Washington -- Americans are struggling to pay medical bills and accumulating medical debt at an increasing rate, according to a survey released Thursday.

"A perfect storm of negative economic trends is battering working families across the United States," said the survey by the Commonwealth Fund, a private foundation that supports independent research on healthcare.

"Healthcare costs are climbing much more rapidly than incomes or the growth in the overall economy," said Sara Collins, assistant vice president of the foundation and one of the authors of the study. As gas and food prices have soared and real estate values have fallen, the federal minimum wage is now $3 an hour lower, in real terms, than it was 40 years ago, the study said.

"What is notable is how these problems are spreading up the income scale," Collins said.

The survey showed that the healthcare gap between poor and moderate-income families is narrowing, and that even middle- and high-income groups are going without medical insurance at some time during the year.

More than two in five adults in the 19-to-64 age group reported problems paying medical bills or had accumulated medical debt in 2007, up from one in three in 2005. Their difficulties included being unable to afford medical attention when needed, running up medical debts, dealing with collection agencies about unpaid bills, or having to change their lifestyle to repay medical debts.

Healthcare costs are limiting expenditure on daily necessities. Of those facing mounting medical bills, 39% used all their savings, 30% incurred large credit card debt, and 29% said medical bills left them unable to pay for basic necessities such as food, heat or rent.