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EKG Detective: Sinus arrest vs. sinus exit block

Both sinus arrests and sinus exit blocks are characterized by an irregular rhythm associated with pauses; which is it?

Welcome back to the EKG Detective. This column is dedicated to illustrating the benefits of utilizing deductive logic as a method for interpreting EKG tracings. For this month鈥檚 article, we will be looking at the difference between sinus arrest and sinus exit block.

If you need a refresher on inductive and deductive logic, check out our introductory article.

Throughout this series, we will be using the EKG Detective Interpretation Checklist (see Figure 1). This checklist is intended to prompt providers through five sequential elements associated with basic EKG interpretation while working through the specific criteria for each element:

  1. Rhythm regularity
  2. Rhythm rate
  3. P-wave criteria
  4. PR interval
  5. QRS criteria

EKG rhythms will be eliminated as we identify criteria within the EKG tracing until there is only one probable interpretation. We will use this checklist to illustrate how deduction is used to interpret an ECG tracing. More practically, it can be used as an EKG interpretation job aid.

EKG Detective.png

Fill out the form on this page to download your copy of the EKG Detective Interpretation Checklist.

Sinus arrest and sinus exit block

For this article, we will be looking at sinus arrest and sinus exit block to illustrate the principle behind the EKG Detective (see Figure 2).

Figure 2 - Sinus Arrest & Sinus Exit Block.jpg

  • ECG Category 1: Rhythm regularity
    • Both sinus arrests and sinus exit blocks are characterized by an irregular rhythm associated with pauses.
      • To categorize a pause, you will see a gap in the rhythm, and the R-to-R intervals before and after the gap will be regularly spaced (see Figure 3). With the R-to-R intervals being consistently spaced before and after the gaps, both of our example EKGs meet the criteria for a pause.
      • As there are pauses, we can eliminate any rhythms not associated with pauses.
Figure 3 - Pauses.jpg

  • There are no ectopic beats because none of the complexes have abnormal P-wave criteria that are typically associated with PAC(s) or PJC(s), nor do they have different looking QRS complexes that are associated with PVC(s) (see Figure 4 for all eliminated rhythms from regularity of rhythm).
May 2024 Fig 4.png

  • ECG Category 2: Rhythm rate
    • Sinus arrests and sinus exit blocks can present with any heart rate, so no other rhythms can be eliminated due to the rate associated with these rhythms.
  • ECG Category 3: P-wave criteria
    Working through the 鈥淪pecific Criteria鈥 for P-waves:
    • Do the P-waves appear to be saw-toothed and/or flutter waves? As sinus arrests and sinus exit blocks do not have flutter-waves, we should eliminate atrial flutter.
    • Are the P-waves discernible? As P-waves are discernible in sinus arrests and sinus exit blocks, we should eliminate atrial fibrillation because it has no discernible P-waves.
    • Are there three or more different looking P-waves? As sinus arrests and sinus exit blocks have just one P-wave morphology, we should eliminate wandering atrial pacemaker.
    • Is there more than one P-wave for every QRS complex?鈥 As sinus arrests and sinus exit blocks have one P-wave for every QRS, we should eliminate all the heart blocks except for sinus arrest with 10 and sinus exit block with 10.
    • Are the P-waves in front or after the QRS complex? P-waves in sinus arrests and sinus exit blocks are in front of the QRS. Based upon identification of this criteria, none of the remaining rhythms should be eliminated (see Figure 5 for all the eliminated rhythms from P-wave criteria).
May 2024 Fig 5.png

Figure 5.

  • ECG Category 4: PR interval
    • As PR intervals for sinus arrests and sinus exit blocks are typically between 0.12-0.20 seconds and constant, we should eliminate sinus arrest with 10 and sinus exit block with 10 (see Figure 6).
May 2024 Fig 6.png

Figure 6.

There is no need to move onto EKG Category 5 鈥 QRS Criteria, because the only remaining choices are the sinus arrest and sinus exit block we have been working from.

Sinus arrest vs. sinus exit block pauses

To differentiate between sinus arrest and sinus exit block, you need to march the underlying R-to-R intervals across the pause. For a sinus arrest, the R-to-R interval does not match up when the rhythm resumes after the pause; whereas the R-to-R interval for a sinus exit block does match up when the rhythm resumes after the pause (see Figure 7).

Summary

This example illustrates how deductive logic is used to interpret sinus arrest and sinus exit block. For sinus arrest, the QRS complex interval does not match up after the pause; but for sinus exit block, the QRS complex interval does match up after the pause.

See you next month, and remember it is always better to practice as a clinician rather than a technician.

ECG Interpretation: Books & Resources
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An Illustrated Study Guide For Students
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Learn how to become an EKG Detective.

Bob Matoba, M.Ed., EMT-P is an associate professor at the College of Central Florida in Ocala. Bob鈥檚 career has spanned almost every aspect of the EMS profession, first as an EMT and paramedic for private ambulance companies, EMS coordinator for medical oversight, EMS system consultation in the private and public sector, all the way to the EMS chief for a metropolitan fire department. He has made it his mission to educate clinicians, rather than technicians. Bob is a monthly columnist for 91视频.com and has been a featured and contributing author for EMS World Magazine and JEMS.
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