Hospitals in the U.S. territories see more deaths after heart attacks, heart failure and pneumonia than the states, according to a study out Tuesday.

Researchers said the healthcare gap is not explained by patients being sicker or hospitals lacking medical facilities, but could be due to a lack of federal funding.

"We were no less than stunned to see the wide disparity in terms of the quality of care in the territories," said Dr. Marcella Nunez-Smith, whose findings are published in the Archives of Internal Medicine.

"For every heart attack, we are going to have two more people die in the territories than in the states."

The territories include Puerto Rico, Guam, the U.S. Virgin Islands, the Northern Mariana Islands and American Samoa, with more than four million inhabitants total.

While the population exceeds that of many states, Nunez-Smith, at Yale School of Medicine in New Haven, Connecticut, said the islands have long been ignored in the national debate.

"The U.S. territories have not been part of the conversation we have been having nationally about making sure that the quality of care is high for all minorities," she told Reuters Health. "This study is a call to action."

The researchers tapped into three years' worth of Medicare data, which mainly covers patients 65 years and older, from 57 hospitals in the territories and nearly 4,800 hospitals in the states.

They found marked differences when they compared death rates for the three leading causes for hospital admissions in this age group—even after they adjusted the results based on how sick patients were to begin with.

For heart attacks, which kill about half a million Americans every year, close to 19 percent of patients died within a month in the territories, compared to 16 percent in the states.

The death rates were 12.3 percent versus 10.8 percent for those with heart failure, and 14.9 percent versus 11.4 percent for cases of pneumonia.

While hospitals in the territories were less likely to follow treatment guidelines and also treated fewer patients, those differences couldn't explain the increased death rates.

"In the territories, unfortunately the quality of care appears to be lower than in the states," said Nunez-Smith, whose work is the first to compare large-scale health disparities between U.S. territories and states.

It's unclear what accounts for the findings. According to Nunez-Smith, the islands' remoteness is unlikely to be the problem, because there was no disparity when the team looked at Hawaii.

Rather, lack of federal funding may be at play, the researchers say. For instance, 2003 Medicare spending per beneficiary was just $2,800 in the territories, compared to $6,300 in the states.

Because Medicare reimbursement makes up a substantial part of hospital revenues, Nunez-Smith said, this difference "is going to have implications above and beyond Medicare patients."

A commentary in the journal broadens the discussion to also include Medicaid reimbursement. The government pays states back for up to 83 percent of their Medicaid expenditures, but it reimburses the territories for no more than 50 percent.

What's more, writes Dr. Nilsa Gutierrez, federal funding is capped at a specific dollar amount in the territories, while it is open-ended in states.

"The unintended negative effects of these policy differences are evident, and their impact on access to care and treatment, services, and delivery system infrastructure are measurable," notes Gutierrez, of the U.S. Centers for Medicare & Medicaid Services.

Most of the territories lack rehabilitation and hospice services, for instance, as well as nursing homes and home care.

In a U.S. state, Nunez-Smith said, such conditions wouldn't be tolerated.

"I live in Connecticut," she told Reuters Health. "If I published a paper that said, this is what healthcare is like in Connecticut, great attention would be paid."