Washington ? The Federal Reserve left a key short-term interest rate at a 45-year low on Wednesday but dropped a promise it had been making since August to keep rates low “for a considerable period.”

The wording change was enough to jolt financial markets, sending stock prices plunging, even though private economists said they believed the Fed still planned on keeping rates unchanged for most of this year.

The Dow Jones industrial average, which had been in positive territory before the Fed’s afternoon announcement, lost more than 140 points in trading.

Bond prices dropped as well, sending interest rates set by the market sharply higher.

Short-term rates tied to Fed actions did not move at all because the Fed left its target for the federal funds rate, the interest that banks charge on overnight loans, unchanged at 1 percent, where it has been since last June. That means commercial banks’ prime rate, the benchmark rate for millions of consumer and business loans, remains unchanged at 4 percent, the lowest rate since 1959.

The adverse market reaction occurred because the Fed dropped the phrase it had included in its past four policy statements — that it believed low inflation and slack utilization of resources gave it the leeway to keep rates low “for a considerable period.”

Instead, the central bank, still citing the low inflation and slack resource utilization, said it believed “it can be patient” in deciding to raise rates.

While the wording change was subtle, Wall Street worried that by dropping the phrase “for a considerable period,” the central bank was getting closer to a series of rate increases to make sure the rebounding economy does not trigger inflation.

Still, private economists said the statement as a whole did not indicate the central bank was edging closer to a rate increase.

Trader Daniel Burke, center, shouts an order in the Eurodollar futures pit at the Chicago Mercantile Exchange. He worked in the pit Wednesday, after the Federal Reserve left a main short-term interest unchanged at 1 percent. The rates were lowered to their current level in June and haven't changed since then.