Chapter 22

Quick and decisive management is of paramount importance in the treatment of cardiac rhythm disturbances in the ED. Cardiac arrhythmias are common and can present as dizziness, palpitations, syncope, or sudden death. Arrhythmias can be caused by a primary cardiac event, such as ischemia, by a genetic abnormality of the conduction system, or be the result of a vast number of pathologic processes, including sepsis, metabolic derangements, toxicologic, or traumatic.

Through understanding of cardiac anatomy and electrophysiology (EP), the ECG, and the patient’s clinical status, decisions can be made quickly and aggressively to stabilize the patient and uncover the underlying diagnosis to provide definitive management.

###### Figure 22-1.

A. Cardiac conduction system. B. Vasculature. (Reproduced with permission from Fauci AS, Kasper DL, Braunwald E, et al: Harrison’s Principles of Internal Medicine, 17th ed. © 2008, McGraw-Hill, New York.)

The heart consists of three types of specialized tissue: (1) pacemaker cells that undergo spontaneous depolarization and can initiate an electric impulse (this property is called automaticity); (2) Purkinje cells that conduct electrical waves more rapidly than other cardiac cells; and (3) contractile cells, which contract when electrically depolarized.

The sinus [sinoatrial (SA)] node is the dominant cardiac pacemaker unless disease or drugs depress its activity. The SA node is near the junction of the superior vena cava and right atrium. Blood supply is from the sinus node artery, which arises from the right coronary artery in about 55% of individuals or from the left circumflex artery in the other 45%. Sympathetic and parasympathetic nerves are the primary controls of the heart rate and innervate the SA node. The normal sinus discharge rate is 60 to 100 beats/min. Specialized atrial conduction tracts (anterior, middle, and posterior internodal tracts) propagate the electric impulse through the atria from the SA node and the atrioventricular (AV) node, thus activating atrial contraction.

The atria and ventricles are insulated electrically from each other by the fibrous connective tissue of the AV ring (annulus fibrosus). Normally, electrical impulses from the atria can reach the ventricles only by passing through the AV node and infranodal conducting system.

The AV node is under the surface of the right atrial endocardium. The AV node receives its blood supply from the right coronary artery in 90% of individuals or from the left circumflex artery in the other 10%. This accounts for the common occurrence of AV conduction disturbances with acute inferior myocardial infarctions (MIs). The AV node is innervated by sympathetic and parasympathetic fibers. It has two important electrophysiologic characteristics: a slow conduction velocity and a long refractory period. The slow conduction velocity through the AV node allows time for atrial contraction to give an extra 10% ...

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