ECG Blog #306 — Alternating Beats

The ECG in Figure-1 — was obtained from a 60-year old woman, who presented to the ED (Emergency Department) with intermittent shortness of breath over the past week. No chest pain. The patient has a history of diabetes and hypertension.

• How would YOU interpret the ECG in Figure-1?
• Why are there "alternating" beats?

Figure-1: 12-lead ECG obtained from a 60-year old woman with intermittent dyspnea (but no chest pain).

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NOTE: Today’s tracing is another ECG “Quick Case” ( = EQC) — in that I’ll provide a more “time-efficient” account of my thought process (with goal toward expediting your interpretation within seconds rather than minutes)! Relevant links are at the bottom of the page.

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MY Thoughts on the ECG in Figure-1:

The technique for recording today's ECG provides us with a continuous look at the rhythm — but the leads switch halfway through.

• The advantage of this technique — is that it provides us with a simultaneous look at the first 9 beats in each of the 6 limb leads — and then, a simultaneous look at beats #10-thru-17 in each of the 6 chest leads.
• The disadvantage — is that we do not get to see the rhythm strip recorded entirely from the perspective of any one lead.

Even before applying the Ps, Qs, 3R Approach for assessing the rhythm (as described in ECG Blog #185) — I was "struck" by the picture of "alternating" beats in Figure-1. By this I mean — that the shape of each QRST complex changes every-other beat. In this sense — this is a bigeminal rhythm.

• I reviewed the concept of "Bigeminy" — and the common bigeminal rhythms in ECG Blog #243. Given the obvious widening of every-other-beat in Figure-1 — the principal diagnostic considerations would be distinction between: i) Ventricular bigeminy (in which every-other-beat is a PVC); ii) Atrial or junctional  bigeminy (in which every-other-beat is a PAC or PJC — with QRS widening resulting from either preexisting bundle branch block or aberrant conduction) — vs — iii) Sinus rhythm with a conduction defect such as bundle branch block occurring every-other-beat.

PEARL #1: As is so often the case — the simple act of labeling all P waves is often revealing (Figure-2).

• RED arrows in Figure-2 show that the underlying rhythm is a regular sinus tachycardia at ~120/minute. Doesn't the PR interval look to be the same in front of each of the 17 beats on today's tracing?

PEARL #2: If the reason for the alternating beats in Figure-2 is atrial, junctional or ventricular bigeminy — then the wider beats should be "premature" (ie, PACs, PJCs and PVCs are all characterized by their early occurrence before the next expected sinus beat).

• Therefore — the KEY to diagnosing the etiology of the rhythm in Figure-2, is to carefully measure the R-R interval from the beginning of a normally-conducted sinus beat — until the beginning of a wider sinus-conducted beat ( = the R-R interval marked "A" in both limb leads and chest leads).
• Then compare this interval "A" — to the R-R interval marked "B", which extends from the onset of a wider sinus-conducted beat — until the next normally-conducted (narrower) beat.
• In both limb leads and chest leads — interval "A" is precisely equal to interval "B". And since the PR interval preceding all beats in this tracing is the same — this confirms that all beats in Figure-2 are sinus-conducted with the same PR interval. The only thing changing — is that the QRS complex becomes wider every-other-beat — because all even-numbered beats in Figure-2 ( = beats #2,4,6,8,10,12,14,16) are being conducted with LBBB (Left Bundle Branch Block). 

Figure-2: I have labeled all P waves from Figure-1 — and compare the R-R intervals of alternating beats (See text).

Intermittent Bundle Branch Block:

We are used to seeing conduction defects (ie, RBBB, LBBB, IVCD, hemiblocks) occur with every beat. On occasion — conduction defects may be "rate-related" (usually in association with an increase in rate — in which the QRS widens when the rate accelerates to a certain amount — and then narrows again after the rate slows down).

• Conduction defects can also be intermittent. Usually this occurs with a "fixed" interval of time between beats that conduct normally, and wider beats that manifest the conduction defect (ie, most often showing the intermittent conduction defect  every 2nd, every 3rd, or every 4th beat). This is the situation with today's tracing — in which we see LBBB conduction every-other-beat.
• On occasion — the intermittent conduction defect may show random alternation between normal and impaired conduction, with no "fixed" interval between narrow and wider beats (See My Comment in the June 25, 2020 post in Dr. Smith's ECG Blog).

What We Can Learn from Intermittent BBB Conduction!

Because conduction defects alter the sequence of ventricular depolarization — the sequence of ventricular repolarization will also be changed! As a result — it will always be more challenging to evaluate ST-T wave changes in association with a conduction defect (especially with LBBB — which alters the initial vector of ventricular depolarization).

• PEARL #3: Today's tracing offers the unique opportunity to see the effect that LBBB may have in each of the 12 leads of an ECG. To facilitate visualizing this effect — I first color in BLUE the odd-numbered beats which are conducted normally (Figure-3). Doing so allows us to appreciate the even-numbered beats — which are conducted with LBBB.
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• NOTE: QRS morphology for the even-numbered beats in Figure-3 is typical for LBBB in the limb leads (ie, monophasic, all upright R wave in high-lateral leads I and aVL). The straight descent with predominant negativity for the anterior leads is typical for LBBB — although lead V6 lacks the monophasic R wave usually expected with typical LBBB.
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• PEARL #4: With LBBB — the finding of very deep S waves in one or more of the anterior leads (ie, deeper than 25-30 mm) suggests LVH. This is seen is leads V1 and V2 (which manifest S waves of 30 and 25 mm, respectively).
• Because the left ventricle enlarges not only to the left, but also posteriorly with LVH — some patients with LBBB will not manifest an all-upright R wave until we arrive at a lead more lateral than lead V6 (ie, a lead V7 or V8). However, given how typical the morphology of even-numbered beats is for LBBB in both high-lateral and anterior leads — I'd assess the conduction defect in this tracing consistent with LBBB.
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• As discussed in ECG Blog #204 and ECG Blog #282 — ST-T waves for the even-numbered beats in Figure-3 that are conducted with LBBB morphology do not suggest acute infarction.

