The axis is defined as the direction of spread of depolarisation through the ventricles, as seen from the front. It is affected by hypertrophy, conduction defects, etc.

vertical leads

By knowing the directions in which the leads ‘look' at the heart, the cardiac vector can be ascertained:

  • If a lead is predominantly positive (R larger than S), it is close to the vector.
  • If a lead is predominantly negative (S larger than R), it is pointing in the opposite direction to the vector.
  • If a lead shows little activity (R and S of similar size), it is perpendicular to the vector.

The normal axis is between -30 and +90 – leads I, II and III usually being positive as the vector is in their direction.

  • If the axis shifts to the right (greater than 90), lead I becomes predominantly negative.
  • If the axis shifts to the left (less than -30), lead III becomes predominantly negative.

Axis deviations of themselves are seldom significant, but when taken with other signs, can be pointers towards right or left hypertrophy; also note that a change to the right may indicate pulmonary embolism, while a change to the left can indicate conduction defects.

Horizontal leads

The QRS complex shows a progression from mainly negative in lead V1 to mainly positive in V6. The transition point is where R and S waves are equal and indicates the position of the septum – usually at V3/V4. However, right ventricular hypertrophy causes clockwise rotation – shifting the transition point to V4/V5 or even V5/V6.

SHORTCUT – if the sum of R in V5 or V6 and S in V1 is greater than 35mm, this indicates left ventricular hypertrophy.