ECG Interpretation
Read a rhythm strip or a 12-lead ECG, choose the rhythm — and for some cases the next step — then check. Some cases then ask you to find the key wave on the tracing. The answer shows the tracing annotated beat by beat, a systematic read and the guideline behind it.
Tracings are computed by the TracingsMD physiology engine — simulated, not patient recordings, and not for diagnosis. This is educational practice, not a clinical examination.
Interpretation · 27 cases
New to ECG reading? Start at Beginner, and use Theory or the Basics lessons when a term is unfamiliar. Cases come in random order, and the same rhythm can return as a different engine recording — the rate and timing change, so you read the ECG, not a remembered picture. Not sure? Use “Show answer” to see the explanation without scoring.
P wave (navy)P wave not conducted (red)flutter wave (purple)shaded QRS, amber when 120 ms or wider (whole-heart width across all leads, so it can run past the end you see in one lead)PR bracket, msteal bracket = RR interval, msQT bracket with the same beat's QTcPVC / PAC label = early beatvertical tick = pacing spike (red: no capture; gray: in the refractory period)red outline = ST elevation ≥ 0.1 mV at J+60 msblue dashed outline = ST depression ≥ 0.08 mV
Find it on the tracing
Measure it yourself
Attempt complete
Your score reflects the answers checked in this attempt. It does not assess clinical competence.
Case by case
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One setting covers Theory and Interpretation.
How records are kept
A case counts as missed if any of its answers was wrong. Each case keeps its latest result and, after a miss, when to review it: the next day, then 3 and 6 days after each correct review, until three reviews in a row are correct. Reviews and retries mix in one look-alike rhythm for each missed case, picked from that case’s wrong answers; which answer you chose is not saved. Answers shown without scoring are not saved. Tracing tasks count on this page only and are never saved; measurements are not scored or saved. Clearing removes both sets' records on this device. Turning saving off deletes device records and keeps this page's temporary records until you leave.
How these cases are made
Every tracing is recorded output of the TracingsMD physiology engine (Lead II and other leads at 500 samples per second, 25 mm/s, 10 mm/mV) — no patient recordings and no generative AI. Annotations are placed from the engine's own beat events: where it started each P wave and QRS complex, which P waves it did not conduct, and the intervals it measured. Measurements are model values, not clinical measurements. QRS width is the whole ventricular activation, from the earliest start to the latest end across all leads, as QRS duration is measured on a 12-lead ECG; in a single lead the last part can be nearly flat, so a wide QRS can look 20–35 ms narrower there. QT is taken from one beat of the underlying rhythm with a typical RR interval (never an early beat or the beat after a pause), and the table uses the same beat as the QT bracket. P waves that fall inside a QRS complex are outlined in gray: the engine times them, but they cannot be seen.
A “Find it on the tracing” task is judged by the wave your mark lands on, using the same engine events as the annotations. The first and last 0.35 s of the strip are not judged, nor (when you look for a P wave) any QRS–T, because a wave there can be cut off or hidden. “Measure it yourself” (PR or QT on one magnified beat) is never scored: it shows your value beside the engine’s, because careful readers measuring the same beat by hand often differ by 10–30 ms.
Each case has one or more recordings of the same rhythm at different rates. Answers follow the references listed with each case; management answers follow the AHA 2025 adult advanced life support algorithms, as on the ACLS Scenarios page. Patterns are teaching examples; real tracings vary more, and a single lead cannot resolve every rhythm.