I have been told that the chance of survival decreases about 10% per minute of delay before defibrillation. My father collapsed at a shopping centre that did not have a defibrillator. Someone immediately called an ambulance and started performing CPR but the ambulance took IIRC about 9 minutes to arrive and my father died. If there is a defibrillator on site and the ambulance has already been called but you cannot see it yet, then I would suggest that you use the defibrillator. If there are two people available to help, then even more so, as one person can fetch it and get it ready whilst the other person is doing CPR.
If your heart stops beating, your chance of survival is about 5%.
I was there when one of my hockey team fell over during a hockey tournament in the Netherlands. He got CPR and mouth-to-mouth resuscitation for the 10 minutes it took for the ambulance to arrive, but he didn't survive.
One of my friends - a medical professor - had better luck. He fell over in a university commencement procession and got his CPR from the the professor of anesthetics, who was just behind him in the procession - it broke sever al ribs, but kept his brain working.
He had another heart attack more recently - half-way up a bell tower while he was bell-ringing. Getting him out was tricky, and got into the local TV news, but he survived that too (though the other bell-ringers did insist th at he give up bell-ringing).
It's been awhile since I've studied this, but here's what I remember...
In VFib, the normal cardiac rhythm is taken over by a fast cyclic loop - with an excitatory wave circling the heart (often called the "circus movement"). As with any excitable tissue, there's a refractory period -- i.e. it can only fire just so many times/second. The classic defibrillation method essentially excites the entire heart (nerve and muscle) at the same time, which gets the whole heart into this refractory period. This (temporarily) stops the excitatory wave.
The important thing is that there's some way to "restart" the heart. If the normal source (the sino-atrial [SA] node) fails, other parts of the heart can take over, hopefully helped along by CNS control. If the VFib was due to some pathological tissue it can return (the patient is likely toast in that case). Also the heart can be so fatigued from VFib (remember it's doesn't work as a pump, so oxygen perfusion is gone) that restarting the heart may be difficult. CPR can be a life- saver as it can keep some blood flowing even while in fibrillation. (Good for the brain too!)
The technology of defibrillators has greatly improved over the years, as has our understanding of some of the physiology of VFib, and drugs to address some of our biochemical problems, but the flaws present in our bodies are mostly unchanged.
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