There was this interesting discussion on CPR and defib in response to the question "Why can't we defibrillate the heart within 1 minute after ventricular fibrillation by electroshock?".

Now I was wondering "How long can the brain cope without O2?".

And to specify this question to a realistic situation: "How long can the brain survive a cardiac arrest without (gross) neurological symptomatic consequences?"

I would define "gross symptomatic consequences" as "noticeable and debilitating" in one way or another to the subject. This opposed to minute changes in, e.g., motor response latencies measured in a laboratory task designed to pick up the slightest results of asphyxia.

As background: I did these CPR and defib courses and the first thing they tell you to do is (1) call 911 / 000 whatever alarm number and (2) arrange a partner in crime to help with CPR; or in case of a defib situation, (call alarm nr), make your way to the defib, unpack, apply electrodes, look at the manual, casually wait for instructions/warning lights etc. But..but... there is a person here without oxygen supply to his brain! And I know professional staff should be brought to the scene, but what struck me most is that starting up the defib (which inherently takes time) is more important than CPR (which is immediate)? In other words, loosing precious minutes is advantageous over diving straight away into CPR and pumping blood around?

  • $\begingroup$ Fibrillation is very hard to detect clinically. It can be confirmed only on ecg (as far as my knowledge extends). So unless the patient is at high risk of fibrillation, generally cpr is the first aid done $\endgroup$
    – One Face
    Jan 15, 2015 at 15:36

2 Answers 2


From Boron and Boulpaep textbook of Medical Physiology, second edition, p.289:

Because of falling ATP levels in the brain, consciousness is lost within 10 seconds of a blockade in cerebral blood flow. Irreversible nerve cell injury can occur after only 5 minutes of interrupted blood flow.

If conscious is lost within 10 seconds of blockade in cerebral blood flow, I guess a person will not be conscious to do any action in only 10 seconds!

Brain ischemia is divided into

  1. Focal ischemia which signifies stroke.
  2. Global ischemia which occurs in cardiac arrest.

Permanent neural injury means that once perfusion is restored, the symptoms don't relieve as the part of brain which caused those symptoms becomes dead irreversibly.

For a list of symptoms check this page.


If we consider another case, there is blood flow, but no new oxygen is coming into the system, the brain cells may begin to die after about four minutes [1]. However, this depends on the person not the amount of time. Consider free divers and more specifically Tom Sietas. These men and women, in this sport, can go well beyond the 5 minute mark with no debilitating brain damage. Now, Tom Sietas holds the world record for static apnea with the time of 22mins and 22sec under water without air and yes he survived. Here is a news article about Tom but it goes on to say Pearl divers go about 7mins without air.

  • 1
    $\begingroup$ @ChrisStronks I would guess, with these people (free divers), since their body is trained, they could go into cardic arrest and last longer than 5 mins but this isn't a testable scenario. $\endgroup$
    – dustin
    Jan 16, 2015 at 1:03
  • $\begingroup$ Interesting answer, +1. However, I think we are indeed talking about exceptional, trained cases here, which would make an interesting new question as to how they exactly cope! $\endgroup$
    – AliceD
    Jan 16, 2015 at 1:06
  • 1
    $\begingroup$ @ChrisStronks I know they are exceptions but we can't say 4-5mins is a hard limit. It all depends on the person. In the case of Tom, I could die four times before him. $\endgroup$
    – dustin
    Jan 16, 2015 at 1:07
  • 2
    $\begingroup$ I think such training will enable the lungs to retain more oxygen (well developed lungs that expand fully) and the circulatory system is better adapted by increasing hemoglobin concentration (this increases the blood carrying capacity). It has nothing to do with cerebral perfusion. Cut blood supply to brain no one can stop symptoms from occurring. $\endgroup$
    – One Face
    Jan 16, 2015 at 1:26
  • $\begingroup$ @CRags I said let's consider a case with blood flow and no new oxygen. $\endgroup$
    – dustin
    Jan 16, 2015 at 1:28

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