Some variation in the sinoatrial (SA) node discharge rate is common; however, if the variation exceeds 0.12 second 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 0.12 second between the shortest and longest P–P interval (Fig. 2-1). 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 (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 pause is less than fully compensatory. PACs are associated with stress, fatigue, alcohol use, tobacco, coffee, chronic obstructive pulmonary disease (COPD), digoxin toxicity, coronary artery disease, and may occur after adenosine-converted paroxysmal supraventricular tachycardia (PSVT). PACs are common in all ages, often in the absence of significant heart disease. Patients may complain of palpitations or an intermittent “sinking” or “fluttering” feeling in the chest.
Premature atrial contractions (PACs). A. Ectopic P′ waves (arrows). B. Atrial bigeminy.
Emergency Department Care and Disposition
Discontinue precipitating drugs (alcohol, tobacco, or coffee) or toxins.
Treat underlying disorders (stress or fatigue).
PACs that produce significant symptoms or initiate sustained tachycardias can be suppressed with agents such as β-adrenergic antagonists (eg, metoprolol 25 to 50 milligrams Po 3 times daily), usually in consultation with a follow-up physician.
Sinus bradycardia occurs when the SA node rate becomes slower than 60 beats/min. The ECG characteristics of sinus bradycardia are (a) normal sinus P waves and PR intervals, (b) 1:1 AV conduction, and (c) atrial rate slower than 60 beats/min. Sinus bradycardia represents a suppression of the sinus node discharge rate, usually in response to 3 categories of stimuli: (a) physiologic (vagal tone), (b) pharmacologic (calcium channel blockers, β-blockers, or digoxin), and (...