when reading about defibrillators: nothing can serve as a substitute for a defibrillator, because nothing you have laying around can produce the 4,000V necessary to stop a human heart

reading everything else about electricity: voltage has nothing to do with stopping your heart, it’s amperage

Amperage doesn’t exist, it’s a spook. “It’s not the voltage, it’s the amperage” is folk wisdom that has the grain of something true to it, but the lesson it intends to teach is lost to the ages. (Well I know what it intends to teach, but that’s due to training, not because the saying still teaches anything)

  • Qualifications for understanding the dangers of electricity: I am an electronics engineer. I have worked as a consultant on automated electrical systems, and during this time I have taken certificates in working with live high-power electrical systems. Those certificates are old and gathering dust, I may be misremembering some of this, it has been literally years.
  • Qualifications for understanding defibrilation: I have an up-to-date CPR certificate lying around. That one is not dusty, I renewed it this spring.
  • Neither of these two things specifically qualify me to tell you exactly how defibrillation works, but nobody else knows either [citation: go check wikipedia. Seriously, we don’t know how defibrillation works, it just does.]

Electricity acts on your body in three ways:

First: Electric current, running through anything, will heat up that thing. If sufficient electric energy is deposited inside of you, you will burn up from the inside.

Second: Muscles, reacting to an electric signal, will contract. If they do this for long enough, or violently enough, they may pull themselves apart.

Third: Muscles, reacting to an electric signal, will contract. If those muscles are responsible for, oh, say, breathing, or blood circulation, you may die from oxygen deprivation. In addition, the biological systems responsible for regulating their contraction and relaxation may become disturbed so that, after the electricity is powered off, your systems remain in a blocked or dysfunctional condition.

Defibrillation is a variation on the third.

(Sidenote: Defibrillation does not work on a stopped heart. TV lied to you)

Defibrillation works for two specific problems with the heart that can both be phrased as “it’s beating weird.” (There are also ways your heart can beat wrong that defibrillation will not help with.)

If your heart is beating way too fast, you can give it a steady pulsating beat at a slower rhythm and the module in your heart that is responsible for setting the rhythm may be pulled along.

If the four chambers of your heart are all beating simultaneously, or in the wrong order, your heart will not act correctly as a pump. By applying a correct rhythm, the module that is sending out signals in the wrong order may, again, be pulled along and return to normal functionality.

How much voltage do you need for this? How much energy? The answer is a question: Where are you asking from?

If you’re asking from outside the body, you need enough voltage to power a signal through the chest and across the heart, and at the heart it needs to be powerful enough to overpower the dysfunctioning module and control the heart until the module picks up the correct rhythm from reading the heart’s motion. You also need enough power that you can sustain this voltage.

If you’re asking from inside the body because you are, for example, a pacemaker or Literally Magic, the amount may well be less. I wouldn’t know, I’m not a doctor or pacemaker manufacturer – CPR is applied from the outside.

Returning to the thing you wrote in the OP: Nothing can substitute for a defibrillator because nothing you have lying around produces the exact sinus rhythm of a functioning, well-beating heart. I can definitely take some of the components I have lying around and fake up a 4kV jolt, but that wouldn’t do anything except ruin everybody’s day.

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