This article pertains to: LEGACY API (V1) INFORM (V2)
Heart rate variability is the time between each of a person's heart beats.
While there are some devices on the market that capture this data, it is not a supported metric in the Validic Inform API.
Some sources, such as Garmin and Fitbit, may have devices that capture this data, but those vendors have not yet added the metric to their API.