ECG Blog #307 — No Symptoms, But a Slow Rate

The ECG in Figure-1 — was obtained from a man in his 40s, who was referred to the ED (Emergency Department) for a "slow rhythm". The patient was asymptomatic at the time this ECG was recorded.

• How would YOU interpret this rhythm?
• What are your diagnostic considerations?

Figure-1: 12-lead ECG and long lead rhythm strip recorded on an asymptomatic man in his 40s.

My Thoughts on Figure-1:

Intuitively applying the Ps, Qs, 3R Approach for assessment of the rhythm in Figure-1 (See ECG Blog #185) — my initial impression of this tracing (that I made within the first ~15 seconds of seeing this ECG) — was the following:

• There is a slow, supraventricular (narrow QRS) rhythm .
• This rhythm is irregular.
• There is some variation in QRS morphology (seen best in the long lead V1 rhythm strip). 
• P waves are present — and at least some of these P waves are not conducting.
• In a quick overall glance at this 12-lead tracing — there do not appear to be acute ST-T wave changes (at least nothing that might require prompt cath or thrombolytic treatment).
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• NOTE: The clinical goal of this quick initial look at today's tracing is meant to see IF there are any priorities that need to be immediately addressed. That said — since this patient was asymptomatic (therefore hemodynamically stable) — it is likely that he has been in this rhythm for at least some period of time — and by definition, this provides us with a moment for more careful analysis.

PEARL #1: In applying the Ps, Qs, 3R Approach — I have found the simple act of identifying all P waves to be invaluable for guiding me toward the right diagnosis.

• Using calipers greatly facilitates identifying all of the P waves (including those partially or totally hidden within the QRS or ST-T wave of certain beats).
• The P waves are small everywhere in Figure-1. That said — they are BEST seen in the long lead II rhythm strip. Setting calipers at the P-P interval between any 2 P waves that are clearly seen in the long lead II rhythm strip — readily allows you to "walk out" this interval throughout the rhythm strip (RED arrows in Figure-2).

PEARL #2: It is common to see slight irregularity in the P-P interval in association with slow sinus rhythms (regardless of whether or not some degree of AV block is present). Awareness of this sinus arrhythmia facilitates finding sinus P waves that may be partially hidden (ie, such as the earlier-than-expected P wave seen immediately after the QRS complex of beat #6 in Figure-2).

CHALLENGE: Look at the RED arrows in Figure-2 that highlight all sinus P waves in this rhythm strip:

• Are any of these P waves conducted to the ventricles? IF so — Which P waves in Figure-2 appear to be conducted?
• Are there any P waves that you know are not being conducted?

Figure-2: I've added RED arrows to highlight sinus P waves.

ANSWER: 

For clarity in Figure-3 — I've modified the colors of the arrows in the long lead rhythm strip (from Figure-2) — to highlight P waves that most probably are not conducting.

• The 3 P waves highlighted by the YELLOW arrows in Figure-3 are definitely not conducting. The first 2 of these arrows (ie, the YELLOW arrows in front of beats #1 and #4) — highlight P waves with a PR interval that is too short to conduct.
• The 3rd YELLOW arrow in Figure-3 highlights a P wave that occurs immediately after beat #6. We know that this P wave is also not conducting — because it clearly falls within the absolute refractory period.
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• Of the remaining 6 P waves — the 2 P waves highlighted by the PINK arrows (that occur nearly midway between beats #3-4 and between #5-6) — are highly unlikely to be conducted, because this would require an exceedingly long PR interval.
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• RED arrows highlight the remaining 4 P waves. Each of these remaining 4 P waves could be conducting.

PEARL #3: Perhaps the BEST clue that a P wave is being conducted to the ventricles — is when the same PR interval is seen before several beats.

• In Figure-3 — the PR interval preceding beats #2, 5 and 7 is identical. Therefore — each of the P waves in front of these beats is being conducted to the ventricles. I measure this PR interval as slightly more than 1 large box in duration (ie, ~0.22 second) — so, consistent with 1st-degree AV block.
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• QUESTION: Isn't the PR interval preceding beat #3 slightly longer than the PR interval before beat #2?

