12 Lead ECG Interpretation

Presenter Information

• Leslie L Davis, PhD, RN, ANP-BC, FAANP, FACC, FAHA
• Associate Professor, UNC Chapel Hill School of Nursing
• Email: [email protected]
• No disclosures relevant to this presentation.

Objectives for Workshop

• Review principles behind the 12 views on the ECG and alignment with frontal, augmented, and precordial leads.
• Interpret axis deviation on the 12 Lead ECG if present.
• Identify pathological events causing abnormalities on the 12 Lead ECG.
• Utilize a systematic approach for interpreting 12 Lead ECGs with case studies.

Systematic Interpretation of 12 Lead ECGs

• Step 1: Determine rate, intervals, and rhythm
  • Lead II or V1 are best for this analysis.
  • A rhythm strip can be helpful.

Conduction System of the Heart

• Basic Elements of the ECG
  • P Wave: Atrial depolarization (right then left); small and rounded, usually upright in most leads.
  • PR Interval: 0.12 - 0.20 seconds (3-5 small squares); measures time from start of atrial to start of ventricular depolarization.
  • QRS Complex: Usually <0.12 seconds; ventricular depolarization from the bundle of HIS to ventricular myocardium.
  • T Wave: Ventricular repolarization; usually rounded and upright, can be inverted or biphasic.
  • QT Interval: Time from start of ventricular depolarization to end of refractory period; normal <0.40 seconds, corrected QT (QTc) varies with heart rate.
  • U Wave: May be present, follows the T wave; indicates repolarization of papillary muscles or Purkinje fibers.
  • ST Segment: End of ventricular depolarization to start of repolarization; normally isoelectric.

ECG Paper and Heart Rate Calculation

• ECG Paper Measurements:
  • Horizontally: Small box = 0.04 sec, Large box = 0.20 sec
  • Vertically: Small box = 1 mm, Large box = 5 mm
• Heart Rate Calculation:
  • Step 1: Find an R wave on a bold line.
  • Step 2: Use sequence 300 - 150 - 100 - 75 - 60 - 50
  • Step 3: Find next R wave and estimate rate.

12 Views of the Heart

• Frontal Leads (Vertical Plane):
  • I, II, III (bipolar)
  • aVR, aVL, aVF (unipolar, augmented)
• Standard Limb Leads:
  • Lead I: RA (-) to LA (+)
  • Lead II: RA (-) to LL (+)
  • Lead III: LA (-) to RL (+)
• Augmented Limb Leads:
  • aVL: Left arm positive
  • aVR: Right arm positive
  • aVF: Feet positive

Precordial Leads (Horizontal Plane)

• Six Precordial Leads (Chest Leads):
  • V1: 4th ICS, right of sternum
  • V2: 4th ICS, left of sternum
  • V3: Between V2 and V4
  • V4: 5th ICS, midclavicular line
  • V5: Between V4 and V6
  • V6: 5th ICS, midaxillary line

Combining the 12-Leads

• Includes:
  • 3 Limb leads (I, II, III)
  • 3 Augmented leads (aVR, aVL, aVF)
  • 6 Precordial leads (V1-V6)

Case Study 1

• Details for case study 1 will be analyzed in further steps (rate, intervals, rhythm, etc.).