What Happens to Your Body When a Defibrillator Is Used on You

It’s a common misconception that defibrillators—whether it’s the old school “paddles,” modern automated “sticky pads,” or the implanted variety—restart hearts that have stopped beating. Blame Hollywood for this one: a lot of depictions of defibrillators in action show them being used on “flat-lining” patients. In reality, a defibrillator stops a dysfunctional, quivering heartbeat—a so-called “fibrillating” heartbeat— so the heart can (hopefully!) restore its “regular” rhythm on its own (with the help of CPR and possibly adrenaline). So don’t be fooled by what TVTropes calls the “Magical Defibrillator”—shocking a stopped heart won't do any good.

The shock from an automatic implantable cardiac defibrillator (ICD) is notoriously painful. The device is just a bit smaller than a deck of cards but it packs a wallop. ICDs are implantable in people with certain heart conditions - basically, if doctors deem you at risk for ventricular arrhythmias, they might suggest a permanent ICD implant. The University of Michigan’s Frankel Cardiovascular Center advises patients that “people say it feels like being hit in the back with a baseball bat” when it fires. The intense pain, however, “only lasts for a second” and is preferable to, y’know, dying.

The need varies from patient to patient, but manual defibrillators generally work by generating about 300 joules of electrical current, or about “as much as a 100 watt incandescent lamp uses in three seconds.” Only part of that current, however, flows through the heart. Most of it is “dissipated through the resistance of the skin and the rest of the body.”

Ever notice how they always rub the paddles together in the movies? It’s not to build up a charge or anything: that actually comes from the practice of applying conductive gel to the paddles and then rubbing them together for an even application (these days, sticky pads are far more common than paddles anyway). The traditional request for those in the room to "clear" away from the patient ("Clear!") before administering the shock, by the way, still holds true when using hand-held paddles but is considered largely unnecessary when using sticky pads (far less risk of shock).

Cardiologists say that getting hit by manual defibrillation paddles when awake would feel like “being kicked by a mule in the chest.” So it’s a good thing that a so-called “appropriate session” of defibrillation happens when the patient is unconscious. Do patients remember the shock later? Does the pain linger? Nope: experts say “they won’t ever feel or remember the shock.”

The American Heart Association says that defibrillating shocks “typically cause a powerful tetanic contraction of skeletal muscles.” Translation: muscles near your skeleton—most importantly the ribs in this case—go through a powerful, sustained contraction. Since CPR and defibrillation go hand-in-hand, it’s important for medical professionals to time their chest compressions correctly. Otherwise, as the AHA notes, the victim could suffer rib fractures and the “rescuer” could suffer wrist or arm fractures.

Jackie Todd told The Atlantic in 2015 that when she was 14, her implanted defibrillator fired after she ran across her high school campus to catch a screening of the second Harry Potter movie and overworked her heart. The implant saved her life, but felt like “someone had shoved metal doors violently into [her] back,” knocking the wind out of her. To make things worse, the first shock sent her heart rate skyrocketing, so another jolt was automatically administered. The second one made her pass out: “Her vision went white, her knees buckled, and she collapsed at the top of a stairwell, unconscious.”