Ok, so I posted a 12 Lead ECG on Facebook this past Monday, June 2nd, 2014, which Â generated some interesting comments and thoughts…
Â This ECG was obtained from a 67 year old male, complaining of difficulty breathing for the past 2 hours, and presented diaphoretic, with no other signs of hypoperfusion.
Past medical history:
- Congestive Heart Failure (CHF)
- Blood Pressure: 110/88 mmHg
- Heart Rate: Between 80 -94 beats/min and regular
- Respiratory Rate: 18 breaths/min
- SpO2: 97%
Now to the fun part…
As most noticed, it is indeed a paced rhythm, with unipolar leads which cause a bigger and more noticeable “pacer spike”, preceding every QRS. The QRS Â are wide in appearance, with an arrow on the bottom of the ECG pointing to the pacing origin.
Some interpreted this as a RBBB due to the dominant R wave in V1, however, this finding in the presence of a pacemaker, suggest LV or septalÂ pacing, with a superior axis, moving away from the inferior leads, as well as the lateral leads, due to impulses originating and moving away from the apex, which causes the axis to shift towards the right upper quadrant of the Hexaxial Reference System, at approximately -118 degrees.
But there is more to it than just a paced rhythm.
Some suggested that since the rhythm is paced,Â no other diagnosis could be made, but this is not the case here, but, that’s ok, this is why we discuss this type of cases, to make us better clinicians. Remember, during ischemic events, cardiac tissue may not transmit this potentials properly, whether paced or natural conduction, affecting both depolarization and repolarization phases, which is exactly what we are looking at when evaluating an ECG. A pacemaker can still show ventricular beats and even fail to produce capture.
This is a paced rhythm with Infero-postero-lateral Myocardial Infarction, with 100% RCA occlusion found after Cath Lab activation.
Primary ST-T changes are caused by ischemia and/or infarction, while Secondary ST-T changesÂ are caused by repolarization abnormalities. The later is what we typically see during a normal LBBB or paced rhythm.
Sgarbossa’s Criteria was developed to identify STEMIs in the presence of a LBBB or paced rhythm.
- Concordant ST Segment Elevation > 1 mm in at least one lead = 5 points
- Concordant ST Segment Depression > 1 mm in V1-3= 3 points
- Excessive discordant ST Segment Elevation > 5mm
Discordance: ST-T vector moving away from the terminal portion (the last wave) of the QRS
Concordance: ST-T vector moving the same direction as the terminal portion (the last wave) of the QRS
3 points or more has a 90% specificityÂ for MI. And then there’sÂ Dr. Smith’s Sgarbossa RuleÂ whichÂ replaces the 3rd criterion, increasing both sensitivity and specificity.This replacement looks for an ST/S ratio > -.25 , dividing the ST segment elevation by the depth of that preceding S wave. Â This can also applied to discordant depression.
As you can see, we have > 5 points with 2 different criterion found
How does it apply to both LBBB and paced rhythms?
- A LBBB produces a tall wide R wave in leads I, aVL and V6 since impulses traveling towards these leads, which look at the Left Ventricle (LV),Â are creating a delayed depolarization
- In the meantime, the Right Ventricle (RV) depolarizes normally because the Right Bundle Branch is conducting normally. This produces an rS pattern (smallÂ r wave and deep S wave) in V1-3 because impulsesÂ are moving away from these leads, causing a negative deflection (S wave), but impulses travel from myocyte to myocyte, causing a delayed conduction making the S wave wide.
- A typical paced rhythm, will also have fast RV conduction, as the pacer wires are often implanted on said wall, while the LV is being depolarized slowly
Because of this, a paced rhythm will often present with a LBBB morphology, therefore, this criteriaÂ can apply to both LBBBs and paced rhythms, however, it has been shown to be a predominant factor in LBBBs over paced rhythms. Keep in mind that you may not always see changes while the patient may still be experiencing an acute ischemic event, but with a proper assessment and good ECG evaluation, you may just make a difference in someone’s life.
Although no specific Sgarbossa’s criterion study has been published on a particular pacemaker setting, once the particular electrophysiologicÂ function is understood, abnormalities such as the ones mentioned above can be identified, even if no particular criteria has been specifically developed for that particular scenario.