How to read an ECG systematically: the same order every time

Atrial fibrillation jumps out at first glance. You have your diagnosis, you stop reading, and the ST elevation in V2–V4 goes unnoticed. An error like this comes not from an unknown pattern but from a part of the tracing nobody looked at.

Here is a 10-step order for any 12-lead ECG, with adult normal values and a worked example.

Why an order helps: a first impression is not enough

In a meta-analysis of 78 studies, accuracy before training was 42% in medical students and 74.9% in cardiologists ^[1]; see our ECG course article.

Experienced readers often recognise an ECG at a glance; beginners have not yet seen enough tracings. In an experiment by Ark and colleagues, 48 psychology students new to ECGs learned to diagnose 10 cardiac conditions ^[2]. The group told to "identify all features first" reached 42% accuracy, the group told to "trust your first impression" 41%, and the two groups that combined both, explicitly or implicitly, 56% and 53% ^[2].

So a systematic order does not replace the first impression; it checks it. Sibbald and colleagues had 44 clinicians, novice to expert, read 10 ECGs and then check their readings against a checklist: all levels found and fixed mistakes, and novices gained most ^[3]. Our clinical reasoning article covers both modes of thinking. The rule: note your first impression, then finish every step, even after an obvious finding.

How to read an ECG: 10 steps and adult normal values

Textbook and guideline orders differ slightly, and so do some limits, such as rate. The 2022 Brazilian Society of Cardiology guideline, for example, reports rhythm and rate, P and PR, axis, QRS, then repolarisation ^[4]. What matters is one order, every time. The MedIQ ECG course (lesson 15) teaches this order; the table gives the source of each normal value:

The rate limits in the table are the Brazilian guideline's; the MedIQ course drills use 60–100/min.

Steps 1–2: context and quality

The same ECG means one thing in a 20-year-old athlete and another in a 70-year-old with chest pain, so read the clinical details first. Then check the calibration pulse and the filters: in adults the high-frequency cut-off should be at least 150 Hz and the low-frequency cut-off 0.05 Hz ^[4]. Negative waves in lead I with positive waves in aVR suggest swapped arm electrodes. The main differential is dextrocardia, where P, QRS and T are negative in V6 as well ^[4].

Steps 3–5: rate, rhythm, axis

At 25 mm/s a large box is 0.2 s and a small box 0.04 s, so the rate is 300 ÷ the number of large boxes between two R waves, or 1500 ÷ the number of small boxes. In an irregular rhythm such as atrial fibrillation, count the QRS complexes in 6 seconds and multiply by 10 ^[4].

Sinus rhythm is defined by the P wave, upright in I, II and aVF ^[4]. Whether every P is followed by a QRS and the PR stays constant is a separate question about AV conduction: sinus rhythm can coexist with AV block ^[4]. For the axis, look at I and aVF: a mainly positive QRS in both means 0° to +90°. If I is positive and aVF negative, look at II: a positive QRS there means the axis is not beyond −30°, so it is still normal ^[4].

Step 6: intervals

Machines measure P and QRS globally, from the earliest onset to the latest offset in any of the 12 leads, so their values can exceed a single-lead reading ^[4][7], and a doctor must check them ^[4].

The QT runs from QRS onset to the end of the T wave and is corrected for rate, most often with Bazett's formula: QTc = QT / √RR (RR in seconds). It is less reliable below 60 or above 90/min, where the Framingham or Hodges formula is recommended ^[4]. Postema and Wilde note that Bazett works best at 60–100/min ^[5]. In long QT syndrome, congenital or acquired, a QTc of 500 ms or more carries a markedly higher risk of arrhythmia ^[5]. Limits vary by source: the Brazilian guideline accepts up to 470 ms in women ^[4].

