One doesn't have to read much about CPR for Cardiac Arrest victims to encounter the assertion that the probability of survival decreases about 10 percent per minute. Sometime other numbers are used.
This, it turns out, is an oversimplification.
One common sequence seen in sudden cardiac arrests is:
1. Normal sinus rhythm[doesn't need to be treated], followed by...
2. Supraventricular tachycardia [a shockable rhythm], followed by...
3. Ventricular fibrillation [a shockable rhythm], followed by...
4. Asystole [a non-shockable rhythm]
From Chan, McNally et al (DOI: 10.1161/CIRCULATIONAHA.114.009711) we know that 27.9% of victims in shockable rhythms survive and 4.4% of victims in non-shockable rhythms survive.
Because you as a bystander do not have a heart monitor, you cannot know what the victim's rhythm is. But you do know that chest compression can delay the transition from a shockable rhythm to an unshockable rhythm.
You want that victim to still be in a shockable rhythm when the ambulance arrives with the heart monitor / defibrillator!
You also know from Trenkamp, Perez (DOI: 10.1016/j.ajem.2015.06.070) that 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. You also know from that study 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.