Heart Block

Definition -Heart block is an abnormal heart rhythm where the heart beats too slowly (bradycardia).

In this condition, the electrical signals that tell the heart to contract are partially or totally blocked between the upper chambers (atria) and the lower chambers (ventricles).

Normal conduction pathway- SA node -> atrial muscle -> AV node -> bundle of His -> Left and Right Bundle Branches -> Ventricular muscle

Types-

They are further classified as,

First degree heart block ( first degree AV block)
Second degree heart block (second degree AV block)
Third degree heart block (third degree AV block)

1. First degree heart block ( first degree AV block)-First-degree atrio-ventricular block (AV block), or PR prolongation, is a disease of the electrical conduction system of the heart in which the PR interval is lengthened beyond 0.20 seconds.

Causes of first- degree AV block-

Intrinsic avascular Necrosis (AVN) disease
Acute myocardial infarction (MI), particularly acute inferior wall MI Myocarditis
Electrolyte disturbances (eg, hypokalemia, hypomagnesemia)
Drugs (especially those drugs that increase the refractory time of the AVN, thereby slowing conduction)

First Degree Heart Block (1º)

SA Node – Normal
Normal P wave
AV Node conducts more slowly than normal
Prolonged PR Interval
Rest of conduction is normal
Normal QRS

Significance

Clinical significance - None
Treatment-None
Note – this can progress to 2º or 3º heart block

Second Degree Heart Block (2º)

A. Mobitz Type I (Wenkebach)

B. Mobitz Type II

A. Second Degree Heart Block (2º) Mobitz Type I (Wenkebach)

Conduction through the AV Node – progressively delayed until a drop beat is seen
Second-degree atrio-ventricular (AV) block, or second-degree heart block, is characterized by disturbance, delay, or interruption of atrial impulse conduction through the AV node to the ventricles.

Cause-

Drugs (beta-blockers, calcium channel blockers, amiodarone)
Cardiomyopathy
Rheumatic fever, myocarditis
Varicella-zoster virus infection
Rheumatic diseases
Hypoxia
Hyperkalemia
Hypothyroidism
Inferior wall myocardial infarction

PR Interval prolongs with each beat until a dropped beat is seen The PR Interval is NOT constant
After each dropped beat, the PR interval is normal and the cycle starts again

Significance

Clinical Significance - Slight symptoms e.g.. Lethargy, Confusion
Treatment -Pacemaker if during day &/or symptoms
No treatment if at night

Note – This can progress to 3º Heart Block

B. Second Degree Heart Block (2º) Mobitz Type II

Conduction through the AV node is constant.
PR interval is normal and constant
Occasionally a dropped beat is seen

Significance

Clinical significance – This is more significant disease
Treatment – Pacemaker

Note – This can progress to 3º Heart Block

3. Third Degree Heart Block (3º) (Complete)

Third-degree atrioventricular (AV) block, also referred to as third-degree heart block or complete heart block, is a disorder of the cardiac conduction system where there is no conduction through the atrioventricular node.

Complete failure of the AV Node
No impulses from Sinus Node will pass through to the ventricles Some part if the conducting system will take over as pacemaker of the heart (even a myocardial cell 10-15 bpm)
P wave rate – Normal
Ventricular rate – slow
Ventricular complex may be broad
Idioventricular rhythm
Complete dissociation between P waves & QRS

Significance

Clinical significance - Symptoms LOC, Confusion, Dizziness, Low BP •
Can lead to standstill, VT or VF (stokes Adams)
Treatment - pacemaker

Management

A. Antidysrhythmic Drugs-4 Classes

Class I- Sodium channel blockers (decrease conduction velocity in the atria, ventricles and His-Purkinje system). Ex:-Procainamide, Quinidine, Lidocaine (Xylocaine), Phenytoin.
Class II- Beta adrenergic blockers (decrease automaticity of the SA node, decrease conduction velocity in AV node). Ex: - Atenolol, Metoprolol.
Class III- Potassium channel blockers (Delay repolarization). Ex:- Amiodarone, Dofetilide.
Class IV- Calcium channel blockers (decrease automaticity of the SA node, delay AV node conduction). Ex:- Diltiazem, Verapamil.

B. Defibrillation:- Defibrillation is a therapeutic procedure in which a direct current (DC) electric shock is passed through the heart to depolarize the cells of the myocardium. When the myocardial cells repolarize, the SA node is usually able to resume the role of heart's pacemaker. The machine used for defibrillation and cardioversion called defibrillator.

C. Electrical Cardioversion:- The procedure of electrical cardioversion same as defibrillation delivered by defibrillator. It is the treatment of choice for patient with hemodynamically unstable ventricular or supraventricular tachydysrhythmias.

D. Implantable cardioverter defibrillator (ICD)- If patient is at the risk of future life-threatening dysrhythmias. Cardioverter defibrillator is an effective technology to decrease cardia mortality.

E. Pacemaker:- A pacemaker is an electronic device that stimulate the heart muscle through the electrical waves.

Pacemakers are generally used when a patient has slower than normal impulse formation or a rhythmác disturbance that causes symptom.

Nursing Priorities

Decreased cardiac output related to failure of the heart to pump enough blood to meet metabolic needs of the body as manifested by hypotension.
Acute chest pain related to decreased blood flow to myo-cardium through coronary arteries.
Ineffective tissue perfusion related to decreased cardiac output as manifestation by syncope.
Fatigue related to increased hypoxic tissue and slowed removal of metabolic wastes.
Excess fluid volume
Ineffective breathing pattern
Ineffective tissue perfusion
Activity intolerance
Impaired airway clearance

NURSING MANAGEMENT

Assess the high risk patients
Monitor ECG of the patient
Assess the family history of heart disease
Assess the history of smoking and alcoholism
Monitor lab values frequently especially serum cholesterol levels.
Assess for CAD
Monitor vital signs
Instruct to avoid high fat and oil rich diet