CARDIAC DYSRHYTHMIAS….The older term ‘arrhythmia’ should better be replaced by the new term ‘dysrhythmia’ for obvious reasons. Cardiac dysrhythmia includes all conditions where the cardiac rate and rhythm are altered or there is defect in the conductive system. Dysrhythmias in relation to rate and rhythm may have its origin in the SA node, atrium, AV junction or in the ventricle.
MECHANISMS OF PRODUCTION OF DYSRHYTHMIA
Bradycardia and tachycardia may arise from abnormalities of automaticity arising out of a single cell and disturbances of conduction arising out of interactions between cells. Thus accelerated automaticity, triggered activity and a re-entry phenomenon are main mechanisms for dysrhythmogenesis.
- Accelerated automaticity
In this mechanism in the SA node an abnormal pacemaker impulse competes with the normal pacemaker function. This is seen in sinus tachycardia, escape rhythms and accelerated junctional rhythms.
- Triggered activity
Here the oscillations of transmembrane potential develop at the end of action potential which is due to myocardial injury. At times when these oscillations reach the threshold potential dysrhythmnia is produced. This is usually seen in atrial tachycardia after digitalis toxicity and ventricular dysrhythmias in long QT syndrome.|CARDIAC DYSRHYTHMIAS
- Re-entry Phenomenon
In recent years His bundle electrocardiography has revolutionized our understanding of various types of cardiac dysrhythmias. In fact, it is believed now-a-days that many of the arrhythmias occur as a result of re-entry phenomenon. The term re-entry means that the original stimulus re-enters in site of origin which may be located in SA node, Atria, AV node, His Purkinje system.When there are two pathways of conduction between any two points A and C in the myocardium, an impulse at the point A can easily be conveyed along segments B1 and B2 with equal velocity and will reach at the point C in equal times. But due to some reason or other if the arc B2 is having a long refractory period and B1 has a normal refractory period, then the original impulse at the point A can be propagated along the arc B1 but not along B2 as it is still refractory. When the impulse reaches the point C through B1 segment, if by this time the segment B2 becomes no longer refractory and ready to conduct impulse then the same impulse will be conveyed along the arc B2 in the opposite direction and will come at the point A and in this way it goes on continuing like this.CARDIAC DYSRHYTHMIAS