Submitted by Dawn on Mon, 09/02/2024 - 21:14
The Patient: This 72-year-old woman called EMS because of a sudden onset of breathlessness and anxiety. She had a history of COPD (asthma), CHF, and Type II diabetes. We do not know her medications or any other history. She was found to have bilateral breath sounds with "minimal" expiratory wheezing. She was alert and very anxious. Her initial pulse rate was recorded at around 60 bpm and irregular. A systolic BP was heard at 140 mm Hg, but the paramedic could not hear a pulse after that. She was given oxygen via CPAP (Continuous positive airway pressure). The first ECG at 15:50 was recorded during this assessment. After appearing to improve, she became neurologically altered, and her level of consciousness varied during the call. She was turned over to emergency department staff conscious and able to speak, but had a cardiac arrest subsequently. The paramedics were unable to obtain followup information regarding the outcome.
ECG at 1550: The first QRS on the recording has no associated P wave, and is presumed to be an escape beat, probably junctional, with an interventricular conduction delay (QRS .12 sec.). This is a right bundle branch block pattern with left anterior fascicular block (bifascicular block). The second QRS is about the same width, but with a different morphology and discordant T waves, so probably ventricular. The third QRS is very much like the first, except that it appears to be conducted from the preceding P wave. For the next five seconds, there are only P waves, which are regular at about 130 bpm. The three-beat pattern seen at the beginning repeats itself near the end.
This ECG shows evidence of severe conduction blocks. The wide QRS complexes indicate interventricular blocks. In this case, some are probably premature ventricular contractions and some are sinus beats with bifascicular block. Even more worrisome is the intermittent loss of AV conduction. This can be called "intermittent trifascicular block", or "intermittent ventricular standstill". This is not a "third-degree AV block", because there are signs of AV conduction, but it is very close. With two of the three main fasicles of the left bundle branch blocked initially, it only takes a block in the remaining fascicle to produce a complete lack of AV conduction. Of course, there are no pulses during the time of ventricular standstill. The really concerning part of this situation is the lack of an ESCAPE RHYTHM. This is a good time for a temporary pacemaker, either transcutaneous or, if available, transvenous.
ECG at 1603: This ECG was obtained enroute to the hospital. The patient is once again alert and anxious. There is some artifact which hampers evaluation, but there are two P waves for every QRS complex. The atrial rate is about 120 bpm and the ventricular rate is about 60 bpm. The non-conducted P waves are buried in the T waves of the preceding beats. There is a right bundle branch block pattern with left anterior fascicular block, as we saw in the first ECG.
ECG at 1605: Now, the patient has become unresponsive to voice. We see nothing but P waves for the entire ten-second strip.
RHYTHM STRIP at 1606: Now there is a tachycardia with the same pattern of RBBB and LAFB. The rate is a bit over 120 bpm. It is difficult to determine the P-QRS relationship because of the artifact. There is one beat with a different QRS morphology (8) that comes in after an interruption of the tachycardia. It appears to be conducted from a P wave, but it is followed by two non-conducted P waves and an escape beat(9). The SpO2 graph indicates a pulse between 7 and 8.The patient felt better and was able to talk with the emergency department physician. The patient suffered a cardiac arrest subsequent to this strip, and we do not know the outcome.
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Comments
Masquerading BBB into Ventricular Standstill
Today’s case by Dawn is instructional. The patient suffered cardiac arrest. We unfortunately do not know the treatment interventions en route to the hospital, nor do we know the ultimate outcome. That said — soul-searching retrospective analysis can be insightful.
I’ll add to Dawn’s excellent commentary with a few additional thoughts.
The term, “trifascicular block” is a misleading one that is no longer recommended (Surawicz et al — JACC: 53(11):976-981, 2009 — https://www.jacc.org/doi/abs/10.1016/j.jacc.2008.12.013). Asystole is a “complete” trifascicular block. An “incomplete” trifascicular block could be any combination of a bifascicular block plus first-degree block — or — that might not be a “trifascicular” block at all, if the reason for the prolonged PR interval is disease in the AV node and not in the remaining fascicle.
Instead of “trifascicular block” — intermittent ventricular standstill (as mentioned by Dawn) is a much more accurate descriptive term. To emphasize — despite minimal conduction, there ARE 2 sinus-conducted beats in the initial 15:50 tracing — so this is intermittent ventricular standstill and NOT complete AV block (as any sinus conduction at all negates the definition of “complete” AV block).
It could be easy to overlook the 2:1 AV block in the 2nd (16:03) tracing — because (as Dawn mentions) — the 2nd P wave is hidden within the preceding T wave. The KEY is to look at all 12 leads on the ECG — with the 2:1 AV block being easiest to recognize by the equally spaced 2 negative deflections in lead V1. And, if we measure the P-P interval in the initial 15:50 tracing — it is IDENTICAL to the distance between those 2 negative P wave deflections within each R-R interval in the 2nd (1603) tracing.
The other important finding to recognize in the 2nd (16:03) tracing is MBBB ( = Masquerading Bundle Branch Block). MBBB is identified by the presence of QRS widening in the presence of at least some sinus conduction, in which QRS morphology is consistent with RBBB conduction in the chest leads — but LBBB conduction in the limb leads (especially when there is a marked leftward axis). The clinical significance of MBBB — is that it identifies a group of patients with very severe underlying heart disease — who have a much higher predisposition for developing severe heart block (needing a pacemaker) — and who have an extremely poor long-term prognosis (See my ECG Blog #419 — for more on MBBB — https://tinyurl.com/KG-Blog-419).
The reason I highlight the diagnosis of MBBB — is that if this patient during sinus rhythm (when last seen by her clinician) had this same MBBB morphology — it may have served as a warning to carefully assess the patient for potential need of prophylactic pacing.
Ken Grauer, MD www.kg-ekgpress.com [email protected]