EKG Rhythm Strip Practice: How to Read an ECG Strip
Updated October 26, 2026
Reading a rhythm strip is the skill most EKG technician students worry about, and it’s tested directly on the NHA CET exam under EKG Analysis and Interpretation. The good news: every strip is read the same way, in the same order. Learn the five steps and the normal values below, then try the free practice questions.
The five steps
Check these in order on every strip:
- Rate. Is it 60 to 100 beats per minute?
- Rhythm (regularity). Are the R-to-R intervals the same across the strip?
- P waves. Is there one upright P wave before every QRS complex?
- PR interval. Is it 0.12 to 0.20 seconds?
- QRS width. Is it narrower than 0.12 seconds?
If all five answers are yes, it’s normal sinus rhythm.
Normal values and the grid
| Measurement | Normal |
|---|---|
| Heart rate | 60 to 100 beats per minute |
| PR interval | 0.12 to 0.20 seconds (3 to 5 small boxes) |
| QRS complex | Less than 0.12 seconds (under 3 small boxes) |
| One small box (at 25 mm/sec) | 0.04 seconds |
| One large box (5 small boxes) | 0.20 seconds |
Counting the rate. For a regular rhythm, divide 300 by the number of large boxes between two R waves (or 1,500 by the number of small boxes). For an irregular rhythm, count the QRS complexes in a 6-second strip and multiply by 10.
Common rhythms at a glance
| Rhythm | What to look for |
|---|---|
| Sinus bradycardia | Normal sinus rhythm, but the rate is under 60 |
| Sinus tachycardia | Normal sinus rhythm, but the rate is over 100 |
| Atrial fibrillation | Irregularly irregular rhythm, no clear P waves, a wavy baseline |
| Atrial flutter | “Sawtooth” flutter waves between the QRS complexes |
| First-degree AV block | Every P wave conducts, but the PR interval is longer than 0.20 seconds |
| Second-degree AV block, type I (Wenckebach) | The PR interval gets longer and longer until a QRS is dropped |
| Second-degree AV block, type II | The PR interval stays the same, but some QRS complexes are dropped |
| Third-degree (complete) AV block | P waves and QRS complexes march independently of each other |
| Premature ventricular contraction (PVC) | An early, wide, bizarre QRS with no P wave before it |
| Ventricular tachycardia | A fast run of wide QRS complexes, usually regular |
| Ventricular fibrillation | Chaotic, irregular waves with no identifiable QRS |
| Asystole | A flat line: no electrical activity |
Always check the patient, not just the monitor. A flat line can be a loose lead, and shaking can look like a dangerous rhythm (artifact). Ventricular fibrillation, pulseless ventricular tachycardia and asystole are emergencies: report them immediately.
Free rhythm strip practice questions
Each question describes a strip the way you’d measure it.
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A strip shows a regular rhythm at 72 beats per minute, an upright P wave before every QRS, a PR interval of 0.16 seconds and a QRS of 0.08 seconds. What is the rhythm?Answer: BThe rate is between 60 and 100, the rhythm is regular, every QRS has a P wave, the PR interval is within 0.12 to 0.20 seconds and the QRS is narrow. Every value is normal, so it's normal sinus rhythm.
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On a regular rhythm, there are 4 large boxes between two R waves. What is the heart rate?Answer: CFor a regular rhythm, divide 300 by the number of large boxes between R waves: 300 ÷ 4 = 75 beats per minute.
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A strip shows an irregularly irregular rhythm with no identifiable P waves and a wavy baseline. The QRS complexes are narrow. What is the rhythm?Answer: AAn irregularly irregular rhythm with no clear P waves and a fibrillating baseline is atrial fibrillation. Atrial flutter shows regular sawtooth waves, and sinus arrhythmia still has a normal P wave before every QRS.
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Every P wave is followed by a QRS, the rhythm is regular at 68 beats per minute, and the PR interval is constant at 0.28 seconds. What is the rhythm?Answer: DA PR interval longer than 0.20 seconds, the same on every beat, with every P wave conducted, is a first-degree AV block. No beats are dropped, which rules out second- and third-degree blocks.
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The PR interval gets longer with each beat until a P wave appears with no QRS after it, then the pattern repeats. What is the rhythm?Answer: AA PR interval that lengthens progressively until a QRS is dropped is the pattern of second-degree type I, also called Wenckebach or Mobitz I. In type II, the PR interval stays the same before the dropped beat.
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In a 6-second strip of an irregular rhythm, you count 9 QRS complexes. What is the approximate heart rate?Answer: CFor an irregular rhythm, count the QRS complexes in 6 seconds and multiply by 10: 9 × 10 = 90 beats per minute.
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A monitor suddenly shows a chaotic, irregular waveform with no identifiable QRS complexes. What should the EKG technician do first?Answer: BThe waveform looks like ventricular fibrillation, a life-threatening emergency, but it could also be artifact from a loose lead or movement. Check the patient right away. If the patient is unresponsive, call for help and follow emergency procedures.
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What does one small box on standard EKG paper represent, at a paper speed of 25 mm/sec?Answer: AAt 25 mm/sec, each small (1 mm) box is 0.04 seconds, and each large box, made of 5 small boxes, is 0.20 seconds. Five large boxes make 1 second.
Keep practising
The CET exam has 100 scored questions in 2 hours, and Analysis and Interpretation is 24 of them. The rest cover safety, patient care and EKG acquisition (lead placement, artifacts and equipment), so practise all three areas.
Pass your EKG technician exam on the first try
- All 190 questions on safety and compliance, patient care, EKG acquisition and rhythm analysis
- An explanation for every answer, with the AHA recommendation, OSHA rule or guideline behind it
- Unlimited full 100-question practice exams, with each domain in its real share