Mechanisms of Arrhythmias
Abnormal impulse formation
Abnormal impulse formation can result in abnormal frequency, as in symptomatic sinus bradycardia, but often the problem is an abnormal location of impulse formation, as is the cause in an ectopic pacemaker.
The sinus node contains pacemaker cells that have spontaneous firing capacity. This is called normal automaticity. Abnormal automaticity occurs when other cells start firing spontaneously, resulting in premature heartbeats. All cardiac cells have spontaneous firing capacity, but at only at a very slow heart rate. Therefore, during a normal heart rate, they will never have the chance to show off their firing capacity. However, in pathologic conditions, such as during extreme bradycardia, other cells can take over and cause for example an AV-nodal heart rate.
During triggered activity heart cells contract twice, although they only have been activated once. This is often caused by so called afterdepolarizations (early or delayed afterdepolarizations EADs / DADs) caused by electrical instability in the myocardial cell membrane. A typical example of this is Torsade de Pointes.
Conduction delay can cause a slow heart rate, as happens during AV conduction blocks. If conduction delay occurs more distally in the heart, i.e. within the ventricles, the QRS complex will widen and a left or right bundle branch block can be seen on the ECG.
Re-entry is a common cause of arrhythmias. Ventricular tachycardia and AV-nodal re-entry are typical examples. Re-entry can occur when a conduction path is partly slowed down. As a result of this, the signal is conducted by both a fast and a slow pathway. During normal sinus rhythm this generally does not cause problems, but when an extrasystole follows rapidly upon the previous beat, the fast pathway is sometimes still refractory and cannot conduct the signal. Now the following sequence results in re-entry.
- The atrial signal coming from above is conducted by the slow pathway.
- As the signal, going through the slow pathway, reaches the end of the fast pathway, it finds this pathway able to conduct.
- The signal is conducted through the fast pathway up to the beginning of the slow pathway, which by that time is able to conduct.
- This circle is perpetuated and a signal generator is created. In the case of AV-nodal re-entry this will typically generate a signal at a frequency of 180-250 bpm.