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Day 3: SA and AV Nodal Conduction Abnormalities

  1. Intracardiac electrograms

    1. Conduction disturbances in the surface ECG have their genesis in specific locations in the conduction system.




    2. Surface ECG disturbances are more clearly appreciated by concomitant analysis of the intracardiac electrogram.

    3. Components of the intracardiac electrogram

      1. Sinoartrial (SA) node

        1. There is no surface ECG representation of SA nodal depolarization; a recurrent, normal axis P wave implies that the SA node is responsible.

        2. Careful recordings from a tiny area in the upper right portion of right atrium have demonstrated SA nodal activity preceding atrial depolarization.

      2. Atria

        1. Atrial depolarization produces the P wave on the surface ECG.

        2. The P wave axis is demonstrative of the direction of atrial depolarization.

      3. Atrioventricular (AV) node

        1. The AV node is responsible for most of the delay between the P wave and the QRS complex.

        2. On the intracardiac electrogram, the delay in the AV node is represented by the P wave to His bundle spike interval (the A—H interval).

        3. Disturbances of AV nodal conduction result in prolongation of the A—H interval.




      4. His bundle

        1. There is no surface representation of His bundle activation; it is implied by a succeeding QRS complex.

        2. On the intracardiac electrogram, careful positioning of an electrode cam demonstrate a small deflection coincident with the activation of the His bundle.

        3. The time between the His bundle spike and the QRS complex is the H—V interval.

        4. The sum of A—H and H—V intervals equals the PR interval.

      5. Bundle branches

        1. Depolarization of the right and left bundles produce the QRS complex on the surface ECG.

        2. Defects of bundle branch conduction were discussed on Day 2.

  2. AV conduction abnormalities

    1. First degree AV block (Day 3-01) (Day 3-02)

      1. In first degree AV block, the PR interval > 200 msec.

      2. The PR interval is dependent on heart rate, so that at very slow rates, a PR interval > 200 may be normal.

      3. First degree AV block is almost always due to a prolongation of the A—H interval.

    2. Second degree AV block

      1. Type I (Wenckebach) (Day 3-03) (Day 3-04)

        1. In second degree AV block type I, there is progressive prolongation of the PR interval until there is a dropped QRS complex.

        2. The Wenckebach phenomenon usually produces group beating of the QRS complexes.

        3. In the His bundle electrogram, there is progressive prolongation of the A—H interval until there is no His spike produced.

        4. The H—V interval is usually normal.

      2. Second degree AV block type II (Day 3-05) (Day 3-06)

        1. In second degree AV block type II, there are regular P waves with an occasional loss of the QRS complex.

        2. The PR interval does not change before the conducted beats.

        3. On the His bundle electrogram, this type of block is usually associated with an intermittent failure of H—V conduction.

    3. Third degree AV block (Day 3-07) (Day 3-8)

      1. In third degree AV block, there is complete failure of conduction ...

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