Oct 27, 20223 min read

The Zombie Heart

Updated: Oct 3

Bradyarrhythmias Explained for the Cardiac Electrophysiology Pro

Bradyarrhythmia:

Depressed Automaticity

There are TWO mechanisms of bradyarrhythmias that can turn your sweet patient into a zombie.

Failure of impulse generation
Failure of impulse propagation

Failure of impulse generation is due to insufficient generation of intrinsic signals by the

SA node.

Failure of Impulse Generation

An insufficient number of electrical impulses by the SA node is the failure of impulse generation and is the most common cause of bradyarrhythmia. Sick sinus syndrome is an example of failure of impulse generation.

Examples of Failure of Impulse Generation:

Sinus bradycardiaSinus pauses
Sick sinus syndrome

Failure of Impulse Propagation

Failure of impulse propagation is caused by delay or block of an electrical impulse through the SA or AV Node.

Examples of Failure of Impulse Propagation:

Sinoatrial exit block
First-degree heart block
Second-degree heart block
- Mobitz Type I
- Mobitz Type II
Third-degree heart block

When the SA node does produce a sufficient heart rate, bradycardia may still exist as a product of the failure of impulse propagation. This occurs when the electrical impulse fails to conduct, usually through the SA or AV node, known as heart block.

Mechanism of Bradyarrhythmia:

Dependent on the degree of delay or conduction block

To appreciate impulse failure, let’s start with the basics of anatomical and functional pathophysiology associated with bradycardia.

SINUS NODE - failure of impulses generated by the SA node
INTRA-ATRIAL TRACTS - impaired signal propagation
AV NODE - abnormal conduction velocities and refractory periods - impairing conduction
HIS BUNDLE - abnormal conduction velocities and refractory periods - impairing conduction

ECG CONCEPTS: From Limb to Limb

Buy Now

Q. How do you diagnose failure of impulse generation?

A. Look at the starting line!

ASSESSMENT: SA Node

This can be helpful in patients with suspected sick sinus syndrome (SSS) or inappropriate sinus bradycardia. Some of the key findings would be prolonged SNRT > 1500ms and would be diagnostic in patients with symptoms suggestive of bradycardia:

Syncope
Lightheadedness

This would also be a diagnostic finding in symptomatic patients with suspected sinus node dysfunction. Keep in mind that an EP Study assessment of sinus node function may be considered when the diagnosis remains uncertain after noninvasive evaluations.

Conduction Assessment Pacing Protocols Revealed: The Zombie Heart Part II

Sinus Node Recovery Time (SNRT)
Corrected Sinus Node Recovery Time (CSNRT)
Autonomic Testing
Sinoatrial Conduction Time (SACT)

Failure of impulse propagation can occur anywhere along the conduction system. From the AV Node down to the fascicles. As failure occurs further down the line, more zombie-like behavior may be seen. These can be very slow heart rates as the backup pacemaker cells have to take over.

Even though impulse generation is the leading culprit in brady arrhythmias, the failure may fall farther down the chain of command. In other words, someone is running the show, but the message is not being delivered in a timely manner or at all.