The first-line in defense against cardiac arrest? Your hands.
According to the American Heart Association, less than 1/3 of cardiac arrest victims get the help they need in the moments immediately following the incident. That's because most bystanders are either too afraid to help, or they simply don't know how. In reality, all you really need is your own two hands.
If you suspect someone is suffering from cardiac arrest, first call 911. Then, push hard and fast in the center of the chest until help arrives. There is no need to worry about causing harm - your actions can only help. The more people that know about CPR, the more lives we can save.