Complete heart block

1. First-degree AV block
   • AV conduction is delayed, producing a prolonged PR interval (> 0.20 sec).
   • It rarely causes symptoms.
2. Second-degree AV block
   • Mobitz type I block (‘Wenckebach’s phenomenon’):
     • There is progressive lengthening of the PR intervals, culminating in a dropped beat.
     • The cycle then repeats itself.
   • Mobitz type II block:
     • The PR interval of conducted impulses remains constant but some P waves are not conducted.
3. Third-degree (complete) AV block
   • AV conduction fails completely
   • The atria and ventricles beat independently (AV dissociation)
   • Ventricular activity is maintained by an escape rhythm arising in the AV node or bundle of His (narrow QRS complexes) or the distal Purkinje tissues (broad QRS complexes).

* Davidson’s Essentials of Medicine, 2nd Edition Page: 224

Note: Normal physiology – AV nodal delays

• The impulse, after traveling through the internodal pathways, reaches the A-V node about 0.03 second after its origin in the sinus node.
• Then there is a delay of another 0.09 second in the A-V node itself before the impulse enters the penetrating portion of the A-V bundle, where it passes into the ventricles.
• A final delay of another 0.04 second occurs mainly in this penetrating A-V bundle.
• Thus, the total delay in the A-V nodal and A-V bundle system is about 0.13 second.
• This delay, in addition to the initial conduction delay of 0.03 second from the sinus node to the A-V node, makes a total delay of 0.16 second before the excitatory signal finally reaches the contracting muscle of the ventricles.

Figure: The approximate intervals of time in fractions of a second between the initial onset of the cardiac impulse in the sinus node and its subsequent appearance in the A-V nodal system.

* Guyton and Hall Textbook of Medical Physiology, 13th Edition Page: 125

Recurrent syncope due to episodes of ventricular asystole is called Stokes-Adams attacks.

 * Pre-exam preparation for medicine, HN Sarker

It is the brief attack of syncope or blackout in a patient with complete heart block due to ventricular asystole.

Stokes-Adam attack may also occur in Mobitz type II heart block, ventricular tachycardia or fibrillation, sinoatrial disease.

Short Cases in Clinical Medicine, ABM Abdullah Page: 105

Stokes-Adams attacks due to complete heart block.

 * Pre-exam preparation for medicine, HN Sarker

1. Congenital
2. Acquired
   • Idiopathic fibrosis
   • MI/ischaemia
   • Inflammation
     • Acute (e.g. aortic root abscess in infective endocarditis)
     • Chronic (e.g. sarcoidosis, Chagas’ disease)
   • Trauma (e.g. cardiac surgery)
   • Drugs (e.g. digoxin, β-blocker)

* Davidson’s Principles and Practice of Medicine, 22nd edition Page: 593

Congenital heart block usually occurs, if the mother is suffering from SLE due to the presence of anti­Ro (SSA) antibody, which crosses the placenta and causes congenital heart block.

Short Cases in Clinical Medicine, ABM Abdullah Page: 104

The causes of Stokes-Adams attacks are:

• Complete heart block
• Mobitz type II second-degree AV block
• Sinoatrial disease.

 * Pre-exam preparation for medicine, HN Sarker

The features of complete heart block are:

• Symptoms
  • It may be asymptomatic or recurrent syncope (Stokes- Adams attacks)
• Signs
  • Bradycardia—Slow (25–50/min) and regular largevolume pulse
  • Intermittent cannon waves in JVP
  • Variable intensity of first heart sound.

 * Pre-exam preparation for medicine, HN Sarker

• May be asymptomatic
• Dizziness, sudden loss of consciousness or recurrent syncope (Stokes-Adam attack).

Short Cases in Clinical Medicine, ABM Abdullah Page: 105; Short and Long Cases in Clinical Medicine, HN Sarker Page: 69

• Syncope or blackout with or without preceding dizziness.
• During attack patient is unconscious, looks pale and may have convulsion.
• If asystole persists, there may be cyanosis, pulse is absent, incontinence of urine, pupil is fixed and dilated, plantar is extensor.
• Usually consciousness recovers rapidly followed by flushing.

Short Cases in Clinical Medicine, ABM Abdullah Page: 105

• Pulse
  • Bradycardia, 20-40 beats/min ( 40 beats/min), high volume, does not increase by exercise or injection atropine.
• Blood pressure
  • High systolic, normal diastolic and high pulse pressure.
• Neck vein
  • Cannon waves (large ‘a’ wave) may be present.
• Heart sounds
  • Variable intensity of first heart sound.
• Murmur
  • Systolic flow murmur (due to increased stroke volume).

Short Cases in Clinical Medicine, ABM Abdullah Page: 104

• Pulse rate is high in congenital complete heart block and does not require treatment.
• If the block occurs more proximally in AV node (narrow complex escape rhythm).

Short Cases in Clinical Medicine, ABM Abdullah Page: 105

Read details on the topic JVP at Examination of cardiovascular system

Variable intensity of first heart sound is due to loss of AV synchrony.

Short Cases in Clinical Medicine, ABM Abdullah Page: 104

Systolic flow murmur is due to increased stroke volume.

Short Cases in Clinical Medicine, ABM Abdullah Page: 104

The findings are:

• Complete AV dissociation (complete dissociation between P and QRS)
• Slow ventricular rate (25–50/min)
• PR interval variable but PP interval and RR interval fixed.

 * Pre-exam preparation for medicine, HN Sarker

Figure: Complete (third-degree) AV block. There is complete dissociation of atrial and ventricular complexes. The atrial rate is 80/min and the ventricular rate is 38/min.

Figure: First-degree AV block. The PR interval is prolonged and measures 0.26 s.

Figure: Second-degree AV block (Mobitz type I – Wenckebach’s phenomenon). The PR interval progressively increases until a P wave is not conducted. The cycle then repeats itself. In this example, conduction is at a ratio of 4 : 3, leading to groupings of three ventricular complexes in a row.

Figure: Second-degree AV block (Mobitz type II). The PR interval of conducted beats is normal but some P waves are not conducted. The constant PR interval distinguishes this from Wenckebach’s phenomenon.

* Davidson’s Principles and Practice of Medicine, 22nd edition Page: 571, 572

Permanent pacemaker is the treatment of chronic complete heart block whether symptomatic or asymptomatic.

 * Pre-exam preparation for medicine, HN Sarker

• Permanent pacemaker ( even after a single syncopal attack).
• During attack CPR should be done.

Short Cases in Clinical Medicine, ABM Abdullah Page: 105

Right bundle branch block

• Normal variant
• Right ventricular hypertrophy or strain, e.g. pulmonary embolism
• Congenital heart disease, e.g. atrial septal defect
• Coronary artery disease

Left bundle branch block

• Coronary artery disease
• Hypertension
• Aortic valve disease
• Cardiomyopathy

* Davidson’s Principles and Practice of Medicine, 22nd edition Page: 573

Right bundle branch block. Note the wide QRS complexes with ‘M’-shaped configuration in leads V, and V2 and a wide S wave in lead I.	Left bundle branch block. Note the wide QRS complexes with loss of the Q wave or septal vector in lead I and ‘M’-shaped QRS complexes in V5 and V6.

* Davidson’s Principles and Practice of Medicine, 22nd edition Page: 573