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Atrial premature beats

APBs result from ectopic stimuli i.e. these beats arise from somewhere in either the left or right atrium but not in the SA node. After an atrial depolarization, the stimulus that spread normally through the His-Purkinje system into the ventricles gives a normal QRS complex.

APBs have the following major features:  
1. The atrial depolarization is premature, occurring before the next normal P wave is due.
2. The QRS complex of the APB is often preceded by a visible P wave that usually has a slightly different shape and/or different PR interval from the P wave seen with normal sinus beats. The PR interval of the APB may be either longer or shorter than the PR interval of the normal beats. In some cases, the P wave may be buried in the T wave of the preceding beat.
3. After the APB, a slight pause generally occurs before the normal sinus beat resumes. This usually slight delay is due to “resetting” of the SA node pacemaker by the premature atrial stimulus. This slight delay contrasts with the longer, “compensatory” pause often (but not always) seen after VPBs.
 4. The QRS complex of the APB is usually identical or very similar to the QRS complex of the preceding beats. This finding contrasts with VPBs, in which the QRS complex is usually very wide because of abnormal depolarization of the ventricles.
 5. Rarely, APBs result in aberrant ventricular conduction, so that the QRS complex is wider than normal. E.g. APBs associated with right bundle branch block aberrancy. APBs with left bundle branch block aberrancy may also occur.
6. Sometimes when an APB is very premature, the stimulus reaches the AV junction just after it has been stimulated by the preceding normal beat. Because the AV junction, like all other conductive tissues, requires time to recover its capacity to conduct impulses, this premature atrial stimulus may reach the junction when it is still refractory. In this situation the APB may not be conducted to the ventricles and no QRS complex appears. The result is a blocked APB. The ECG shows a premature P wave not followed by a QRS complex. After the blocked P wave, a brief pause occurs before the next normal beat resumes. The blocked APB therefore produces a slight irregularity of the heartbeat.

Two APBs occurring consecutively are referred to as an atrial couplet. Sometimes, each sinus beat is followed by an APB. This pattern is referred to as atrial bigeminy.

Clinical significance:
APBs are very common. They may occur with a normal heart or virtually any type of organic heart disease. Thus the presence of APBs does not necessarily mean that an individual has cardiac disease. In normal people, these premature beats may be seen with
1) emotional stress,
2) excessive intake of caffeine,
3) the administration of sympathomimetic agents (epinephrine, isoproterenol, theophylline).
4) hyperthyroidism.

APBs may produce palpitations and patients may complain of feeling a skipped beat or an irregular pulse. APBs may also be seen with various types of structural heart disease. Frequent APBs are sometimes the forerunner of atrial fibrillation or flutter or other atrial tachyarrhythmias.


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