ECG : A Brief Overview

An ECG is a record of the electrical activity of the heart. It could be
ordered in many situations, some being...

-- Arrhythmias
-- Heart blocks
-- Myocardial infarction
-- Cardiomyopathy
-- Metabolic changes
-- Certain systemic diseases
-- Pericarditis
-- Drug toxixity
-- As part of pre anasthetic checkup

There are three types of electrocardiographs:

1. String galvanometer: It involves recording on a photographic plate which
   has to be subsequently developed.

2. Radioamplifier: The commonest variety.

3. Oscilloscope: A continuous monitoring on a fluoroscent tube.

4. A holter recorder.

ECG PAPER: It is a continuous roll of paper impregnated with graphite.
           Each small square on X-axis = 0.04 seconds.
           Each small square on Y-axis = 0.1 mV.

Certain artefacts may appear: They may be due to local effects of A/C
current, hiccoughs, movement by the patient, patient may be in contact
with a metallic surface, patient may have smoked.

Before applying the electrodes the ECG paste is used which contains
electrolytes and also abrassive material which removes the horny layer
of the skin.

THE EINTHOVEN TRIANGLE:
( a schematic representation)

                             Lead I
                    RA   _____________   LA
                         \           /
                          \         /
               Lead III    \       /     Lead II
                            \     /
                             \   /
                              \ /

                               LL

RA: Right Arm
LA: Left Arm
LL: Left Leg

LL is at the highest potential.
RA is at the lowest potential.

Bi-polar leads:

The total potential amounts to 0. Thus...

     Lead I + Lead II + Lead III = 0

Einthoven said that Lead II had an opposite vector as compared to the
other leads thus the new equation would be:

     Lead I + (-Lead II) + Lead III = 0

Which can be written as:

     Lead I -Lead II + Lead III = 0

               OR

     Lead I + Lead III = Lead II

Unipolar leads:

All the three ( RA, LA, LL ) are connected to make one lead which is
passed through a resistance of 5000 ohms. This is now called the
indefinite electrode. With its reference the potentials in the other
electrodes are noted.

With such usage it was seen that the potential was very low thus a new
terminology - augmented limb leads was introduced. With this technique
the potential difference was enhanced.

Now called as Cl, Cr, Cf : C ----> Chest.

Thus we record: Vr, Vl, Vf: V ----> called the vector.

We may introduce an esophageal lead called E.
Thus...

  E 15-25 (put 15-25 cm from the cannines) : positioned behind the atria.

  E 25-35 (put 25-35 cm from the cannines) : positioned behind AV groove.

  E 35-50 (put 35-50 cm from the cannines) : indicating ventricular activity
          especially in posterior ventricular myocardial infarction.

Certain orthogonal leads (all perpendicular to each other) are also used
called X Lead, Y Lead, Z Lead.

Pre-Cordial Leads:

  V1 -----> Placed in the 4th inter costal space on the right side.

  V2 -----> Placed in the 4th inter costal space on the left side.

  V3 -----> Placed midway between V2 and V4.

  V4 -----> Placed in the 5th inter costal space, the mid clavicular line,
            on the left side.

  V5 -----> Placed in the 5th inter costal space, the anterior axillary
            line, on the left side.

  V6 -----> Placed in the 5th inter costal space, the mid axillary line,
            on the left side.

  V7 -----> Placed in the 5th inter costal space, the posterior axillary
            line, on the left side.

  V8 -----> Placed in the 5th inter costal space, the posterior scapular
            line, on the left side.

  V9 -----> Placed in the 5th inter costal space, the lateral spinal line,
            on the left side.