The normal ECG shows the graphical tracings of the electrical activity in the heart, displayed in twelve leads. The pattern and shape of these ECG deflections are due to the (inter)action of dipole vectors with lead leads and limb vectors.
In simple terms an ECG tracing consists of
1. a recording (graph) paper that moves at a constant speed
2. a pen (recording or writing device) that has two states:
a. when there is no electrical activity a straight line (baseline or isoelectric line) is inscribed on the recording paper
b. when the ECG detects cardiac electrical activity the pen records a series of curves or deflections rising above or descending below the baseline.
ECG Recording Paper
The basic features of the paper that records an ECG are:
1. a grid pattern that resembles graph paper
2. a horizontal axis that measures distance intervals (millimetre [mm])or time intervals (millisecond [msecond])
3. a vertical axis that measures distance intervals (millimetre [mm]) or voltage intervals (millivolt [mV])
The basic unit of the recording paper for ECGs is a square with a length of 1 millimetre (mm). Twenty five of these units form a “box” that is five mm square. The boundary of each box is marked by a line that is slightly wider than the lines forming the basic unit. The ECG paper is formed by assembling these boxes.
The vertical and horizontal axes of the ECG paper have basic measuring units of 1 mm and 5 mm.
The vertical axis (or voltage axis) is calibrated in volts. The usual (standard) calibration is set so a vertical distance of 1 centimetre (cm) corresponds to one thousandth of a volt (i.e. to 1 millivolt [mV]). A vertical height of 1 mm represents a voltage of 0.1 mV. The vertical amplitude of complexes can be measured in mm as well as mV e.g. a QRS complex can be described as having a vertical amplitude of 15 mm and 1.5 mV.
Sometimes the cardiac complexes are too tall for measurement when standard calibration is used. In this case half-standard calibration is used, with the calibration adjusted so that a vertical distance of 1 mm represents 0.2 mV.
Measurement on the horizontal axis can be width (in mm) or time (in seconds or milliseconds). The horizontal width of the smallest square is 1 mm. The time unit measured by the horizontal squares depend on the speed of the ECG paper. The usual paper speed is 25 mm/second i.e. there are 25 small squares in one second. The time interval measured by a horizontal 1 mm small square is thus 1/25 second i.e. 0.04 seconds (or 40 milliseconds). If the paper speed is increased to 50 mm/second, the horizontal distance of one small square measures 1/50 second i.e. 0.02 seconds (20 milliseconds).