Optical head-mounted displays are an emerging digital technology in domains like healthcare where computer usage in mobile contexts is required but hand-held devices are not particularly suited for practical reasons (e.g. hygienic regulations). As the work of paramedics, nurses and physicians is not only physically but also mentally challenging, the promise of hands-free interaction alone will not ensure efficient and safe usage. Rather, various aspects of cognitive ergonomics have to be carefully considered. Best practices for designing wearable interactive systems have to be evaluated. Based on the results of different user-centered system design projects and studies with members of Emergency Medical Services, nurses and clinical physicians, we will discuss optical head-mounted displays with respect to human-computer interaction. Interaction design as well as lessons learned concerning tasks and workflows will be summarized. Transmodal consistency is introduced as a general design principle for digital technologies supporting multiple input and output modalities like touch, gestures and speech.