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Day 6: Ectopic Arrhythmias and Triggered Activity

  1. Ectopy—a disorder of impulse formation

    1. Mechanisms of ectopic arrhythmias

      1. Ectopic arrhythmias require:

        1. Default—slowing of the normal dominant sinus pacemaker which allows a slower focus to take control, or

        2. Usurpation—an acceleration of a lower pacemaker which takes control by virtue of being faster than the sinus rate

      2. Disorders of the sinus node, such as SA arrest, SA exit block, or excessive vagal tone may allow a lower focus to take control by default

      3. A variety of factors, including digitalis toxicity, hypoxia, electrolyte disturbances, ischemia, or chronic lung disease may stimulate an ectopic focus to accelerate and usurp control from the SA node

    2. Properties of ectopic arrhythmias

      1. Ectopic arrhythmias usually start and stop gradually (non-paroxysmally)

      2. They are not usually initiated by a premature beat

      3. They may be somewhat irregular

      4. They are not terminated by vagal maneuvers, although AV block may be increased

      5. AV block of varying degrees is frequently present (particularly if digitalis toxicity is the cause)

      6. These arrhythmias are usually quite resistant to treatment with standard class I or III agents

      7. Catheter ablation may be effective if a causative agent cannot be identified or treated

  2. The major ectopic arrhythmias

    1. Wandering atrial pacemaker

      1. Mechanisms and causes

        1. There are three or more ectopic atrial pacemakers

        2. This arrhythmia is typically seen in young healthy persons, particularly athletes

        3. The etiology is uncertain

      2. Heart rate—the heart rate is 60–100 and is usually irregular

      3. ECG morphology (Day 6-01)

        1. There are at least three P wave morphologies with varying PR intervals

        2. There is usually moderate variation in the heart rate

    2. Multifocal atrial tachycardia

      1. Mechanisms and causes

        1. Caused by multiple ectopic atrial foci

        2. Chronic lung disease is typically the underlying clinical abnormality, although it can also occur in the setting of hypoxia, electrolyte abnormalities, acid-base disturbances, and ischemia (i.e., frequently in the intensive care setting)

      2. ECG morphology (Day 6-02)

        1. There are at least three P wave morphologies with varying PR intervals

        2. The rate is 100–140

        3. There is typically 1:1 AV conduction

        4. This arrhythmia is frequently confused with atrial fibrillation; the distinction is an important one since management is usually very different

    3. Ectopic atrial rhythms

      1. Mechanisms and causes

        1. A single ectopic atrial focus accelerates and usurps control from the sinus node, or the sinus node slows down and allows an ectopic focus to appear

        2. Digitalis toxicity, electrolyte abnormalities, ischemia, hypoxia, and chronic lung disease are typical causes

      2. ECG morphology (Day 6-03) (Day 6-04)

        1. The P waves are of the same morphology but have an abnormal axis, indicating their ectopic origin

        2. The atrial rate may be slightly irregular

        3. AV block of varying degrees is sometimes present (particularly if digitalis toxicity is the cause)

        4. Atrial tachycardia with AV block should be considered a manifestation of digitalis toxicity until proven otherwise (Day 6-05)

        5. The atrial rate in atrial tachycardia is usually 140–200

        6. Atrial tachycardia may be confused with atrial flutter, but the latter is usually faster ...

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