If you've got one of Lezyne's comically feature-laden GPS units (which, if you don't, you should probably ditch the overpriced video game console on your bar mount and pick one up), then you're probably in the market for a heart rate monitor. Keep it all kosher with Lezyne's own HR Flow Sensor, a Bluetooth-enable monitor that lets you examine the minutiae of your ticker when the pressure's on. The HR Flow Sensor works with any head unit that speaks Bluetooth, and an LED indicator keeps you abreast of the power situation in the replaceable battery.