How Long Does Brain Activity Last After Cardiac Arrest?

cardiac arrest in the rain
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It's easy to assume when someone falls unconscious that she is not aware of anything that is happening around her. We all know that's not really how it works. Anyone who's ever fallen asleep in a room with other people talking knows that some of it comes through the haze of sleep.

But what about cardiac arrest? Is that too much for the brain?

Brain Activity and Cardiac Arrest

The common medical understanding is that cardiac arrest patients become unconscious within 20 seconds of the loss of blood flow (the heart stops). That's not quite the same thing as losing "all brain activity." It just means the brain is incapable of keeping you awake.

In my own experience, the 20-second figure seems to be about right. I've had two patients go into sudden cardiac arrest while talking to me. In both cases, the patients remained completely conscious for several seconds after their heart rhythms changed to ventricular fibrillation.

Both of these cardiac arrest patients, as well as another patient I happened upon in the emergency department, only lost consciousness (passed out) once I asked them if they were all right. I don't know why I asked; I could clearly see their hearts were no longer pumping blood.

All three patients survived their cardiac arrests without any problems and I learned never to ask patients how they're feeling. (Not really what I learned, but would you blame me?)

What to Do About It

All brain activity is thought to be over by about 3-4 minutes from the moment the heart stops, which is one reason why we want to start CPR as quickly as possible. It's also why hands-only CPR is good for victims of sudden cardiac arrest. It's easy: push on the chest fast and hard while someone else calls 911.

If you're by yourself, call 911 first then start pumping. Make it quick, though.

Not only does the brain stop working as it runs out of oxygen and sugar (brought to the brain by blood flow supplied by the heart), blood gets trapped in the brain until it starts flowing again. That stale blood is accumulating acids, free radical oxygen molecules, and other toxins while it sits there.

As soon as you start pumping on the chest and pushing the stale blood around, you're going to bathe the brain in those toxins. The less time those toxins have to build up, the better. It's almost as important to flush those toxins out as it is to bring fresh nutrients and oxygen in.

No matter how you look at it, the quicker you start CPR, the better.

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