Normal Sinus Rhythm With Junctional Escape Beats: ECG Findings

Figure 1

Figure 2

Figure 3

A 68-year-old woman with hypertension complains of intermittent dyspnea and light-headedness. She is asymptomatic during the evaluation. Vital signs are normal, but an irregularly irregular pulse is noted on examination as well as on the telemetry monitor. The 12-lead ECG is shown here; the ECG machine printout reads "atrial fibrillation." The patient has no history of this arrhythmia.

Do you agree with the computer-generated diagnosis? Which of the following do you think best explains the ECG findings?

• Atrial fibrillation.
• Normal sinus rhythm with premature junctional contractions.
• Normal sinus rhythm with junctional escape beats.
• Second-degree atrioventricular block, Mobitz type I.
• Normal sinus rhythm with premature atrial contractions.

(Answer and Discussion on next page.)

Figure 1

Figure 2

Figure 3

Answer: Normal sinus rhythm with junctional escape beats

What The ECG Shows

The ECG shows an irregularly irregular rhythm that is slightly bradycardic but nearly normal in rate (Figure 1). There appear to be P waves preceding some, but not all, beats (all except 5 and 8). Beats 5 and 8 have P-like deflections immediately preceding the QRS complexes, but these are too close to the QRS complexes to conduct.

INITIAL STEPS IN RHYTHM ANALYSIS

The first step in deciphering a tricky narrow-QRS-complex rhythm is to ask for a 12-lead ECG, if patient stability allows, since atrial activity may be obscure in certain leads. Imagine trying to determine in this patient whether P waves were present if you were given only a single-lead rhythm strip from lead III or lead V₁!

If the rhythm diagnosis remains obscure after reviewing the 12-lead tracing, it is best to generate a rhythm strip using the 12-lead ECG machine, incorporating the leads that best demonstrate atrial activity in that patient. Here, these would include leads II and V₅.

It is also often helpful to obtain a previous ECG for comparison. This strategy is frequently used to evaluate T-wave patterns in patients with possible cardiac ischemia, but it is underutilized in settings such as this. A previous ECG will demonstrate what the patient's P waves normally look like and thus help determine whether minor, difficult-to-classify "atrial" deflections are indeed artifact or coarse atrial fibrillation rather than P waves.

Lastly, the ECG machine may be manipulated to increase the size of all the complexes, thus making minute P waves larger. This is referred to as increasing the "standardization," or calibration, of the tracing to "double standard" (ie, 20 mm/mV, rather than the default of 10 mm/mV). Increasing the calibration to double standard results in the doubling in size of all cardiac waveforms, as well as of the plateau-shaped waveform at the extreme left of each row of waveforms on the tracing. Normal standardization is reflected by a plateau-shaped waveform that is 2 large boxes in amplitude (or 20 small squares, as in these tracings); "double standard" shows a waveform 4 large boxes (or 40 small squares) in amplitude.

DECIPHERING THE IRREGULAR RHYTHM

"Regularly irregular" versus "irregularly irregular." An irregular, narrow-QRS-complex rhythm evokes a wide differential diagnosis (Table).¹ To narrow the differential, first determine whether the rhythm is irregularly irregular or regularly irregular. The rhythm in this tracing (shown in Figure 1 with the ventricular beats/ QRS complexes numbered on the lead II rhythm strip at the bottom) appears to be irregularly irregular. If it were regularly irregular, several ECG diagnoses would move up in the differential diagnosis: namely, normal sinus rhythm (NSR) with a regular bigeminal, trigeminal, or quadrigeminal pattern, and second-degree atrioventricular (AV) block in a fixed pattern (eg, 2-to-1 [meaning 2 atrial beats for every ventricular beat], 3-to-1, and so forth). Because the tracing shows an irregular pattern of irregularity, all entities in the differential diagnosis are still viable.

References:

REFERENCE:

1.

Harrigan RA. Dysrhythmias at normal rates. In: Chan TC, Brady WJ, Harrigan RA, et al.

ECG in Emergency Medicine and Acute Care.

Philadelphia; Elsevier Mosby; 2005:31-35.