Apple Watch never checks for heart attacks
Basically, this is really just a disclaimer that Apple Watch cannot detect heart attacks. 

According to Apple:


  
    Things you should know
  
  
  
  Apple Watch cannot detect heart attacks. If you ever experience chest
  pain, pressure, tightness, or what you think is a heart attack, call
  emergency services immediately.  
  Apple Watch is not constantly looking for AFib. This means Apple Watch
  cannot detect all instances of AFib, and people with AFib may not get
  a notification.
  If you’re not feeling well, you should talk to your doctor even if you
  don’t get a notification. Symptoms such as a rapid, pounding, or
  fluttering heartbeat, dizziness, or fainting, can indicate a serious
  condition.    
  Do not change your medication without talking to your
  doctor.
  In some instances, the notification may indicate the presence of an
  irregular heart rhythm other than AFib.
  


Source: Heart rate notifications on your Apple Watch, Apple.