ECG Rhythm and Heart Block Simulator

What is an ECG rhythm simulator?

An ECG rhythm simulator draws a rhythm strip from a known timetable of heartbeats, so you can watch each rhythm form and measure it the way you would measure a real tracing. This one draws lead II on standard paper, at 25 mm/s and 10 mm/mV, for normal sinus rhythm, sinus bradycardia and tachycardia, first-degree, second-degree and complete heart block, atrial fibrillation, atrial flutter, and premature atrial and ventricular complexes. A ladder diagram under the strip follows every impulse from the atria, through or into the AV node, to the ventricles. The first number to read from any strip is the rate, HR = 60 / RR, which at the standard paper speed becomes HR = 1500 / small squares between two R waves.

It is a teaching model of textbook rhythms, not a diagnostic tool, and it cannot interpret a real ECG. The patterns follow the definitions of the 2018 ACC/AHA/HRS bradycardia guideline by Kusumoto and colleagues, and the waves are the ones the cardiac cycle simulator draws under its Wiggers diagram, so the two tools show the same beat.

Reading ECG paper

At 25 mm/s each small square, 1 mm, is 1 / 25 = 0.04 s, and each large square of five small ones is 5 × 0.04 = 0.2 s. Vertically, at 10 mm/mV, a small square is 0.1 mV and a large one 0.5 mV. The step at the start of each row is the calibration pulse, 1 mV drawn 10 mm high and 0.2 s wide, the check that the gain and the speed are standard. Five large squares make a second, so a 6 second strip is 30 large squares long.

Rates come from the spacing of the R waves. With the RR interval in large squares the rate is 300 / large squares, and in small squares it is 1500 / small squares. Both are 60 seconds divided by the interval, because a minute of paper is 300 large squares or 1500 small ones. When the rhythm is irregular no single interval stands for the rate, so count the QRS complexes in 6 seconds and multiply by 10.

Using the simulator

Choose a rhythm from the menu. The fields below it change to the ones that rhythm uses, each limited to the values its definition allows: sinus bradycardia goes no higher than 59, and first-degree block no lower than a PR of 210 ms. Press Play and the strip sweeps from left to right in real time, as a bedside monitor does, writing over the previous sweep a few squares ahead of the cursor. The paper holds two rows, read like lines of text. The scrubber moves through the minute by hand, and the speed menu slows the sweep to a tenth of real time.

Two tools work on the strip itself. Drag across it and calipers read the distance as time, small squares and a rate, the way you would march out P waves on paper; or select it and step one beat at a time with the arrow keys, and the line under the rhythm’s name gives that beat’s PR and RR. Triangles along the top of the paper mark every P wave or flutter wave, in the conducted colour when it reached the ventricles.

The readouts give the rates, the conduction ratio, the PR, QRS, QT and QTc, the RR range and the regularity. The plot draws every RR, PR and PP interval of the minute, where Wenckebach’s staircase and the scatter of atrial fibrillation are easiest to see.

Worked example: normal sinus rhythm at 75 bpm

The simulator opens on normal sinus rhythm at 75 beats a minute, with a PR interval of 160 ms and a QRS of 80 ms. A reader measures what the readouts show:

• RR interval: 60 / 75 = 0.8 s, which is 0.8 / 0.04 = 20 small squares, or 4 large ones.
• Rate from the paper: 300 / 4 = 75 or 1500 / 20 = 75 beats a minute.
• PR interval: 160 ms, 0.16 / 0.04 = 4 small squares, inside the normal 120 to 200 ms.
• QRS: 80 ms, 2 small squares, narrow because it is under 120 ms.
• QT: Bazett’s formula with a corrected QT of 400 ms gives QT = 0.40 × √0.8 = 0.3578 s, just under 9 small squares, and correcting it back gives QTc = 0.3578 / √0.8 = 0.400 s.
• The 6 second count: the first QRS starts at 0.36 s, so the first 6 seconds hold 8 of them and 8 × 10 = 80. The method counts whole complexes, so it is good only to about 10 beats a minute.

