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.
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
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
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% ...