Premature Beats
Premature Atrial Complex
Premature atrial complexes (PACs) arise from an ectopic focus anywhere in the atria, including the coronary sinus and pulmonary veins. Most often, PACs are asymptomatic and incidentally discovered on the 12-lead electrocardiogram. When three or more consecutive PACs from the same atrial focus occur at a rate greater than 100 bpm, the rhythm is called atrial tachycardia.
ECG Appearance
The P wave of a PAC occurs prematurely and has a different morphology than the sinus P wave. A PAC may occur in isolation or recur in a pattern of bigeminy or trigeminy.
Noncompensatory Pause: PACs simultaneously conduct anterograde to the AV node and retrograde to the sinus node. Retrograde conduction from a premature atrial beat results in depolarization of the sinus node. This effectively resets the timing of the sinus rhythm that follows the PAC. A pause follows the PAC because the conduction system and/or myocardium is still in a refractory state from the PAC. Subsequent firing from the reset sinus node results in the usual P morphology. Because the initial sinus rhythm has been interrupted, the distance between the two sinus beats straddling the PAC will not be twice the distance of the R-R interval.
Depending on the refractory state of the AV node from the most preceding sinus impulse, these PACs have several possible fates:
Nonconducted PACs
A PAC is blocked when it encounters the AV node still in its refractory period (corresponds to label A in Fig. 5.1). The P wave of a blocked PAC can be very subtle, especially if it is buried in the previous T wave. A pause still follows the nonconducted PAC. When the block occurs repeatedly, these pauses can separate QRS complexes into grouped beats.
 FIGURE 5.1 ECG appearance of a blocked PAC. |
Nonconducted PACs in a pattern of atrial bigeminy could be mistaken for sinus bradycardia.1 P waves of a PAC may appear only as subtle deformations of preceding T waves.
Conducted PACs
Conducted Normally
A grouped beating pattern produced by conducted PACs in a pattern of atrial bigeminy could be mistaken for second-degree block.
 FIGURE 5.2 ECG appearance of a conducted PAC. |
Conducted with PR Prolongation
An ectopic atrial impulse may encounter the AV node in its relative refractory period. Conduction is delayed somewhere within the AV node or bundle of His and PR prolongation results. Conduction then proceeds down the bundle branches resulting in a normal-appearing QRS complex.
Conducted with Aberration
An ectopic atrial impulse may encounter the right bundle branch in its absolute refractory period. The right bundle branch tends to have a longer refractory period than the left bundle branch. QRS complexes are wide and have an RBBB pattern. PACs with aberrant conduction could be confused with PVCs, especially when the P wave of a PAC is buried within the preceding T wave.
 FIGURE 5.3 ECG appearance of a conducted PAC with aberrancy. |
Premature Junctional Complex
The AV junction includes the AV node and the bundle of His. Premature junctional complexes (PJCs) are less common than premature atrial and ventricular complexes. Nonsustained PJCs are considered benign.
ECG Appearance
Narrow QRS Complex
Anterograde conduction from a junctional focus results in simultaneous activation of left and right ventricles. The QRS complex of a PJC is narrow.
Inverted P Wave
Premature impulses arising from the AV junction can travel in retrograde fashion to activate the atria. Atrial conduction occurs in an inferior-to-superior direction. P waves of junctional impulses are inverted in the inferior limb leads II, III, and aVF.
Narrow P Wave
Because both left and right atria are activated simultaneously from the junctional focus, P waves tend to be narrow.2
Depending on the relative conduction speeds of the anterograde ventricular conduction and retrograde atrial conduction, this P wave may occur before the QRS complex, buried within the QRS complex, or after the QRS complex.
 FIGURE 5.4 Different appearances of premature junctional beats. |
Retrograde conduction to the atria may or may not reset the sinus node. The next sinus beat may occur on time (fully compensatory pause) as if no premature junctional beat had occurred. If the sinus node is reset, the next sinus beat may occur earlier than expected (noncompensatory pause).
Accelerated Junctional Rhythm
When consecutive junctional impulses occur at a rate greater than the intrinsic rate of the AV junction (40-60 bpm), the rhythm is called accelerated junctional rhythm.
 FIGURE 5.5 Accelerated junctional rhythm with retrograde P waves following QRS complexes. Retrograde P waves can also be buried in QRS complexes or appear before the QRS complexes. |
 FIGURE 5.6 Anterograde and retrograde conduction coming from a junctional focus. |
Rates greater than 60 bpm are above the upper normal intrinsic rate of the AV junction; the term nonparoxysmal junctional tachycardia is often used interchangeably with accelerated junctional rhythm when the rate of a junctional rhythm exceeds 60 bpm.
AV dissociation can occur in accelerated junctional rhythm. A junctional focus may be able to conduct anterograde to the ventricles but fail to conduct retrograde to the atria. When this is the case, atrial conduction is controlled by the sinus node and ventricular conduction by the AV junction.
An important cause of accelerated junctional rhythm is digitalis toxicity.
Premature Ventricular Complex
A premature ventricular complex (PVC) represents an impulse that originates in the ventricular myocardium. PVCs are common, and the prevalence of PVCs is age dependent.
PVCs are associated with structural heart disease but occur in the absence of any identifiable heart disease and rarely cause symptoms. They are most often asymptomatic and incidentally found on a 12-lead electrocardiogram, but they can present as chest discomfort, palpitations, and light-headedness. In patients with a high burden of PVCs, they may cause left ventricular dysfunction and PVC-induced cardiomyopathy that may be reversible with catheter ablation or pharmacologic suppression.
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