7.3 Classification of arrhythmias
Rhythm disturbances can be classified according to:
- Rate: The normal range of heart rate in a quiet, relaxed horse, at rest is
- bpm. Dysrhythmias may be faster than normal (tachydysrhythmias), or slower than normal (bradydysrhythmias). Bradydysrhythmias may result in clinical signs such as weakness or syncope when they result in an abnormally low cardiac output, but they seldom cause signs of congestive heart failure (CHF). Tachydysrhythmias are often due to severe cardiac disease and may result in, or be associated with, signs of CHF. Measurement of the heart rate and assessment of rhythm, are important parts of the interpretation of an ECG (see section 4.1.4).
- Origin and conduction: Dysrhythmias are further defined as supraven(originating from the SA node, atria, or AV node and junctional tissues) or ventricular in origin. The origin of an arrhythmia can usually be determined by careful interpretation of the waveforms and their relationships on the ECG. The origin of the arrhythmia makes up the essential part of the terminology of arrhythmias.
- Timing: When isolated, abnormal depolarisations are seen, they can be determined to be premature (they occur before the normal timing for the P wave or QRS complex) or escape complexes (they occur late, to rescue the ventricles from asystole).
- Frequency: Abnormal complexes may occur as intermittent single comin groups, or may be persistent.
- Cause: Arrhythmias can be roughly divided into physiological or pathological types. Physiological arrhythmias may result from alterations in autonomic tone. Pathological arrhythmias may be related to abnormalities such as valvular or myocardial disease, or electrolyte disturbances. In addition arrhythmias are often categorised as being caused by abnormal impulse formation, or by abnormal conduction. In fact, this classification is a gross simplification, because the pathogenesis of arrhythmias is incompletely understood and is extremely complex. Some arrhythmias may be induced or abolished by exercise or other procedures such as eyeball pressure (a vagal manoeuvre).