Arrhythmias Overview

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Overview of Arrhythmias

  • Normal cardiac conduction begins in the pacemaker of the heart known as the sinoatrial node. An impulse is formed in the sinoatrial (SA) node and a wave of depolarization travels to the atrioventricular (AV) node then to the bundle of His, and finally through the bundle branches into the Purkinje fibres. The cardiac rhythm is regulated by the sympathetic and parasympathetic nervous systems.

  • Arrhythmias (a.k.a. dysrhythmias) comprise any irregularities of the heart rate, rhythm, site or conduction of heartbeat development.

  • The presence of an arrhythmia does not always indicate disease.

  • Pathological arrhythmias=arrhythmias that occur as the result of a pathological process.

  • Physiological arrhythmias=arrhythmias that occur in a normal animal and are not part of a pathological condition. Horses normally have physiologic arrhythmias that are harmless and are eliminated with exercise.

  • Arrhythmias are detected by using electrocardiography (ECG) and evaluating the P, QRS, and T waves.

P=atrial depolarisation

QRS=ventricular depolarisation


Questions to ask when looking at an ECG:

1. Is the ECG of good quality?

2. What is the heart rate?

3. Is there a normal relationship between P and QRS waves?

4. Is the heart rhythm normal or irregular? If irregular, is the rhythm regularly irregular or irregularly irregular?

5. Does the mean electrical axis and deflection magnitude indicate chamber enlargement?

Classification of Different Arrhythmias

  • Change in heart rhythm: tachyarrhythmia (fast heart rhythm); bradyarrhythmia (slow heart rhythm)

  • Change in heart rhythm regularity: fibrillation (irregular rhythm)

  • Origin of rhythm disturbance: Ventricular arrhythmia (originate in the ventricles); Supraventricular arrhythmia (originate from the atria or AV node)

Causes of Arrhythmias

1. Electrolyte Imbalances

2. Pathological processes affecting the heart

3. Pathological processes affecting the circulatory system and other organs

3. Toxin or Drug Reactions

4. Acid-Base Disturbances

5. Over stimulation of the autonomic nervous system

6. Hypoxia, Hypovolemia, Hypothermia

Arrhythmias Overview Learning Resources
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