Every P wave is upright and followed by a QRS after the same PR, the rate is between 60 and 100, and every RR interval is the same: normal sinus rhythm. Pick sinus bradycardia and the rate drops to 50, below the conventional 60; pick sinus tachycardia and it rises to 120. The QTc calculator goes further into the QT, comparing Bazett’s correction with three others at any rate.

Reading the ladder diagram

A ladder diagram, also called a Lewis diagram after the cardiologist Thomas Lewis, draws conduction against the same time axis as the strip. The top tier, A, is the atria; the middle tier, AV, is the AV node and the bundle of His; the bottom tier, V, is the ventricles. A sinus impulse is a short line across the A tier at the start of its P wave, a sloping line through the AV tier, and a line across the V tier at the start of its QRS. The slope through the AV tier is the PR interval, written beside it in ms where there is room, so a long PR draws a shallow line.

An impulse that is not conducted stops partway through the AV tier with a short bar across its end. Where it stops follows where the block usually is: halfway down for a block in the AV node, as in Mobitz I, and near the bottom for a block below it, as in Mobitz II. A dot marks a beat that starts somewhere other than the sinus node: in the A tier for a premature atrial complex, in the V tier for a premature ventricular complex or a ventricular escape beat, and at the foot of the AV tier for a junctional escape beat. A line from such a dot back towards the atria that ends in a bar is retrograde conduction that went nowhere.

First-degree AV block and Mobitz I

In first-degree AV block every P wave is conducted, but slowly: the PR is longer than 200 ms, one large square, and the same in every beat. Nothing is dropped, so the rhythm stays regular at the sinus rate. The simulator opens it at a PR of 280 ms, 7 small squares.

Second-degree AV block means some P waves are not followed by a QRS. In Mobitz type I, also called Wenckebach, the PR lengthens from beat to beat until one P wave is not conducted, and the cycle starts again from the shortest PR. Pick Mobitz I from the opening settings and the groups are 4:3, four P waves for three QRS complexes:

• PR intervals: 160, 160 + 80 = 240 and 240 + 40 = 280 ms, and then a P wave with no QRS.
• RR intervals: each is the PP interval plus that step in the PR, 800 + 80 = 880 and 800 + 40 = 840 ms, so they shorten towards the pause.
• The pause around the dropped beat: 2 × 800 − (280 − 160) = 1480 ms, less than two PP intervals.
• Ventricular rate: three QRS complexes every 3.2 s, so 75 × 3 / 4 = 56.25 beats a minute.

Those three signs, the shortening RR, the pause shorter than two cycles and the beats arriving in groups, are the footprints of typical Wenckebach periodicity. The block is usually in the AV node, and Mobitz I is common in healthy athletes and during sleep, when vagal tone is high.

Mobitz II, 2:1 block and complete heart block

In Mobitz type II the PR is the same before and after the P wave that is not conducted. The block is usually below the AV node, in the His-Purkinje system, the QRS is often wide from a bundle branch block that comes with it, and it can progress without warning to complete heart block. That is why the 2018 guideline recommends a permanent pacemaker for acquired Mobitz II that has no reversible cause. At 4:3 from the opening settings the RR intervals are 800, 800 and then exactly 2 × 800 = 1600 ms across the dropped beat. Widen the QRS to 120 ms or more to see the pattern it most often comes with.

When every second P wave is dropped the strip shows 2:1 block, and there is no pair of conducted PR intervals in a row to compare, so it cannot be called type I or type II from the strip alone. A narrow QRS points to the AV node and a wide one to the conduction system below it, which is how the ladder decides where to draw the block.

In third-degree, or complete, heart block no P wave is conducted at all. The P waves keep the sinus rate, a slower escape rhythm drives the ventricles, and the two are unrelated: march out the P waves and they pass straight through the QRS complexes and T waves. From the opening settings the atria run at 75 and a junctional escape at 44, with a narrow QRS; a ventricular escape, typically 20 to 40 a minute, has a wide QRS and a T wave pointing the other way. Heart block after an inferior heart attack is usually at the AV node, because in most people the artery to the AV node branches from the right coronary artery, which you can trace in the heart explorer.