Figure-3: I've labeled the P waves in today's rhythm with 2 additional colors.

Putting It All Together:

We have essentially "solved" the arrhythmia in Figure-3:

• The underlying rhythm is sinus bradycardia and arrhythmia (with an overall ventricular rate ~50/minute). The PR interval of sinus-conducted beats is prolonged to 0.22 second — so there is 1st-degree AV block.
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• Some form of 2nd-degree AV block is present — because the on-time P waves highlighted by PINK arrows in Figure-3 fail to conduct despite having more than adequate opportunity to do so.
• The type of 2nd-degree AV block is Mobitz I (AV Wenckebach) — because in the one group of beats in which there are 2 consecutive P waves that do conduct, the PR interval progressively increases until a beat is dropped (ie, the PR interval before beat #3 is longer than the PR interval before beat #2 — and then the next on-time P wave highlighted by the PINK arrow after beat #3 is not conducted).
• Other features consistent with the Mobitz I form of 2nd-degree AV block in Figure-3 include: i) Statistics = The clinical reality that over 90-95% of all 2nd-degree AV blocks are of the Mobitz I type; ii) The QRS complex is narrow (whereas it is usually wide with Mobitz II); and, iii) Sinus-conducted beats manifest 1st-degree AV block (whereas it is more likely that the PR interval of conducted beats with Mobitz II will be normal).
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• Clinically — The importance of distinguishing between the Mobitz I and Mobitz II forms of 2nd-degree AV block — is that patients with Mobitz II are much more likely to need a pacemaker (See LINKS and the ADDENDUM below for more on the ECG diagnosis and clinical implications of the AV blocks).

What Then are Beats #1, 4 and 6?

We have emphasized that the P waves highlighted by YELLOW arrows in front of beats #1 and #4 have a PR interval that is too short to conduct. No P wave immediately precedes beat #6. Therefore — none of these beats are sinus-conducted.

• Since the QRS complex of beats #1, 4 and 6 is narrow — these beats are not ventricular in etiology.
• By the process of elimination, since beats #1,4,6 are neither sinus-conducted nor ventricular in etiology — they must be "escape" beats arising either from the AV node or from the Bundle of His. Because the R-R interval preceding beats #4 and #6 is long (ie, ~10 large boxes in duration — corresponding to an escape rate of ~30/minute) — these beats probably originate from "lower down" in the conduction system (ie, from the Bundle of His).
• In support of a His origin for these escape beats — is the decidedly different QRS morphology of beats #1, 4 and 6 in the long lead rhythm strips. While QRS morphology of junctional escape beats may differ slightly from QRS morphology of sinus-conducted beats — there usually is not as marked of a difference in morphology as we see in Figure-3.

PEARL #4: Although it is fairly easy to tell which P waves are (and are not) being conducted to the ventricles in Figure-3 — it is sometimes quite difficult to determine this. In such cases — recognizing slight variation in QRS morphology may provide an important clue that indicates which beats represent non-conducted "escape" beats — and, knowing this may greatly facilitate diagnosis of the rhythm etiology (See ECG Blog #63 for an example of this Pearl in action). 

PEARL #5: It is important to realize that there is transient AV dissociation in Figure-3. That is — None of the P waves highlighted by the YELLOW arrows are related to neighboring QRS complexes (because none of these P waves are conducted to the ventricles).

• This serves to illustrate that AV dissociation is not the same thing as complete AV block. The fact that no less than 4 of the P waves in Figure-3 are conducted to the ventricles rules out the possibility of complete AV block (in which none of the P waves would be conducted to the ventricles).
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• For more on distinction between AV dissociation vs complete AV block — See ECG Blog #191.

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The Laddergram:

For clarity — I've drawn a laddergram in Figure-4 of today's case, to illustrate the mechanism of this arrhythmia.

Figure-4: Laddergram of today's case. Mobitz I 2nd-degree AV block is diagnosed by progressive increase in the PR interval (from beat #2-to-beat #3) — until the P wave after beat #3 is blocked. Beats #1, 4 and 6 are "escape" beats. Although I drew these "escape" beats as originating from within the AV Nodal Tier — they may well originate from a lower level in the conduction system (ie, from the Bundle of His).

