Cardiac arrest rhythms
Lethal rhythms: the EKG strips you cannot get wrong
A lethal rhythm is one in which the heart is not pumping blood, or is about to stop. Left alone it ends in death within minutes. There are four rhythms of cardiac arrest: ventricular fibrillation, pulseless ventricular tachycardia, asystole and pulseless electrical activity. A fifth, torsades de pointes, is a kind of ventricular tachycardia that most courses and exams list beside them.
Criteria from the American Heart Association guidelines and published ECG standards · Every strip checked by measurement · Sources and method · Updated November 15, 2026 · For education only
The lethal EKG rhythms at a glance
| Rhythm | What the strip shows | Pulse | Shock it? | First moves |
|---|---|---|---|---|
| Ventricular fibrillation | A chaotic, shapeless line. No P waves, no QRS complexes, nothing that repeats. | Never | Shockable | CPR, and a shock as soon as a defibrillator is there. |
| Pulseless ventricular tachycardia | Wide complexes of one shape, regular, faster than 100 and usually 150 to 250. | Sometimes. Without one it is cardiac arrest. | Shockable | No pulse: CPR and a shock, exactly as for VF. |
| Torsades de pointes | Fast wide complexes whose height swells and shrinks, as if the trace were twisting around the baseline. | Often lost within seconds | Shockable | No pulse: defibrillate. IV magnesium is the specific treatment. |
| Asystole | A flat or nearly flat line. No complexes at all. | Never | Not shockable | CPR and epinephrine. Confirm it in a second lead. |
| Pulseless electrical activity | Any organized rhythm, even one that looks normal. | None, by definition | Not shockable | CPR, epinephrine, and a search for the cause. |
The last two columns are a study summary, not a protocol. In a real arrest follow the current American Heart Association cardiac arrest algorithms and your facility’s policies.
Each lethal rhythm on a strip
Six seconds of lead II at the standard paper speed. Read each one the way the notes under it do.
1Ventricular fibrillation
No complexes to count or measure
Look for a QRS complex and you will not find one. The line wanders up and down with no two waves alike. Early in an arrest the waves are tall (coarse VF); as the heart runs out of energy they flatten (fine VF), and that is when it gets mistaken for asystole.
2Pulseless ventricular tachycardia
208 per minute, QRS 0.16 s
Three questions settle it: is it wide (0.12 s or more), is it regular, is it faster than 100? Then leave the monitor and feel for a pulse. The same strip belongs to a patient who is talking to you and to one who is in cardiac arrest, and only the pulse says which.
3Torsades de pointes
About 221 per minute, amplitude rising and falling
It is a ventricular tachycardia, but the complexes are not all one shape. Follow their tips: they grow, shrink to almost nothing and grow again, pointing up for a few beats and then down. It comes from a long QT interval, which is why magnesium and not an ordinary antiarrhythmic is the treatment.
4Asystole
No electrical activity
A flat line has two explanations, and the patient tells you which. Awake and talking: an electrode has come off. Unresponsive with no pulse: asystole. Before accepting it, check the leads, look in a second lead and turn up the gain, because fine VF can hide in a line this flat.
5Pulseless electrical activityNO PULSE
Rate 81, PR 0.13 s, QRS 0.09 s, and no pulse
This strip is not a mistake. Pulseless electrical activity can look like anything, including normal sinus rhythm: the electricity is organized and the heart is not pumping. No strip question can show it, which is why exams that ask you to name rhythms from paper leave PEA to the scenario questions.
Shockable and not shockable
A defibrillator does not start a heart. It stops one: the shock silences every cell at the same instant, so that the heart’s own pacemaker has a chance to take over again. That is why it works on the two rhythms in which the ventricles are full of disorganized or runaway electricity, and on nothing else.
- Shockable
- Ventricular fibrillation and pulseless ventricular tachycardia, torsades included. There is electricity to stop.
- Not shockable
- Asystole: there is nothing to stop. PEA: the electricity is already in order, and the problem lies elsewhere.
The patient, then the monitor
Every one of these rhythms is confirmed at the bedside and not on the screen. Is the patient responsive? Is there a pulse? A flat line in someone who is talking is a loose electrode. A wide fast rhythm in someone with a pulse is treated one way, and without a pulse another. A perfect sinus rhythm in someone with no pulse is PEA.
A strip exam can only show you the paper, so it asks for the four that paper can show: V-fib, V-tach, torsades and asystole. On the floor, the pulse check comes before the name.
