What are the red flags on an electrocardiogram (ECG)?

Red Flags on Electrocardiograms (ECGs)

ST-segment elevation with reciprocal changes is the most concerning ECG finding, as it has a 94% positive predictive value for acute myocardial infarction and requires immediate intervention. 1

Primary Life-Threatening ECG Findings

ST-Segment Abnormalities

• ST-segment elevation ≥1mm in 2+ contiguous leads

  • Most concerning when accompanied by reciprocal ST depression in anatomically opposite leads 1
  • Indicates transmural myocardial injury/infarction
  • Requires immediate reperfusion therapy evaluation
  • False positives can occur with:
    • Left ventricular hypertrophy
    • Left bundle branch block
    • Pericarditis
    • Early repolarization pattern

• ST-segment depression ≥0.5mm in 2+ contiguous leads 2

  • May indicate subendocardial ischemia
  • Particularly concerning when horizontal or downsloping

T-Wave Abnormalities

• Deep T-wave inversions ≥1mm in 2+ contiguous leads 2
  • Especially concerning in anterior (V2-V4) or lateral (I, aVL, V5-V6) leads
  • May indicate ischemia, cardiomyopathy, or pulmonary embolism
  • Note: T-wave inversions in V1-V3 may be normal in black athletes and adolescents <16 years 2

Conduction Abnormalities

• Complete left bundle branch block (new onset)

  • QRS ≥120ms with predominantly negative QRS in V1 and upright notched/slurred R wave in leads I and V6 2
  • May mask underlying STEMI
  • New LBBB with appropriate clinical context should be treated as STEMI equivalent

• Profound intraventricular conduction delay

  • QRS duration ≥140ms 2
  • May indicate severe conduction system disease

Arrhythmias

• Ventricular tachyarrhythmias

  • Couplets, triplets, non-sustained ventricular tachycardia 2
  • Indicate electrical instability

• Profound bradycardia

  • Heart rate <30 beats/min or sinus pauses ≥3 seconds 2
  • May lead to hemodynamic compromise

• High-grade AV blocks

  • Mobitz Type II second-degree AV block
  • Third-degree (complete) heart block 2
  • May require urgent pacing

Other Critical ECG Findings

Q-Wave Abnormalities

• Pathological Q waves
  • Q/R ratio ≥0.25 or duration ≥40ms in 2+ contiguous leads (excluding III and aVR) 2
  • In anterior MI, associated with larger infarct size, higher prevalence of heart failure, and increased mortality 3
  • Q-wave regression over time is associated with improved left ventricular function 4

QT Interval Abnormalities

• Prolonged QT interval

  • QTc ≥470ms (male), ≥480ms (female), or ≥500ms (marked prolongation) 2
  • Risk for torsades de pointes and sudden cardiac death
  • May be congenital or acquired (medications, electrolyte abnormalities)

• Short QT interval

  • QTc <340ms
  • Associated with risk of sudden cardiac death

Brugada Pattern

• Type 1 Brugada pattern
  • Coved ST-segment elevation ≥2mm with downsloping ST-segment followed by negative T-wave in ≥1 lead in V1-V3 2
  • Associated with risk of ventricular fibrillation and sudden death

Pre-excitation

• Ventricular pre-excitation (WPW pattern)
  • PR interval <120ms with delta wave and wide QRS (≥120ms) 2
  • Risk of rapid conduction of atrial arrhythmias

Electrolyte and Metabolic Abnormalities

• Hyperkalemia

  • Tall, peaked T waves
  • Widened QRS complex
  • Flattened or absent P waves
  • Sine wave pattern in severe cases
  • Can mimic STEMI 2

• Hypokalemia

  • U-wave prominence
  • ST-segment depression
  • T-wave flattening

• Hypercalcemia

  • Shortened QT interval

• Hypocalcemia

  • Prolonged QT interval

Clinical Context Considerations

Pitfalls in ECG Interpretation

• False positives for ischemia 2:

  • Early repolarization (especially in young athletes)
  • Left ventricular hypertrophy
  • Bundle branch blocks
  • Pericarditis/myopericarditis
  • Brugada syndrome
  • J-point elevation syndromes
  • Ventricular pre-excitation
  • Cardiomyopathies
  • Metabolic disturbances
  • Certain medications (tricyclic antidepressants, phenothiazines)

• False negatives for ischemia 2:

  • Prior MI with Q-waves and/or persistent ST elevation
  • Right ventricular pacing
  • LBBB
  • Lateral wall MI (may be "electrically silent") 5

Important Clinical Applications

• All ECGs should be interpreted by a qualified physician, not solely by computer algorithms 2, 6
• Serial ECGs are essential for evolving conditions like acute MI 2
• Compare with previous ECGs when available 2
• ECG findings must always be interpreted in clinical context 6

Algorithmic Approach to ECG Red Flags

1. Assess for immediate life threats:

   • ST-segment elevation with reciprocal changes
   • New LBBB
   • Ventricular tachyarrhythmias
   • High-grade AV blocks

2. Evaluate for ischemic changes:

   • ST-segment deviations
   • T-wave inversions
   • Pathological Q waves

3. Check for conduction abnormalities:

   • Bundle branch blocks
   • Fascicular blocks
   • AV blocks

4. Assess for arrhythmias:

   • Bradyarrhythmias
   • Tachyarrhythmias
   • Irregular rhythms

5. Look for electrolyte/metabolic disturbances:

   • QT interval abnormalities
   • T-wave morphology changes
   • U-wave prominence

By systematically evaluating these key areas, clinicians can identify critical ECG findings that require immediate attention and intervention to improve patient outcomes.