Triggered activity differs from automaticity in that it is not consistently spontaneous and can result in either a single or repetitive firing of myocardial cells caused by re-excitation. Late potentials during Phase 3 or 4 of the cardiac action potential can trigger afterdepolarizations, which can lead to ventricular arrhythmias. Reentry involves an impulse traveling through two pathways in the heart with different refractory periods, with the impulse reentering the first pathway after the second pathway recovers.

1. Like automaticity, triggered activity involves the leakage of positive ions into a cell, resulting in new action potentials. However, unlike automaticity, Triggered Activity is not consistently spontaneous. It may be either a single or repetitive firing of a myocardial cell, or group of cells, caused by re-excitation. Ion leakages that occur late in Phase 3 or early in Phase 4 (after cell recovery has begun) are called afterdepolarizations, or late potentials. Afterdepolarizations can be the ‘trigger’ that cause ventricular tachyarrhythmias.

2. For reentry to occur, the following events must take place: A premature impulse occurs in the reentrant circuit at a time when Pathway A (with the short refractory period) is ready to accept the impulse, and Pathway B (with the long refractory period) is still repolarizing from the previous depolarization. The impulse slowly travels through Pathway A and reaches Pathway B just as Pathway B completes it repolarization and is no longer refractory, which means it is ready to accept a stimulus. The impulse travels through Pathway B in a retrograde direction and reenters Pathway A. The impulse is conducted antegrade through Pathway A, and the circuit continues. Reentry does not display a “warm up” or “cool down” period.

3. Once the membrane reaches -40 to -45mV the charge falls quickly to the resting level. The cell membrane once again becomes polarized and the cycle begins again. (Ellenbogen, Kenneth A. & Wood, Mark A. Basic Concepts of Pacing. Cardiac Pacing & ICDs, 3 rd Edition. Malden, MA: Blackwell Science, Inc., 2002: 48-49.) Also see: Fogoros, Richard N. Electrophysiologic Testing, 3 rd Edition. Blackwell Science, Inc. 1999.

4. Once the membrane reaches -40 to -45mV the charge falls quickly to the resting level. The cell membrane once again becomes polarized and the cycle begins again. (Ellenbogen, Kenneth A. & Wood, Mark A. Basic Concepts of Pacing. Cardiac Pacing & ICDs, 3 rd Edition. Malden, MA: Blackwell Science, Inc., 2002: 48-49.) Also see: Fogoros, Richard N. Electrophysiologic Testing, 3 rd Edition. Blackwell Science, Inc. 1999.

5. Atrial fibrillation can also result from the rapid discharge of impulses from one or many ectopic (non-sinus) sites in the atria. The ectopic cells (called foci) depolarize independently of the sinus node and disrupt the normal sinus rhythm. Multifocal firing takes place at multiple atrial ectopic sites. The cells produce many depolarization waves that activate different areas of the atrial myocardium at different times. AF occurs because the myocardial cells do not contract and relax rhythmically, in normal synchronization with the sinus node. *Note: Because a mechanism of AF may be Mulifocal Firing, some would argue that AF is a disorder of Impulse Formation (abnormal automaticity), rather than Impulse Conduction (multiple wavelets of reentry). Some argue still that while ectopic or mulifocal firing may begin AF, it is reentry that sustains it. For purposes of laying out rhythm disorders in this presentation, AF has been identified as a disorder of Impulse Conduction due to how it is sustained. May it be understood, however, that it could arguably be classified under both disorder descriptions.

6. WPW is characterized by: Short PR interval (120 ms or less) indicating that the impulse did not travel the path through the AV Node. QRS is wide (110 ms or greater) again implying the impulse did not travel through the normal conduction system; and An obvious delta wave as a result of early conduction. A delta wave looks like a gradual onset of the QRS complex (above graphic) and the QRS is >110 ms. The PR interval is typically less than 120 ms. As mentioned, the most common SVT in WPW is orthodromic, which creates a narrow complex QRS. Approximately 7%-10% of WPW is antidromic, which creates a wide complex QRS (exaggeration of delta wave).

7. Ventricular Premature beats that form patterns are classified according to the number of normal ventricular beats that occur between premature beats. Bigeminy – PVC every other beat; Trigeminy – PVC every third beat; or Quadrigeminy - PVC every fourth beat.

8. Second-Degree AV block is characterized by intermittent failure of atrial depolarizations to reach the ventricle. There are two patterns of second-degree AV block. Type I is marked by progressive prolongation of the PR interval in cycles preceding a dropped beat. This is also referred to as Wenckebach or Mobitz Type I block. The AV node is most commonly the site of Mobitz I block. The QRS duration is usually normal.