5 largest medical device companies are Johnson & Johnson, GE Healthcare, Siemens, Medtronic and Philips Healthcare. http://investingnews.com/daily/life-science-investing/medical-device-investing/top-medical-devices-stocks-johnson-ge-healthcare-siemens-medtronic-philips/
(One of my friends works as an engineer working with hyperbaric chambers. Have a look at this interesting link. But it's not like the top 5.) http://nypost.com/2015/12/01/why-this-new-york-giant-sleeps-in-an-18k-hyperbaric-chamber/