Waves and intervals are named with capital Latin letters — P, Q, R, S, T, the PR interval, the QRS complex, the QT interval — and the lowercase c in QTc stands for corrected, that is, corrected for heart rate. Speed is written mm/s and gain mm/mV.
The square is a unit of paper, not of time: it means 40 ms only at 25 mm/s, and at 50 mm/s the same square is already 20 ms. And QT is not QTc: the first is measured directly on the strip, the second is calculated by a formula, and for one and the same strip different formulas give different numbers.
The pen draws two heart cycles over and over on the graph paper, starting with an ochre calibration pulse ten squares high. The second R wave pierces the letter T right through, above the strip an ochre caliper stretches from one R to the next, below it a plum caliper measures the QT interval from the start of Q to the end of the T wave, and the T wave itself lights up plum from time to time.
Quantity data sheet
sweep · gain · square sheet 1/1 · rev. 2026-09ECG paper 25 mm/s · 10 mm/mV · QTc
Non-SI unit of recording · quantity: time and voltage on the strip, 1 square = 40 ms × 0.1 mV
The millimetre that beats
When a cardiologist lays a ruler on the strip, they measure not millimetres but time and voltage: the paper runs under the pen at 25 millimetres per second, and the pen deflects ten millimetres for every millivolt. So one small square of the grid is at once 40 milliseconds horizontally and 0.1 millivolt vertically, and from the number of squares between two R waves the doctor names the heart rate at a glance.
The most capricious stretch of the strip is the QT interval, from the start of the ventricular complex to the end of the T wave. It shortens when the heart beats faster, and in 1920 the physiologist Henry Bazett proposed dividing it by the square root of the RR interval, so that people with different heart rates could be compared. The formula proved simple and therefore immortal, although at a fast rhythm it clearly overcorrects, and because of this quite a few healthy people received a report saying “prolonged QT”.
The sheet prints a live strip, measures it with calipers, and shows what happens to the square at another speed and to QTc with another formula.
01 · Definition
speed · gain · intervals · correctionECG paper is graph paper on which the electrocardiograph draws the potential difference between electrodes as a function of time. The scale is set by two conventions: a paper speed of 25 mm/s and a gain of 10 mm/mV, so a horizontal millimetre equals 40 ms and a vertical one 0.1 mV; every fifth line is thicker and bounds a large square of 200 ms and 0.5 mV.
The square turns paper into an instrument. The distance between two R waves gives the RR interval, and from it the heart rate; the width of the QRS complex shows how fast the excitation spread through the ventricles, and the QT interval shows how long their whole electrical systole lasts, from the start of depolarisation to the end of repolarisation.
The QT interval lengthens at a slow rhythm and shortens at a fast one, so it is referred to a rate of 60 per minute, that is, to an RR interval of one second. Bazett divided QT by the square root of RR, Fridericia in the same year by the cube root, and later the linear corrections of Framingham and Hodges appeared; they all coincide at sixty beats and diverge the more, the further the heart rate is from that point.
How many squares in one beat
The strip crawls from right to left at the chosen speed, and the pen at the right edge draws a heart with the set heart rate; on the left, an ochre rectangle is the calibration millivolt. At each new beat the ochre caliper at the top stretches from one R wave to the next, and the plum one below from the start of Q to the end of T, and above them it is written how many millimetres that is on paper and how many milliseconds in the heart. The QTc slider sets the heart's own property, and the machine draws the strip from it honestly, as living physiology does.
Torsades de pointes on the strip
When ventricular recovery drags on, a premature impulse can land in the vulnerable window at the end of the T wave, and then a “pirouette” appears on the strip — torsades de pointes, a ventricular tachycardia whose complexes now grow, now shrink, as if the strip were twisting around the line. The risk rises markedly at a QTc above 500 ms, and that is exactly why dozens of drugs — from antibiotics to antihistamines — are tested for QT prolongation, and several of them were withdrawn from the market in the 1990s when they turned out to prolong the interval in healthy people.
02 · Conversion
25 mm/s · 50 mm/s · amplitudeFrom squares to milliseconds and millivolts
Enter a length on the strip in millimetres or squares, a time in milliseconds or a heart rate, and the sheet converts one into another for both paper speeds. The third tab reads the same number as the height of a wave and converts millimetres into millivolts at different gains.
A rule of thumb: the heart rate equals 300 divided by the number of large squares between R waves, or 1500 divided by the number of small ones — but only at 25 mm/s.
