RESEARCH TRIANGLE PARK, N.C. — Traffic on the initial public offering exit ramp for venture capital-backed companies picked up a great deal in the second quarter. However, the increase may not be there for long.

That’s the word from Thomson Financial and the National Venture Capital Association in their latest update on IPOs and other exits.

Some 19 venture-backed companies executed IPOs in the second quarter, Thomson and the NVCA said Wednesday. That’s nearly double the number (10) of venture-backed IPOs in the first quarter.

The second-quarter IPOs generated more than $2 billion — up from $540 million in the first quarter. The amount of money raised was the most since 27 IPOs generated nearly $3 billion in the fourth quarter of 2004.

“The growth in IPO volume is encouraging, but we certainly are not out of the woods yet,” said Mark Heesen, president of the NVCA. “We will need to see these levels continue to increase throughout the rest of the year to characterize the U.S. public markets as available to venture-backed companies.

“These companies now have a multitude of exit and liquidity options — acquisitions, foreign exchanges, and buyout rounds,” he added. “A U.S. IPO is still not a viable option for many of these emerging enterprises. We not only need to see more companies going public, but more companies going public successfully if we expect others to follow suit.”

The exit ramp for venture-backed firms through acquisitions was much busier, with 86 deals. Of those, 34 had a disclosed value of $3.2 billion, Thomson and the NVCA said. The numbers were down from 101 total deals and 47 with a disclosed value of $5.38 billion in the first quarter.

Vonage Holdings, a provider of Internet telephony services, pulled off the largestworth $531 million. That was the largest venture-backed IPO since October of 2004.

Eight technology companies went public for $1.4 billion.

Northstar Neuorsciences executed the largest life science IPO at $106.5 million. Nine life science IPOs were worth more than $452 million.

Another 41 venture-backed companies are “in registration” with the Securities and Exchange Commission for IPOs. That’s up from 24 in the first quarter, according to Thomson and the NVCA.

Three companies backed by U.S. venture capitalists chose to go public on foreign exchanges.