Heart Rhythms Part 6: Blocks

A heart block is a disease in the electrical system of the heart. This is different than coronary arterial disease. The muscle tissue and vascular system is ok but the electrical impulses are not traveling in a normal fashion through the heart.

There are 3 main types of blocks, conveniently, first, second and third degree blocks. Second degree blocks are then broken down into two subtypes. We'll get to those in a minute.

First Degree Blocks 

More correctly called a first degree atrioventricular block, it is a condition in which the PR interval (the length between the beginning  P wave and the beginning of the QRS complex) is lengthened beyond 0.20 seconds.

What that means is that the electrical impulse generated in the SA node is delayed by the AV node before going on to the ventricles. Management includes identifying and correcting any possible electrolyte imbalances or withholding any offending medications.

Second Degree Blocks

A second heart block is a conduction block between the atria and ventricles. A second-degree AV block is when one or more (but not all) of the atrial impulses fail to make it to the ventricles due to impaired conduction.

There are two types of second degree heart blocks, types 1 and 2. A type 2 heart block is considered to be a much more malignant block than a type 1.

Second Degree Heart Block Type 1

A second degree heart block type 1 is also known as a Mobitz 1 or a Wenckebach. 

A Wenckebach is characterized on the EKG by having an increased amount of time between the P wave and the QRS complex until a QRS complex is dropped, or missed. This is because of a diseased AV node.

The arrows point to the P waves. Not the increasing distance between the P waves and the QRS complexes.

Second Degree Heart Block Type 2

A second degree heart block type 2 is also called a Mobitz 2 block. A Mobitz 2 is characterized on the EKG by having intermittent QRS complexes dropped without a change in the PR interval. When this happens in a regular pattern the number of P waves to QRS complexes is stated as in 3:1, meaning 3 P waves for every QRS complex. 

A second degree type 2 block is caused by a a disease of the His-Purkinje System. It may also rapidly progress into a complete heart block.

Treatment may require an implanted pacemaker.

Third Degree Blocks

A third degree heart block is also known as a complete heart block. A complete heart block is a medical condition in which the electrical impulse generated in the SA node does not propagate to the ventricles. On the EKG this is characterized by having  regular P waves and regular QRS complexes but they are independent of one another. Essentially there are two separate heart rhythms on the EKG. One for the upper chambers of the heart and one for the lower ones. Usually the rate of the P waves will be higher than that of the QRS complexes. Some of the P waves will be buried behind the QRS complexes.

The most common cause is coronary ischemia. Treatment is an implanted pacemaker.

Heart Rhythms Part 1: Basic Anatomy
Heart Rhythms Part 2: Sinus Rhythms
Heart Rhythms Part 3: Junctional Rhythms
Heart Rhythms Part 4: Ventricular Rhythms
Heart Rhythms Part 5: Premature Beats
Heart Rhythms Part 6: Heart Blocks