Heart block

Definition: Heart block is a condition in which the conductive tissue of the heart fails to conduct impulse normally that results in altered rhythm of the heart. A heart block is a disease in the electrical system of the heart. This is opposed to coronary artery disease, which is disease of the blood vessels of the heart. While coronary artery disease can cause angina (chest pain) or myocardial infarction (heart attack), heart block can cause lightheadedness, syncope (fainting), and palpitations.

Classifications:

1.      SA block.

2.      AV block.

·         First-degree heart block.

·         Second-degree heart block.

·         Third-degree heart block.

3.      Bundle branch block (Hemi block):

·         Right bundle branch block (RBBB)

·         Left bundle branch block (LBBB).

4.      Purkinjee block.

Aetiology:

[A]Acquired:

·         Focal fibrosis.

·         Rheumatic carditis.

·         Coronary artery disease.

·         Syphilitic carditis.

·         Digitalis toxicity.

·         Atrial fibrillation.

[B]Congenital: VSD.

Clinical features:

First degree heart block: Every impulse from atria is conducted to ventricle with delay.

·         No symptoms.

·         No sign.

·         Only diagnosed by ECG.

Causes:

·         Rheumatic carditis.

·         Diptheric carditis.

·         Digitalis toxicity.

·         Other causes of myocarditis.

·         After MI (esp infection MI)

ECG shows: Prolonged PR interval (>0.2 sec)

Second degree heart block: In this condition dropped beats occure because some impulses from the atrial fail to get through the ventricules.

·         Mobitz type-I: Here progressive prolongation of P-R interval occurs until a drop beat.

·         Mobitz type-II: Here P-R interval is constant, but some P waves are not conducted. Pulse is slow and regular.

Third degree heart block: (Complete heart block) When A-V conduction fails completely, the atrial & ventricles beat independently (Av dissociation). Ventricular activity is maintained by an escape rhythm arising in the bundle of His (narrow QRS complexes) or the distal conducting tissue (Broad QRS complexes)

Etiology of complete heart block:

1.      Congenital.

2.      Acquired.

·         Idiopathic fibrosis.

·         MI/ischemia.

·         Inflammation:

Acute (e.g. aortic root abscess in infective endocarditic)

Chronic (e.g. Sarcoidosis)

·         Trauma (e.g. Cardiac surgery)

·         Drugs (e.g. Digoxin,β-blocker)

Clinical features:

Symptoms:

1.      May be asymptomatic.

2.      Chest pain.

3.      Breathlessness.

4.      Extreme fatigability.

5.      Stroke Adam’s syndrome.

Signs:

1.      Pulse: Bradycardia (<40/min) which does not ↑ on exercise.

2.      BP: Systolic=high, Diastolic=Low, wide pulse pressure.

3.      JVP: Rasied and occasionally showing cannon wave.

4.      Heat: Varying intensity of the first heart sound.

5.      ECG findings:

·         P and QRS complexes are independent.

·         P-P interval regular with atrial rate 60-50%.

·         R-R interval regular with ventricular rate 25-50/min.

·         P-R interval irregular.

·         QRS complex: narrow in AV & wide in purkinje block.

Management:

[A]AV blocks complicating acute MI:

a)      If the patient remains well no treatment is required.

b)      In first degree heart block: no treatment but the patient must be close monitored.

c)      In 2^nd degree heart block: Atropine (0.6 mg i/v repeated as necessary). If this fails: Temporary pacemaker.

d)     In 3^rd degree (complete) heart block:

·         Bed rest and oxygen inhalation.

·         Atropine (0.6 mg i/v, repeated as necessary).

·         Inj. Isoprenaline (1-5 mg) in 500 ml of 5% DA i/v.

·         To prevent recurrence: Use long acting isoprenaline (30mg) 4 times daily or

·         Correction of acidosis by 7.5% NaHC.

e)      Complete heart block after infraction:

·         Atropine (0.6 mg) i/v to increase the heart rate.

·         Temporary pacemaker, or

·         Corticosteroid in bedside for 2 weeks.

·         Glucose & insulin to improve cardiac function.

f)       Chronic AV block:

·         No treatment: for asymptomatic first degree or Mobitz type-I 2^nd degree AV block.

·         Permanent pacemaker: for asymptomatic Mobitz type-II 2^nd degree or complete heart block.

Reference:

1.      Davidson’s Principle and Practice of Medicine, 21st edition.

2.      Wikipedia the free encyclopedia.