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ECG Rhythm Lab
Start with three-lead ECG fundamentals, core rhythms and the New to ECGs assessment. Pro will add the complete rhythm library, advanced monitoring lessons and higher-level assessments. Instructor controls are planned for SimLab.
Delta ECG Rhythm Lab
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Learn to read ECGs, from zero
What the tracing is, what each wave means, a five-step method for reading any strip, and a guided strip to work through. Educational only.
What an ECG actually is
Every heartbeat starts as an electrical signal that spreads through the heart muscle and makes it squeeze. Electrodes on the skin pick up that signal, and the ECG draws it as a line moving left to right over time. The flat parts, where no signal is moving, sit on the isoelectric line, the baseline you measure everything against. Paper and screen run at 25 mm per second, so distance across is time and height is voltage.
Why the letters P, Q, R, S, T
One heartbeat makes a repeating set of bumps. Willem Einthoven, who built the first practical ECG, labelled them with letters starting at P rather than A, leaving room in the alphabet in case earlier waves turned up. The names stuck. P is the atria depolarizing, the QRS is the ventricles depolarizing (the big spike, and the beat you feel as a pulse), and the T is the ventricles repolarizing, resetting for the next beat.
What each part is telling you
The P wave is the atria firing. The QRS is the ventricles firing, the main pumping chambers, so it is tall. The T wave is the ventricles recovering. Between them the trace returns to the isoelectric line. When one piece is missing, extra, wide, or in the wrong place, that is the clue: no P wave, a wide QRS, or a T that has flipped all mean something specific.
One beat, labelled
P is the atria firing, the QRS is the ventricles firing, the T is the ventricles resetting, and the flat baseline between them is the isoelectric line.
The intervals, and why they matter
PR is the trip from atria to ventricles (a long PR is a block). QRS width is how fast the ventricles fire (wide means trouble). QT is the whole reset (long QT risks torsades). The ST segment sits on the baseline; lifted or dropped means injury.
The five-step method
The same five checks, in the same order, on every strip.
- 1Rate. Count it with the sequence method. Under 60 is slow, over 100 is fast.
- 2Rhythm. Is the R-to-R spacing even? Regular, irregular, or regular with a pattern.
- 3P waves. Is there one upright P before every QRS in lead II?
- 4PR interval. From the start of P to the start of QRS, normally 120 to 200 ms.
- 5QRS width. Narrow means the beat came from above; wide means the ventricles, or a block.
Guided strip
Here is a strip. Answer one factor at a time and get feedback on each, then name the rhythm. Work through all 6 and the method becomes automatic.
Walk a strip, one step at a time
Step 1. Rate: by the sequence method (300, 150, 100, 75, 60, 50), what is the rate?
Factor 1 of 5Educational only. Simulated waveforms for training, not for clinical use.
Where rhythms come from
The library files every rhythm by the site that produces it. Pick a family and see the piece of wiring it belongs to.
SA node
The pacemaker with the fastest natural rate, high in the right atrium. Every sinus rhythm starts here, and the P wave is its signature.
Library family: Sinus. The further the pacemaker falls down this wiring, the slower and wider the rhythm it produces, which is the whole ladder in one sentence.
Placing the electrodes
Pick the lead set you are running, then tap an electrode and tap where it belongs. Wrong spots say so, and say why.
The standard diagnostic tracing. Ten electrodes, twelve views, and the one that lets you speak about territory: anterior, inferior, lateral.
Holding V1. The first landmark is found by touch: the sternal angle, then down to the 4th intercostal space.
Why the order is V1, V2, V4, V3, V5, V6
V3 is not a landmark, it is a midpoint: it is defined as halfway between V2 and V4, so V4 has to be on the chest before V3 can be placed at all. V5 and V6 are the same idea running outward, each defined by staying level with V4 rather than by a space of their own. Put on the ones the anatomy fixes, then fill the gaps between them, and you never guess and never move one twice.
