Understanding Heart Block

Overview of Heart Block

• Heart block is an interruption in the heart's electrical conducting system.
• It is not a problem with the blood supply.
• It is an electrical issue, requiring an electrician instead of a plumber.

Normal Heart Contraction

• Heart contraction begins with a wave of depolarization across the myocardium.
• The SA node generates an impulse that spreads across the atria.
• The AV node transmits the impulse to the ventricles.
• The impulse travels down the bundle of His and up the Purkinje fibers.
• This causes synchronized contraction, first of the atria, then the ventricles.

Atrioventricular Nodal Block

• This involves a disruption between the atria and ventricles.
• It's described as "upstairs not talking to downstairs."
• It can result from elderly fibrosis or ischemia, where the AV node is starved of blood.

First-Degree Heart Block

• Impulse transmission from atria to ventricles is delayed at the AV node.
• An ECG will show a prolonged PR interval, which is the distance between the P wave and the R wave.
• A normal PR interval is 0.12-0.20 seconds (3-5 small ECG squares).
• In first-degree heart block, the PR interval is longer than 0.2 seconds (one big ECG square).
• Every P wave is still followed by a QRS complex.
• The impulse makes it through, but with a delay.

Second-Degree Heart Block

• The atria contract normally.
• Not every impulse reaches the ventricles, some are not transmitted.
• P waves are regular.
• A QRS complex occasionally drops a beat.
• Mobitz Type I (Wenckebach): The PR interval increases until a QRS complex is dropped.
• Mobitz Type II: The PR interval remains constant, but QRS complexes are still occasionally dropped..
• 2:1 Block: Only one out of every two atrial impulses (P waves) is conducted to the ventricles (QRS complex).

Third-Degree Heart Block (Complete Heart Block)

• The impulse is completely blocked.
• The atria and ventricles do not communicate.
• An accessory pacemaker initiates ventricular contractions (escape rhythm).
• There is no relationship between P waves and QRS complexes on the ECG.
• Atrial and ventricular rates can be calculated separately.
• The ventricular rate is typically slower.

Summary of Heart Blocks

• First-degree: Delay, shown by a prolonged PR interval.
• Second-degree: Dropped beat, includes Mobitz Type I, Mobitz Type II, and 2:1 block.
• Third-degree: A complete breakdown of the relationship, with no communication between atria and ventricles.