ECG: the first step

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Projection of the "cardiac vector"

The deflection of the electrocardiograph needle is proportional to the component size of the "cardiac vector" in the direction of the lead.

A 3-D "cardiac vector" at a certain instant, for example, during ventricular depolarization, is projected to a plane and forms a 2-D (plane) vector. The 2-D vector is further projected to a linear lead, and forms a 1-D vector. The size of the 1-D vector is the voltage, which is represented by the deflection of the electrocardiograph needle.

In summary, projection can be illustrated as in this figure.

Achievement Quiz

  1. When a 3-D "cardiac vector during ventricular depolarization" is projected to a plane, it loses the component perpendicular to the plane, and forms a 2-D "cardiac vector during ventricular depolarization".
  2. When a 2-D "cardiac vector during ventricular depolarization" is projected to a lead, it loses the component perpendicular to the lead, and forms a 1-D vector.
  3. The deflection in ECG is the 1-D component of the "cardiac vector during ventricular depolarization" in the direction of the lead.
  4. When the "cardiac vector during ventricular depolarization" is to the lead, the deflection in the ECG needle is large.
  5. When the "cardiac vector during ventricular depolarization" is to the lead, the deflection in the lead is zero.
  6. When the "cardiac vector during ventricular depolarization" is zero, the ECG needle deflection is also zero.