Millions of Americans likely to develop and die from heart disease live in cardiology deserts — areas of the country without a single heart specialist to care for them.

New research published Monday in the Journal of the American College of Cardiology finds that nearly half of all counties in the U.S. lack a practicing cardiologist.

Most of those counties are rural, with residents who tend to be sicker in general with complex medical problems.

“The counties that do not have cardiologists probably need this type of specialty care even more,” said the study’s senior author, Dr. Haider Warraich, a cardiologist at Brigham and Women’s Hospital in Boston.