What is second degree heart block?

What is second degree heart block?

First degree heart block and complete heart block or third degree heart block have been covered earlier. What remains is second degree heart block or more precisely, second degree atrioventricular block. Please note that these are blocks in the electrical conduction within the heart and are heart rhythm disorders. They are quite different from the more familiar blocks in the blood vessels of the heart which cause heart attack. Still, electrical blocks can occur as a complication of a heart attack occasionally and they may co-exist. Electrical system of the heart has also been discussed in detail earlier.

In first degree AV block, all the signals from the sinus node are conducted to the lower chambers or ventricles, with an extra delay. Complete AV block is a situation in which none of the signals are conducted to the ventricles. Second degree AV block is in between, some signals are conducted while others are not. Sinus node is the natural pacemaker of the heart, located in the upper right chamber or right atrium. It gives out regular electrical signals for the orderly contraction of heart chambers. Normally, the signals are delayed slightly in the AV node at the junction between the upper and lower chambers. This is to ensure completion of contraction of upper chambers before the lower chambers start contracting.

Second degree AV block is detected in the electrocardiogram or ECG, which is the recording of the electrical activity of the heart. In the ECG, P waves represent the electrical activity of the upper chambers and QRS complexes represent the electrical activity of the lower chambers. When the interval between the two is prolonged, that is prolonged PR interval, it is first degree AV block. In addition, if some of the P waves do not get conducted to the lower chambers and there is skipping of one or more QRS complexes, it is second degree AV block. In third degree or complete heart block, none of the P waves get conducted to the ventricles. Then the ventricles are controlled by signals from AV node or the ventricles themselves.

There are two types of second degree AV blocks, known as type I and type II. Obviously, type II is more dangerous than type I and more likely to progress to complete heart block. In type I, there is progressive prolongation of PR interval, followed by skipping of a QRS. It is known as Wenckebach phenomenon after the person who described it. Classification into types I and II are named after Mobitz as Mobitz type I and type II. In Mobitz type II, there is no progressive prolongation of PR interval, but occasional QRS complexes are skipped. It may be associated with conduction disturbances in the branches of His bundle, which take signals to the ventricles from the AV node.

Usually the disease process in Mobitz type I block is in the AV node, above the His bundle. But in Mobitz type II, the disease is usually below His bundle, that is why it is associated with bundle branch block patterns on ECG. When the block is higher up, a subsidiary pacemaker with higher rate can arise from the lower part of the AV node if the block progresses to complete heart block. This produces a narrow QRS complex. In Mobitz type II, as the block is lower down, if it progresses to complete heart block the subsidiary pacemaker arises from the ventricles, which has a lower rate and more unstable rhythm. That is why Mobitz type II is considered to be more dangerous than Mobitz type I.