The Case:
An 85-year-old woman with past medical history of inferior MI s/p VVI pacemaker for atrial fibrillation with slow ventricular response comes in to the ED with acute chest pain. An ECG is obtained on arrival (Figure 1) (1).
Interpretation:
Rate: 60 bpm; Rhythm: ventricular paced rhythm; Axis: left axis deviation (I: pos, II: neg., aVF: neg) Intervals: PR: n/a, paced; QRS: 163, wide; QT: 466, borderline/prolonged; P-Waves: n/a, paced; QRS Complex: RBBB morphology (RSR’ in V1, wide S wave in V6); ST Segment/T-waves: concordant ST segment elevation in V2-V5
Based on the presenting ECG, how can one tell what type of pacer the patient has in place? Does this patient have an OMI? What criteria can we use to detect OMI in right and left ventricular paced rhythms?
Discussion:
The ECG findings of RBBB in the setting of a paced rhythm indicate that the patient has a LEFT ventricular paced rhythm and the findings of concordant ST segment elevation in V2-V5 are highly suggestive of OMI. The patient did well post-catheterization and was ultimately discharged from the hospital.
The ECG features of a paced rhythm depend on the pacemaker type, pacing mode, lead placement, and regular electrical activity of the heart. Common features of all pacemakers include a short pacing impulse usually less than 2 milliseconds appearing as a vertical line. Ventricular pacing ECG features vary depending on whether there is right or left ventricular pacing. Left ventricular pacing results in ECG features that are similar to the complete right bundle branch block pattern (RBBB) and right ventricular pacing results in ECG features that are similar to the complete left bundle branch block (LBBB) (2).
Traditional ECG criteria for occlusive myocardial infarction (OMI) cannot be applied to ventricular paced rhythms and diagnosis of OMI/NOMI can be challenging. The Smith modified Sgarbossa criteria are currently the accepted standard for detecting OMI in patients with a LBBB on ECG (3). These criteria are a follows:
Concordant ST segment elevation in ANY lead >1mm;
Concordant ST segment depression in leads V1-V6 >1mm;
Discordant ST segment elevation greater than 25% of the S-wave in ANY lead.
As patients with right ventricular paced rhythms demonstrate a LBBB pattern on ECG, recent guidelines have suggested using the Smith modified Sgarbossa criteria for detection of OMI (4). One recent study demonstrated a sensitivity of 81% and specificity of 96% for diagnosis of occlusion myocardial infarction in the presence of right ventricular paced rhythms (5).
Figure 2. Paced rhythm with LBBB and 1 mm of concordant ST Elevation in lead V5. Adapted from Dr. Smith's EKG Blog. Source: https://hqmeded-ecg.blogspot.com/2019/01/a-70-something-with-ventricular-paced.html
Detecting ischemia in RBBB can be even more challenging as no standard criteria have been developed for detection. In normal RBBB, ST segments and T-waves should be discordant from the QRS, notably with some ST depression in V1-V3 (6). If concordant ST segment elevation, an isoelectric ST segment, or excessive ST segment discordance is observed on ECG in leads V1-V3, a high suspicion for OMI should be maintained in both unpaced and left epicardial paced rhythms demonstrating RBBB (6-7).
Figure 3. Paced rhythm with RBBB and ST elevation in V2-V3. Adapted from ECG Guru. Source: https://www.ecgguru.com/ecg/acute-mi-right-bundle-branch-block-and-atrial-pacing
Take Away Points:
For patients with RIGHT ventricular paced rhythms, the ECG should mimic LBBB and the Smith modified Sgarbossa criteria should be utilized to detect ischemia.
For patients with LEFT ventricular paced rhythms, the ECG should mimic RBBB and leads V1-V3 should be analyzed for ST segment concordance, excessive discordance, or even an isoelectric ST segment to detect ischemia.
Authored by Michael Hohl, MD.
References:
McLaren, J. (2022, October 11). Pacer mnemonic approach to pacemaker patients: ECG cases: EM cases. Emergency Medicine Cases. Available at: https://emergencymedicinecases.com/ecg-pacer-approach-pacemaker/
Surawicz, Borys and Lawrence E. Gering. “Chapter 26 – Electrocardiography of Artificial Electronic Pacemakers.” (2008). doi: 10.1016/B978-141603774-3.10026-7.
Smith SW, Dodd KW, Henry TD, Dvorak DM, Pearce LA. Diagnosis of ST-elevation myocardial infarction in the presence of left bundle branch block with the ST-elevation to S-wave ratio in a modified Sgarbossa rule. Ann Emerg Med. 2012 Dec;60(6):766-76. doi: 10.1016/j.annemergmed.2012.07.119. Epub 2012 Aug 31. PMID: 22939607.
Di Marco A, Rodriguez M, Cinca J, Bayes-Genis A, Ortiz-Perez JT, Ariza-Solé A, Sanchez-Salado JC, Sionis A, Rodriguez J, Toledano B, Codina P, Solé-González E, Masotti M, Gómez-Hospital JA, Cequier Á, Anguera I. New Electrocardiographic Algorithm for the Diagnosis of Acute Myocardial Infarction in Patients With Left Bundle Branch Block. J Am Heart Assoc. 2020 Jul 21;9(14):e015573. doi: 10.1161/JAHA.119.015573. Epub 2020 Jul 4. PMID: 32627643.
Dodd KW, Zvosec DL, Hart MA, Glass G 3rd, Bannister LE, Body RM, Boggust BA, Brady WJ, Chang AM, Cullen L, Gómez-Vicente R, Huis In 't Veld MA, Karim RM, Meyers HP 3rd, Miranda DF, Mitchell GJ, Reynard C, Rice C, Salverda BJ, Stellpflug SJ, Tolia VM, Walsh BM, White JL, Smith SW; Electrocardiographic Diagnosis of Acute Coronary Occlusion Myocardial Infarction in Ventricular Paced Rhythm Using the Modified Sgarbossa Criteria. Ann Emerg Med. 2021 Oct;78(4):517-529. doi: 10.1016/j.annemergmed.2021.03.036. Epub 2021 Jun 23. PMID: 34172301.
Horton CL, Brady WJ. Right bundle-branch block in acute coronary syndrome: diagnostic and therapeutic implications for the emergency physician. Am J Emerg Med. 2009 Nov;27(9):1130-41. doi: 10.1016/j.ajem.2008.09.039. PMID: 19931763.
Triska J, George J, Rector G, Alam M, Smith SW, Meyers HP, Birnbaum Y. Acute Coronary Occlusion in a Patient With Prior Known Right Bundle Branch Block: Another Chink in the Armor for the ST-Elevation Myocardial Infarction Criteria. Ann Emerg Med. 2023 Aug;82(2):219-221. doi: 10.1016/j.annemergmed.2022.12.006. PMID: 37479399.