Matt Duch, a portfolio manager at Calvert Investments, said there had been an increase in borrowers seeking to add high leverage and "payment-in-kind toggle" deals, which give borrowers the option to pay lenders with more debt rather than cash.
"The strong demand for low-rated high yield could be fostering a generation of paper with subpar structures," he said. "That could definitely be a problem in the future should defaults pick up or financing suddenly become less available."
More from the Financial Times:
Sell-off as markets expect early Fed move
Foreign investors rapidly sell Treasuries
Other demand for Treasuries may wane
Some of the most aggressive deal structures are being proposed in the loan market, which is used for financing leveraged buyouts. There have now been 61 consecutive weeks of inflows into mutual funds and exchange-traded funds specializing in loans, according to Lipper, adding up to $39.1 billion of new money chasing investment opportunities. Some $11.6 billion has been added since June 19.
Because loans offer a floating rate of interest, they are seen by investors as insulated from the risks of rising rates as the Fed tapers QE.
(Read more: Dovish or hawkish? Fed message could get markets flying)
The US software group BMC, which was acquired by a private equity consortium led by Bain Capital and Golden Gate Capital, this month sold $5.86 billion of debt with only light covenant protections for investors and with an unusual provision giving more freedom to sell assets before repaying lenders.
The use of "covenant-lite" loans has re-accelerated since Mr Bernanke's remarks, accounting for 58 percent of all loan issuance so far this month. That puts August on course to be the second highest month ever, after January 2013, according to S&P Capital IQ.