What is the step-by-step process for interpreting an Electrocardiogram (EKG)?

Systematic Approach to ECG Interpretation

A systematic approach to ECG interpretation is essential for accurate diagnosis, with all ECGs requiring verification by a qualified physician regardless of computer interpretation, as incorrect readings can significantly impact patient morbidity and mortality. 1

Step 1: Assess Rate and Rhythm

• Heart Rate: Count the number of QRS complexes in a 6-second strip and multiply by 10
  • Normal: 60-100 beats/min
  • Bradycardia: <60 beats/min
  • Tachycardia: >100 beats/min 1
• Rhythm: Evaluate regularity of R-R intervals
  • Check for P waves before each QRS complex
  • Assess if P waves are uniform in morphology
  • Determine if PR interval is constant

Step 2: Evaluate Intervals and Durations

• PR Interval: Measure from beginning of P wave to beginning of QRS complex
  • Normal: 120-200 ms (3-5 small squares)
  • Prolonged PR (>200 ms): First-degree AV block
  • Short PR (<120 ms): Pre-excitation syndromes 1
• QRS Duration: Measure width of QRS complex
  • Normal: <120 ms (3 small squares)
  • Widened QRS (≥120 ms): Bundle branch blocks, ventricular rhythms 1
• QT Interval: Measure from beginning of QRS to end of T wave
  • Calculate corrected QT (QTc) for heart rate
  • Normal QTc: <450 ms in men, <460 ms in women 1

Step 3: Analyze Axis and Voltage

• Determine Electrical Axis:
  • Normal axis: -30° to +90°
  • Left axis deviation: -30° to -90°
  • Right axis deviation: +90° to +180°
  • Extreme axis deviation: -90° to ±180°
• Check for Hypertrophy:
  • Left ventricular hypertrophy: Sokolow-Lyon criteria (S in V1 + R in V5 or V6 >35 mm) or Cornell criteria (R in aVL + S in V3 >28 mm for men, >20 mm for women) 1
  • Right ventricular hypertrophy: R in V1 >7 mm or R/S ratio in V1 >1 1

Step 4: Examine ST Segments and T Waves

• ST Segment Evaluation:
  • ST elevation: ≥0.1 mV (1 mm) in all leads except V2-V3
  • For V2-V3: ≥0.2 mV (2 mm) in men ≥40 years, ≥0.25 mV in men <40 years, ≥0.15 mV in women 2
  • ST depression: ≥0.05 mV (0.5 mm) in two contiguous leads
• T Wave Assessment:
  • Normal T wave: Upright in leads I, II, V3-V6; inverted in aVR
  • T wave inversion: May indicate ischemia, strain, or normal variant 2
  • T wave alternans: Alternating T wave amplitude, may indicate electrical instability 2

Step 5: Look for Signs of Ischemia/Infarction

• Identify location based on lead groups:
  • Anterior: V1-V4
  • Lateral: I, aVL, V5-V6
  • Inferior: II, III, aVF
  • Posterior: Tall R waves and ST depression in V1-V2 1
• Look for pathological Q waves: >40 ms or >25% of R wave amplitude 1
• Assess for evolving changes: Hyperacute T waves, ST elevation, Q waves, T wave inversion

Step 6: Identify Additional Abnormalities

• Conduction Abnormalities:
  • AV blocks: First-degree, second-degree (Mobitz I/II), third-degree
  • Bundle branch blocks: RBBB, LBBB
  • Fascicular blocks: Left anterior/posterior fascicular block
• Chamber Enlargement:
  • Atrial enlargement: P wave abnormalities
  • Ventricular hypertrophy: Voltage criteria and repolarization changes
• U Wave Abnormalities: Most evident in leads V2 and V3, abnormal if prominent 2

Common Pitfalls to Avoid

1. Relying solely on computer interpretation: Computer algorithms have significant error rates (40.7% for acute MI, 75% for high-degree AV blocks) 1
2. Misplacement of electrodes: Especially V1/V2 placed too high, leading to false patterns 1
3. Failure to compare with previous ECGs: Serial changes are often more important than a single tracing
4. Ignoring clinical context: ECG findings must be interpreted in light of the patient's clinical situation 1
5. Pattern recognition without understanding: Focus on understanding pathophysiology rather than memorizing patterns 1

Maintaining Competency

• Interpret at least 500-800 ECGs under supervision to develop competency 1
• Maintain proficiency by interpreting at least 100 ECGs yearly 1
• Participate in continuing education and workshops to improve interpretation skills 1

By following this systematic approach, clinicians can improve their accuracy in ECG interpretation, leading to better clinical decisions and improved patient outcomes.