Cardiac arrhythmias
A cardiac arrhythmia is a disturbance of the normal rhythm of the heart.
Many arrhythmias are asymptomatic unless myocardial function is compromised.
Normal sinus rhythm is not exactly regular as there are fluctuations in
autonomic tone with respiration. On inspiration the parasympathetic tone falls
and the heart rate increases, conversely on expiration parasympathetic tone
rises and the heart rate decreases. This variation is normal and is referred to
as sinus arrhythmia.
Cardiac arrhythmias can be classified according to whether they are
bradycardias (<60 bpm) or tachycardias (>100 bpm).
Bradycardias may be due to disorders of the sinus node or the
atrioventricular (AV) node:
·
Sinus bradycardia and sinus node
disease (also called sick sinus syndrome).
·
Atrioventricular block where the
atria may be acting normally, but the AV node does not conduct the impulses
normally to the ventricles.
Tachycardias are also subdivided according to their origin:
·
Sinus tachycardia.
·
Supraventricular tachycardia
including atrial or junctional (AV nodal) tachycardias.
·
Ventricular tachyarrhythmias such
as ventricular tachycardia and ventricular fibrillation are often secondary to
ischaemic myocardial damage. Torsades de pointes is a distinctive type of
ventricular tachycardia associated with a long Q–T interval with a
characteristic ECG.
A useful clinical division is between narrow complex tachycardias, which
are due to supraventricular (atrial or junctional) tachycardias, and broad
complex tachycardias, which are most often ventricular in origin. However, in
patients with bundle branch block and in cases where the rapid rate of
supraventricular tachycardias causes transient bundle branch block, broad
complex tachycardias can be supraventricular in origin.
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