ECG basics

So basically, ECG is  an electrical representation of heart activity on ECG paper. 

Here are some important postulates of ECG paper:

The needle of galvanometer travels 300 big squares in 1 minute or 60 seconds. 

That makes 0.2 seconds for each big square. 

In turn each big square is divided into 5 small boxes /squares.

So we get each small box counting about
0.04 seconds. 

So let's review all calculations:

1 Big square = 0.2 seconds.
1 small square = 0.04 seconds. 

Each P wave travels about 2 and 1/2 small  squares making upto 0.1 second.

The segment PQ (Sometimes we use PR) again takes 2 and 1/2 small squares making upto 0.1 second.

QRS complex used another 2 and 1/2  small squares making another 0.1 second. 

While S-T segments uses 10 small squares forming using 0.4 seconds.

Note: We normally consider 2 and 1/2 small squares but there can be 2 or 3 small squares. 

~Ojas