The PM4 includes built-in heart rate capabilities that work with Garmin, Suunto, or ANT+HR chest belts. The PM4 can also work with Polar chest belts, provided you also have the Concept2 heart rate receiver and cable.
For more information, visit:
Heart rate monitoring
How to Use Polar Equipment with your PM4
How to Use Your Garmin or Suunto Equipment with your PM4