Okay, in my story a character can use magic. Their friend has been stabbed in the chest. Could magically slowing down their heart or body help save them?
closed as off-topic by TrEs-2b, DaaaahWhoosh, Hohmannfan, Josh King, JDługosz Oct 31 '16 at 22:23
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Not really. Reducing their heart rate would drop their blood pressure which would initiate hypoxia (low oxygen) and tissue damage/cell death. Your intent, to reduce the loss of blood through the laceration, would be undermined by the loss of blood pressure. You would basically be expediting the low blood pressure state that leads to death. One reason our heart rate increases when injured (or in a fight in general) is to overcome the reduction in blood supply due to blood loss. As it leaks out, an increased heart rate (along with increased vascular muscle tone) can help sustain a blood pressure sufficient for us to finish the fight or get away to safety prior to collapse.
Penetrating chest trauma can do several things. It can lacerate or sever a major blood vessel, which is basically a death sentence, it can puncture a lung which can lead to collapse or a tension pneumothorax (which won't be mitigated by a low heart rate), or it can cause oozing internal bleeding from lesser vessels that eventually fill up the space around a lung and compress it, again not something a low heart rate will really prevent.
Hibernation or some sort of low heart rate comatose state would only really work if the person could be hyperoxygenated PRIOR to being placed into that state, which seems unlikely if they just got stabbed. Plus there would need to be a mechanism for slowing cellular activity to reduce oxygen use/waste production lest tissue become acidic and start to become ischemic. A low heart rate/low blood pressure state is useful when you are sewing a torn vessel closed (if it can't be clamped off), but otherwise it won't do much for you.
An edge case where it MAY be useful is if the heart wall itself is pierced and blood starts to fill the pericardial sac, leading to cardiac compression, aka cardiac tamponade. In this case, slowing the heart rate may slow filling of the sac, allowing the heart to continue beating, albeit at a lower rate, until the blood in the sac can be drained (usually by needle decompression) and ultimately opened and repaired.
Obviously if you are utilizing magic that allows you to ignore cellular damage then just lowering the heart rate to reduce blood pressure could slow blood loss until the person could receive some other kind of magical or mundane healing to repair the injury.
While it would be invaluable for stabilizing them for transport to a hospital, on its own it would be unlikely to save them.
We can divide the lethality of a wound into two factors: blood loss and direct trauma. Blood loss is what you are thinking of. If you lose too much blood, the rest of the body suffers and dies (in particular, the brain). In this sense, it would be effective. It might even give the blood around the would time to coagulate and stop the bleeding.
However, we can't ignore the fact that your character has been stabbed in the chest. There are plenty of more direct issues to pay attention to. If that stab happens to strike a major artery, it is likely impossible to stop the bleeding via coagulation without cutting off bloodflow to large parts of the body. A hit to the heart most likely do enough damage that the heart would not be able to operate after the blood coagulates.
A few wounds may actually be helped by this. If your character was stabbed in the lung, it seems reasonable that you could slow the blood until it stops the bleeding. Many people live with only one lung, so it's absolutely reasonable that they could keep going if they had to lose a lobe of a lung, or even a whole lung. I do not, however, know about the long term infection risk therein. You would introduce a large amount of bacteria, and you would quickly have a good sized body of necrotic tissue where your lung was. These could be fatal on their own.
For stabilization, however, this would be incredibly powerful. If hospitals exist in your world, Tezra's answer is a real life application of your magic. The thing to remember, however, is that you're just stabilizing them, not curing them. It's up to the surgeon to do the actual curing (such as stitching together arteries).
I can't find what its called (link for related article), but this IS kinda a thing. The idea is so slow the body to a near death state, do the fix, than restore them (this surgery has to be preformed in like 30 seconds though or you get brain damage).
In your case, you would be trying to trigger a a sort of hibernation, which will buy the hero time, but they will still need to be treated relatively quickly (I want to say it buys them about an hour or two. Don't remember where I heard that though)