How do you read scarring on an Electrocardiogram (EKG) with a paced rhythm?

Interpreting Sgarbossa Criteria on EKG with Paced Rhythm

The Sgarbossa criteria cannot be reliably applied to standard ventricular paced rhythms because pacing-induced depolarization fundamentally alters QRS morphology and ST-segment vectors, masking or mimicking ischemic changes. However, modified approaches focusing on proportional ST-segment deviation and comparison with baseline paced ECGs can improve diagnostic accuracy.

Why Standard ECG Interpretation Fails in Paced Rhythms

• Ventricular pacing creates wide QRS complexes with discordant ST-T wave changes that mimic or obscure ischemic patterns, making traditional ECG criteria for myocardial infarction unreliable 1
• Computer algorithms fail to recognize pacing spikes in 18.4% of cases and frequently misinterpret paced beats as intrinsic beats, leading to false diagnoses of myocardial infarction 2
• Left bundle branch block, right bundle branch block, and ventricular pacing can all mask signs of ischemia or injury on the ECG, requiring alternative diagnostic strategies 3

The Sgarbossa Criteria: Original Application

The original Sgarbossa criteria were developed for detecting acute MI in the presence of left bundle branch block, not specifically for paced rhythms. The three criteria are:

• Concordant ST elevation ≥1 mm in leads with positive QRS complex (5 points - most specific)
• Concordant ST depression ≥1 mm in V1-V3 (3 points)
• Discordant ST elevation ≥5 mm in leads with negative QRS complex (2 points)

A score ≥3 points suggests acute MI, but these criteria have limited sensitivity in paced rhythms 3, 4.

Modified Approach for Paced Rhythms

Proportional ST-Segment Analysis

• Measure the ratio of ST-segment deviation to the amplitude of the preceding S-wave or R-wave rather than using absolute millimeter cutoffs
• A ratio of ST elevation to S-wave depth ≥0.25 (25%) in leads with discordant ST segments suggests acute ischemia with higher sensitivity than the original 5mm cutoff 5

Comparison with Baseline Paced ECG

• Always compare the current paced ECG with previous paced ECGs if available, as new ST-segment changes in a paced rhythm may reveal acute ischemia that would otherwise be attributed to the pacing pattern 3
• The absence of prior ECGs significantly limits diagnostic accuracy in paced rhythms 2

Detecting Scar/Prior MI in Paced Rhythms

Q-Wave Analysis Limitations

• Q waves on ECG correlate poorly with actual myocardial scar burden, with only 66% sensitivity and 85% specificity compared to cardiac MRI with late gadolinium enhancement 6
• In paced rhythms, Q-wave analysis is essentially impossible as the pacing spike initiates ventricular depolarization from an abnormal location 2, 7
• Pathological Q waves have high false positivity (32% of Q waves represent non-infarct pathology) and frequently miss true myocardial infarction 6

Alternative Markers in Paced Rhythms

• QRS fragmentation in lateral leads (I, aVL, V5-V6) suggests basal-lateral scar, though this is uncommon (20%) and primarily seen in non-ischemic cardiomyopathy 5
• S/R ratio ≥0.25 in lead V6 distinguishes non-ischemic from ischemic scar with 77% specificity, but this requires absence of bundle branch block or pacing artifacts 5
• These markers are unreliable during active ventricular pacing due to altered depolarization patterns 2, 7

Recommended Diagnostic Algorithm for Suspected ACS with Paced Rhythm

Step 1: Immediate Actions

• Obtain serial high-sensitivity cardiac troponins immediately - troponin is the most sensitive test for myocardial injury and is not affected by pacing 3
• Repeat ECG with any change in symptoms (chest pain recurrence, dyspnea, hemodynamic instability) even if the paced rhythm persists 3

Step 2: ECG Analysis

• Look for concordant ST elevation ≥1mm (most specific finding even in paced rhythms)
• Calculate proportional ST deviation (ST elevation/S-wave depth ratio ≥0.25 suggests ischemia)
• Compare with prior paced ECGs to identify new ST-segment changes 3

Step 3: Advanced Imaging

• Cardiac MRI with late gadolinium enhancement is the preferred non-invasive method to identify and characterize myocardial scar, with far superior accuracy compared to ECG 1, 4, 8
• MRI can distinguish transmural from non-transmural scar and quantify scar burden (>5% of LV mass indicates significantly increased arrhythmic risk) 4, 8
• Quantified percent scar tissue >17% of left ventricle predicts Q waves with 90% sensitivity and specificity, but this requires MRI, not ECG 8

Step 4: Risk Stratification

• Never rely on a single normal or non-diagnostic paced ECG to exclude ACS - this is the most dangerous error in chest pain evaluation 3
• Continue serial ECGs and troponins until ACS is definitively excluded 3

Critical Pitfalls to Avoid

• Do not attempt to apply traditional STEMI criteria to paced rhythms - the wide QRS and discordant ST-T waves will lead to false positives and false negatives 1, 2
• Computer-generated ECG interpretations require revision by cardiologists in 61.3% of paced ECG cases - never rely solely on automated readings 2
• Hyperkalemia and other metabolic derangements can produce dramatic ECG changes even during paced rhythm (including QRS widening and sine waves), so consider electrolyte abnormalities in the differential 7
• Pacing in scarred myocardium is associated with 5-fold worse outcomes (pump failure and sudden cardiac death) compared to pacing in non-scarred tissue, emphasizing the importance of scar detection before device implantation 1

When Sgarbossa Criteria May Still Help

• In patients with biventricular pacing (CRT) who have underlying sinus rhythm with fusion beats, some native QRS morphology may be preserved, allowing partial application of Sgarbossa criteria 1
• During periods of pacing failure or loss of capture, when intrinsic rhythm emerges, standard ECG criteria become applicable 2