How can i discuss a 3lead ECG and a electrical axis of the heart? also how does morphology* affect cardac axis

Question

morphology= e.g size and shape of person -small, tall slim, fat.

Answer 1

The basic 3 lead ECG (which is the original form of the ECG prior to the development of the aVL aVR and aVF and all the anterior chest leads V1-V6) comprises the detection of a potential difference between the electrodes placed on R arm, L arm and L leg.

In days of yore, the subjects would sit with their feet and hands in bowls of solute and a voltmeter would detect the tiny flickers of electricity.

The potential differences are

I - Left arm to Right arm (LA - RA)
II - Left leg to Right arm (LL - RA)
III - Left leg to Left arm (LL - LA)

Conveniently, the number of L's tells us if it is I, II or III

I explain to students that they can either learn some rules or do some vector mathematics.

The rules are: "Left Out", "Right In"

If the QRS in I points up and the QRS in III points down then that's "out" and hence left axis deviation. This is to do with the arrangement of I, II and III on the ECG paper

I......^
II
III.....v

If the QRS in I points down and the QRS in III points up then that's "in" and hence represents right axis deviation

I......v
II
III.....^

These rules are actually a simplification of the proper way to calculate the cardiac electrical axis.

The right way to do it is by drawing Einthoven's triangle and performing a vector sum of I, II and III. In this manner you can determine the direction of the majority of the cardiac electrical activity.

Draw an equilateral triangle - point down. This is Einthoven's triangle. Horizontally across the chest is the PD between LA and RA (ie I) and the diagonals are the LL-RA (or II) and LL-LA (or III) potential differences.

If you move III across so it comes off the same origin as I and II you wind up with a tri-axial origin and you can plot out the magnitude and direction of the QRS in I, II and III on the relevant axes. Adding these vectors will result in the cardiac axis.

._
/\
triaxial origin (cf hexaxial reference system)

See the wiki page

The morphology of the person does not have so much of an influence on the axis as the morphology of the heart and chest cavity. Taller thinner people will have more vertical hearts and the axis will tend towards a vertical up/down axis (90 degrees on the triaxial or hexaxial reference system). Shorter fatter people will have a shorter wider chest and their axis will tend towards the horizontal (0 degrees on the triaxial or hexaxial reference system). In both cases, the normal range still applies (up to -15 degrees through to 90 degrees is considered normal).