Before anything else: what is it we are actually measuring?
Voltage is always a difference
There is no such thing as voltage at one point. A meter always reports one point minus another. That is why every ECG lead needs two ends — and why the answer can come out negative.
A Q wave and an R wave are not different kinds of event — they are the same kind of event, moving in opposite directions relative to that lead. Swap the two electrodes and every up becomes a down.
How big is the heart's signal?
Log scale — every gridline is ten times the one below.
That millivolt comes from charged atoms crossing a membrane.
The ventricular action potential
Tap a phase. Current in the body is carried by ions, not electrons.
Every cell makes a tiny arrow. Here is how millions of them become one trace.
1 · What a vector is
Some things are just a number — temperature, weight. Others need a direction as well — wind, a push, and the heart's electricity. Anything with both a size and a direction is a vector, and we draw it as an arrow.
2 · Two arrows make one
Two patches of muscle firing together make two arrows. The body cannot report them separately. It reports only their sum, found by laying them tip to tail.
3 · Millions of cells, one arrow
Each patch of muscle below is firing its own little arrow. Drag the spread to change how well they line up, and watch what survives in the total.
4 · What one lead can see
A lead is a line of sight. It cannot see the whole arrow — only the part lying along its own direction. That part is the arrow's shadow on the lead.
5 · Shadows become the trace
The arrow changes every millisecond. Take its shadow on one lead at each instant, plot that as a height, and the trace draws itself.
The electrode translates ions into electrons
Ions move charge in the body. Electrons move charge in a wire. The electrode is the point where one becomes the other.
This is the same chemistry as a torch battery, and for the same reason.
At the silver surface, chloride ions from the gel stick to the metal and release electrons into it: Ag + Cl⁻ → AgCl + e⁻. Electrons pile up in the metal, chloride is used up in the gel, and that separation of charge is a voltage. It settles at a steady value and stays there.
So the pad is a weak battery you did not ask for, wired in series with the patient. The machine receives heartbeat + battery added together, and cannot tell them apart from one wire alone.
Drag the slider. The red line is heartbeat + battery. Watch it lift away from zero while its shape never changes.
The amplifier subtracts a constant to bring the line back to zero, then magnifies what is left. Watch the same signal go through both steps.
Types of electrode
The sensor you stick on decides how clean the trace can ever be. Pick one and watch what it does to the signal.
Impedance — how hard it is for the signal to get out
Impedance is the resistance the signal meets between heart and amplifier. Almost all of it lives in the dead outer layer of skin. Change the conditions and watch it move.
The amplifier cancels interference only if both electrodes see it identically. Give one a different impedance and the cancellation breaks.
Ten electrodes on the skin. Twelve leads on the paper. They are not the same thing.
Electrode vs lead
Placement
Tap any electrode. Ribs, sternal angle and reference lines drawn to scale.
1 Check each site — bone, scar and heavy hair all give a poor signal.
2 Clean the skin. Wipe with alcohol and let it dry, or abrade lightly.
3 Dry the skin, then press the electrode down from the centre outward.
4 Attach the leadwires last, and let them hang loose so they do not tug.
How each lead is built
| I | LA − RA | bipolar · 0° |
| II | LL − RA | bipolar · +60° |
| III | LL − LA | bipolar · +120° |
| aVR | −(I + II)/2 | augmented · −150° |
| aVL | I − II/2 | augmented · −30° |
| aVF | II − I/2 | augmented · +90° |
Everything here happens to the real voltage, before a single number exists.
Overview — the whole journey
The complete path from one cell to the printed trace. Tap any point to see what the signal looks like there. The rest of this module walks the same path in detail.
About 1 mV of heartbeat, buried under a far larger offset and room noise.
What arrives at the wire
The starting point for everything in this module: about 1 mV of heartbeat, sitting on a much larger steady offset, with room noise on top. Every stage that follows is an attempt to keep the first part and discard the rest.
Two wires in, one difference out. Noise shared by both cancels; the heartbeat survives and is magnified about a thousandfold.
Why every lead needs two wires
Mains hum reaches both electrodes equally. Since a lead is a subtraction, that shared noise cancels itself out. This is the single most important trick in the whole machine.
From here on the signal becomes numbers, and stops being a voltage.
A fixed hardware filter sitting just before the converter. It removes anything too fast to be sampled, because that particular damage cannot be undone afterwards.
Anti-alias — the one filter you cannot switch off
Some things attached to a patient are far faster than anything the heart does: electrosurgery, radio pickup, switching noise from nearby equipment. This filter blocks them before the converter. Here is why it has to.
The machine samples 500 times a second. The gold dots are the only moments it looks.
A 600 Hz source reaching the converter, with the filter on and off.
Hardware, before the converter
Always on
Cannot be undone
Software, after the converter
You switch it on
Can be undone
How often the machine glances at the voltage. Glance too rarely and the peak falls between glances.
