Pacemaker Basics - ECG Blog

Created by team@ekgdx.com on Jan. 26, 2024

Today's article is about the pacemaker basics. 

Analysis of the paced electrocardiogram (ECG) is important to the follow-up evaluation of patients with implanted pacemakers. The complexity and variability of pacemaker algorithms make the diagnosis of paced ECGs considerably more challenging compared to the interpretation of typical ECGs. 

In order to achieve an adequate interpretation of a paced ECG, it is imperative to know the basics. That is why I am going to dedicate this article to such an important topic. I will explain it with simple steps and classic examples so you can understand better. I hope you find it useful.

I consider it important to highlight that the ECG tracings were created using the unique EKGDX technology.

Let’s talk about the steps.

Step 1: Identify the chamber(s) paced.

Determine the relationship between pacing spikes and P waves, as well as QRS complexes: A spike occurring before the P wave usually indicates atrial pacing, while a spike occurring before the QRS complex typically indicates ventricular pacing. Let's look at the classic examples.

 

 

 

Step 2: Identify the timing intervals from two consecutively paced beats.

A-A interval in the setting of atrial pacing

 

V-V interval in the setting of ventricular pacing

 

A-V and V-A intervals in the setting of dual-chamber pacing

 

Step 3: Identify the chamber(s) sensed.

Atrial pacemaker: Adequate atrial sensing is confirmed when intrinsic atrial activation (native P wave) is followed by either (1) a native P wave occurring at an interval shorter than the A-A interval or (2) an atrial-paced beat that occurs after an interval equal to the A-A interval.

A-A intervals in the setting of atrial pacemaker 

 

Ventricular pacemaker: For proper ventricular sensing: It is confirmed when intrinsic ventricular activation (native QRS complex) is consistently followed by either (1) a native QRS complex occurring at an interval shorter than the V-V interval or (2) a ventricular-paced beat occurring after an interval equal to the V-V interval.

V-V intervals in the setting of ventricular pacemaker 

 

Dual-chamber pacemaker: Atrial sensing is confirmed when intrinsic atrial activation (native P wave) is consistently followed by either (1) a native QRS complex occurring at an interval shorter than the A-V interval or (2) a ventricular-paced beat occurring at an interval equal to the A-V interval.

A-V intervals in the setting of dual-chamber pacemaker 

 

Dual-chamber pacemaker: Ventricular sensing is confirmed when intrinsic ventricular activation (native QRS complex) is consistently followed by either (1) a native P wave occurring at an interval shorter than the V-A interval or (2) an atrial-paced beat occurring at an interval equal to the V-A interval.

V-A intervals in the setting of dual-chamber pacemaker 

 

Step 4: Identify pacemaker malfunction.

Sensing abnormalities: Undersensing occurs when, based on timing intervals, pacing spikes that should have been inhibited by a native P wave or QRS complex were not. This leads to a paced beat that appears earlier than expected.

For ventricular pacing, undersensing is evident when a native QRS complex is followed by a ventricular-paced beat at an interval less than the V-V interval.

Undersensing in the setting of ventricular pacing

 

Oversensing is identified when, according to timing intervals, pacing spikes that should have been initiated after a native P wave or QRS complex were not. This leads to a paced beat that appears later than expected. In the case of ventricular pacing, oversensing occurs when a native QRS is succeeded by a ventricular-paced beat at an interval significantly greater than the V-V interval.

Oversensing of the T wave may occur, wherein the T wave is mistakenly sensed as the QRS complex.

Oversensing T wave 

 

Failure to capture (Atrium or Ventricle) occurs when a pacing spike is released but fails to stumilate the atrium or ventricles. In other words, pacing spikes are not followed by a P wave or QRS complex.

Failure to capture the ventricle

It's important to note that the appearance of ventricular-paced beats in a surface EKG does not necessarily rule out the presence of a dual-chamber pacemaker. Ventricular-paced beats could result from either a single-chamber ventricular pacemaker or a dual-chamber pacemaker in which ventricular spikes are timed to follow P waves (DDD pacemaker).

I share the content of this article in Twitter (X) and it received more than 100,000 views in less than 24 hours 😳.

Remember: This article is focused on the basics. The world of pacemakers is as deep as the ocean, but this knowledge is necessary to understand the most complex scenarios.

WELL, that's it for today's article. I hope it has been useful to you. If you have a friend or colleague who has asked you about this topic, send them this article. See you at the next one.

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Thanks for reading this blog.

Dr. Roig

 

Pacemaker Basics