ECG Blog #290 (84a) - Errors in ECG Interpretation

The ECG shown in Figure-1 was sent to me for my opinion. All I was initially told — was that the chest X-ray and Echo from this middle-aged man were normal — and that correct electrode lead placement was verified.  

• How would YOU interpret this tracing?

Figure-1: ECG that was sent to me with limited clinical information (See text).

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NOTE: Some readers may prefer at this point to listen to the 9:00 minute ECG Audio PEARL before reading My Thoughts regarding the ECG in Figure-1. Feel free at any time to refer to My Thoughts on this tracing (that appear below ECG MP-84a).

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Today’s ECG Media PEARL #84a (9:00 minutes Audio) — Reviews my observations on some Common Errors in ECG Interpretation (Part-1) — with attention to the cardiac rhythm. 

My THOUGHTS on ECG #1:

As always — I approached the ECG shown in Figure-1 systematically — beginning with assessment of Rate and Rhythm (See ECG Blog #205 for my Systematic Approach to 12-Lead ECGs): 

• QUESTION #1: Did YOU Notice that a simple sinus rhythm is not present in Figure-1 — because the P wave at the beginning of the long lead II rhythm strip is not upright? (YELLOW arrows in Figure-2).
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• QUESTION #2: Did YOU Notice in Figure 2, that the P wave becomes positive after beat #7?

Figure-2: I've labeled P waves in ECG #1. Did YOU Notice that the first 6 P waves in the long lead II rhythm strip are negative?

Today's Audio Pearl reviews a number of Common Errors in ECG Interpretation. One of the most prevalent errors (even among experienced clinicians) — is the failure to begin interpretation by spending the less than 5 seconds it takes to seek out a long lead II rhythm strip — and routinely look in front of each QRS complex to see IF it is preceded by an upright P wave in lead II.

• IF the P wave in lead II is not upright in front of neighboring QRS complexes — then (assuming there is no dextrocardia or lead misplacement) — the rhythm is not sinus. 
• Note that although the P wave is negative in each of the inferior leads for the first 6 beats — the P wave is positive in simultaneously-recorded lead aVL for beats #4,5,6. This tells us that atrial activity is traveling backward (retrograde) with respect to the inferior leads — and towards high lateral lead aVL, which defines this as either a Low Atrial or slightly accelerated Junctional Rhythm (See My Comment in the January 28, 2019 post of Dr. Smith's ECG Blog for distinction between these 2 rhythms).

QUESTION #3: What happens in Figure-2 to the P wave in the long lead II rhythm strip after beat #6?

• Advanced QUESTION #4: Can you explain why this change in P wave morphology after beat #6 occurs? 

ANSWER:

The P wave in front of beat #7 in lead II becomes tiny and isoelectric (RED-YELLOW arrow in Figure-3). After beat #7 — the P wave becomes positive in lead II, and remains so for the rest of the tracing.

• The change to a positive P wave with a constant and normal PR interval preceding beats #8-thru-13 indicates resumption of sinus rhythm.
• Verification that the RED-YELLOW arrow in front of beat #7 is in fact pointing to a P wave — is forthcoming from looking in front of beat #7 in the simultaneously-recorded long lead V1 rhythm strip. I suspect that the tiny, isoelectric deflection in front of beat #7 in the long lead II represents an atrial fusion beat at the time of transition from the low atrial (or junctional) rhythm seen for the first 6 beats — to the sinus rhythm that takes over after beat #7.

The reason WHY sinus rhythm resumes after beat #7 is illustrated in Figure-3, in which I have measured (in milliseconds) the R-R interval for each of the beats in this tracing:

• Although subtle — the rhythm in the long lead rhythm strip is not regular. This is difficult to determine without the use of calipers!
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• PEARL #1: Calipers are not needed for interpretation of many rhythms. The reason calipers are so helpful in today's case — is that they allow you to quickly determine that the reason sinus rhythm resumes after beat #7, is that the sinus rate speeds up enough to overtake the slightly slower low atrial (or junctional) rhythm. A more extended period of monitoring would probably reveal an underlying sinus arrhythmia — with periodic slowing sufficient to allow an ectopic atrial (or slightly accelerated junctional) focus to intermittently take over the rhythm. As opposed to a usurping rhythm — intermittent sinus slowing (with exchange between another pacemaking site) is much more likely to represent a benign phenomenon.

Figure-3: I've labed sinus P waves with RED arrows. Note that the R-R interval decreases for a few beats after beat #7. This allows sinus rhythm to take over (See text).

Let's Complete Our Interpretation of Today's Tracing:

Having assessed the Rate and Rhythm in Figure-3 — Let's complete our systematic interpretation of this 12-lead tracing.

