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Premature complexes arise before the sinus node (or other pacemaker) with increased automaticity from the site producing the premature conduction. In other words, these do not result from dysfunction of the sinus node but from increased activity of other pacemaker sites. Premature contractions are classified by their pacemaker site. Sometimes, these premature complexes reach the sinus node and depolarize it, causing it to reset; this causes the next P wave to be delayed ("post-extrasystolic pause" or compensatory pause). These premature complexes can also occur in repeating patterns. Bigeminy is characterized by premature conduction every other beat and trigeminy by premature conduction every 3rd beat. Couplets are 2 consecutive premature complexes and triplets are 3 of them.
Premature Atrial Complexes
Premature Atrial Complexes
A premature atrial complex (PAC) results from an ectopic atrial site. Therefore, the P wave has an unusual morphology. These are differentiated from PVC's by the associated P waves (PVC's do not have associated anterograde P waves though may have retrograde P waves). Conduction to the AV node and ventricles depends on the timing of the PAC and the refractory period of the His-Purkinje system. If the His-Purkinje system has completed the refractory period, the QRS complex can appear as it would for the sinus beats. However, if they arise early enough, the bundle branches may have not completed repolarization and have aberrant conduction; this is usually in the form of a RBBB as the right bundle is often slower to complete repolarization than the left bundle. Sometimes, these P wave conductions are not carried to the ventricles if none of the His-Purkinje system has completed its refractory period. This will show an abnormal P wave morphology without a QRS complex ("blocked PAC"). Sometimes, multiple ectopic atrial sites cause PAC's, resulting in various P wave morphologies. Furthermore, these can occur often enough to confuse providers with other causes of irregularly irregular narrow complex tachycardias. Although PAC's sometimes produce palpitations, they are rarely the result of any significant pathology. Common causes of PAC's are the typical causes of increased automaticity: increased adrenergic activity (stress, caffeine, lack of sleep, medications...) or electrolyte abnormalities. Sometimes, however, PAC's can trigger re-entrant tachydysrrhythmias (afib, SVT) in those that are predisposed already.
Examples:
PAC with aberrant conduction in aVF. PAC with normal conduction in II.
Note the abnormal P wave that is early compared to the sinus P waves with QRS complex that is wide in aVF.
Non-conducted PAC hidden in T wave
Note that the T wave has a larger amplitude than the other T waves present in this rhythm strip (because of the addition of the PAC). The PAC is not followed by a QRS complex ('blocked'). This is the cause of the pause noted on the strip (delay to next beat-'compensatory pause').
Normal sinus rhythm with frequent PAC's.
This is easily confused for atrial fibrillation or MAT. Note the sinus rhythm occurring with the black and red arrows. The underlying sinus rate all has the same P wave morphologies that are consistent with a sinus origin and the rate is not tachycardic with these. However, the conducted PAC's (green arrows) have different P wave morphologies, occur early, and have aberrant conduction.
Premature Junctional Complexes
Premature Junctional Complexes
Premature junctional complexes commonly occur for the same reasons as PAC's, though also commonly occur with digoxin toxicity. They are much less common than PAC's or PVC's. They are early beats with narrow complexes that may or may not have a P wave (if present, would be a retrograde P wave). Usually, the QRS complexes appear very similarly to the 'native' QRS complexes, though may be slightly different (usually just barely wider). Repolarization (T wave) is commonly somewhat altered, which can be another way to differentiate a junctional complex from a sinus complex. However, these may be associated with a compensatory pause as the retrograde conduction from PJC's (or PVC's below) can stimulate the atria; this results in an additional required repolarization period, so the next beat may appear late (though, again, is from a compensatory pause). Occasionally, this can trigger accelerated junctional rhythms as the AV undergoes repolarization and subsequent depolarization more quickly than the sinus node that has received the impulse later and, therefore, starts repolarization and subsequent depolarization later. This usually occurs if there is some increased automaticity to increase the spontaneous depolarization of the AV node.
Example:
Premature junctional beats
Note the lack of associated P waves and QRS complex appears similar to native complexes except slightly wider.
Premature Ventricular Complexes
Premature Ventricular Complexes
PVC's usually result from one area of the ventricles and are characterized by wide QRS complexes. The site of origin can be determined from the morphology of the QRS complex; for example, a PVC with a RBBB morphology usually arises from the left ventricle. They usually have discordant ST-T changes and are typically followed by a compensatory pause (as the ventricular conduction system is 'reset' by the PVC and the refractory period restarted). This compensatory pause produces the next beat after an interval that is equal to double the preceding R-R interval. Retrograde conduction of the atria can occur, producing retrograde P waves. The ST-T changes follow the rule of appropriate discordance. In other words, if there is concordance or excessive discordance, one could diagnose ischemia within PVC's. 'Frequent PVC's' are defined by 5 or more per minute. As with PAC's, PVC's can arise from multiple ventricular sites and, therefore, have different QRS morphologies.
PVC's are usually not a result of significant pathology and their etiology is usually from what was previously mentioned. However, they can trigger re-entrant tachydysrhythmias (ie VT or SVT). They are not considered to be benign if they produce an R on T phenomenon in the setting of a prolonged QTc, which usually precipitates ventricular arrhythmias. They are a common cause of palpitations.
Examples:
Ventricular bigeminy
Multifocal PVC's
Note that the markers also display the rule of the next beat occurring with a doubling of the prior R-R interval
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