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KNOWLEDGE COMPETENCIES

  1. Identify electrocardiogram (ECG) characteristics and treatment approaches for each of the following advanced dysrhythmias:

    • Supraventricular tachycardias (SVTs)

    • Wide QRS beats and rhythms

  2. Using the 12-lead ECG, determine the following:

    • Bundle branch blocks

    • QRS axis

    • Patterns of myocardial ischemia, injury, and infarct

  3. Identify ECG characteristics of single- and dual-chamber pacemakers during normal and abnormal functioning.

  4. Identify ECG characteristics of Brugada syndrome (BrS) and long QT syndromes.

THE 12-LEAD ELECTROCARDIOGRAM

The 12-lead electrocardiogram (ECG) records electrical activity as it spreads through the heart from 12 different leads, which are in turn recorded by electrodes placed on the arms and legs, and in specific spots on the chest. Each lead represents a different “view” of the heart and consists of two electrodes. A bipolar lead has two poles—one positive and one negative. A unipolar lead has one positive pole and a reference pole that is a point in the center of the chest that is mathematically determined by the ECG machine. The standard 12-lead ECG consists of six frontal plane limb leads that record electrical activity traveling up/down and right/left in the heart, and six precordial leads that record electrical activity in the horizontal plane traveling anterior/posterior and right/left. Limb leads are recorded by electrodes placed on the arms and legs, and precordial leads are recorded by electrodes placed on the chest (Figure 18-1).

Figure 18-1.

(A) Limb electrodes can be placed anywhere on arms and legs. Standard placement is shown here on wrists and ankles. (B) Chest electrode placement. V1 = fourth intercostal space to right of sternum; V2 = fourth intercostal space to left of sternum; V3 = halfway between V2 and V4 in a straight line; V4 = fifth intercostal space at mid clavicular line; V5 = same level as V4 at anterior axillary line; V6 = same level as V4 at midaxillary line.

A camera analogy makes the 12-lead ECG easier to understand. Each lead of the ECG represents a picture of the electrical activity in the heart taken by the camera. In any lead, the positive electrode is the recording electrode or the camera lens. The negative electrode tells the camera which way to “shoot” its picture and determines the direction in which the positive electrode records. When the positive electrode sees electrical activity traveling toward it, it records an upright deflection on the ECG. When the positive electrode sees electrical activity traveling away from it, it records a negative deflection (Figure 18-2). If the electrical activity travels perpendicular to a positive electrode, no activity is recorded. The standard 12-ECG records three bipolar frontal plane leads (leads I, II, and III) and three unipolar frontal plane leads (aVR, aVL, and aVF). In addition, there are six unipolar precordial leads: V1, V2, ...

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