ECG criteria
- Assess signs of ventricular pacing: wide QRS (usually > 120 ms) with morphology typical of pacing, pacemaker spikes if visible on surface ECG, device data and previous ECGs; absence of a visible spike does not rule out pacing
- Concordant ST elevation ≥ 1 mm in a lead with a positive (upward) paced QRS is the most specific sign of acute coronary occlusion
- Concordant ST depression ≥ 1 mm in V1–V3 is also a pathological sign
- Discordant (opposite in direction to QRS) ST elevation — assess using the ratio of absolute values: |ST|/|S| ≥ 0.25 at the J point (Smith criterion; in signed form with positive ST and negative S this corresponds to ST/S ≤ −0.25); evaluated only in leads with a measurable S wave and preferred over a fixed threshold of ≥ 5 mm (original Sgarbossa criterion); auxiliary criterion, evidence base for paced rhythm is smaller than for LBBB
- Compare with the patient's previous paced ECGs — dynamics are more important than absolute values
- Consider clinical presentation (pain, biomarkers) — ECG in paced rhythm has low sensitivity; a negative result does not rule out MI
What it looks like on ECG
During ventricular pacing (most often from the right ventricular apex), the QRS complex is wide, with a directionally discordant ST segment and T wave — this is a normal variant for a paced rhythm (analogous to "secondary" ST-T changes in bundle branch block). Therefore, standard ST elevation or depression criteria applied to a native narrow QRS do not work.
The modified Sgarbossa (Smith) criteria are used, originally developed for left bundle branch block and considered auxiliary in paced ventricular rhythm:
| Criterion | Cut-off | Points (orig. Sgarbossa) | ||||
|---|---|---|---|---|---|---|
| Concordant ST elevation (QRS and ST in the same direction) | ≥ 1 mm in any lead | 5 | ||||
| Concordant ST depression in V1–V3 | ≥ 1 mm | 3 | ||||
| Discordant ST elevation (QRS and ST in opposite directions) | ≥ 5 mm (orig.) or | ST | / | S | ≥ 0.25 at the J point (Smith criterion) | 2 |
The Smith criterion is assessed only in leads with a measurable S wave: the absolute magnitude of ST elevation at the J point is compared with the absolute depth of the preceding S wave; with discordant elevation, ST is positive and S is negative, so in signed form the ST/S ratio is ≤ −0.25.
A threshold of ≥ 3 points on the original Sgarbossa scale has high specificity mainly in the LBBB population in which the scale was developed. In ventricular pacing, the Sgarbossa and Smith criteria are auxiliary signs of probable acute coronary occlusion: their sensitivity and specificity are less studied, a negative result does not rule out MI, and a positive result does not replace clinical and biochemical diagnostics. Reciprocal changes are harder to interpret and less systematic during pacing, but in the presence of a convincing clinical picture, they can support the diagnosis of acute occlusion.
Mechanism
The pacemaker lead in the right ventricle causes depolarization that does not follow the normal His-Purkinje system, but proceeds slowly from apex to base — hence the wide QRS. With such a distorted sequence of excitation, secondary, predominantly discordant ST-T changes usually form — this is electrophysiologically predictable, as in bundle branch block. However, the severity and direction of these changes may vary with a different lead position, fusion complexes, loss of capture, and cardiac resynchronization therapy. In acute ischemia, regional repolarization vectors change additionally: depending on the localization and type of ischemia, ST may shift concordantly with the QRS direction (clear sign of pathology) or exaggerate baseline discordance (excessive discordant elevation). This shift does not boil down to a simple dichotomy of "subendocardial ischemia causes elevation, transmural causes another pattern": subendocardial ischemia usually manifests as ST depression, and pronounced elevation is typically associated with a regional epicardial repolarization gradient during transmural injury.
Causes
- STEMI or equivalent of acute coronary occlusion in a patient with a previously implanted single-chamber or dual-chamber pacemaker with ventricular pacing
- NSTE-ACS: Sgarbossa ECG criteria are not designed for it, NSTEMI diagnosis requires troponin dynamics and signs of ischemia (clinical, ECG, imaging), not just biomarkers
- Ventricular pacing in cardiac resynchronization therapy (biventricular pacing) creates a different, often more complex depolarization vector with potential fusion complexes; Sgarbossa criteria are developed and studied mainly for LBBB and right ventricular pacing, their use in CRT is limited
Differential diagnosis
- Baseline ST-T discordance in paced rhythm — a normal variant for pacing; distinction — absence of ST concordance and ST elevation below the threshold (|ST|/|S| < 0.25), but failure to meet the criterion does not prove normalcy
- Infarction with left bundle branch block — the same Sgarbossa/Smith criteria, mechanism is similar, but the cause of wide QRS is different (blockade rather than pacing)
- Pacemaker syndrome — clinical hypotension and symptoms due to AV dyssynchrony, not ischemic ST changes
- Pacemaker capture or sensing failure — changed morphology or rate of stimuli, not the ST segment
- Fusion complexes — partially native, partially paced QRS, ST morphology may be intermediate and not obey Sgarbossa criteria
Clinical significance and management
Paced ventricular rhythm makes standard ECG diagnosis of STEMI unreliable: Sgarbossa and Smith criteria are relatively specific but insufficiently sensitive, so a negative result does not confirm a normal state and does not rule out occlusion or MI. With persistent suspicion, clinical presentation, ECG dynamics, troponin, and comparison with previous tracings of the same patient during pacing are required.
