A picture of moving electricity
An ECG does not photograph the heart and it does not measure how hard the heart squeezes. It records voltage — tiny electrical differences picked up by stickers on the skin — and plots that voltage against time. Every heartbeat begins with a wave of depolarization, an electrical change that sweeps cell to cell through the heart muscle and tells it to contract. The ECG is simply that travelling wave, drawn on paper.
The single most useful rule for beginners is this: when the electrical wave moves toward a recording lead, that lead's line goes up; when the wave moves away, the line goes down. The whole tracing is just many leads watching the same wave from different positions and reporting whether it is coming or going.
Reading the grid
ECG paper moves at a fixed speed, so distance across the page is time, and height is voltage. At the standard settings, each small square is 0.04 seconds wide (40 milliseconds) and each large square — five small ones — is 0.20 seconds. Vertically, each large square is 0.5 millivolts. Once you trust the grid, you can read any duration or amplitude straight off the page with a ruler or just your eye.
Standard ECG paper (speed 25 mm/s, gain 10 mm/mV):
Horizontal (time):
1 small square = 1 mm = 0.04 s (40 ms)
1 large square = 5 mm = 0.20 s (200 ms)
5 large squares = 1.00 s
Vertical (voltage):
1 small square = 1 mm = 0.1 mV
1 large square = 5 mm = 0.5 mV
Worked example:
A QRS complex spans 2.5 small squares wide.
Width = 2.5 x 0.04 s = 0.10 s = 100 ms -> normal (<120 ms).Twelve views of one heart
The full study is the 12-lead ECG. Despite the name, only ten stickers are used; the machine combines them mathematically into twelve different views of the same electrical event. Six look at the heart in the vertical plane of the chest and limbs; six look across it in the horizontal plane from the front of the chest. Because each lead watches from a different angle, the same heartbeat looks tall in one lead and inverted in another — and that disagreement is exactly what lets you locate where a problem sits.
So the ECG you will learn to read is one short story told twelve ways. The waves themselves — covered next — come from the conduction system firing in a set order. Master that order, and the tracing stops being a tangle of lines and starts being a sentence you can read.