I've heard this is (or has been?) used as a method of capital punishment, but I wouldn't recommend it. Hyperkalemia (high blood potassium) includes symptoms like abdominal pain, nausea, diarrhea/vomiting (on the mild/moderate side), and chest pain, palpitations, weakness on the more severe side. If you were to do this you would definitely want sedation of some kind. Antiemetics would be nice as well for comfort, but not necessary as the potassium in this circumstance would be injected and so throwing up would not be a reason the method would fail.
I'm wondering if it's that easy?
It's not instantaneous. IV is the quickest way to get meds into your blood, but even then the time to effect is at least 30-60 seconds for it to spread through your system. Then at that point you also have to consider how the medication works and how long it takes to have an effect. Potassium doesn't really just "stop the heart", but moreso it disrupts how it works. Your heart works by different electrolytes diffusing across membranes, and this transfer of chemical energy results in generation of electricity. This is how signals are generated and conducted through your heart, and then contraction of heart muscle is triggered. Dumping a lot of potassium into the system would pretty quickly destabilize your heart's function, but it still wouldn't be instantaneous. Could be anywhere from maybe 1-3 minutes to >30 depending how much potassium you have to give yourself and other factors like underlying cardiovascular health.
Could you just get liquid potassium chloride and I inject into your own vein?
Nope. Potassium chloride is not something you can just go in and buy. It's a very dangerous medication, but elemental potassium is also extremely reactive so chemicals like potassium chloride which are probably not too difficult to turn into elemental potassium are controlled for that reason as well. At least where I live, you can't even buy undiluted acetic acid (vinegar) or bleach in stores. Suppliers who sell industrial grade cleaners or concentrated chemicals won't just cell to anyone, and the vinegar you buy from grocery stores is prediluted to something like 5-15%.
Anyone have any medical experience or thoughts on this?
There are a lot of roadblocks
1. You would likely run into issues even sourcing it. It's not used recreationally, and otherwise I'm not sure why there would be people buying it illegally. There's probably not a lot of supply of KCl, which would mean if you even did come by it being sold illegally it would probably be quite expensive. I could only really see someone being able to steal it from a "legal" source if you worked in a hospital, university lab, or some sort of chemical plant/refinery. Even then, these places will keep inventory lists. Hospitals especially, if a vial is noticed missing I can guarantee it would be tracked to you. Hospitals nowadays will use retina scanning, fingerprinting, or at the very least IDs and codes to sign out meds. You would be dated and times as signing out a medication from that specific drawer, and you would definitely be questioned.
2. Can you cannulate (place an IV) in yourself confidently? This would be a roadblock for everyone. Even trained healthcare providers who are able to cannulate others would struggle to do this on themselves, it's a two-handed procedure.
3. Figuring out dosages. You could look into it, but I sort of doubt there's a lot of public information on this. Previous commentor noted an LD50, but I'm not sure I would want to do this as it would only give you 50-50 odds of succeeding. Are dosages used in euthanasia available. What other medications/interventions are performed in that setting? Are you able to apply that protocol to yourself? If you are sedated you would not be lucid for the whole procedure. Even lucid surgeons and physicians make mistakes, how confident are you in yourself to do all this while not fully lucid for at least part of the procedure?
4. The method itself would be painful. Potassium burns when injected IV. It is technically a salt, and salts are generally not pleasant when injected. Beyond this there are the symptoms I mentioned previously. All of them are unpleasant, but people generally struggle to stay calm when they have chest pain and palpitations. Would SI be an issue for you?
5. This is obviously a less common method. In both suicide and as a medical form of euthanasia, to my knowledge it's not very common. Only place I've heard of it is in correctional medicine in the US, but all other countries to my knowledge use different forms of execution, and countries that allow physician-assisted suicide use different (more comfortable) methods.
I would say if you had a healthcare professional performing this procedure on you with sedation, antiemetics, other comfort measures, etc. It would be a good way to die. Doing it on yourself in an uncontrolled environment, possibly without antiemetics or sedation? Probably wouldn't be too keen on it.
Not proofread and no sources referenced, please excuse mistakes.