Editor’s note: The EKG Detective will be a monthly column dedicated to illustrating the benefits of utilizing deductive logic as a method for interpreting ECG tracings. The column will highlight and review all the basic ECG interpretations, before transitioning into a monthly interpretation challenge. See you next month and remember, it is always better to practice as a clinician rather than a technician.
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’s article, we will be looking at premature atrial contractions (PACs).
If you need a refresher on inductive and deductive logic, check out our introductory article.
Fill out the form on this page to download your copy of the EKG Detective Interpretation Checklist.
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:
- Rhythm regularity
- Rhythm rate
- P-wave criteria
- PR interval
- 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.
Premature atrial contractions
For this article, we will be looking at premature atrial contractions to illustrate the principle behind the EKG Detective (see Figure 2 for example of PAC).
Premature atrial contractions are given their designation because they occur early in the cardiac cycle. Prematurity associated with PACs is determined by measuring the R-to-R intervals of the underlying rhythm with the PAC and/or PACs falling prior to the next expected beat (see Figure 3 for an example of prematurity in the cardiac cycle).
For this specific article, we’ll only be focusing on premature atrial contractions and not the underlying rhythms within the EKG tracings.
Ectopic beats
All normal beats originate from the sinoatrial (SA) node. The term “ectopic” means an abnormal place or position. Thus, the term “ectopic beat” means any beat that originates outside of the SA node. PACs originate ectopically somewhere in the right or left atria, but not within the SA node.
EkG Category 1: Rhythm regularity
As mentioned, PACs occur early in the cardiac cycle, which causes irregularity within the rhythm.
Identifying PACs within the EKG Category 1: Rhythm regularity
- By the nature of where they occur within the cardiac cycle, PACs have a tendency to look out of place as it pertains to the overall ECG tracing.
- Focusing on the morphology of the P-waves is critical for identifying PACs. As P-waves for PACs originate from an ectopic focus, they will look different from the P-waves in the underlying rhythm. A different P-wave morphology, when compared to the underlying rhythm’s P-waves, is the first of three criteria required to identify a PAC (see Figure 4 for different P-wave morphology).
ECG Category 2: Rhythm rate
Again, we are not focusing on underlying rhythms, so the rate category will not apply for this article. As we aren’t looking at underlying rhythms, we will eliminate all the underlying rhythms and go directly to the “Ectopic Beats” section within the EKG Detective Checklist (see Figure 5).
- Ectopic Beats Category for P-wave criteria:
- The second of three criteria required to identify PACs is to determine whether the P-wave is located before the QRS complex. P-waves for PACs will be located in front of the QRS complex (see Figure 6).
- The final criteria of three required to identify a PAC is measuring the PR interval. The PR interval for PACs will be 0.12 seconds or greater (see Figure 6).
- With these final two P-wave criteria, we can eliminate premature junctional contractions (PJCs) within the Ectopic Beats section of the EKG Detective checklist – because PJCs have P-waves that are in different locations than PACs and/or the PR intervals are less than 0.12 seconds (see Figure 7).
- Ectopic Beats Category for QRS complex criteria
- Premature ventricular contractions (PVCs) can be eliminated because PVCs do not have P-waves and their QRS complexes are wide and bizarre (see Figure 8).
At this point in the deductive process, the only remaining choice is the premature atrial contraction we have been working from.
Identifying premature atrial contractions
This example illustrates how deductive logic is used to interpret PACs. At a glance, premature atrial contractions can be a challenge to identify. Focus on comparing P-waves within the underlying rhythm to the P-wave if there is a single premature atrial contraction or P-waves if there are multiple premature atrial contractions. The PACs will have a different looking P-wave with PR intervals measuring 0.12 seconds or greater.
See you next month, and remember it is always better to practice as a clinician rather than a technician.
