Test Your Interpretive Skills

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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.

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Answer: Normal sinus rhythm with junctional escape beats

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.

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.

"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.

Is there a P wave? The next step is to look for an underlying sinus rhythm (ie, look for underlying P waves). This turns out to be the key to deciphering this rhythm. First, look for a consistent R-R interval; here it occurs between beats 2 and 3, 3 and 4, 6 and 7, and 9 and 10. This represents the underlying sinus rhythm; each of these complexes is preceded by a P wave. When a consistent P wave can be identified, atrial fibrillation is ruled out—even if the rhythm is irregularly irregular; true P waves are never evident in atrial fibrillation.

Next, look at the relationship of any outliers (here, ventricular complexes 5 and 8) to the underlying rhythm. If the outliers come late (ie, the R-R interval preceding the outliers is longer than the underlying R-R interval), then they represent junctional escape beats: impulses that originate in the AV node (junctional beats) after a sinus pause. If the outliers come early (ie, the R-R interval preceding these beats is shorter than the underlying R-R interval), then they represent premature beats: either atrial or junctional premature beats if the QRS complex width is less than 0.12 seconds (ie, if the QRS complex is not widened).

Here, the R-R interval that precedes complexes 5 and 8 is longer than the underlying R-R interval. Thus, the outliers represent junctional escape beats, and the ECG diagnosis in this patient is NSR with junctional escape beats.

If these beats had been early, the diagnosis would have been NSR with premature junctional contractions because the PR interval for beats 5 and 8 is less than 0.12 seconds, making it too short for there to be conduction from the atrium to the ventricle. If the PR interval had been 0.12 seconds or longer, then any premature beat would have been atrial in origin, and the correct rhythm diagnosis would have been NSR with premature atrial contractions.

Is there grouped beating? At first glance, this patient's lead II rhythm strip seems to show clusters of ventricular depolarizations: ventricular beats 1 through 4, 5 through 7, and 8 through 10 (see Figure 1). Classically, such "grouped beating" (a periodicity to the QRS complexes, so that they appear to be bunched) suggests second-degree AV block.

In Figure 2, the QRS complexes appear in groups of 3; however, this is not second-degree AV block. Notice that the third QRS complex in each group in Figure 2 is premature; this is not characteristic of AV block. Moreover, there is no evidence of any P waves that are not followed by a QRS complex, the hallmark of second- degree AV block. Instead, Figure 2 shows NSR with supraventricular (atrial or junctional) trigeminy: ie, every third beat is premature.

For second-degree AV block to be diagnosed, a P wave must be evident and the associated QRS complex must be "dropped," or absent. Because the conduction block is at or near the AV node in second-degree AV block, the P wave occurs as usual (reflecting normal sinoatrial node activity), but it is not followed by a QRS complex (reflecting the delay in conduction at or near the AV node). These findings are evident in Figure 3, which demonstrates second-degree AV block, type I. Note also that in second-degree AV block the P-P interval before the dropped beat is relatively constant. It may be long (greater than 0.20 seconds) or normal, but it should be constant.

Despite the computer interpretation of the ECG, this patient did not have atrial fibrillation. She was admitted to the telemetry unit for cardiac workup. Her subsequent ECGs continued to show no signs of ischemia, and her cardiac enzymes remained normal. Her symptoms having resolved, the patient was discharged for further outpatient workup.