- ECG (EKG) Paper.

ECG Paper.

The output of an ECG recorder is a tracing on a graph or a standard  paper  traveling at a rate of 25 mm/s.

This paper is divided into large squares and each large square is divided into small squares

Time is represented on the x-axis, and voltage, or amplitude is represented on the y-axis.

Look at the previous  picture describing the time and voltage for a traditional ECG tracing. Notice, for example, how one small 1 mm x 1 mm square represents  0.04 seconds in time and 0.1 millivolts in amplitude.

The electrical activity detected by the electrocardiogram machine is measured in millivolts.
Machines are calibrated so that a signal with amplitude of 1 mv moves the recording stylus vertically 1cm or 2 large squares.  

Look for the baseline. It is the line that would be perfectly straight and horizontal if not for those vertical deflections. This is called the isoelectric line.

The isoelectric line is considered to have an amplitude of zero. Anything above the isoelectric line is "positive" deflection; below the line is "negative" deflection.

The direction of the deflection on the electrocardiogram depends on whether the electrical impulse is travelling towards or away from a detecting electrode.

By convention, an electrical impulse travelling directly towards the electrode produces an upright (“positive”) deflection relative to the isoelectric baseline, whereas an impulse moving directly away from an electrode produces a downward (“negative”) deflection relative to thebaseline.

When the wave of depolarisation is at right angles to the lead, an equiphasic deflection is produced. 

 References:-

1. Clinical electrocardiography a simplified approach 7th ed,Ary L.Goldberger, CH2,P.g 9.
2. ECG made easy, John R.Hampton 7th ed,CH1, p.g 6.
3. ABC of clinical electrocardiography steve meek,Francis Morris 2nd ed, Ch1, P.g1.2
4. Only EKG Book You'll Ever Need,Thaler, Malcolm S., The 5th Edition p.g.17,18