SA and AV Nodal Conduction Abnormalities




Day 3: SA and AV Nodal Conduction Abnormalities



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  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 from the atria to the ventricles.



      2. The atrial rate is always faster than the ventricular rate.



      3. The escape rhythm may arise from the junctional area, in which case its rate will typically be 40–60, or it may arise from a ventricular focus with a rate of 20–40.


         


        DAY 3-01



         


        DAY 3-02



         


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        DAY 3-04



         


        DAY 3-05



         


        DAY 3-06



         


        DAY 3-07



         


        DAY 3-08



         



      4. Junctional escape rhythms have a narrow QRS complex (unless there is an accompanying bundle branch block), but ventricular rhythms will be wide (QRS > 120 msec).



      5. Third degree block is one form of A-V dissociation (see later in this chapter).



      6. There may be slight variation in the P-P intervals, with the P waves which surround a QRS complex being slightly closer together than those which do not (ventriculophasic sinus arrhythmia).



    4. Summary of AV block




      1. First degree AV block is usually caused by a prolongation of the A—H interval.



      2. Second degree AV block type I (Wenckebach) is caused by progressive prolongation of the A—H interval.



      3. Second degree AV block type II is usually caused by an intermittent failure of H—V conduction.



      4. Third degree AV block is usually caused by a complete failure of H—V conduction.



      5. In general, A—H prolongation is a benign clinical event, while abnormalities of H—V conduction represent serious clinical situations that usually require permanent pacing.


     



     



  3. AV dissociation




    1. AV dissociation is present when there are independent atrial and ventricular rhythms.



    2. Types of AV dissociation




      1. By default




        1. In this case, there is a failure of conduction from a higher pacemaker, so that a lower pacemaker takes over



        2. Third degree block is the principle example of this form of AV dissociation.



      2. By usurpation (Day 3-9)




        1. In this case, a lower pacemaker speeds up and usurps control from the higher pacemaker by virtue of being faster.



        2. Ventricular tachycardia (70% of which has AV dissociation) is an example of this form.


     


    DAY 3-09



     



  4. SA block




    1. The four types of conduction abnormalities associated with the AV node also exist for the SA node; however first degree, second degree type II, and third degree SA block cannot be identified on the surface ECG. (Day 3-10) (Day 3-11) (Day 3-12)



    2. Second degree SA block type I produces an identifiable pattern of group beating on the ECG, with P waves of the same morphology and unchanging PR intervals. (Day 3-13) (Day 3-14)


     



     



  5. Group beating of QRS complexes and examples of AV and SA nodal block. (Day 3-15) (Day 3-16)(Day 3-17) (Day 3-18) (Day 3-19) (Day 3-20) (Day 3-21) (Day 3-22) (Day 3-23)




DAY 3-10



DAY 3-11



DAY 3-12



DAY 3-13



DAY 3-14



DAY 3-15



DAY 3-16



DAY 3-17



DAY 3-18



DAY 3-19



DAY 3-20



DAY 3-21



DAY 3-22



DAY 3-23



Sample Tracings



ECG 1



ECG 2



ECG 3



ECG 4



ECG 5



ECG 6



ECG 7



ECG 8



ECG 9



ECG 10



ECG 11



ECG 12



ECG 13



ECG 14



ECG 15



ECG 16



ECG 17



ECG 18



ECG 19



ECG 20



SA and AV Nodal Conduction Abnormalities




Interpretations of Sample Tracings



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ECG 1



Atrial rate: 65



Ventricular rate: 65



Rhythm: Sinus rhythm with second degree SA block type I



P wave: Normal



PR interval: 200 msec



QRS complex:



 Axis: 0°



 Duration: 95 msec



 Voltage: Normal



 Morphology: Normal



ST segment: Normal



T wave: Normal



QT interval: 420 msec



U wave:



Diagnosis: Sinus rhythm with second degree SA block type I (note the identical P waves and the unchanging PR intervals)



ECG 2



Atrial rate: 60



Ventricular rate: 45



Rhythm: Sinus rhythm with second degree AV block type I



P wave: Normal



PR interval:



QRS complex:



 Axis: 30°



 Duration: 80 msec



 Voltage: Normal



 Morphology: Normal



ST segment: Normal



T wave: Nonspecific changes



QT interval: 450 msec



U wave:

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  4. Chamber Abnormalities and Intraventricular Conduction Defects

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Jan 13, 2019 | Posted by in CARDIOLOGY | Comments Off on SA and AV Nodal Conduction Abnormalities

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