ECG Pointers: A Paced STEMI

Author: Lloyd Tannenbaum, MD (Emergency Medicine Resident, San Antonio, TX) // Edited by:  Jamie Santistevan, MD (@jamie_rae_EMdoc – EM Physician, Presbyterian Hospital, Albuquerque, NM); Manpreet Singh, MD (@MPrizzleER – Assistant Professor of Emergency Medicine / Department of Emergency Medicine – Harbor-UCLA Medical Center); and Brit Long, MD (@long_brit  – EM Attending Physician, San Antonio, TX)

Welcome to this edition of ECG Pointers, an emDOCs series designed to give you high yield tips about ECGs to keep your interpretation skills sharp. For a deeper dive on ECGs, we will include links to other great ECG FOAMed!

The Case: You’re working a busy shift in the ER when nursing rolls back a 64-year-old man with chest pain and shortness of breath.  He states that this started acutely this morning, and he has a lot of pressure in his chest. His past medical history is significant for a pacemaker placed several years ago.  He is completely pacer dependent. His ECG is shown below. How do you diagnose a STEMI in a patient who is paced?

This ECG shows a paced rhythm.  You can identify the pacing from the pacer spikes, which are seen running across the bottom of the ECG, under the rhythm strip shown for lead II.  You can also see the pacer spikes before each QRS complex. This ECG is tachycardic, with a rate of 114 and a wide QRS complex with a QRS duration of 143 msec.  ECG compliments of Dr. P. Kjell Ballard.

What are The Sgarbossa Criteria?

The Sgarbossa criteria are used to diagnose a STEMI in patients with a left bundle branch block (LBBB).  LBBBs and paced rhythms look similar, but can we also use the Sgarbossa Criteria to diagnose a STEMI in a paced rhythm?  To begin, let’s quickly review the classic Sgarbossa Criteria:

1. Concordant ST Segment Elevation >1mm in any lead (5 points)
2. Concordant ST Segment Depression >1mm in leads V1, V2, or V3 (3 Points)
3. Discordant ST Segment elevation >5mm in any lead (2 Points)

To be diagnostic of an acute MI, you need >3 points.  That gives 96% specificity but only 36% sensitivity [1].

For a visual representation of the criteria, please see below [2]:

For more FOAMed on Sgarbossa’s criteria, Life in the Fast Lane has a tremendous write up.

Interestingly, Dr. Stephen Smith, of Dr. Smith’s ECG Blog published a modification to the 3rd criteria to make it more sensitive for acute myocardial infarctions. This modification seeks to replace excessively discordant ST elevation with a rule defined by proportional ST-segment elevation to S-wave depth (ST/S ratio) for better diagnostic utility, with a value of < -0.25 suggestive of acute MI. To put it another way, this ratio is suggestive of MI when a single lead has excessively discordant ST elevation as defined by 25% of preceding S-wave.  Dr. Smith recently published an interesting case of MI in a paced rhythm. See this case where the original Sgarbossa criteria were falsely reassuring in acute MI, as well as this Rebel EM post which contains a full investigation of that criteria’s modification.

While the third criteria is the least reliable for diagnosing an acute MI in patients with a LBBB, there have been 2 papers that make note of the fact that the third criteria may be the best one for diagnosing an acute MI from a patient with a paced rhythm [3, 4].

The first paper was published by Sgarbossa et al in 1996 and found this criteria to have a sensitivity of 53% and a specificity of 88%. Of note, this study only included 17 patients.  The authors looked at all 41,021 participants of the GUSTO-1 trial to see how many of them had a pacemaker, which was 32.  Unfortunately, only 17 participants out of the 41,021 enrolled qualified for their study, but their findings were statistically significant with a p value of 0.025 [3].

The second paper is a retrospective chart review of patients with a ventricularly paced ECG and diagnosis of an acute MI.  These authors were looking to further validate the above work of Dr. Sgarbossa. They were able to find 57 ECGs that qualified for evaluation, and their results are similar to Dr. Sgarbossa’s in that the third criterion (discordant STE >5mm) is the most helpful for diagnosing an acute MI in patients with a paced rhythm.  They report a specificity of 99% but a sensitivity of only 10% [4].

There are multiple case studies that cite frustration with a lack of a definitive method to diagnose a STEMI in a patient with a paced rhythm [5,6,7].  Das and McGrath have an excellent case study that shows an ECG with both significant discordance and concordant ST elevation. Their ECG is shown below, and the full text can be found here: https://www.ncbi.nlm.nih.gov/pubmed/27091803.

ECG A is the patient’s baseline ECG.  ECG B shows significant discordance (>5mm) in V2 and V3 and concordant ST elevation >1mm in V4 [8].

So, based on the evidence, what conclusions can we draw?

Right now, there are not very good data or scores to diagnose a STEMI from a paced ECG.  At this time, it appears that the third of the original Sgarbossa criteria is the most specific for an acute MI with specificity ranging from 88% – 99%, but with a low sensitivity (10%-32%).  The MSC display promise for diagnosis of acute MI with a paced rhythm. As with all high risk cardiac patients, remember to get serial ECGs and look for dynamic changes. If your clinical suspicion is high, also consider applying modified Sgarbossa criteria if the original criteria are not met. If in doubt, discuss the case and your concerns with a cardiologist.

Dr. Smith advocates utilizing this algorithm, which incorporates the MSC:

Case Conclusions:

A code STEMI was called on this patient, as lead V2 meets criterion 2 of Sgarbossa’s original criteria, as there is concordant ST depression >1 mm.  The patient was taken emergently to the cath lab and found to have a 100% blockage in his LAD.

References:

1. Sgarbossa E. et al.. Electrocardiographic diagnosis of evolving acute myocardial infarction in the presence of left bundle-branch block. GUSTO-1 (Global Utilization of Streptokinase and Tissue Plasminogen Activator for Occluded Coronary Arteries) Investigators. NEJM. 1996. 334(8):481-7
2. Cai Q et al. The Left Bundle-Branch Block Puzzle in the 2013 ST-Elevation Myocardial Infarction Guideline:  From Falsely Declaring Emergency to Denying Reperfusion in a High-Risk Population. Are the Sgarbossa Criteria Ready for Prime Time? Am Heart J 2013. PMID: 24016487
3. Sgarbossa EB, Pinski SL, Gates KB, et al. Early electro- cardiographic diagnosis of acute myocardial infarction in the presence of ventricular paced rhythm. Am J Cardiol 1996;77: 423-4.
4. Maloy, K, et al. Sgarbossa Criteria are Highly Specific for Acute Myocardial Infarction with Pacemakers. Western Journal of Emergency Medicine. September 2010. 9 (4) 354-357.
5. Sefa, N and Sawyer, K. Smith-Modified Sgarbossa Criteria and Paced Rhythms: A Case Report. J. Emerg Med. 2016 Nov;51(5):584-588.
6. Ilicki, J et al. Sgarbossa criteria used to identify cardiac ischemia in patient with ventricular paced rhythm. Journal of electrocardiology. 2018 Sept – Oct. 51 (5) 830-832.
7. Grautoff, S. Right Bundle Branch Block, Left Bundle Branch Block, Pace Maker in Case of Acute Coronary Syndrome – is the ECG of any Value? Dtsch med Wochenschr 2017; 142(02): 123-129
8. Das, D and McGrath, B. Sgarbossa criteria for acute myocardial infarction. CMAJ. 2016 Oct 18;188(15):E395. Epub 2016 Apr 18.