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  1. Correctly identify key elements of electrocardiogram (ECG) waveforms, complexes, and intervals:

    • P wave

    • QRS complex

    • T wave

    • ST segment

    • PR interval

    • QT interval

    • RR interval

    • Rate (atrial and ventricular)

  2. Compare and contrast the etiology, ECG characteristics, and management of common cardiac rhythms and conduction abnormalities:

    • Sinus node rhythms

    • Atrial rhythms

    • Junctional rhythms

    • Ventricular rhythms

    • AV blocks

  3. Describe the indications for, and use of, temporary pacemakers, defibrillation, and cardioversion for the treatment of serious cardiac dysrhythmias.*

*While different professional organizations may prefer the use of the term arrhythmia versus dysrhythmia, they are synonymous. This chapter uses dysrhythmia, the exception being the use of arrhythmia in selected professional statements, protocols, and practice statements.


Continuous monitoring of cardiac rhythm in the critically or acutely ill patient is an important aspect of cardiovascular assessment. Frequent analysis of electrocardiogram (ECG) rate and rhythm provides for early identification and treatment of alterations in cardiac rhythm, as well as abnormal conditions in other body systems. This chapter presents a review of basic cardiac electrophysiology and information essential to the identification and treatment of common cardiac dysrhythmias. Advanced cardiac dysrhythmias, and 12-lead ECG interpretation, are described in Chapter 18, Advanced ECG Concepts.


The electrical impulse of the heart is the stimulus for cardiac contraction. The cardiac conduction system is responsible for the initiation of the electrical impulse and its sequential spread through the atria, atrioventricular (AV) junction, and ventricles. The conduction system of the heart consists of the following structures (Figure 3-1):

Figure 3-1.

The conduction system of the heart.

Sinus node: The sinus node is a small group of cells in the upper right atrium that functions as the normal pacemaker of the heart because it has the highest rate of automaticity of all potential pacemaker sites. The sinus node normally depolarizes at a regular rate of 60 to 100 times/min.

AV node: The AV node is a small group of cells in the low right atrium near the tricuspid valve. The AV node has three main functions:

  1. Its major job is to slow conduction of the impulse from the atria to the ventricles to allow time for the atria to contract and empty their blood into the ventricles.

  2. Its rate of automaticity is 40 to 60 beats/min and can function as a backup pacemaker if the sinus node fails.

  3. It screens out rapid atrial impulses to protect the ventricles from dangerously fast rates when the atrial rate is very rapid.

Bundle of His: The bundle of His is a short bundle of fibers at the bottom of the AV node leading to the bundle branches. Conduction velocity accelerates in the bundle of His and the impulse is transmitted to both bundle branches.


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