Static HKMetadataKeyHeartRateEventThreshold 

pub unsafe static HKMetadataKeyHeartRateEventThreshold: &'static NSString

Available on crate feature HKMetadata only.

Represents the threshold heart rate that triggered a heart rate event.

The expected value is an HKQuantity object with a compatible unit (e.g. count/min). This key will be set on heart rate event samples of type HKCategoryTypeIdentifierHighHeartRateEvent and HKCategoryTypeIdentifierLowHeartRateEvent.

See also Apple’s documentation