ECG setup: capture a trace you can trust
A clean trace and correct lead placement decide whether the ECG can be believed before anyone interprets it. This trains the hands: skin prep, where every electrode goes, and the placement traps that change the diagnosis. Educational and vendor neutral, always second to your current protocols.
A clean trace before anything else›
Artifact fakes and hides pathology. Most bad ECGs are a preparation problem, not a heart problem.
- Dry the skin. Sweat and moisture break the electrode contact; wipe the site and let it dry fully before the electrode goes on.
- Degrease with an alcohol wipe and let it evaporate. Lotion, diaphoresis, and oils all lift electrodes.
- Clip, do not shave. Excess hair stops contact, but a razor nick bleeds and irritates; clip close with scissors or clippers.
- Abrade lightly. A few passes with a rough gauze or the abrasive on the electrode drops skin resistance and sharpens the trace.
- Fresh electrodes with moist gel. Dried-out gel is a common cause of wandering baseline; check the expiry and the seal.
- Keep the patient warm and still. Shivering and muscle tension add fine artifact that mimics atrial activity; a cold patient makes a noisy strip.
- Place over bone and flat tissue, not big muscle bellies, and keep leads off pacer pockets and wounds.
Limb leads: limbs, not the trunk (usually)›
Where the limb electrodes sit changes the diagnosis, not just the picture.
- Diagnostic 12-lead uses true limb placement: on the wrists and ankles, over bone, away from muscle.
- Torso placement (Mason-Likar), shoulders and hips, cuts motion artifact for monitoring and stress tests, but shifts the axis and can create or erase Q waves and ST changes.
- So: use trunk placement for continuous monitoring, and true limb placement for the ECG you are going to call a STEMI on.
- Right and left arm reversal flips lead I and is a classic trap: look for a negative P and QRS in lead I with a normal V6.
The precordial leads: V1 to V6›
Find the landmark first. Guessing the fourth intercostal space is the most common placement error, and it shifts R-wave progression and ST readings.
- Find the sternal angle (Angle of Louis), the ridge where the 2nd rib joins. Slide off it to the 2nd intercostal space, then walk down to the 4th.
- V1: 4th intercostal space, right sternal border. V2: 4th intercostal space, left sternal border.
- V4: 5th intercostal space, midclavicular line. Place V4 before V3.
- V3: midway between V2 and V4.
- V5: anterior axillary line, horizontal with V4. V6: midaxillary line, horizontal with V4.
- V4 to V6 follow a horizontal line, not the rib. On a patient with breasts, place the electrode under the breast tissue, not on it.
When 12 is not enough: 15-lead›
Two blind spots hide on a standard 12-lead. A few extra electrodes uncover them.
- Right-sided V4R: mirror V4 onto the right chest (5th intercostal space, right midclavicular line). ST elevation there means right ventricular infarction, which changes fluid and nitrate decisions in an inferior STEMI.
- Posterior V7 to V9: V7 posterior axillary line, V8 tip of the scapula, V9 left paraspinal, all level with V6. ST elevation there is a posterior MI, often paired with tall R and ST depression in V1 to V3.
- Get a right-sided and posterior look on every inferior STEMI and whenever V1 to V3 show ST depression without a clear anterior cause.
- The machine still prints these as V4 to V6. Move the electrodes, then write V4R, V8, and V9 on the paper by hand.
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Simulated waveforms for education and training only. Not a medical device, diagnostic tool or substitute for clinical protocols and medical direction.
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Vendor neutral on purpose: we teach ECGs and patient assessment, not devices. What you learn here reads the same on whatever monitor or AED your service runs.
A free study aid from Delta Emergency Support Training. This is a simulator, not a diagnostic tool, and not medical direction. Simulated tracings are idealised; real patients are noisier. Always follow your own protocols and medical oversight.
Rhythms come up in EMR training. See the course.Emergency Medical Responder