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Electrocardiogram

Notes

Electrocardiogram

Electrocardiogram (ECG) Notes

ELECTROCARDIOGRAM: sum of the heart's action potentials; direction of each wave on ECG reflects direction in which current moves through heart
12 electrodes: detect the heart's action potentials
12 leads: unique perspectives of heart's action potentials; connect (+) electrodes to (–) electrodes
3 standard limb leads (bipolar)
3 augmented limb leads (bipolar)
6 chest leads (unipolar)

3 STANDARD LIMB LEADS
Lead I: right arm (negative electrode) → left arm (positive electrode)
Lead II: right arm (negative electrode) → left leg (positive electrode)
Lead III: left arm (negative electrode) → left leg (positive electrode)
Lead II = Lead I + Lead III

3 AUGMENTED LIMB LEADS: one on each arm and left leg
6 CHEST LEADS: around the heart

READING AN ECG:

  1. SA node fires
  2. P wave: atrial depolarization
    *smaller than QRS complex because atria less muscular than ventricles
  3. QRS complex: ventricular depolarization
    *masks electrical impulse of atrial repolarization (occurs simultaneously)
  4. T wave: ventricular repolarization (ventricles finish contracting and emptying)
    *end of T wave = end of ventricular ejection phase

Segments:
PR segment: AV nodal delay
ST segment: start of ventricular contraction (after ventricular depolarization completes)
TP segment: ventricular filling phase, cardiac muscle cells completely repolarized, SA node fires again