II. Definitions

  1. Electrocardiogram (EKG)
    1. Heart electrical activity over time as recorded graphically by an electrocardiograph

III. Background

  1. See Cardiac Electrophysiology Anatomy
  2. Interpretation requires patient age, EKG indication
  3. Images
    1. heartElectrical.jpg
    2. ekg.png

IV. Precautions

  1. Standard EKG speed and amplitude is nearly always the best setting
    1. Although EKG gain can be increased and EKG speed can be slowed, the result is often more difficult to interpret
  2. Do not rely on computerized EKG Interpretation (aside from interval measurement)
    1. There is no standardization for software quality across EKG manufacturers and accuracy varies widely
    2. Computer interpretations miss STEMIs in up to 23-41% of cases
    3. Atrial Fibrillation is a frequent False Positive on computer interpretations
    4. Pacemaker rhythms are frequently misinterpreted by computers (STEMI False Positives and False Negatives)
    5. References
      1. Orman and Mattu in Herbert (2018) EM:Rap 18(3): 1-2
      2. Schlapfer (2017) J Am Coll Cardiol 70(9): 1183-92 [PubMed]
      3. Willems (1991) N Engl J Med 325(25):1767-73 +PMID:1834940 [PubMed]

V. Components: EKG Paper

  1. Record speed: 25 mm/sec
  2. Small square (1 mm): 0.04 seconds
  3. Large square (5 mm): 0.20 seconds
  4. Standard EKG contains distance markers that are 15 large boxes wide (3 seconds in duration)

VI. Components: Leads

  1. General
    1. Electrical cardiac signals traveling toward positive lead are upright on EKG
    2. Electrical cardiac signals traveling away from positive lead are negative or oriented down on EKG
    3. Most leads are primarily positive (P Wave and QRS), except aVR and V1
  2. Limb leads (bipolar, vector to positive lead)
    1. ekgLimb.png
    2. Lead I: Left arm (+) - Right arm (-), 0 degrees
    3. Lead II: Left leg (+) - Right arm (-), 60 degrees
    4. Lead III: Left leg (+) - Left arm (-), 120 degrees
  3. Augmented Limb Leads (unipolar, vector to positive lead)
    1. ekgAugmented.png
    2. Lead aVR: Right arm (+) - Left arm/leg (-), -150 degrees
    3. Lead aVL: Left arm (+) - Right arm/Left leg (-), -30 degrees
    4. Lead aVF: Left leg (+) - Right arm/left arm (-), 90 degrees (directly down, toward feet)
  4. Precordial Leads (unipolar chest)
    1. ekgPrecordial.jpg
    2. Vector from posterior (combined, negative limb leads) to anterior (chest positive, precordial leads)
      1. QRS grows gradually more upright in transition from V1 to V6 (see R Wave Progression)
      2. Ventricular depolarization passes from subendocardial to subepicardial, mostly toward anterior chest
    3. Lead V1: Right sternal border (Right Ventricle)
    4. Lead V2: Left sternal border
    5. Lead V3: Medial Breast (Septum)
    6. Lead V4: Nipple
    7. Lead V5: Lateral Breast
    8. Lead V6: Lateral chest wall (Left Ventricle)

VII. Evaluation: Approach

  1. Assess EKG Validity
  2. Rate and Rhythm: "Watch your P's and Q's and the 3R's"
    1. P Waves
      1. Not sinus rhythm if P Waves absent, or not upright in Lead II
    2. QRS Complex wide or narrow?
      1. Narrow Complex Tachycardia or Wide Complex Tachycardia
      2. Bundle Branch Block
    3. Rate (see EKG Rate)?
      1. Tachycardia
      2. Bradycardia
    4. Regularity of Rhythm?
      1. Atrioventricular Block
      2. Atrial Fibrillation
      3. Multifocal Atrial Tachycardia
    5. Relationship between P Waves and QRS Complex
      1. Atrioventricular Block
      2. Sick Sinus Syndrome
  3. EKG Axis
    1. EKG-Axes.png
    2. Cardiac depolarization normally moves from upper right to lower left
  4. Intervals (prolonged?)
    1. PR Interval
    2. QRS Complex
    3. QT Interval
  5. Hypertrophy?
    1. Left Ventricular Hypertrophy
    2. Right Ventricular Hypertrophy
    3. Right Atrial Enlargement
    4. Left Atrial Enlargement
  6. Infarction?
    1. Q Waves
    2. R Wave Progression
    3. ST Segments
    4. T Waves
  7. Reference
    1. Grauer (2001) 12 Lead EKGs, KG/EKG Press, Gainesville

VIII. Interpretation: Specific Circumstances

  1. See Electrocardiogram in Myocardial Infarction
  2. See EKG Changes in Syncope due to Arrhythmia
  3. See Electrocardiogram in Atrial Fibrillation
  4. See EKG Changes During Pregnancy
  5. See Hyperkalemia Related EKG Changes
  6. See Hypokalemia Related EKG Changes
  7. Ventricular Tachycardia
    1. Northwest axis (opposite Nl axis)
    2. V1-V6 positive QRS Complex
    3. See AV dissociation
  8. Pericarditis
    1. See Electrocardiogram in Pericarditis
    2. Stage I: Diffuse ST inc all leads except aVR, V1
    3. Stage II: ST Segments temporarily normalize
    4. Stage III: Diffuse ST depress +/- T inversion
    5. Stage IV: Gradual normalization of ST and T Waves
  9. Pulmonary Embolism
    1. See Electrocardiogram in Pulmonary Embolism
    2. Right Strain Pattern (Classic PE, but rarely seen)
      1. S1 (deep S wave in lead I)
      2. Q3 (Q Wave in lead III)
      3. T3 (inverted T Wave in lead III)
  10. Atrial Fibrillation or Atrial Flutter with Rapid Ventricular Rate
    1. See Electrocardiogram in Atrial Fibrillation
    2. At a rapid rate (e.g. 140-150 bpm), may be difficult to differentiate AFib/Flutter from ST, PSVT
      1. Increase EKG "paper speed" to 50 mm/sec (rapid) to spread out complexes (P Waves may be seen)
      2. Mattu (2020) Crit Dec Emerg Med 34(4): 18

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