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Sinus Arrhythmia

Some variation in the sinoatrial (SA) node discharge rate is common; however, if the variation exceeds 120 milliseconds between the longest and shortest intervals, sinus arrhythmia is present. The electrocardiogram (ECG) characteristics of sinus arrhythmia are (a) normal sinus P waves and PR intervals, (b) 1:1 atrioventricular (AV) conduction, and (c) variation of at least 120 milliseconds between the shortest and longest P–P interval (Fig. 2-1). If two or more different P wave morphologies are present, atrial ectopy, wandering atrial pacemaker, or another competing nonsinus focus may be present. Sinus arrhythmias are affected primarily by respiration and are most commonly found in children and young adults, disappearing with advancing age. Occasional junctional escape beats may be present during very long P–P intervals. No treatment is required.

Premature Atrial Contractions

Premature atrial contractions (PACs) have the following ECG characteristics: (a) the ectopic P wave appears sooner (premature) than the next expected sinus beat; (b) the ectopic P wave has a different shape and direction; and (c) the ectopic P wave may or may not be conducted through the AV node (Fig. 2-2). Most PACs are conducted with typical QRS complexes, but some may be conducted aberrantly through the infranodal system, typically with a right bundle branch block pattern. When the PAC occurs during the absolute refractory period, it is not conducted. Since the sinus node is often depolarized and reset, the interval between normal P waves before and after the PAC will not be twice the existing P to P interval, creating a shorter pause than a fully compensatory pause (unlike that seen after most premature ventricular contractions). PACs are associated with stress, fatigue, alcohol use, tobacco, coffee, chronic obstructive pulmonary disease (COPD), digoxin toxicity, and coronary artery disease, and may occur after adenosine-converted paroxysmal supraventricular tachycardia (PSVT). Patients may complain of palpitations or an intermittent “sinking” or “fluttering” feeling in the chest. PACs are common in all ages, often in the absence of significant heart disease, but can precipitate sustained atrial tachycardia, flutter, or fibrillation under certain circumstances.

Figure 2-2

Premature atrial contractions (PACs). A. Ectopic P′ waves (arrows). B. Atrial bigeminy.

Emergency Department Care and Disposition

  1. Discontinue precipitating drugs (alcohol, tobacco, or coffee) or toxins.

  2. Treat underlying disorders (stress or fatigue).

Premature Ventricular Contractions

Clinical Features

Premature ventricular contractions (PVCs) are due to impulses originating from single or multiple areas in the ventricles. The ECG characteristics of PVCs are as follows: (a) a premature ...

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