Chapter: Medicine and surgery: Cardiovascular system

Cardiac arrhythmias

A cardiac arrhythmia is a disturbance of the normal rhythm of the heart.

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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