On TV it appears that every time a heartbeat monitor flat-lines they pull out the paddles of a manual external defibrillator (MED) and give a few shocks, often explicitly cranking up the output when an initial charge doesn't restore a heartbeat.

My technical understanding of this is that a MED is only indicated when the cause of the heart "stopping" is some form of fibrillation or arrhythmia. Granted, maybe "off-screen" is an electrocardiograph the EMT is referencing. But if not, would an EMT ever use a MED if the only thing he knew was that a person had no pulse?

Or, is it the case that a heart will only stop due to either fibrillation or arrhythmias that a MED can reverse?

What if a subject does have a steady heartbeat: Can a MED stop a beating heart?

I suppose that if a subject has no pulse one might argue that the MED can't hurt. But is a MED a substitute for CPR? E.g., if you had a MED and no better information on the patient would you apply both MED shocks and CPR, or what is the current best practice?

Back to TV depictions: They almost always show the subject of MED arching his back for roughly a full second when the charge is applied. I've never seen a real MED application, but my understanding is the pulse is only supposed to last 12ms. Also, since it is applied across the chest, the skeletal muscles most prone to contract would presumably be the chest and upper abdominals, which would (if anything) cause the exact opposite contraction, and only for an instant.

1 Answer 1


Your technical understanding is correct and television is fiction. In fact, watching shows involving CPR and defibrillation is a source of both amusement and frustration for most medical professionals because it is almost always portrayed wildly inaccurately.

Defibrillation is effective against only ventricular fibrillation (VF or V-Fib, which is when the heart is beating too irregularly for effective blood circulation to occur) and pulseless ventricular tachycardia (VT or V-Tach, which is when the heart is beating too fast for effective circulation). Automated defibrillators simply will not shock other rhythms, including asystole ("flat line"). No one trained to use a manual defibrillator would do so either.

Yes, a defibrillator can definitely stop a beating heart. However, if that heart started out healthy and hasn't been injured, then it's quite likely it will restart on its own due to the autorhythmicity of cardiac cells. This is why cardioversion can be used to treat arrhythmias with relative safety. A cardioversion is simply a defibrillation of the heart timed to coincide precisely with a safe point in the heart rhythm.

Interestingly, there was even one case of an unsuccessful suicide attempt using a defibrillator. Had the nurse followed his training and applied the paddles to his chest instead of his head, he might have succeeded.

Although the "it can't hurt" logic is tempting, the fact is that inappropriate defibrillation can hurt. Defibrillation works by depolarizing the entire heart, which interrupts the arrhythmia and provides an opportunity for autorhythmicity to restart a normal rhythm. If the patient is in asystole the heart is already depolarized, so defibrillation will accomplish nothing except perhaps depolarizing cells that were in the process of repolarizing, thereby eliminating the already very small chance of restoring a heartbeat.

Defibrillation is also ineffective against pulseless electrical activity (PEA). In PEA, the heart's conduction system is functioning normally but some other cause is preventing it from pumping blood. Hypovolemia and hypoxemia are the most common causes of PEA. The only effective treatment for PEA is to find and correct the cause.

If the patient is in some rhythm other than VF or VT, defibrillation may stop their heart, or put them into VF or VT. It simply isn't done no matter what you might see on television.

  • Thanks -- your answer prompted this related question to which I'd love to get your response. One other point of clarification: So will you practically never find a MED without an ECG? I.e., if you're "in the field" and find a subject in VT then you can't distinguish it from asystole without an ECG, correct? I guess now you're more likely to have AEDs. But based on your answer it sounds like a MED should never be used without an ECG to determine that it is warranted.
    – Lysander
    Commented May 22, 2016 at 14:20
  • 1
    @Lysander I don't think a MED has ever been made that wasn't combined with an ECG as a single machine. So yeah, you'd never use one without the other.
    – Carey Gregory
    Commented Jun 1, 2016 at 5:10

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