What would be the thing to do if a child has sudden cardiac arrest while playing a sport on the field?

CPR... If anyone, including a child, has a sudden cardiac arrest, 911 should be immediately called and CPR initiated. The 911 operators can talk you through it if you are untrained. An automatic external defibrillator, or aed, should be applied according to the directions if one is available. This device will deliver a "shock" to the heart, if warranted. This can all be life-saving.
Swift action. Put in play your abc's (airway, breathing and circulation) as fast as possible. If you determine that the child has a clear airway, is not breathing and has no pulse initiate CPR right away and instruct one specific individual to call 911 (don't just say, "someone call"). Should there be a defibrillator available delegate the use of the device to someone who knows how to use it at once.

Related Questions

What can you do for someone who has a sudden cardiac arrest while playing soccer?

Call 911. Call for help. While help arrives, you can start cpr. If there is a aed(automatic external defibrillator) available, have someone who is qualified, use it asap. Read more...
CPR. First, call 911. If the person has no pulse, you would initiate cpr. Classes are available to train you in the technique but the 911 operators can also talk you through it so CPR can be done while awaiting for the 911 rescue personell to arrive at the scene. Read more...