins released from reperfusion injury or sympathetic stimulation → ectopic foci.
- Extracellular potassium affects heart rate:
- Hypokalemia → faster depolarization.
- Hyperkalemia → reduces resting membrane potential → slows conduction velocity and heart rate.
- Intracellular calcium ion concentration affects the Na+/Ca++ exchange pump.
- Abnormal automaticity refers to site of depolarization in non-pacemaker tissue.
- Re-entry refers to second depolarization when part of impulse is delayed by passage through diseased tissue (due to hypoxia or fibrosis).
- After-potentials are oscillations in resting membrane potential following repolarization which may reach threshold potential and trigger an impulse.
- After-potentials are enhanced by adrenergic stimulation, digitalis toxicity and increased intracellular calcium.
Timecourse
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.
- Rials S J, Wu Y, Ford N et al (1995) Effect of left ventricular hypertrophy and its regression on ventricular electrophysiology and vulnerability to inducible arrhythmia in the feline heart. Circulation 91 (2), 426-430 PubMed.
- Kowey P R, O'Brien R, Wu Y et al (1992) Effect of gallopamil on electrophysiologic abnormalities and ventricular arrhythmias associated with ventricular hypertrophy in the feline heart. Am Heart J 124 (4), 898-905 PubMed.
- Boyden P A, Tilley L P, Albala A et al (1984) Mechanisms for atrial arrhythmias associated with cardiomyopathy - a study of feline hearts with primary myocardial disease. Circulation 69 (5), 1036-1047 PubMed.