The agency is tasked with promoting the products to medical device manufacturers and anaesthetists working in critical care.
Sphere Medical's patented technology has the scope to improve outcomes among critically ill patients by giving clinicians an accurate picture of each patient's status.
De Facto will aim to build Sphere's reputation through media targeted at medical device companies and the medical journals read by anaesthetists.
De Facto was selected after a four-way pitch.
In addition to media relations, De Facto will also be helping Sphere Medical with its digital marketing.
Strategic direction for the Sphere Medical account will be provided by co-founder Kevin Payne, with associate director Hilary Latham handling day-to-day activities.