Sampling
Zoomed onto the QRS, because that is the fastest thing on the trace and the first to be lost. Grey is the real signal, gold marks each moment the ADC looked, navy is all it kept.
Every reading must land on one of a fixed set of steps. Anything smaller than one step disappears.
What an ADC actually does
ADC = analog-to-digital converter. A voltage is a smooth thing; a computer can only store numbers. The ADC turns one into the other, and it does exactly two jobs.
Step size is simply the gap between two marks: range ÷ number of marks. Nothing can be reported between two marks, so every reading must jump to the nearest one.
Rounding error is how far the reading had to jump to get there. It can never be more than half a step.
Grey is the true signal. Navy is what the ADC stores. There is a genuine 0.1 mV ST elevation in this beat. Change the bit depth here and watch the navy trace break into steps.
Every sample had to jump to the nearest mark. This red line is the size of each jump: what the machine stored, minus what was really there. Magnified so it is visible at all.
Now feed it one voltage and watch it land on a mark.
Quantisation
The ADC outputs whole steps only. Fewer bits, coarser steps.
These run on the stored numbers, so each is optional and can be undone. That is the opposite of stage 3.
Filters you can switch — all digital
These run on the stored numbers, after the ADC, and every one of them is optional. That makes them the opposite of the anti-alias filter in the overview above, which is fixed in hardware and runs before the numbers exist.
Grey is the signal before filtering. Navy is what you are left with. The square pulse on the left is the 1 mV calibration mark, and it is the quickest way to see what a filter has done.
Inject artifact
An "artifact" is anything on the trace that did not come from the heart. Three are common enough to recognise on sight. Switch one on, see what it looks like, then find the filter that removes it — and notice what that filter costs you.
Not part of drawing the trace at all. A separate copy is deliberately distorted so that beats become easy to detect.
Where rectification actually happens
Not in the displayed trace — rectifying that would destroy polarity, and polarity carries the diagnosis (a Q and an R differ only in sign). Rectification lives in the QRS detector, a parallel path used to find beats.
The signal is clean. Now: what does the paper it is printed on actually mean?
Live strip
The square pulse on the left is the 1 mV calibration mark.
Paper speed
Horizontal time scale only. Read it off the strip — never assume 25.
Gain
Vertical amplitude only. Half-standard when complexes are too tall to fit.
Here is the engine. Every trace in this simulator is a projection of this one rotating vector.
The heart vector loop
At any moment the heart's electricity is one arrow. Over a beat the tip of that arrow draws a loop. Drag the slider to watch it move.
The three QRS vectors
The loop is not one arrow — it is three, firing one after another. Change any of them and the loop reshapes.
Top slider of each pair sets direction, bottom sets size.
Projection makes the lead
Each bar shows how much of the arrow that lead can see. That number is exactly the height of the trace at the gold line.
Rotate the loop
Turn the whole loop and every lead changes shape together. This is what axis deviation means.
Full 12-lead
All twelve come straight from the loop above. None of these shapes are drawn by hand.
With a real trace on real paper, rate becomes arithmetic.
Drawn at the paper speed set in the Paper module: 25 mm/s. QT shortens with rate automatically (Bazett).
6-second method
For irregular rhythms. At 25 mm/s, 6 s = 150 mm = 30 large boxes.
Box counting — regular rhythms
Gold calipers mark one R–R interval.
Conduction blocks
If one pathway is blocked, that part of the ventricle is reached late and by slow muscle-to-muscle spread. A new, late vector appears — and the whole 12-lead changes with it. Picking a block overrides the sliders above.
Play the loop. The dot is where the vector is now; the number is milliseconds since the QRS began.
What it does to all twelve leads
The same block, shown everywhere. The normal beat is drawn faintly behind each one for comparison.
Why is a ventricular beat so big?
A VPC is usually taller and wider than the patient's own QRS. Both facts come from the same cause.
A ventricular beat starts in one ventricle and crawls across muscle. The two ventricles fire one after the other, not together, so there is far less to cancel and a much larger vector survives. The slow muscle-to-muscle spread is also what makes it wide. Big and wide are the same fact seen twice.
The first 40 ms tells you where the beat started
A beat that gets into the His–Purkinje system early begins fast. One that has to crawl through muscle begins slowly. Compare the shaded start and end of each.
Now break the loop and see what the twelve leads report.
Infarct location → injury vector
Move the infarct around the heart. The red arrow is the injury current. It always points outward through the damaged wall.
Stage
ST across all 12 leads
Green = elevation, violet = reciprocal depression, grey = near-perpendicular.
Quiz
0 / 0Question 1
Now put it together. Same engine, but you call it.
Case
0 correctScroll it sideways. Second marks are along the bottom; the shaded band is 6 seconds.
Work through it in order
Answer each step. The tool checks you before moving on, so you learn the sequence, not just the answer.