What causes cardiac arrhythmias?
Cardiac Arrhythmias occur when the heart beats improperly, as a result of incorrect impulse generation, (at the Sino-Atrial (SA) node) or impulse conduction. The heart organ broadly differentiates into two types of cell, pacemaker cells, and non-pacemaker cells. Pacemaker cells beat with their own independent rhythm, in synchrony with their neighbours (a heart attack results when these cells get out of synch with each other). The non-pacemaker cells require stimulus to beat (or
contract), this stimulus comes from the pacemaker cells.
Incorrect impulse generation is the fault of the pacemaker cells, which are beating either too slow, too fast, or not totaly in rhythm (asynchrous). Impulse conduction problems are caused because the nervous signal from the pacemaker cells (located primarily at the SA node) fails to reach the non-pacemaker cells correctly. This problem falls into two further categories, a nodal block whereby the signal from the SA node fails to reach the AV node, and a re-entry pathway.
A re-entry pathway occurs when a section of nervous tissue (which conducts the impulse) is damaged in some manner (eg/ Physical trauma, Cardiac infarction (Heart attack)). Part of the tissue only conducts the impulse in a single direction, in the example shown left this is in the opposite direction to the genuine impulse direction. This will set up a loop in the nervous tissue, as the impulse keeps going round and round the junction, stimulating the non-pacemaker cells to contract, and ultimatley disturbing the rate of heartbeat.
Regardless of the specific arrhythmia, the pathogenesis of arrhythmias falls into one of three basic mechanisms. These include enhanced or suppressed automaticity, triggered activity, or re-entry. Automaticity is a natural property of all myocytes. Ischemia, scarring, electrolyte disturbances, medications, advancing age, and other factors may suppress or enhance automaticity in various areas. Suppression of automaticity of the sinoatrial node can result in sinus node dysfunction and sick sinus syndrome. Sick sinus syndrome is still the most common indication for permanent pacemaker implantation.
In contrast to suppressed automaticity, enhanced automaticity can result in multiple arrhythmias, both atrial and ventricular. Triggered activity occurs when early afterdepolarizations and delayed afterdepolarizations initiate spontaneous multiple depolarizations precipitating ventricular arrhythmias. Examples of this include torsades de pointes and ventricular arrhythmias due to digitalis toxicity.
Finally, probably the most common mechanism of arrhythmogenesis results from re-entry. Requisites for re-entry include bi-directional conduction and uni-directional block. "Micro-" level re-entry occurs with VT from conduction around the scar of myocardial infarction and "macro-" level re-entry occurs via conduction through manifest (Wolff-Parkinson-White syndrome—WPW) or concealed accessory pathways. |
