Often newspapers and magazines make a statement that implies that the probability of surviving a cardiac arrest is a x% per minute declining ramp. This, it turns out, is an oversimplification.
In many cases it's more accurate to think of it as three steps of decreasing survival probability:
The starting point is just before the arrest, and your chance of living through the day is essentially 100%.
The first step down is frequently a drop down to ~28% chance of surviving.
The second step down drops your chance of surviving to a little less than 5%
The third step down drops that number to 0%.
The timing of that second step can be delayed by immediate chest compression that lasts until the ambulance crew takes over. This does not apply in all cardiac arrests, but you won't have a heart monitor with you and you won't know for sure that the time of the second step can be moved or not, but everybody with a cardiac arrest needs chest compression, so you're not hurting anything. It's just that if the victim is in the first step down area, it really helps to start immediately and not stop.
For those who wonder where the percentages came from, check Chan, McNally et al (DOI: 10.1161/CIRCULATIONAHA.114.009711). It tells us that 27.9% of victims with shockable rhythms survive and 4.4% of victims in non-shockable rhythms survive.
Also see Trenkamp, Perez (DOI: 10.1016/j.ajem.2015.06.070): five out of six people cannot perform guideline-compliant compression with their hands for ten minutes, a reasonable average response time for the ambulance crew. This study shows that four times as many people can perform ten minutes of compression when they use the heel of their foot.
When you see a sudden cardiac arrest, you need to immediately need to begin heel compression while calling 911 on your cell phone. If you don't have a cell phone, you'll have to call 911 first. Your vital mission is to keep the victim in a shockable rhythm. For a demonstration of how to perform heel compression, go to www.slicc.org/ClassVideo and click on the Adult CPR file.