The patient, a man in his early twenties, hobbled into the E.R. on a Wednesday morning, anxious and gasping, his shirt covered in blood. Minneapolis in the nineteen-eighties was experiencing an increase in violent crime that would later earn it the nickname Murderapolis; at Hennepin County Medical Center, the city's safety-net hospital, stabbings and gunshot wounds had become commonplace. Doctors there had treated dozens of patients with wounds to the chest, and the outcomes had been dismal: roughly half had died, and many survivors suffered brain damage. The chest contains the heart, the lungs, and the body's largest blood vessels. The challenge for a doctor is figuring out which, if any, organs have been injured, since each must be treated differently. For decades, medical texts had advocated using a stethoscope for this task: in theory, doctors could use a patient's breath pattern to detect a collapsed lung, or hear the muffled sounds of a heart filling with blood. But in r