ECG Basics Part 1: Start here Lesson 6 of 22
Your first rhythm gallery
Use the five steps on each strip, then check the one clue that gives the rhythm its name.
Every strip below is lead II at 25 mm/s. Work through the five steps from Lesson 5: Reading a rhythm strip in five steps before reading the clue.
Ventricular tachycardia, ventricular fibrillation and asystole on a screen mean: check the patient now. The ECG cannot tell you whether there is a pulse. This page teaches recognition, not treatment.
Sinus rhythms
Normal sinus rhythm
Clue: every one of the five steps is normal.
Sinus bradycardia
Clue: sinus rhythm, but slower than 60 /min.
Sinus tachycardia
Clue: sinus rhythm, but faster than 100 /min.
Atrial rhythms
Atrial fibrillation
Clue: irregularly irregular R–R intervals with no P waves.
Atrial flutter
Clue: saw-tooth flutter waves, here about 300 /min, with the QRS at a fixed fraction of them. In this example every fourth flutter wave conducts (4:1), so the ventricular rate is 75 /min. A regular ventricular rate near 150 /min suggests 2:1 flutter.
AV blocks
First-degree AV block
Clue: the PR interval is longer than 5 small boxes (0.20 s), the same every beat, and every P wave conducts.
Second-degree AV block, Mobitz I (Wenckebach)
Clue: the PR interval gets longer beat by beat until one P wave has no QRS after it.
Second-degree AV block, Mobitz II
Clue: the PR interval stays the same, then a P wave suddenly has no QRS after it. In patients the QRS in Mobitz II is often wide; this model example shows a narrow QRS.
Third-degree (complete) AV block
Clue: the P waves and the QRS complexes each march at their own regular rate, with more P waves than QRS complexes and no fixed PR interval. Here a narrow junctional escape rhythm keeps the ventricles going.
Ventricular beats and rhythms
Premature ventricular complexes (PVCs)
Clue: an early, wide, oddly shaped QRS with no P wave before it, usually followed by a pause. Its T wave points the opposite way to its QRS.
Ventricular tachycardia
Clue: fast, regular and wide: three or more ventricular beats in a row at over 100 /min (here about 180 /min).
Ventricular fibrillation
Clue: chaotic waves with no identifiable QRS complexes.
Asystole
Clue: a flat line. Check the patient at once; at the same time make sure the flat line is not technical: electrodes attached, gain not turned down, and the same flat line in a second lead. P waves with no QRS complexes at all is called ventricular standstill.
All thirteen at a glance
| Rhythm | Rate | Regular? | P waves | PR | QRS |
|---|---|---|---|---|---|
| Normal sinus rhythm | 60–100 | Yes | Normal, one per QRS | 0.12–0.20 s, constant | Narrow |
| Sinus bradycardia | < 60 | Yes | Normal, one per QRS | Normal | Narrow |
| Sinus tachycardia | > 100 | Yes | Normal, one per QRS | Normal | Narrow |
| Atrial fibrillation | Varies | Irregularly irregular | None; uneven baseline | None | Usually narrow |
| Atrial flutter | Atria about 300; ventricles often 150, 100 or 75 | Usually | Saw-tooth flutter waves | None | Usually narrow |
| First-degree AV block | Any | Yes | One per QRS | > 0.20 s, constant | Usually narrow |
| Mobitz I | Any | Grouped beats | Some not followed by a QRS | Lengthens, then a dropped beat | Usually narrow |
| Mobitz II | Any | Irregular at dropped beats | Some not followed by a QRS | Constant in conducted beats | Often wide (narrow in this model) |
| Complete AV block | Slow escape, 20–60 | P regular, QRS regular | More P waves than QRS | No relationship | Narrow (junctional) or wide (ventricular escape) |
| PVC | Early beat | Interrupts the rhythm | None before the PVC | — | Wide; T opposite |
| Ventricular tachycardia | > 100 | Yes | Not related, often hidden | — | Wide |
| Ventricular fibrillation | Cannot be counted | Chaotic | None | — | No QRS |
| Asystole | — | — | None | — | None |
Key points
- Use the same five steps on every strip, then look for the one clue.
- No P waves and irregularly irregular: think atrial fibrillation.
- For AV blocks, march out the P waves and ask which ones conduct.
- Wide, fast and regular: think ventricular tachycardia, and check the patient.
Check yourself
The QRS complexes are irregularly irregular and you cannot find any P waves. What is the most likely rhythm?
Answer: atrial fibrillation.
P waves come regularly at 75 /min, QRS complexes come regularly at 40 /min, and the PR interval is different every time. What is this?
Answer: third-degree (complete) AV block.
An early, wide QRS with no P wave before it is followed by a pause. What is it?
Answer: a premature ventricular complex (PVC).
For more practice with hidden answers, try Learn. In the live simulator, a PVC is a layer: open normal sinus rhythm, then add a PVC from the Ectopy tab.
References
- 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.
- 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.
- Page RL, et al. 2015 ACC/AHA/HRS guideline for adult supraventricular tachycardia. Rhythm definitions and atrial flutter patterns.
- ACC/AHA/ESC atrial fibrillation guideline, 2006, §II. ECG definition and the effect of AV conduction on ventricular regularity; used here for pattern description, not treatment recommendations.
- Al-Khatib SM, et al. 2017 AHA/ACC/HRS guideline for management of patients with ventricular arrhythmias and the prevention of sudden cardiac death. Circulation 2018;138:e272–e391. Used for the definition of ventricular tachycardia.
- American Heart Association. Adult cardiac arrest algorithm, 2025. VF/pulseless VT and asystole/PEA categories.
SIMULATED · NOT FOR DIAGNOSIS · TracingsMD ECG Basics