Etiology

• Primary or secondary.
• Physiological or pathological.

Predisposing factors

General

• Systemic Inflammatory Response Syndrome (SIRS) Systemic Inflammatory Response Syndrome or sepsis .
• Hypoxia Hypoxemia.
• Myocardial wall abnormality, especially myocarditis Myocarditis and other cardiomyopathies Cardiomyopathy.
• Amyloidosis.
• Drugs.
• Systemic/metabolic diseases especially GI disease.
• Neurological abnormality, ie 'brain-heart syndrome'.

Specific

Bradycardia

• Hyperkalemia Sodium and potassium: overview.
• Hypercalcemia Calcium and phosphorous: overview.
• Drugs (sedatives, esp alpha-2 agonists, anti-dysrhythmics).
• Severe upper respiratory infections.

Tachycardia

• Myocardial disease.
• Drugs:
  • Anesthetic agents.
  • Atropine (parasympatholytics).
  • Anti-dysrhythmics.
• Electrolyte imbalance:
  • Hypokalemia (Often seen in cattle that have been inappetent for a few days) Hypokalemia.
  • Hypocalcemia (Milk Fever) Hypocalcemia.
    • Hypomagnesemia (Grass staggers) Hypomagnesemia.
• Acidosis Subacute ruminal acidosis.
• Nervousness/adrenaline.
• Fever.
• Pain i.e. severe lameness.

Pathophysiology

• Conduction abnormalities or development of ectopic foci   →   trigger dysrhythmias.

• 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.

• Myocardial disease (myocarditis, fibrosis or hypoxia), electrolyte imbalances, SIRS or sepsis, reperfusion injury/ e.g. diseases like E. coli mastitis or sympathetic stimulation   →   ectopic foci.

• 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 and increased intracellular calcium.

• The most common physiological dysrhythmia found in cattle is sinus dysrhythmia. Occasionally first and second degree atrio-ventricular blockade are identified either on auscultation or electrocardiogram. These will all resolve with physical activity or an increase in heart rate.

• (sinus tachycardia, sinus bradycardia).
• Atrial premature contractions (APCs).
• Atrial fibrillation/flutter.
• Atrial tachycardia.
• Sinoatrial block or SA arrest.
• Atrial standstill.
• Functional escape rhythms.

• Second degree AV block – can be physiological or pathological.
• Third degree AV block.

• Ventricular premature contractions (VPCs).
• Ventricular tachycardia.
• Atrioventricular dissociation.
• Accelerated idioventricular rhythm.
• Escape rhythms.