Figure-3: I've colored in BLUE the odd-numbered beats that manifest normal conduction. This facilitates assessment of the even-numbered beats — which manifest LBBB conduction (See text).

PEARL #5: Assessment of acute ST-T wave changes is best made by identifying the normally-conducted sinus beats in a tracing. To facilitate this assessment — I've colored in YELLOW the even-numbered beats in Figure-4, which are conducted with LBBB.

• While true that it will at times be possible to identify acute ST-T wave changes in beats conducted with bundle branch block — I always begin by focusing on ST-T wave assessment of normally conducted beats, as it's usually much easier to spot abnormal findings in sinus-conducted beats.
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• Note in Figure-4 — that there is diffuse ST depression (ie, in almost all leads — most marked in leads V3-thru-V6) in the normally-conducted beats. This occurs in association with ST elevation in these odd-numbered beats in lead aVR.

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PEARL #6: Recognition of the ECG pattern seen in Figure-4 for the normally-conducted beats — in which there is diffuse ST segment depression (usually present in at least 7-8 leads) + ST elevation in lead aVR — should immediately suggest the following Differential Diagnosis:

• Severe Coronary Disease (due to LMain, proximal LAD, and/or severe 2- or 3-vessel disease) — which in the right clinical context may indicate ACS (Acute Coronary Syndrome).
• Subendocardial Ischemia from another Cause (ie, sustained tachyarrhythmia; cardiac arrest; shock/profound hypotension; hypoxemia; GI bleeding; anemia; "sick patient"; etc.).

To EMPHASIZE: This pattern of diffuse Subendocardial Ischemia does not suggest acute coronary occlusion (ie, it is not the pattern of an acute MI) — but rather ischemia due to the above differential diagnosis!

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Figure-4: I've colored in YeLLOW the even-numbered beats that manifest LBBB conduction. This facilitates assessment of the odd-numbered beats — which are normally conducted (See text).

FINAL Comparison:

I complete my discussion of this deceptive rhythm — by isolating in Figure-5, a direct comparison between normally-conducted beats vs beats conducted with LBBB:

• I find it insightful to directly compare QRS and ST-T wave morphology of the normally-conducted sinus beats (ie, beats #3 and 11) — vs — beats conducted with LBBB (ie, beats #4 and 12).

Figure-5: Direct comparison between normally-conducted beats vs beats conducted with LBBB.

Putting It All Together:

Today's tracing was obtained from a 60-year old woman, who presented to the ED with intermittent dyspnea (but no chest pain) over the past week. The patient had a history of diabetes and longstanding hypertension.

• The rhythm in today's tracing is sinus tachycardia at ~120/minute. Every-other-beat is conducted with LBBB. The very deep anterior S waves in beats conducted with LBBB suggests LVH. The marked and diffuse ST depression, with ST elevation in lead aVR — suggests diffuse subendocardial ischemia.
• As suggested above in Pearl #6 — diffuse subendocardial ischemia could be due to severe coronary disease — or — to some other cause. The patient's age, co-morbidities (ie, diabetes, hypertension) and symptoms (dyspnea, albeit without chest pain) — clearly predispose to coronary disease. IF there is no heart failure or other potentially treatable disorder — cardiac cath may be needed to clarify the anatomy. 

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Acknowledgment: My appreciation to Hafiz Abdul Mannan Shahid (from Lahore, Pakistan) for the case and this tracing.

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Relevant ECG Blog Posts to Today’s Post: 

• See ECG Blog #185 — for review of the Systematic Ps, Qs, 3R Approach to Rhythm Interpretation.
• ECG Blog #205 — Reviews my Systematic Approach to 12-lead ECG Interpretation.
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• ECG Blog #204 — Reviews a user-friendly approach to the ECG Diagnosis of conduction defects (ie, LBBB — RBBB — IVCD).
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• ECG Blog #282 — Reviews application of modified-Smith-Sgarbossa Criteria for evaluation acute MI with LBBB (as well as ECG diagnosis of LVH with LBBB).
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• ECG Blog #271 — Reviews the ECG diagnosis of diffuse subendocardial ischemia.
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• ECG Blog #198 — An Irregular WCT (LBBB or IVCD).
• ECG Blog #162 — LBBB with obvious STEMI.
• ECG Blog #146 — LBBB with Acute ST-T Wave Changes.
• ECG Blog #204 — Assessment of Sinus Rhythm and a Wide QRS (due to IVCD).
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• The January 31, 2022 post in Dr. Smith's ECG Blog — Reviews subtle signs of acute OMI in a patient with LBBB (Please see My Comment at the bottom of the page).
• The June 25, 2020 post in Dr. Smith’s ECG Blog — in which I review a case of Sinus Rhythm with Intermittent RBBB.