What is complete heart block? Cardiology Basics

What is complete heart block? Cardiology Basics

When natural pacemaker impulses from the sinoatrial node fails to reach the ventricles and activate them, it is known as complete heart block. The block could be at the level of the atrioventricular (AV) node or below that.

Sinus node is a subepicardial structure located in the upper part of right atrium near the orifice of the superior vena cava. Signals from sinus node travel to the AV node through three internodal pathways. They are Wenckebach, Thorel and Bachmann bundles. Bachmann bundle gives a branch to the left atrium also.

AV node is located near the atrioventricular junction in the lower part of right atrium. It is a subendocardial structure, located at the apex of the triangle of Koch. Triangle of Koch is bounded by the tendon of Todaro, coronary sinus orifice and septal leaflet of tricuspid valve. AV node is a slow conducting tissue which causes the predominant portion of the physiological AV delay.

Bundle of His originates from the lower part of the compact AV node and traverses the AV junction. Normally atria and ventricles are electrically isolated and conduction occurs only through the bundle of His. Bundle of His is fast conducting specialised conduction tissue.

Bundle of His divides into right and left bundle branches which take signals to the corresponding ventricles. Left bundle branch has an anterior and posterior division, meant for the corresponding regions of the left ventricle.

If the signals conducted down from the sinus node are blocked above the bundle of His, it is called supra Hisian complete heart block. If it is below the bundle of His, it is known as infra Hisian complete heart block. Sometimes the block can be intraHisian as well. InfraHisian block has a higher risk as the subsidiary pacemaker in that case will be ventricular, with lower and unsteady rate.

SupraHisian complete heart block is recognized on the ECG by a narrow QRS rhythm, with relatively higher rate. This is because the subsidiary focus arises from the lower part of AV junction with higher automaticity. In infraHisian complete heart block, the subsidiary focus has a lower rate in the range of 20-40 beats per minute and has a wide QRS complex.

Heart rate becomes very low in complete heart block and the person may feel dizzy or even may become unconscious for a short period. Sometimes they can develop life threatening ventricular arrhythmias including polymorphic ventricular tachycardia and ventricular fibrillation. Cardiac arrest due to ventricular asystole may also occur. These are more likely in infraHisian complete heart block than in supraHisian complete heart block.

Complete heart block can occur congenitally. Then it is called congenital complete heart block. Congenital complete heart block can be associated with certain diseases in the mother like systemic lupus erythematosus or SLE, also called just Lupus. In this condition, maternal auto-antibodies cross over to the fetus in utero and cause damage to the fetal conduction system. Congenital complete heart block can also occur in levo-transposition of great arteries. Congenital complete heart block is usually supraHisian and has a higher ventricular rate with narrow QRS complex.

Several acquired diseases like myocardial infarction, myocarditis, spirochetal diseases like Lyme disease, infective endocarditis and age related degeneration of conduction system can cause complete heart block. In complete heart block due to inferior wall myocardial infarction, the involvement is usually supraHisian. Complete heart block due to anterior wall myocardial infarction is usually infraHisian, with poorer prognosis. In anterior wall infarction with complete heart block, usually there is a large myocardial infarction with propensity for left ventricular dysfunction.

Complete heart block is usually recognized in the ECG. P waves and QRS complexes will be regular, with atrial rate higher than the ventricular rate. PR interval will be totally varying, indicating atrioventricular dissociation. ECG shown here has all these features along with QT interval prolongation, which gives a risk for polymorphic ventricular tachycardia known as torsades des pointes. Clinically they will have low heart rate and intermittent cannon waves in the jugular venous pulse. Cannon waves occur when the atrial contraction occurs at a time when the ventricles are in systole, with closed atrioventricular valves.

Complete heart block in the fetus is detected by fetal echocardiography. There will be fetal bradycardia and rate of atrial contraction will be higher than that of ventricular contraction. In labour, fetal bradycardia may be mistaken as fetal distress if it is not detected earlier. Getting an ECG of the fetus is near to impossible, though ECG with scalp electrodes during labour is feasible. Another option when facility is available, is fetal magnetocardiography for documentation of complete heart block.

Complete heart block associated with acute myocardial infarction may be reversible. This is more likely in inferior wall infarction with supraHisian block. Only a temporary pacemaker to support the heart rate may be needed. Temporary pacemaker is kept outside the body and leads introduced through subclavian, jugular or femoral veins, into the right ventricle. If complete heart block is permanent and irreversible, due to other causes, like degeneration of conduction system, a permanent pacemaker is needed.

Permanent pacemakers are battery driven devices which give regular electrical signals to the heart, to increase its rate when needed. They are implanted under the skin, usually in the left infraclavicular region and connected to the heart using leads introduced through a subclavian vein puncture. Single chamber ventricular and dual chamber pacing can be used in complete heart block. In dual chamber pacing, right atrium is paced first, followed by the right ventricle after a short programmable AV delay. In those with associated left ventricular dysfunction due dyssynchrony of left ventricular contraction, another lead is placed in a tributary of the coronary sinus to pace the left ventricle as well. That is known as cardiac resynchronization therapy.