The cost to protect the debt of U.S. companies surged to the highest level in more than a year on concern that the European sovereign crisis is worsening.

The Markit CDX North America Investment Grade Index of credit-default swaps, which investors use to hedge against losses on corporate debt or to speculate on creditworthiness, increased 5.8 basis points to a mid-price of 115.8 basis points as of 5:09 p.m. in New York, according to index administrator Markit Group Ltd. That's the highest level since July 2010.

Investor confidence in corporate debt deteriorated as central banks struggle to prevent a recession, with Federal Reserve Chairman Ben S. Bernanke vowing yesterday to keep borrowing costs at an all-time low through mid-2013 to revive a recovery that's "considerably slower" than expected.