Six mix-ups that cost marks, and worse
Fine VF and asystole
Fine fibrillation keeps moving: a low, irregular wobble that never settles. Asystole is flat. Raise the gain and check a second lead. If the movement is real, it is VF, and VF is shocked.
V-tach and SVT
Both are fast and regular. Measure the QRS. Narrower than three small squares points to SVT; three or more is treated as ventricular tachycardia until someone proves otherwise.
V-tach and a slow wide rhythm
Wide complexes are not V-tach unless they are fast. Count the rate first: 20 to 40 is an idioventricular rhythm, 40 to 100 is accelerated idioventricular rhythm, and only over 100 is ventricular tachycardia.
Torsades and VF
Torsades keeps a beat you can count while its height rises and falls in a pattern. In fibrillation there is no beat and no pattern.
Artifact and VF
A patient brushing their teeth, shivering or being moved can fill the screen with chaos. Fibrillation does not happen in someone who is awake and looking at you. Check the patient and the electrodes.
A normal strip and PEA
The one mix-up no measurement can fix. An organized rhythm with no pulse is PEA, and it is treated as cardiac arrest whatever the monitor shows.
Practice: lethal or look-alike?
Eight strips. Some are lethal rhythms and some are the rhythms that get called lethal by mistake. Count the rate and measure the QRS before you answer.
These eight are fixed. For lethal rhythms and their look-alikes on strips that never repeat, open the lethal rhythm practice.
Strip 1 of 8
Lead II · 25 mm/s · 6 s
Why exams ask for 100% on these
A hospital dysrhythmia exam usually lets you miss a few strips: the pass mark is commonly 80 or 85%. Many add a second rule for the lethal rhythms. Miss one of them and the exam is failed, even with a total above the pass mark, because on a monitored unit that is the one mistake nobody has time to correct.
Our practice test is marked the same way: every full-length paper contains each of the four, and the report shows them on a line of their own.
Questions about lethal rhythms
What are the lethal rhythms?
The rhythms that produce no effective heartbeat and end in death unless they are treated at once. Four are the rhythms of cardiac arrest: ventricular fibrillation, pulseless ventricular tachycardia, asystole and pulseless electrical activity (PEA). Torsades de pointes, a form of ventricular tachycardia, is usually listed with them.
What is the most lethal rhythm?
All of the arrest rhythms end in death within minutes if nothing is done, so none of them is safe to rank second. Asystole has the worst outlook, because there is no electrical activity left for a shock to reset. Ventricular fibrillation and pulseless ventricular tachycardia have the best chance of survival, but only if CPR and a shock come quickly, which is why they are the ones where seconds count most.
What are the 7 lethal rhythms?
There is no official list of seven. Study guides that count seven commonly take the five on this page (ventricular fibrillation, ventricular tachycardia, torsades de pointes, asystole and PEA) and add two slow rhythms that can stop producing a pulse: third-degree (complete) heart block and idioventricular rhythm. Some lists put agonal rhythm or second-degree type II block in their place. Learn the list your own course or exam uses.
Which lethal rhythms are shockable?
Ventricular fibrillation and pulseless ventricular tachycardia, which includes torsades de pointes with no pulse. Asystole and PEA are not shockable: they are treated with CPR, epinephrine and a search for a reversible cause.
Is ventricular tachycardia always a lethal rhythm?
No. A patient in ventricular tachycardia can have a pulse and a blood pressure, and is then treated with cardioversion or medication, not with CPR. It is counted as lethal because it can lose its pulse or turn into ventricular fibrillation without warning, and because without a pulse it is cardiac arrest.
Is third-degree heart block a lethal rhythm?
It is dangerous, but it is not one of the arrest rhythms. In complete heart block the ventricles still beat, slowly, on a backup pacemaker. Some hospital lists of life-threatening rhythms add it, along with very slow idioventricular rhythms, so check the list your own exam uses.
What is the difference between asystole and PEA?
What the monitor shows. Asystole is a flat line: no electrical activity. PEA is organized electrical activity that produces no pulse. Both are non-shockable, and the treatment is the same.
Why do dysrhythmia exams require 100% on lethal rhythms?
Because these are the strips where a wrong reading costs a life in minutes. Many hospital exams pass at 80 or 85% overall and add a second rule: every lethal rhythm must be named correctly, or the exam is failed whatever the total score.