evidence: The patient had what seems to be a pervasive but initially overlooked feature of COVID-19: silent hypoxia. Unlike many other respiratory diseases, COVID-19 can slowly starve the body of oxygen without initially causing much shortness of breath. By the time some patients have trouble breathing or feel pressure in the chest—among the symptoms the U.S. Centers for Disease Control and Prevention lists as emergency warning signs—they are already in dire straits. [...] In these cases, the low carbon dioxide levels that result from rapid breathing cause oxygen to bind more strongly to hemoglobin, the protein that carries oxygen within red blood cells. This means more oxygen can be delivered to tissues in need, as long as the heart continues to pump strongly, explains Ottestad, who is also an anesthesiologist with Oslo University Hospital’s air ambulance department. [...] What’s more, the mechanism behind silent hypoxia as seen with COVID-19 may not be unique to the disease, says Penn Medicine’s Baston, who works with patients suffering from particularly severe and rare lung diseases. While uncommon, some of his past patients have become silently hypoxic from other illnesses, such as bacterial pneumonia.