This is the equipment itself: a box with wires and pads that a hospital buys, moves, switches on and repairs. Its full name is the electrocardiograph. Because the machine, the picture it makes and the test all shorten to the same three letters, the words around it carry the difference. Machines get bought, wheeled, plugged in and turned on; pictures get printed and read.

This is the paper or the screen, the thing you can hold and point at. Each heartbeat shows up as a small sharp peak, and the shape and spacing of those peaks are what the doctor reads. A doctor looks at an ECG, prints one, files one, compares two of them. In full, this one is the electrocardiogram; the machine that makes it has a different full name. Both are shortened to the same three letters, so the sentence around it is what tells you whether someone means the picture or the machine.