What do you think?
My interest is both personal and professional, as in my job as a medical examiner this question comes up from time to time.
I would be interested in hearing the opinions both from people who may have special knowledge, such as physicians, etc., as well as anyone else, and I’d be interested in hearing any reasons or explanations for your opinion, particularly if they incorporate empirical data.
For anyone interested, I have put my somewhat lengthy musings on this question in the spoiler box. But I’d really like to get people’s “virgin” impressions before they read what I think.
Thanks.
[spoiler]As said above, I get asked this question from time to time. After looking into it as best I can, by searching medical literature, asking around, etc., I’ve been surprised by the dearth of what I would consider objective/empirical data on the topic.
The most frequent opinion I have heard in forensic pathology - and it’s an opinion that seems pretty consistent – is somewhere around 15-20 seconds. Indeed, a version of that opinion was recently expressed during the Zimmerman trial in the testimony of the well-known and highly respected forensic pathologist, Dr. Vincent Di Maio.
He bumped it down to 10 seconds, but he did say that was a minimum.
But whence that opinion? What data backs it up? He doesn’t say.
The closest he comes in the testimony is relating an anecdote of a case he was involved with where a guy was shot point blank in the chest with a 12 gauge shotgun, such that his heart was “shredded”, and he subsequently ran 65-75 feet before he collapsed. I have heard similar anecdotes from other pathologists.
However, I have looked in several well-known text books, including one written by Di Maio, and the specific issue of consciousness after cardiac arrest is not directly addressed. The closest the books come is to offer the opinion, during discussions of asphyxia due to strangulation, that a person may remain conscious for about 5-10 seconds after both carotid arteries are completely compressed. So, now, whence that 5-10 second timeframe?
It’s a little humorous to research, because many of the textbooks and other sources reference Di Maio’s textbook, Forensic Pathology, and/or other sources that in turn reference Di Maio. In that book, Di Maio’s main reference is a 1943 article (partial abstract here and interesting “revisit” (pdf)here) describing some experiments whereby subjects’ carotid arteries were compressed while they were monitored for, among other things, maintenance of consciousness as reflected by ECG readings and subjects’ recollections after the fact. They reported an average time to loss of consciousness of 6.8 seconds, with the vast majority of subjects losing consciousness in 5-7.5 seconds, and a total range of 4-10 seconds.
So this 70 year old data is apparently where the 5-10 second opinion comes from with reference to neck compression. Now, carotid artery compression is not cardiac arrest, and for that matter neither is a heart-shredding shotgun wound, but those all would result in immediate cessation of (most – not necessarily all, and this may be important) blood flow to the brain, so I suppose the three scenarios may be sufficiently analogous to allow for cross references.
But how does the 5-10 second timeframe with carotid compression jive with a guy running 65-75 feet after a heart shredding shotgun wound?
Well, how long does it take someone to run 70 feet? According to this Wikipedia article, the preferred human walking speed is 1.4 meters/second. That converts to about 4.6 feet/second. So to walk 70 feet at the preferred human walking speed takes about 15 seconds. If we estimate that a person can run at least twice as fast as they walk, that means a person can run 70 feet in about 7.5 seconds. This is right around the empirically derived average of 6.8 seconds until loss of consciousness after carotid artery compression. And if we allow that a person runs a little faster than twice their walking speed, which seems reasonable, the calculated 7.5 seconds decreases even more.
Now, even though I said the three scenarios – cardiac arrest, a heart-shredding shotgun blast, and carotid artery compression - are analogous enough to allow for cross reference, are they really? The main difference I can see between the three is that with cardiac arrest and heart-shredding, not only would blood flow to the brain immediately cease, but blood pressure in the cerebral circulation would immediately and precipitously drop. And probably more so with heart-shredding than with cardiac arrest, as not only is the driving force of blood pressure stopped, but also a huge breach is opened in the circulatory system, resulting in instant massive hemorrhage. With carotid artery compression, blood pressure in the cerebral circuit should not drop as quickly, as the blood that is in the circuit would be trapped (assuming the jugular veins are also compressed). Furthermore, although the carotids supply the vast majority of blood to the brain, there are other smaller arteries that also deliver some blood. So compressing the carotids does not cut off all blood flow to the brain.
These things together would suggest that, if anything, consciousness would be maintained longest with carotid artery compression, then maybe cardiac arrest, and least with heart-shredding.
In any case, what little data there is, as well as Di Miao’s and other people’s own anecdotes, seems more consistent with consciousness being maintained after cardiac arrest significantly less than 10 seconds or so, rather than the oft quoted but seemingly unsupported claim of up to 20 or so seconds, “minimum”.[/spoiler]