• Intervals: The PR interval for sinus-conducted beats #8-thru-13 is normal. The QRS is narrow — and the QTc does not appear to be prolonged.
• Axis: Normal (about +40 degrees).
• Chamber Enlargement: None.

Regarding Q-R-S-T Changes:

• Q Waves: None.
• R Wave Progression: Normal. Transition (where height of the R wave becomes taller than the S wave is deep) occurs normally between leads V2-to-V3. There is an r' (ie, terminal positive deflection) in lead V2 — which probably reflects slight electrode lead misplacement, as an isolated terminal r' in lead V2 but not V1 or V3 does not make anatomic sense. That said — this is unlikely to affect the overall interpretation of todays' tracing.
• ST-T Wave Changes: There appears to be slight J-point ST elevation in each of the inferior leads. Reciprocal ST depression is not seen in lead aVL — and there are no significant ST-T wave changes in other leads.

PEARL #2: The appearance of ST elevation in the inferior leads of Figure-3 reflects the Emery Phenomenon — in which the oppositely-directed atrial repolarization wave (ie, the T of the P wave) produces a "pseudo"-ST elevation effect because of the relatively large size of the negative inferior lead P waves, with short PR interval for the first 6 beats of the tracing.

• Most of the time — the Tp (also known as the "Ta" or atrial T wave) is hidden within the QRS complex. But on those uncommon occasions when a large negative P wave with short PR interval is seen in the inferior leads — the resultant oppositely-directed Tp may simulate acute inferior infarction (See My Comment in the June 3, 2020 post in Dr. Steve Smith's ECG Blog for discussion of the Emery Phenomenon in the context of a case that went to cath because of this "pseudo"-ST elevation).

My Clinical Impression of ECG #1:

The underlying rhythm is sinus arrhythmia — with intermittent takeover by a rhythm manifesting negative inferior lead P waves with a short PR interval (either a low atrial or slightly accelerated junctional escape rhythm). There may be slight electrode lead misplacement of lead V2. There is "pseudo"-ST elevation in the inferior leads during periods of the escape rhythm (due to the Emery Phenomenon) — but there are no acute changes.

CASE Follow-Up:

Additional history on today's patient revealed that the reason this ECG was done — was dizziness and a syncopal episode. On further evaluation — the patient was found to be hypovolemic with orthostatic hypotension — and this was thought to be the cause of his syncope. Treatment was with volume replacement. Echo showed excellent systolic function, without any localized wall motion abnormality.

• My thought on learning this additional history was that this rhythm was probably benign — and given the rate-related dependence of this rhythm shown in Figure-3 — I thought this ectopic rhythm would probably resolve as volume replacement was accomplished.

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Acknowledgment: My appreciation to Hosam Shabib (from Doha, Qatar) for the case and this tracing.

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Additional Relevant Material to Today's Case:

• See ECG Blog #185 — for review of the Systematic Ps, Qs, 3R Approach to rhythm interpretation.
• See ECG Blog #205 — Reviews my Systematic Approach to 12-lead ECG Interpretation.
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• See ECG Blog #196 and ECG Blog #263 and ECG Blog #220 — for Review on assessing the regular WCT rhythm. 
• See ECG Blog #197 and ECG Blog #278 — for Review of the Idiopathic VTs (ie, RVOT VT, Fascicular VT, others).
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• See ECG Blog #211 — for Review of why some beats conduct with Aberration.
• See ECG Blog #42 — for Review of Criteria for distinguishing between VT vs SVT with Aberration or Preexisting BBB.
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• See ECG Blog #240 — for Review on the ECG assessment of the patient with a regular SVT rhythm (including distinction between the various types of SVT reentry).
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• See ECG Blog #214 — for Review of why I emphasize the phrase, "12 Leads are Better than One".
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• See ECG Blog #235 and ECG Blog #286 — for, "Some Simple Steps to Help Interpret Complex Rhythms".
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• See ECG Blog #193 — for Review of the term, "OMI" ( = Occlusion-based MI).
• See ECG Blog #218 and ECG Blog #260 — for Review of WHEN a T-wave is Hyperacute.
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• See ECG Blog #204 and ECG Blog #282 — for Review of a user-friendly approach for the ECG Diagnosis of the Bundle Branch Blocks (RBBB/LBBB/IVCD) in a matter of seconds!
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• For more on distinction between Low Atrial vs Junctional Rhythm — Please see My Comment at the BOTTOM of the page in the January 28, 2019 post in Dr. Smith's ECG Blog.
• For more on the Emery Phenomenon — Please see My Comment at the BOTTOM of the page in the June 3, 2020 post in Dr. Smith's ECG Blog.