With millions of people across the U.S. and the world battling COVID-19 infections, many of them struggling to breathe, ventilators have become a top priority for the health-care workers trying desperately to keep patients alive. And those machines, which help patients breathe or breathe for them, are in startlingly short supply. For doctors, resorting to a ventilator is an extreme measure, used when a patient’s lungs cannot supply enough oxygen on their own.