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CASE Conclusion:

Surprisingly, the 40-ish year old man in today's case was not symptomatic at the time he presented with his initial ECG (which I've reproduced in Figure-5).

• Presumably — the reason he came to the ED, was that someone noted a slow and irregular pulse.

Figure-5: The initial ECG in today's case. P waves are labeled with RED arrows.

WHY Did this Patient Present with Asymptomatic Mobitz I?

It is not common for patients to present with significant bradycardia and 2nd-degree AV block without any symptoms — especially in the younger adult age group of today's patient. As a result — this case presentation should raise a number of questions:

• Was the patient truly asymptomatic? Many patients deny or ignore symptoms — so a careful history in hope of discovering clues to the rhythm etiology is essential.
• The most likely etiology for a bradycardia with Mobitz I, 2nd-degree AV block — would seem to be a myocardial infarction that the patient was somehow unaware of. While the ECG in Figure-5 does not show obvious signs of recent or acute infarction — there are some subtle ECG findings. These include: i) A hint of ST segment coving and elevation in the 1 QRST complex that we see in lead aVL (corresponding to beat #3); and, ii) A taller-than-expected R wave appears in lead V3. These 2 findings could be consistent with recent postero-lateral infarction — though they are in no way definitive. Other leads show no more than nonspecific ST-T wave flattening.
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• Additional considerations for entities that might result in Mobitz I, 2nd-degree AV block are shown in Figure-6. If nothing in a careful history and thorough evaluation of this patient suggests any of these entities — cardiac cath should be considered, looking for underlying "silent" coronary disease that might be amenable to reperfusion.
• If nothing "fixable" is found — a pacemaker may ultimately be needed if there is further slowing of the overall ventricular rate and/or if symptoms develop.

Figure-6: Diagnostic considerations for a patient who presents in AV block (adapted from Mangi et al — StatPearls, 2021).

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Acknowledgment: My appreciation to Mubarak Al-Hatemi (from Qatar) for the case and this tracing.

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For MORE on Diagnosis of AV Blocks/AV Dissociation:

• ECG Blog #185 — Reviews the Ps, Qs, 3R Approach to Rhythm Interpretation.
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• ECG Blog #188 — for Review on How to Read (and Draw) Laddergrams.
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• ECG Blog #63 — Reviews a case of Mobitz I with Junctional Escape.
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• ECG Media Pearl #4 (4:30 minutes Audio) — The AV Blocks & WHEN to Suspect Mobitz I — See ECG Blog #186 —
• ECG Media Pearl #6 (12:00 minutes Video) — ECG Blog #189 — What type of AV Block? Detailed analysis of this challenging arrhythmia (including ECG Video with step-by-step analysis of this complex laddergram).
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• ECG Media Pearl #8 (8:20 minutes Video) — ECG Blog #191 — Distinguishing between AV Dissociation vs Complete AV Block.
• ECG Media Pearl #9 (5:40 minutes Video) — ECG Blog #192 — Reviews the 3 Causes of AV Dissociation.
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• ECG Media Pearl #19 (5:00 minutes Audio) — ECG Blog #202 — How to quickly rule out complete AV Block within seconds!
• ECG Media Pearl #41 (4:00 minutes Audio) — ECG Blog #224 — Reviews HOW to recognize Mobitz I within seconds when there is ongoing Inferior STEMI.

ADDENDUM (5/23/2022):

This 15-minute ECG Video — Reviews the 3 Types of 2nd-Degree AV Block — plus — the hard-to-define term of "high-grade" AV block. I supplement this material with the following 2 PDF handouts.

• Section 2F (6 pages = the "short" Answer) from my ECG-2014 Pocket Brain book provides quick written review of the AV Blocks (This is a free download).
• Section 20 (54 pages = the "long" Answer) from my ACLS-2013-Arrhythmias Expanded Version provides detailed discussion of WHAT the AV Blocks are — and what they are not! (This is a free download).

• Mobitz I ( = AV Wenckebach).
• Mobitz II.
• 2nd-Degree AV Block with 2:1 AV conduction.