6th March 2024

Background

Any aberration of the normal cardiac rhythm is considered a cardiac arrhythmia. This may be because of a change in rate (normal heart rate 60-100 bpm) or rhythm, in terms of regularity or frequency of either atrial and or ventricular impulses. Cardiac arrhythmia can be benign or life-threatening through haemodynamic instability. 

Normal cardiac rhythm refers to electrical activity originating from the sino-atrial (SA) node within the posterior wall of the right atrium. This spreads across the atria via internodal pathways and Bachmann’s bundle, allowing for atrial contraction. The conduction arrives at the atrio-ventricular (AV) node where it is delayed allowing for sufficient ventricular filling. It continues along the bundle of His, the left and right bundle branches; the left divides into two fascicles (anterior and posterior) and the right into one fascicle, to reach the Purkinje fibres resulting in ventricular contraction. 

Any deviation from this, such as disease of SA or AV node, accessory pathways, or ectopic pacemaker activity, will result in a cardiac arrhythmia that may manifest with symptoms of palpitations, breathlessness, syncope/presyncope or cardiac arrest. Common arrhythmias include sinus bradycardia or tachycardia, atrial fibrillation or flutter, SA or AV node disease, re-entry tachycardias, and ventricular tachycardia or fibrillation. Common causes for arrhythmia include:

• Myocardial ischaemia – hypoxia causes inappropriate depolarisation of cardiac cells affecting impulse conduction and/or formation.
• Electrolyte abnormalities – K⁺, Mg²⁺and/or Ca²⁺ disturbance.
• Drug-induced arrhythmias including antiarrhythmic medication – this may be through influence on autonomic activity, electrolyte balance or QT interval.
• Structural causes including accessory pathways and changes secondary to heart failure.

We can assess arrhythmias by detecting the electrical impulses which translate to waveforms on an ECG. Persistent arrhythmias can be detected on a standard ECG, however intermittent arrhythmias require monitoring over a period using a Holter monitor, typically monitoring between 24 hours to a few days. Longer monitoring is possible by means of an implantable loop recorder. More complex arrhythmias can be investigated using electrophysiological studies.