12Lead ECG

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12-Lead ECG
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Cardiac Conduction Terms

• Automaticity hearts ability to initiate
  impulses (prominent in SA node)
• Excitability cells ability to respond to
  electrical impulse
• Conductivity cells ability to transmit
  electrical impulses
• Contractility ability of cardiac fibers to
  contract

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Cardiac Conduction System

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12-Lead ECG Interpretation

• To interpret a 12-lead ECG, look at 5 general
  areas 1. Rate 2. Rhythm 3. Axis
  4. Hypertrophy 5. Infarction

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Rate

• ECG paper speed 25 mm/sec
• 1 small box 1 mm 0.04 sec 1
  large box 5 mm 0.2 sec
• Usual ECG voltage standardization is 10 mm 1
  millivolt
• P-R interval 0.12 - 0.20 sec QRS interval
  0.1 sec

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Rate

• Rate of impulse formation SA
  Node - 60-100 AV Junction - 40-60 Ventricle
  - 20-40
• Determine ECG rates by 3 methods 1. 300,150,100,
  75,60,50 2. of cycles in 6 sec. strip x
  10 3. 300/ of blocks between QRS complexes

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Rhythm

• Categories 1. Sinus Rhythm and its
  Disturbances 2. Atrial Arrhythmias 3. A-V
  Junctional Arrhythmias 4. Ventricular
  Arrythmias 5. Conduction Disturbances
• See handout for arrhythmia summary

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Axis

• Refers to the direction of depolarization which
  spreads throughout the heart to stimulate the
  muscle fibers to contract
• Depolarization is downward and to the patients
  left
• Vector points toward hypertrophy and away from
  infarction
• Observe leads I and AVF for normal vs. axis
  deviation

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Hypertrophy

• Is an increase in the thickness of the wall of a
  heart chamber
• Right atrial hypertrophy. See diphasic P wave
  with tall initial component
• Left atrial hypertrophy. See diphasic P wave
  with wide terminal component

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Hypertrophy

• Right ventricular hypertrophy - R wave
  greater than S wave in V1 - R wave gets
  progressively smaller from V1 to V6 - S
  wave persists in V5 and V6 - Wide QRS

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Hypertrophy

• Left Ventricular Hypertrophy - S wave in V1
  R wave in V5 add up to more than 35 mm -
  Left axis deviation - Wide QRS - T wave
  slants down slowly and returns up rapidly
  (inverted)

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Infarction

• Classical triad of a myocardial infarction 1.
  Ischemia 2. Injury 3. Infarction
• Ischemia - Inverted T waves - T waves
  are usually upright in lead I,II, and V2-V6 ,
  check these for inversion

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Infarction

• Injury - elevated ST segment -
  signifies an acute process, ST returns to
  baseline with time - if ST depression..
  Digitalis or subendocardial infarction
• Infarction - small Q wave may normal in V5
  V6 - abnormal Q wave 0.04 sec or 1/3 of QRS
  height in lead III

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Infarction

• Infarction location - Anterior MI Q in
  V1,V2,V3, or V4 - Lateral MI Q in I and
  AVL - Inferior MI Q in II, III, and AVF -
  Posterior MI large R in V1, Q in V6