Before counting squares, the doctor looks at the calibration pulse at the start of the strip: if it is five millimetres high rather than ten, the machine was set to half gain, and all the waves are in fact twice as tall. The same goes for the speed label in the corner: at 50 mm/s the complexes look wide, and a reader unaware of this will diagnose a bundle branch block where there is none.
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04 · Measuring instruments
string · thermal head · Holter · watchFrom a one-tonne string to a wristband
A silvered thread between magnets
Willem Einthoven stretched an extremely thin silvered quartz thread between the poles of an electromagnet: the heart's current deflected it, and the thread's shadow in a beam of light fell on a moving photographic plate. The apparatus weighed about 270 kilograms and took five people to operate, the patient's arms and leg were lowered into jars of salt solution, and the grid on the record appeared precisely then — it was printed on so that time could be measured on the strip.
Twelve leads in ten seconds
A modern machine digitises the signal from ten electrodes hundreds of times a second, assembles twelve leads from them and prints them with a thermal print head on paper that already carries a printed red grid. The paper speed is now set not by a motor but by software, and therefore the standard requires the paper itself to be ruled exactly: a grid error of half a millimetre per large square shifts every measured interval by ten percent.
Holter's backpack
The biophysicist Norman Holter carried on his back in the late 1940s a backpack weighing about 38 kilograms that transmitted his ECG by radio, and a decade later he shrank the device to a tape recorder on a belt. A day of recording at 25 mm/s is more than two kilometres of strip, so a Holter recording is not printed but viewed on screen in fast-forward, and the square turns into a number of pixels.
Lead I on the wrist
A watch with an electrode records a single lead resembling standard lead I: the back of the case touches one arm, and a finger of the other hand rests on the crown. Half a minute of such a recording is enough to spot atrial fibrillation, and the file the watch hands to the doctor is still drawn on a red grid at 25 mm/s and 10 mm/mV — otherwise the cardiologist could not read it.
05 · Writing rules
formula · unit · speedQTc without the name of the formula cannot be read
Intervals are written in milliseconds or seconds, not in squares, heart rate in beats per minute, QTc is given with its formula, and the description of the strip gives the speed and gain if they are not standard.
The first line on the right gives neither a unit nor a formula, and the second writes the interval in squares, which at an unknown speed can mean 440 as well as 220 ms. In the third, seconds are confused with milliseconds, in the fourth the heart rate is given in hertz, although 75 beats per minute is 1.25 Hz, in the fifth “sec” stands instead of the standard s, and in the last the Russian abbreviation of millisecond, typed in Latin letters, becomes “mc”, which means nothing.
06 · Neighbouring units
millisecond · millivolt · beats per minuteThe ECG square stands at the intersection of three units: the millisecond horizontally, the millivolt vertically, and the beat per minute, which is derived from the RR interval.
Forty milliseconds is one square at 25 mm/s, twenty at 50.
The R wave is usually 0.5 to 2 mV, that is, five to twenty squares.
Sixty divided by RR in seconds; at an RR of exactly 1 s, QT and QTc coincide.
07 · Historical section
archive · from Waller's bulldog to ICH E14How the heart learned to write on graph paper
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How much extra Bazett adds
The plum curve is the difference between QTc by Bazett and by Fridericia for a heart with a true QTc of 400 ms at different heart rates; the horizontal line is zero difference at sixty beats. The red marker runs along the curve from bradycardia to tachycardia.
At one hundred twenty beats Bazett adds almost fifty milliseconds, and a normal strip crosses the threshold of 450, which is why regulators prefer Fridericia in drug trials.
The square as a unit that must be verified every time
The millimetre on the strip has no value of its own: it acquires time and voltage only together with two settings of the machine. That is why every recording begins with a check right on the paper — the machine feeds exactly one millivolt to the input for two hundred milliseconds and draws a rectangle, whose height is used to check the gain and whose width the speed. If the rectangle is ten squares high and five wide, everything else on the strip can be measured in squares.
The standard 25 mm/s became established when the trace was written by a pen on a roll, and that speed was easy to hold with a motor, while a square of 40 ms turned out to be fine enough to see the QRS complex and coarse enough to fit ten seconds on a sheet. In a number of European countries 50 mm/s is preferred, at which the square equals 20 ms and narrow waves read better, so a report that does not state the speed cannot be checked against the strip.
QTc adds one more layer to this paper metrology: it is no longer a measured but a calculated quantity, and its value depends on the chosen formula as strongly as on the strip itself. The ICH E14 guideline on evaluating drugs requires stating the method of correction and comparing the drug with placebo by the change in QTc, and names as the threshold of concern an increase of about five milliseconds — one eighth of a small square.
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