Steps 7–9: P, QRS, ST, T and U

QRS voltage is typically 5–20 mm in the limb leads and 10–30 mm in the chest leads, and the R wave grows from V1 to V6 ^[4]. Look here for pathological Q waves and signs of bundle branch block. If the ST segment deviates from the PR level, note the leads. The guideline takes ST elevation of at least 1 mm in two contiguous limb or left chest leads as a sign of injury. In V1–V3 the limits are higher: ≥ 1.5 mm in women, ≥ 2 mm in men aged 40 or older and ≥ 2.5 mm in men under 40 ^[4]. Judge smaller elevations in the clinical context too. In a normal ECG the T wave is inverted in aVR, and a U wave follows the direction of the T wave ^[4].

Step 10: comparison and conclusion

What is new compared with the previous ECG? Write the conclusion: rhythm and rate, axis, then the findings by importance. Part II of the 2007 recommendations of the American Heart Association (AHA), the American College of Cardiology Foundation (ACCF) and the Heart Rhythm Society (HRS) offers students, teachers and anyone who reads ECGs a shared list of diagnostic statements, including ones for comparing tracings ^[6]. Last, decide urgency: report a dangerous finding to the responsible doctor at once.

Worked example: a normal ECG in 10 steps

This MedIQ library tracing comes from a 49-year-old man in PTB-XL, a dataset annotated by up to two cardiologists ^[8]. This record was validated by a cardiologist, and its label is "NORM — normal ECG". The course overview walked through this ECG briefly; here every step gets its own check. The numbers are the machine measurements shown on the library's "Measurements" card (PTB-XL+, GE Marquette 12SL algorithm).

1. Context: no clinical details, no previous ECG.
2. Quality: 25 mm/s, 10 mm/mV; P upright in I and inverted in aVR, so the arm electrodes are not swapped.
3. Rate: 67/min; RR = 60,000 ÷ 67 ≈ 896 ms, about 22 small boxes.
4. Rhythm: one P wave before each QRS, upright in I, II and aVF (P axis +53°): sinus rhythm, regular.
5. Axis: +32°, normal.
6. Intervals: PR 138 ms, QRS 108 ms, QT 380 ms. The rate is within 60–90/min, so Bazett suits here ^[4]. Check the QTc: 380 ÷ √0.896 ≈ 380 ÷ 0.947 ≈ 401 ms (Bazett); Fridericia gives 394 ms. Both are normal ^[5].
7. P and QRS: normal P wave; no pathological Q waves, rS in V1, transition near V3–V4, normal voltage.
8. ST: isoelectric.
9. T: upright in I, II and V2–V6, inverted in aVR.
10. Conclusion: "Sinus rhythm, rate 67/min. Normal ECG." It matches the dataset label.

The QRS of 108 ms is a global value; calipers in one lead may give slightly less ^[4].

How to practise on MedIQ

Lesson 15 of the MedIQ ECG course, "The 10-step ECG reading", teaches this order. Every tracing in the course opens in the ECG viewer with "Layout" (3×4 + rhythm, 6×2, 12×1, one lead, rhythm strip), "Paper speed" (25 or 50 mm/s), "Gain" (5, 10 or 20 mm/mV), "Calipers" and "March out". The ECG library holds over 30 real ECGs, and most of their interpretations follow nearly this order: rate, rhythm, axis, intervals, P, QRS, ST–T, conclusion.

In the "Full read" drill you read five 12-lead ECGs yourself: you fill in rate, rhythm, PR, QRS, QTc, axis and findings in the "ECG report" form. Straight after checking, you see a mark for each field, the expected findings and an explanation. A rule-based grader marks it, not AI; the free-text conclusion is not graded. Missing a critical finding, such as STEMI or complete heart block, caps the score at 40%. The drill also includes Level B clinical ECGs and synthetic "Model" tracings.

The course is in Beta: checked by MedIQ AI, not yet by a physician, and a real ECG's diagnosis is the dataset's own label. A short overview is on the ECG course page. This article is educational; decisions about real patients follow local protocols and the responsible doctor.