After 15 seasons of fixing patients and breaking hearts, the doctors of ER, the TV drama that launched superstar George Clooney, booked their final appointment on Thursday.

The 332-episode, multiple-award-winning series was to close on NBC with a two-hour special.

Television critics praised ER, set in the emergency room of the fictional Chicago's County General Hospital, for its ground-breaking style, brilliant casts, and staying power.

"It's the longest-running medical drama ever. Just in terms of pure popularity, you have to go back to the 1960s and the glory days of Bonanza to find an NBC hour to match," the Hollywood Reporter said.

Started in 1994 on the basis of a script by Michael Crichton, author of Jurassic Park, ER immediately set pulses racing with an eye-catching cast that included Clooney playing Doctor Doug Ross.

They broke some hearts and warmed others in overlapping story lines capturing the hectic and harrowing life of an ER unit.

Famous for fast pace and moving camera shots, the show "blazed an unprecedentedly realistic depiction of the visceral, pressurised practice of emergency medicine at a big city hospital," The Philadelphia Enquirer said.

For some critics, ER was a victim of its own success, sticking around too long as ratings slid from spectacular to acceptable.

The premiere generated an audience of 23.8 million viewers and four years later, in 1998, a record 47.8 million viewers tuned in. Last week it was still NBC's most watched show, but only with 10.3 million viewers and 20th in weekly primetime rankings.

"It's about time ER is taken off life support," USA Today said.

"Imagine how much more heartfelt the celebration would have been had it departed six or seven years ago, when it was still in sight of its creative heights."

NBC has given nothing away about the final episode, which starts with a retrospective and could feature some of the original cast.

In anticipation of big viewership, the rate for commercials has been hiked to $US425,000 ($A595,780) from $US135,000 ($A189,247) for a 30-second spot, the Hollywood Reporter said.

But it will be hard to top the emotional punch for fans in the one-episode return of Clooney to the show three weeks ago.

"We must admit, the ER finale tonight has something of an anti-climactic feel to it," the Winnipeg Sun lamented. "Even if Clooney jumps out of a closet again, it's kinda 'been there, done that,' ya know?"

Still, ER's heroes are likely to be remembered for giving primetime television a healthy injection.

The show racked up 122 Emmy nominations, winning 22, and few viewers were not touched by dramas like Clooney's Doctor Ross struggling to rescue a boy from a flooding drain.

"A lot of things have to go just right for this kind of success to happen," creator/executive producer John Wells told the Hollywood Reporter.

"There's an alchemy to these things when they work. You wind up looking like a genius, but the truth is you can never replicate it."