?When he retired on August 1, 2011, from Denver’s Saint Joseph Hospital, Dr. Bill Nelson decided it was time to clean out his home office as well. That meant trashing dozens—if not hundreds—of electrocardiogram (EKG) readouts, called tracings, with which he had overfilled his desk drawers. “I had saved 50 years’ worth of tracings,” the doctor says, “but I could still look at them and remember each patient, even see their faces.” To anyone who knows Nelson well, it would come as no surprise that he could so easily look at a bunch of squiggly lines and come up with a miraculous thought.

For more than five decades, Nelson dedicated his career to the study of—and, maybe more importantly, to the teaching of—electrocardiography, which is the interpretation of the electrical activity of the heart over a period of time. Nelson can take one look at an EKG and tell if a patient is having a heart attack, has an enlarged left chamber, or is experiencing a heart rhythm problem. He can even diagnose rarer, genetic conditions like Brugada syndrome, which can cause sudden cardiac death. Nelson can do all of that without effort, but not everyone has the same gift for EKGs that he does. “I do think that every GP should be able to do an EKG,” he says. “Not only is it the only way to diagnose certain things, it’s significantly cheaper than many other tests doctors seem to like to order. But, to be honest, most doctors don’t get the training they should have to do so.” Which is why Nelson has spent so much of his time teaching hundreds upon hundreds of medical residents how to interpret tracings.

Now that he’s retired, Colorado’s medical community will have to learn to read those squiggly lines on its own. Well, sort of. Nelson hopes to volunteer at Saint Joseph—and he’ll continue to maintain his website, where medical students, residents, and even long-time docs can read his textbook (for free!), email questions, and take quizzes from the master.