On what types of electrocardiograms (EKGs) is ST elevation or depression difficult to interpret?

ST Elevation and Depression Interpretation Limitations in ECGs

ST elevation or depression cannot be reliably interpreted in paced rhythms and left bundle branch blocks due to the secondary repolarization changes that occur in these conditions. 1

Why ST Segment Changes Are Difficult to Interpret

Left Bundle Branch Block (LBBB)

• LBBB causes significant secondary ST-T wave changes that make ischemic changes difficult to distinguish from the baseline abnormality 1
• In LBBB, the ST-segment and T-wave vectors are generally directed opposite to the mean QRS vector as a normal physiological response 1
• The 2013 ACC/AHA STEMI guidelines specifically note that LBBB "may interfere with ST-elevation analysis" 1
• The traditional diagnostic criteria for STEMI are not applicable in the presence of LBBB

Paced Rhythms

• Similar to LBBB, ventricular pacing produces secondary repolarization changes that mask or mimic ischemic ST-segment changes 1
• Ventricular pacing (particularly right ventricular pacing) creates a pattern similar to LBBB
• The repolarization changes associated with prolonged ventricular pacing can take hours or days to develop and dissipate 1

Special Criteria for STEMI Detection in LBBB/Paced Rhythms

When standard ST elevation/depression interpretation isn't possible, alternative criteria must be used:

1. Sgarbossa Criteria for LBBB/paced rhythms 1, 2:

   • Concordant ST elevation ≥1 mm in leads with positive QRS
   • Concordant ST depression in leads V1-V3
   • Excessively discordant ST elevation ≥5 mm in leads with negative QRS

2. Modified Sgarbossa Criteria 2, 3:

   • Replaces the fixed 5 mm threshold with a proportional measurement (ST elevation ≥25% of the S-wave depth)
   • Can be applied to both LBBB and paced rhythms

Other ECG Patterns Where ST Interpretation Requires Caution

Right Bundle Branch Block (RBBB)

• Unlike LBBB, RBBB generally does not significantly affect ST-segment interpretation 1
• Standard STEMI criteria can still be applied in RBBB
• The ST-T changes in RBBB are directed opposite to the terminal slow component of the QRS complex, not affecting the initial portion where ST changes of ischemia would be seen 1

Atrial Fibrillation with Rapid Rates

• While atrial fibrillation can cause ST changes (particularly at rapid rates), these changes don't typically prevent interpretation of significant ST elevation/depression 4
• Rapid rates may cause ST depression due to demand ischemia, but this is different from the structural interpretation problems seen in LBBB/paced rhythms

Prior Ablation Procedures

• Ablation procedures themselves don't inherently prevent ST interpretation
• Any resulting conduction abnormalities would need to be evaluated based on the specific pattern (e.g., if ablation resulted in LBBB)

Clinical Implications

• When LBBB or paced rhythm is present, clinicians should:

  • Look for concordant ST changes (same direction as QRS)
  • Consider additional diagnostic methods (echocardiography, cardiac biomarkers)
  • Have a lower threshold for invasive angiography when clinical suspicion is high 1

• For patients with LBBB or paced rhythms presenting with symptoms suggestive of ACS:

  • Obtain serial cardiac biomarkers
  • Consider urgent coronary angiography rather than relying solely on ECG criteria 5

Conclusion

The correct answer is option A: Paced rhythms and left bundle branch blocks. These ECG patterns create secondary repolarization changes that make standard ST elevation/depression criteria unreliable for diagnosing acute myocardial ischemia or infarction.