of people are using Twitter to share and find out information about heart attacks, CPR and other health issues, a new study finds.  Researchers found more than 15-thousand messages in a month on topics including cardiac arrest, resuscitation, and automatic external defibrillators, perhaps better known as AEDs.  About 44-percent of the tweets were about performing CPR or using an AED.  The study is in the journal "Resuscitation."