Atrial fibrillation and atrial flutter

Atrial fibrillation has no P waves. The atria fire chaotically at 350 to 600 a minute, which shows as a fine irregular baseline, and the AV node passes whichever impulse reaches it once it has recovered, so the RR intervals are irregularly irregular. The model makes each RR interval the node’s refractory period plus a random wait for the next impulse: at the opening rate of 100 a minute the mean RR is 60 / 100 = 0.6 s and the refractory period is 0.6 − 0.15 = 0.45 s, so no RR interval is shorter than 450 ms. The 2020 ESC guideline counts an episode as clinical atrial fibrillation once it lasts at least 30 seconds on an ECG.

Atrial flutter is a single wave of activation circling the right atrium at about 300 a minute, which draws the negative sawtooth flutter waves best seen in leads II, III and aVF. The AV node cannot follow every wave, so it conducts a fixed share of them: at 300 a minute, 2:1 conduction gives 300 / 2 = 150 and 4:1 gives 300 / 4 = 75 beats a minute, both regular. Rate-control drugs such as beta blockers and digoxin slow conduction through the AV node, and the pharmacokinetics simulator shows how a drug’s level rises and settles with repeated doses.

Premature beats and the compensatory pause

A premature atrial complex, or PAC, is an early P wave of a different shape, conducted to a normal QRS. It enters the sinus node and resets it, so the next sinus P wave comes one sinus cycle later. From the opening settings, with a PAC every fourth beat at 65 percent of the cycle, the PAC comes 0.65 × 800 = 520 ms after the P wave before it, and the QRS complexes either side of it are 520 + 800 = 1320 ms apart, less than two cycles: the pause is not compensatory.

A premature ventricular complex, or PVC, also called a premature ventricular contraction, is a wide QRS with no P wave before it and a T wave pointing the other way. It does not reset the sinus node. The next sinus P wave arrives on time, finds the AV node still refractory and is hidden in the PVC, so the QRS complexes either side of it are exactly two cycles apart: 520 + 1080 = 1600 ms, the full compensatory pause. One PVC in every two beats is ventricular bigeminy and one in three is trigeminy. A PVC early enough to land on the T wave of the beat before, R on T, is the timing linked with ventricular tachycardia and fibrillation in a vulnerable heart.

What this model leaves out

• Real physiology. Every rhythm is a timetable with fixed intervals, the same for the whole minute. A real sinus rate varies with breathing, so even a normal strip shows some sinus arrhythmia, a real PR and QT change with autonomic tone and drugs, and a real block can come and go.
• Other leads. Only lead II is drawn, so the axis, the shapes of bundle branch block and ST changes, which need a 12-lead ECG, are not shown.
• Atypical patterns. The PR steps in Mobitz I always halve here; in long groups they often do not, and the RR intervals then fail to shorten. PVCs are never interpolated, PACs are never blocked or conducted aberrantly, and there are no fusion beats.
• Fast arrhythmias. Supraventricular tachycardia, ventricular tachycardia, ventricular fibrillation and pre-excitation are not modelled.

Common mistakes

• Calling 2:1 block Mobitz II. With no two conducted beats in a row, the PR cannot be seen to lengthen or to stay the same, so 2:1 block is a category of its own.
• Missing hidden P waves. P waves land on T waves in sinus tachycardia, in complete heart block and after a PVC. March them out with calipers from the ones you can see.
• Taking a regular rhythm near 150 for sinus tachycardia. It is often atrial flutter with 2:1 conduction.
• Forgetting the paper speed. At 50 mm/s, used in some countries, every square is half the time, and a rate read with the 25 mm/s rules comes out at half the true rate.
• Correcting the QT of a wide QRS as if it were narrow. A bundle branch block adds its extra width to the QT, so the QTc reads long even when repolarisation is normal.