Study Notes on Bradycardia and Conduction Disorders

Chapter 13: Bradycardias

Definition

  • Bradycardia is defined as a slow heart rate, specifically when the frequency is less than 60 beats per minute.

  • Clinically, only symptomatic bradycardia is of concern. Bradycardia can occur as a normal phenomenon (e.g., sinus bradycardia in well-trained athletes).

  • If pathological in nature, bradycardia can be caused by disorders affecting the heart's pacemaker and conduction systems.

  • The determination of the need for pacemaker therapy hinges on the presence of symptoms associated with bradycardia.

Diagnosis

  • Electrocardiogram (ECG): Often the primary tool for diagnosis.

  • In non-conclusive cases, further investigations such as long-term Holter monitoring or implantation of a loop recorder may be required.

Symptoms

  • Frequently asymptomatic.

  • Weakness.

  • Reduced exercise tolerance.

  • Dyspnea (shortness of breath).

  • Dizziness.

  • Presyncope (near fainting).

  • Syncope (fainting).

Disorders of Pacemaker Function

  • Sinus Bradycardia: Characterized by a persistently low heart rate due to a slow frequency from the sinus node. While many patients remain asymptomatic, symptoms such as dyspnea during exertion may arise due to chronotropic incompetence (inability of heart rate to increase appropriately during exercise).

  • Sick Sinus Syndrome: An umbrella term encompassing sinus bradycardia, sinus arrest, and SA block.

  • Tachycardia-Bradycardia Syndrome: Involves alternating fast atrial rhythms and sinus arrest or bradycardia. The rapid atrial frequency is often attributed to paroxysmal atrial fibrillation, leading to an “overdrive” effect on the sinus node followed by long pauses when normal sinus rhythm resumes.

Disorders of Conduction

  • Sinoatrial Conduction Disturbances: The conduction of the sinus node’s activity to the atria may be impeded despite a normal sinus rhythm.

    • 1st-degree SA Block: No visible changes on surface ECG. The sinus rate is good, but conduction to surrounding atrial tissue is delayed.

    • 2nd-degree SA Block:

    • Mobitz Type I (Wenckebach Type): Conduction time from the sinus node to the atria progressively increases until one sinus activation fails to conduct. PP intervals shorten until a P wave is dropped, where the longest PP interval is shorter than twice the preceding PP intervals.

    • Mobitz Type II: Regular PP intervals with an occasional dropped P wave; dropped PP interval equals double the preceding PP interval.

    • 3rd-degree SA Block: No P waves visible, indistinguishable from sinus arrest on surface ECG.

  • Atrioventricular (AV) Conduction Disturbances: The principal question regarding AV blocks is the blockage location within the conduction system.

    • Supra-Hisian Block: Located within the AV node; typically causes benign bradycardia. Junctional tissue assumes pacemaking duties at a rate of 40-50 bpm with a narrow QRS complex. Symptoms tend to be mild with a low risk of syncope or sudden cardiac death.

    • Infra-Hisian Block: Below the AV node; causes malignant bradycardia and can result in syncope or sudden cardiac death. Complete blocks in or below the His bundle lead to very slow, wide-QRS ventricular escape rhythms often preceded by long pauses, resulting in hemodynamic instability.

1st-degree AV Block

  • Characterized by a prolonged PR interval, where every P wave is followed by a QRS complex.

  • PQ interval is greater than 200 ms.

  • Causes may include delayed conduction within the atria, AV node, His bundle, or bundle branches.

2nd-degree AV Block

  • Mobitz Type I (Wenckebach):

    • PR intervals progressively lengthen until a P wave fails to be conducted to the ventricles.

    • The increment of PR prolongation decreases progressively; RR intervals shorten.

    • The pause following the non-conducted P wave is less than double the previous RR intervals.

    • Typically caused by supra-His block, largely benign and infrequently leads to syncope.

  • Mobitz Type II:

    • PR intervals remain consistent; occasionally a P wave is not conducted to ventricles.

    • The pause for the non-conducted P wave is equal to twice the preceding RR interval.

    • Typically caused by infra-His block, associated with a poor prognosis and high frequency of syncope and critical bradycardia. Complete blocks can lead to very slow ventricular escape rhythms or even asystole.

2:1 AV Block

  • Every second P wave fails to be conducted to the ventricles.

  • Due to stable block ratio, ventricular activity remains regular.

  • Frequent causes include supra-His block, though infra-His block may also be responsible if a prolonged PR interval is observed in conducted beats.

High-Grade AV Block

  • Multiple consecutive P waves blocked before one is conducted to the ventricles.

  • Development of a stable block ratio (e.g., 3:1, 4:1) or variable block ratio.

  • Typically associated with severe symptoms, necessitating temporary pacemaker implantation.

Third-degree AV Block

  • Complete dissociation between atrial and ventricular activity.

  • No P waves reach the ventricles.

  • The ventricular rate is regular, sustained by escape rhythms from distal regions below the AV node (junctional rhythm at 40-50 bpm with narrow QRS complexes or ventricular rhythm at 30-40 bpm with wide QRS complexes).

  • Note: While second-degree AV blocks typically display irregular ventricular rhythms, third-degree AV block always shows a regular ventricular rhythm.

Paroxysmal AV Block

  • A rare and critical form of AV block characterized by temporary cessation of AV conduction.

  • Typically involves infra-His block leading to delayed appearance of an escape rhythm.

  • Associated with severe symptoms such as syncope.

  • Diagnosis is complex, as resting ECG may appear normal; invasive electrophysiological studies may yield no further insight. An implantable loop recorder may be utilized to capture the AV block during symptomatic episodes.