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1. No specific treatment is usually required, but any underlying conditions should be investigated and treated.

2. Some patients with acute myocardial infarction have an "inappropriate" tachycardia and may benefit from slowing heart rate with b-adrenergic antagonists.

PREMATURE ATRIAL CONTRACTIONS (PACs) PACs originate from ectopic pacemakers anywhere in the atrium other than the sinus node. The ECG characteristics of PACs are that (1) an ectopic P' wave appears sooner (prematurely) than the next expected sinus beat, (2) the ectopic P' wave has a different shape and axis, and (3) the ectopic P' wave may or may not be conducted through the AV node (Fig...24-7). A PAC is not conducted through the AV node if it reaches the AV node during the absolute refractory period and is conducted with a delay (longer P'R interval) during the relative refractory period. Most PACs are conducted with typical QRS complexes, but some may be conducted aberrantly through the infranodal system if they reach a bundle branch during the refractory period. The sinus node is often depolarized and "reset," so that while the interval following the PAC is often slightly longer than the previous cycle length, the pause is less than fully compensatory.

FIG. 24-7. Premature atrial contractions (PACs). Top: ectopic P' waves (arrows). Bottom: atrial bigeminy.

Clinical Significance PACs are common at all ages and often seen in the absence of heart disease. It is generally assumed, although it remains unproven, that stress, fatigue, alcohol, tobacco, and coffee may precipitate PACs. Frequent PACs may also be seen in chronic lung disease, ischemic heart disease, or digitalis toxicity. PACs may precipitate sustained atrial tachycardia, flutter, or fibrillation under the right circumstances.

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