Ask a room of final-year students how they read an ECG and almost all of them will say the same three words: rate, rhythm, axis. Ask the same room to interpret ten strips and accuracy collapses. This is not a straw man — in one study of emergency medicine residents, nearly all reported using rate/rhythm/axis as their method, and accuracy and confidence were both still low.
The phrase is not wrong. It is just far too short to be a method. What follows is the longer version: seven passes, always in the same order, ending with the two things most likely to change what you do in the next ten minutes.
The whole point of a system is that it does not depend on your attention. On a quiet afternoon you will spot the obvious anterior STEMI whatever you do. At 04:00, on your ninth admission, the system is the only thing standing between you and a missed first-degree block that turns out to be a third-degree block two strips later.
Before the seven steps: check the paper
Two numbers make everything else meaningful, and both are printed on the trace.
- Paper speed — 25 mm/s. One small square is 0.04 s. One large square is 0.20 s.
- Calibration — 10 mm/mV. One large square vertically is 0.5 mV. The calibration pulse at the start of the trace should be two large squares tall.
If either is non-standard, every measurement you are about to make is wrong by a fixed factor. Half-standard calibration is the classic trap: voltages look normal when they are in fact doubled, and left ventricular hypertrophy quietly disappears.
1. Rate
If the rhythm is regular, divide 300 by the number of large squares between two consecutive R waves. Memorising the sequence 300, 150, 100, 75, 60, 50 gets you there without arithmetic.
If the rhythm is irregular, that method is meaningless. Count the QRS complexes across a 10-second rhythm strip and multiply by six.
2. Rhythm
Three questions, in order:
- Is it regular? March two R waves out on a piece of paper and walk it along the strip.
- Is there a P wave before every QRS?
- Is there a QRS after every P wave?
Those last two are separate questions and they are the reason AV block is missed. A P wave with no QRS after it is dropped conduction. A QRS with no P wave in front of it is an escape or a ventricular origin. Asking them as one question — "do the P waves and QRSs look related?" — is exactly the shortcut that lets Mobitz II slip past.
3. Axis
For most clinical purposes you need one glance, not a protractor. Look at leads I and aVF:
| Lead I | Lead aVF | Axis |
|---|---|---|
| Positive | Positive | Normal |
| Positive | Negative | Left axis deviation |
| Negative | Positive | Right axis deviation |
| Negative | Negative | Extreme axis |
Axis is rarely the finding that changes management on its own. It earns its place in the sequence because a new axis shift is a strong hint to look harder at everything else.
4. P wave
Best seen in lead II and V1. You are looking for shape and duration: a broad, notched P suggests left atrial enlargement; a tall, peaked P suggests right atrial enlargement. Absent P waves with an irregularly irregular rhythm is atrial fibrillation until proven otherwise.
5. Intervals
| Interval | Normal | What a long one suggests |
|---|---|---|
| PR | 120–200 ms (3–5 small squares) | First-degree AV block; look for dropped beats |
| QRS | < 120 ms (< 3 small squares) | Bundle branch block, ventricular origin, hyperkalaemia |
| QT | Rate-corrected, < 440–460 ms | Drug effect, electrolytes, congenital long QT |
A short PR with a slurred QRS upstroke is a delta wave, and that changes management in atrial fibrillation specifically and urgently.
6. QRS morphology
Height, width and shape. Pathological Q waves — deeper than 2 mm and wider than 40 ms — suggest old infarction. Poor R wave progression across the praecordial leads suggests anterior infarction or lead misplacement, and lead misplacement is far more common than the textbooks imply.
7. ST segment and T waves
This is last in the sequence and first in importance, which is exactly why it goes last: by the time you get here you already know the rate, the rhythm and the conduction, so you can interpret the ST segment in context rather than in isolation.
Measure ST deviation at the J point relative to the isoelectric baseline. Then ask the question that matters most:
Is the change in contiguous leads, and is there reciprocal change? Elevation in II, III and aVF with depression in I and aVL is an inferior pattern. Elevation scattered across unrelated leads with no reciprocal change is far more likely to be pericarditis, early repolarisation, or artefact.
In inferior patterns, compare III with II. Elevation greater in III than in II points towards a right coronary artery occlusion rather than a circumflex one — and that matters, because right ventricular involvement changes how the patient tolerates nitrates.
Practise this sequence
Run the seven steps on a real trace right now
Einthoven gives you timed ECG rounds at true clinical scale — 25 mm/s, 10 mm/mV, on a standard grid — with teaching feedback after every answer. The first three traces need no account.
Try three traces free →How much practice does this actually take?
More than most curricula allow, and less than you might fear. In a study quantifying the learning curve for rhythm-strip interpretation using deliberate practice, students needed on average 112 minutes and 34 practice cases to reach 75% accuracy.
Two things follow from that number. First, competence here is measured in dozens of cases, not dozens of hours of reading. Second, if you have never sat down and done thirty-odd traces deliberately, the problem is probably not that you have failed to understand ECGs — it is that you have not yet done the reps.
Printable companion
ECG Cheat Sheet Pack — 12 reference sheets
The intervals, axis table and ST patterns above, laid out as printable sheets for a pocket or a ward folder. Designed by a board-certified cardiologist. €14.76 · instant digital download.
View on Etsy →The order matters more than the content
Every item above is in any textbook. What a textbook cannot give you is the habit of doing them in the same order every single time, including on the traces that look boring. Boring traces are where the missed findings live, because a normal-looking ECG is the one you stop reading after four seconds.
Pick the sequence. Use it on every trace, including the ones you are sure about. Then do your thirty-four cases.
References
- Surawicz B, Childers R, Deal BJ, Gettes LS, et al. AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram: Part III — Intraventricular Conduction Disturbances. J Am Coll Cardiol. 2009;53(11):976–981. doi:10.1016/j.jacc.2008.12.013
- Rautaharju PM, Surawicz B, Gettes LS, et al. AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram: Part IV — The ST Segment, T and U Waves, and the QT Interval. J Am Coll Cardiol. 2009;53(11):982–991. doi:10.1016/j.jacc.2008.12.014
- Wagner GS, Macfarlane P, Wellens H, et al. AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram: Part VI — Acute Ischemia/Infarction. J Am Coll Cardiol. 2009;53(11):1003–1011. doi:10.1016/j.jacc.2008.12.016
- Hartman ND, Wheaton NB, Williamson K, et al. Quantifying the medical student learning curve for ECG rhythm strip interpretation using deliberate practice. PMC6737266.