The ECG paper runs at 25 mm/sec through the ECG printer. That’s the convention. Every ECG machine works like that. Always.
The ECG machine and the ECG paper are set up by convention in such a way that 300 big squares pass through the pointer of the ECG in one minute. That means all of the electrical activity that goes through the heart in one minute will be recorded on exactly 300 big squares.
300 big squares = 1 minute
300 big squares = 60 seconds.
1 big square = 0.20 seconds.
One big box (5mm) = 0.20 sec.
One big box has 5 small boxes.
One small box (1mm) = 0.04 sec
3 small boxes = 0.12 sec
P wave < 0.11 sec
PR interval ≤ 0.20 sec. Actually, the normal range is 0.12-0.20 sec. PR is prolonged if it’s more than one big box or five small boxes.
QRS interval≤ 0.10 sec. Some sources like the ACLS algorithms say QRS ≤ 0.12 (i.e. ≤ 3 small boxes).
QTc interval 0.33-0.47
P wave = about 2.5 small squares = 0.10 sec
PR segment = about 2.5 small squares = 0.10 secs
QRS interval = about 2.5 small squares = 0.10 seconds.
QT interval = about 10 small squares = 0.4 seconds. This is about what the duration of the QT interval should be.
The isoelectric line = The line between the T wave and the next P wave. Basically, the isoelectric line is the TP segment.