Medical Grade EKG analysis coming to Apple Watch via 'Kardia Band.'
Wednesday, March 16, 2016 at 10:20AM 
The Apple Watch has always been about tracking health and now the first of what we hope are many health-focused third-party accessories is coming. AliveCor announced that it will bring "the first medical-grade EKG band" to the Apple Watch with the "Kardia Band," in addition to a new smartphone app for users of its heart-rate reading devices.
Users can record a single-lead EKG by simply touching Kardia Band’s integrated sensor that communicates with the Watch app, Kardia by AliveCor. The Atrial Fibrillation (AF) Detector then uses Kardia’s automated analysis process (algorithm) to instantly detect the presence of AF in an EKG, the most common cardiac arrhythmia and a leading cause of stroke.
The kardia Band is undergoing approval for clearance by the US Food and Drug Administration and can't come to market until it has been cleared. AliveCor, the company behind the band, expects it to come to market as soon as "late spring." This could be the first of many accessories that address specific health concerns and help monitor users more comprehensively.
Source: Macrumors