ECG Basics Part 1: Start here Lesson 5 of 22

Reading a rhythm strip in five steps

A rhythm strip is one lead over a few seconds. Ask the same five questions every time, then name the rhythm.

Before you start

Look at the patient first. Then check that the strip is readable (electrodes attached, little artifact) and recorded at the standard 25 mm/s.

The five steps

  1. Rate. For a regular rhythm, divide 300 by the number of large boxes between two R waves. For any rhythm, count the QRS complexes in 6 seconds and multiply by 10. Under 60 /min is bradycardia; over 100 /min is tachycardia (Lesson 9: Three ways to count heart rate).
  2. Regular or not? Compare the R–R spacing with a paper edge or calipers: regular, irregular with a repeating pattern, or irregularly irregular (Lesson 10: Regular, or not?).
  3. P waves. Are they present? Is there one before every QRS, and a QRS after every P? Do they all look the same? Are they upright in lead II?
  4. PR interval. Is it 3–5 small boxes (0.12–0.20 s)? Is it the same every beat, getting longer, or unrelated to the QRS?
  5. QRS width. Narrow (under 3 small boxes) means the impulse used the normal wiring below the AV node. Wide (3 small boxes or more) suggests a ventricular origin, a bundle branch block or pacing, among other causes. Width alone does not prove where a beat started (Lesson 17: Narrow vs wide QRS: bundle branch block).

Key points: is it sinus rhythm?

  • A P wave before every QRS and a QRS after every P.
  • P waves upright in lead II and alike.
  • A constant PR interval of 0.12–0.20 s.
  • Rate 60–100 /min = normal sinus rhythm; under 60 = sinus bradycardia (some guidelines use under 50); over 100 = sinus tachycardia.

A worked example

  1. Rate: each R–R interval is 800 ms, 4 large boxes. 300 ÷ 4 = 75 /min.
  2. Regular: every R–R interval is the same.
  3. P waves: one small upright P before every QRS, all alike.
  4. PR: 158 ms, about 4 small boxes, the same every beat.
  5. QRS: 84 ms, about 2 small boxes: narrow.

Name: normal sinus rhythm.

Your turn

Strip 1

Check yourself

  1. Work through the five steps and name the rhythm.

    Answer: R–R 1333 ms, about 6.7 large boxes, so 300 ÷ 6.7 ≈ 45 /min; regular; an upright P before every QRS; PR about 4 small boxes and constant; narrow QRS. Sinus bradycardia.

Strip 2

Check yourself

  1. Work through the five steps and name the rhythm. The small triangles mark the P waves.

    Answer: 75 /min; regular; a P before every QRS and a QRS after every P; PR 278 ms, about 7 small boxes, the same every beat; narrow QRS. Sinus rhythm with first-degree AV block.

The five steps read the rhythm from one lead. The full 12-lead read adds the axis, ST segments, T waves and QT interval: Lesson 19: A repeatable nine-step read.

To practise, open a rhythm in the live simulator and press Hide answers under the monitor: the name and settings disappear while the trace keeps running.

See it move in the live simulatorNormal sinus rhythmSinus bradycardiaFirst-degree AV block

References

  1. Mason JW, Hancock EW, Gettes LS, et al. Recommendations for the standardization and interpretation of the electrocardiogram, Part II: electrocardiography diagnostic statement list. Circulation 2007;115:1325–1332.
  2. Kusumoto FM, et al. 2018 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay. Circulation 2019;140:e382–e482.
  3. Kligfield P, et al. Recommendations for the standardization and interpretation of the electrocardiogram, Part I: the electrocardiogram and its technology. Circulation 2007;115:1306–1324.

SIMULATED · NOT FOR DIAGNOSIS · TracingsMD ECG Basics

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