In case of ischemic clinical presentation, hemodynamic instability, or convincing criteria for acute occlusion (concordant ST elevation, discordant elevation with |ST|/|S| ≥ 0.25), immediate activation of the reperfusion protocol and consideration of emergency coronary angiography are necessary; the decision is made based on the totality of clinical, ECG, and laboratory data without waiting for troponin results in clear instability. The positive criterion itself indicates a high probability of acute coronary occlusion rather than being an MI diagnosis per se — diagnosing infarction requires clinical context and confirmation of acute myocardial injury (troponin dynamics) unless the patient is in a situation requiring immediate reperfusion.
Reprogramming the device with a lower pacing rate to obtain a native rhythm is not performed routinely: in a pacemaker-dependent patient, this can cause pronounced bradycardia, asystole, or hemodynamic deterioration, and the appearance of a native rhythm does not always facilitate interpretation. Such a measure is permissible only with monitoring, involvement of a device specialist, and confirmed safety, usually after urgent patient assessment rather than as part of the standard ACS algorithm.
Common interpretation errors
- Applying standard ST elevation thresholds (as in native narrow rhythm) to a paced QRS — overdiagnoses or underdiagnoses
- Ignoring concordant ST elevation as "just a pacing variant" — missing true MI
- Using only a fixed threshold of ≥ 5 mm without calculating the |ST|/|S| ratio — loss of sensitivity in patients with low-amplitude S
- Refusing to diagnose MI with negative Sgarbossa criteria without considering the clinical picture and biomarkers
- Confusing fusion complexes with true ischemic ST dynamics
1. Which ECG sign is most specific for acute coronary occlusion in the presence of ventricular pacing?
Concordance of ST elevation with the direction of the paced QRS is not explained by pacing itself and indicates a high probability of acute coronary occlusion; diagnosing infarction additionally requires clinical context and troponin dynamics.
2. According to the Smith criterion, discordant ST elevation is considered pathological if:
The Smith criterion (|ST|/|S| ≥ 0.25) is more accurate than the fixed threshold of ≥ 5 mm and increases the sensitivity for detecting probable acute occlusion in validated populations; the final diagnosis requires clinical evaluation and troponin.
3. Why are the modified Sgarbossa criteria applicable to both left bundle branch block and ventricular pacing?
The common electrophysiological basis — an abnormal ventricular depolarization pathway — leads to a similar pattern of secondary ST-T changes, which is why the criteria for diagnosing ischemia are generally the same, although the evidence base for pacing is weaker.
Frequently asked questions
Can myocardial infarction be diagnosed by ECG if the patient has a pacemaker?
The ECG may provide baseline signs: the modified Sgarbossa (Smith) criteria, originally developed for left bundle branch block, are used. Concordant ST elevation indicates a high probability of acute coronary occlusion, but a negative result does not rule out MI in the presence of ischemic symptoms, and the final diagnosis requires troponin and clinical evaluation.
How do the Sgarbossa criteria for paced rhythm differ from those for left bundle branch block?
The ST concordance thresholds and the rule |ST|/|S| ≥ 0.25 are the same because the physiological basis of ST-T discordance is similar: an abnormal ventricular depolarization sequence. However, the evidence base and diagnostic reliability of these criteria in ventricular pacing have been studied less than in LBBB.
What to do if the ECG during pacing is uninformative, but the clinical presentation is suspicious for ACS?
Rely on troponin dynamics, serial ECGs, comparison with the patient's old tracings, and the clinical picture; if suspicion persists, manage the patient according to the ACS protocol up to an invasive strategy.
Are the Sgarbossa criteria applicable to NSTE-ACS in paced rhythm?
These criteria are designed to detect a STEMI-equivalent pattern, rather than NSTE-ACS. To diagnose NSTE-ACS in the setting of pacing, evaluate high-sensitivity troponin dynamics, symptoms, ischemic ECG changes, or imaging, and rule out alternative causes of myocardial injury.
Why is the ST segment initially altered during ventricular pacing even without ischemia?
The lead in the right ventricle causes an unnatural, slow sequence of depolarization, which is why repolarization typically proceeds discordantly to the QRS direction — this is a normal variant for paced rhythm, not a sign of myocardial injury.
ECG Course
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Related terms
More in the section "Ischemia and Myocardial Infarction"
Sources
- Surawicz B., Knilans T. Chou's Electrocardiography in Clinical Practice
- Hampton J. The ECG in Practice
- AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram, 2009
- ESC Guidelines for the management of acute coronary syndromes, 2023
Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.