What are the criteria for a normal electrocardiogram (EKG) reading?

Normal EKG Interpretation Criteria

A normal electrocardiogram (EKG) should meet specific criteria for rate, rhythm, intervals, axis, and waveform morphology across all 12 leads, with interpretation adjusted for age, gender, and body habitus.

Heart Rate

• Normal sinus rhythm: 60-100 beats per minute (bpm) in adults
• Sinus bradycardia (30-60 bpm) is normal in athletes and physically active individuals 1
• Sinus arrhythmia with respiratory variation is a normal finding 1

Rhythm

• Regular rhythm with P waves preceding each QRS complex
• Normal variants may include:
  • Ectopic atrial rhythm
  • Junctional escape rhythm
  • Sinus arrhythmia with respiratory variation 1

Intervals

• PR interval: 120-200 ms
  • First-degree AV block (PR 200-400 ms) may be normal in asymptomatic individuals 1
• QRS duration: <120 ms
• QT interval:
  • Corrected for heart rate (QTc) using Bazett's formula
  • Normal values: <440 ms in men, <460 ms in women 1
  • Best measured in lead II or V5 at heart rates between 60-90 bpm 1

Axis

• Normal QRS axis: +90° to -30° (adult)
• Normal full-term neonate: 55° to 200° 2
  • By 1 month of age, upper limit falls to 160° or less 2

ST Segment

• J-point elevation thresholds (per AHA/ACC/HRS guidelines) 2:
  • Men ≥40 years: 0.2 mV (2 mm) in V2-V3, 0.1 mV (1 mm) in all other leads
  • Men <40 years: 0.25 mV (2.5 mm) in V2-V3
  • Women: 0.15 mV (1.5 mm) in V2-V3, 0.1 mV (1 mm) in all other leads
  • Both genders in V4-V5: 0.05 mV (0.5 mm), except males <30 years: 0.1 mV (1 mm)
  • Both genders in V7-V9: 0.05 mV (0.5 mm)
• J-point depression threshold: 0.05 mV (0.5 mm) in V2-V3, 0.1 mV (1 mm) in all other leads 2

T Waves

• Upright in leads I, II, V3-V6
• Inverted in aVR
• Variable in leads III, aVL, aVF, V1-V2 2
• Age-specific considerations:
  • Children >1 month: T waves often inverted in V1-V3
  • Adolescents ≥12 years and young adults <20 years: T wave may be slightly inverted in aVF and inverted in V1
  • Adults ≥20 years: T wave inverted in aVR; upright or inverted in aVL, III, and V1; upright in I, II, and V3-V6 2
  • Juvenile T-wave pattern (T-wave inversion in V2-V3) normal in individuals <16 years 1

Q Waves

• Small Q waves (<0.03 sec and <25% of R wave amplitude) may be normal in leads I, aVL, V5-V6
• Q wave duration >30 ms is abnormal 2

Normal Variants

1. Early repolarization:

   • J-point elevation
   • ST-segment elevation
   • J waves or terminal QRS slurring in inferior/lateral leads 1

2. Incomplete RBBB:

   • rSR' pattern in V1
   • qRS pattern in V6
   • QRS duration <120 ms 1

3. Increased QRS voltage in thin individuals:

   • SV1 + RV5 or RV6 >3.5 mV (LVH criteria)
   • RV1 + SV5 or SV6 >1.1 mV (RVH criteria) 1

4. Black athlete repolarization variant:

   • J-point elevation
   • Convex ('domed') ST-segment elevation
   • T-wave inversion in V2-V4 1

Abnormal Findings Requiring Further Evaluation

• QTc intervals >440 ms in males or >460 ms in females
• T-wave inversion extending beyond V3 in adults >16 years
• ST-segment depression ≥0.5 mm in two or more contiguous leads
• Pathologic Q waves
• Presence of symptoms (syncope, palpitations, chest pain)
• Family history of sudden cardiac death 1

Common Pitfalls in EKG Interpretation

1. Misinterpreting normal variants: Particularly early repolarization and athlete's heart changes
2. Overlooking age and gender differences: Threshold values for ST elevation differ by age and gender
3. Artifacts: Common in newborns and can include:
   • Limb lead reversal
   • Incorrect chest lead positioning
   • Electrical interference (60 cycles)
   • Movement artifacts 2
4. Improper measurement technique: QT interval should be measured in leads II, V5, and V6 with the longest value being used 2

Remember that EKG interpretation should always consider the patient's clinical context, including age, gender, body habitus, athletic status, and symptoms when determining whether findings are normal or pathological.