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Day 7: Extrasystoles and Preexcitation Syndromes

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  1. Extrasystoles

    1. Mechanisms of extrasystoles

      1. Reentry

        1. Most extrasystoles, particularly if they are monomorphic, bear a constant relationship to the preceding QRS complex (a fixed coupling interval).

        2. The vast majority of these complexes probably represent a reentrant mechanism (each beat represents one trip around a reentrant pathway).

      2. Parasystole

        1. Parasystole occurs when an ectopic focus fires independently of the basic rhythm.

        2. If the parasystolic focus is not reset by the basic rhythm, the focus is said to be protected

        3. There is a constant interval between ectopic depolarizations, but the ectopic focus will manifest itself only whenever it finds the atrium or ventricle not refractory.

        4. Parasystole is very uncommon.

      3. Escape

        1. Escape is a normal phenomenon that occurs when there is a sufficient pause to allow a lower pacemaker to depolarize.

        2. Common examples are junctional or ventricular escape mechanisms.

      4. Unclassified—some extrasystoles do not fall easily into any of these categories and may remain undiagnosed.

    2. Atrial extrasystoles (Day 7-01) (Day 7-02) (Day 7-03)

      1. Most atrial extrasystoles, or premature atrial complexes (PACs), are reentrant.

      2. Atrial extrasystoles are preceded by a P wave that usually has abnormal morphology, indicative of the abnormal direction of atrial depolarization.

      3. The QRS complex is narrow unless there is a preexisting intraventricular conduction defect.

         

        DAY 7-01

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        DAY 7-02

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        DAY 7-03

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      4. Occasionally, the QRS complex may be wide (aberrant) when one of the bundle branches is not fully repolarized.

      5. An atrial extrasystole usually resets the sinus mechanism and, therefore, is not followed by a compensatory pause (see below).

      6. If an atrial extrasystole is very early, the ventricle may be refractory and not depolarize ("the commonest cause of a pause is a nonconducted atrial extrasystole"—Marriott).

    3. Ventricular extrasystoles (Day 7-04) (Day 7-05) (Day 7-06)

      1. Most ventricular extrasystoles are also reentrant

      2. Ventricular extrasystoles are not preceded by a P wave

      3. The QRS complex is obviously wide

      4. Ventricular extrasystoles usually do not reset the atrial rate and are frequently followed by a compensatory pause.

      5. A ventricular extrasystole may cause retrograde depolarization of the AV node, which results in a lengthening of the subsequent PR interval—a phenomenon known as concealed retrograde conduction.

         

        DAY 7-04

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        DAY 7-05

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        DAY 7-06

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  2. Preexcitation

    1. The origin of accessory pathways

       

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      1. In utero, the atria and ventricles are eventually separated by a fibrous plate called the AV ring.

      2. The function of the AV ring is to provide support for the mitral and tricuspid valves and to electrically insulate the atria and ventricles.

      3. The AV node is the only structure that should allow conduction through the AV ring.

      4. Overexuberant separation of the atria and ventricles produces congenital 3° AV block (see Day 4).

      5. If there is incomplete separation, residual muscle fibers may bridge the AV ring and form accessory electrical pathways.

    2. Characteristics of accessory pathways

      1. Accessory pathways usually do not have the conduction delay properties of the ...

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