Premature Beats and Parasystole

This section will provide an overview of the conditions related to premature beats - that is, beats that occur earlier than expected in the milieu of an otherwise regular (usually sinus) rhythm.

This includes:

• Premature atrial complexes (PACs), which are due to early beats originating from the atria. We will cover how PACs can manifest differently based on their site of origin or coupling intervals, and how variable effects on the sinus node can affect the post-PAC pause.

• Premature junctional complexes (PJCs), which are due to early beats originating from the atrioventricular junction. We will cover the different manifestations of PJCs depending on site of origin, coupling interval, effects on the sinus node, and the presence of concealed conduction. We will also cover how PJCs can simulate some other phenomenon, such as atrioventricular blocks, through concealed conduction.

• Premature ventricular complexes (PVCs), which are due to ventricular beats originating from the ventricles. We will again cover the different manifestations of PVCs based on their site of origin, degree of interaction with the supraventricular beat (i.e. fusion), effects on the sinus node, and presence of concealed conduction. We will also discuss the clinical significance of PVCs and what to recognize from review of the ECG.

• Parasystolic rhythms, which are due to the presence of a protected ectopic pacemaker that is able to evade overdrive suppression and coexist with the sinus rhythm. We will discuss the classic presentation of parasystolic rhythms as premature beats with variable coupling intervals, and move onto several non-classic presentations (such as fixed coupling, reverse coupling, parasystolic rhythms with exit blocks or incomplete entrance blocks, and the presence of multiple parasystolic foci).

• Reciprocating beats, or echo beats, which are premature beats related to the presence of multiple AV nodal pathways, and due to an impulse "turning around" within the AV junction and reactivating the same chamber it has just activated. This will be discussed more in detail in the "Dual AV Nodal Physiology" section.

• Patterns of premature beats, such as bigeminal or trigeminal rhythms. We will also specifically delve into mechanisms of "bigeminal rhythms", such as escape-capture, reverse escape-capture, ectopic (re-entrant), echo beats, and grouped beating related to a second degree block in either the sinus or AV node.