It's been a long life and, for the most part, a happy one. But in a matter of weeks, the last life-support machine will finally be switched off in the emergency room (or "ER") of Chicago's County General Hospital, home to the longest-running primetime medical drama in US television history. You may not have seen a whole episode of ER since the '90s, but if you've been watching any television in the meantime, then you've felt its influence.

With its hyper-realistic hospital setting, its hyper-complex narrative and especially its hyper-kinetic aesthetic, ER was the first show of its kind. But it also leaves the airwaves – after a 15-year, 332-episode run – as the last US drama consistently able to attract such a substantial viewership. In its mid-'90s heyday, ER boasted weekly US audiences of 30 million. It was the world's most watched TV drama for 10 years. When the final season signed off in the States last month, it drew the largest audience for a drama since The X-Files finale in 2002; today, even the most popular network shows struggle to reach 20 million.

The last decade has been a golden age for US TV drama, with critically-acclaimed cable shows such as The Sopranos, Mad Men and The Wire complementing quality network fare like The West Wing, 24 and Lost. But in its lifetime ER has also been party to a steep decline in the networks' commercial and cultural power. Since its 1994 debut, ER has won 22 Emmy awards and been nominated for 122: more than any other TV show, ever. Since that first season, however, audiences have fragmented thanks to cable, the internet and the DVD boxset. And the pay-to-view cable channel HBO has stolen not only some of the networks' audience share, but some of their reputation for top-quality drama, too.

ER's fate was tied to that of other scheduled shows in a way we might not understand on this side of the pond. The show premiered three days before Friends, another huge NBC success; and reached its audience peak – a whopping 47.8 million – when an episode aired straight after the finale of Seinfeld in May 1998. It earned itself a permanent place in the schedule: 10pm Thursday, the same sanctified slot formerly reserved for NBC's landmark shows LA Law and Hill Street Blues.

ER did for the medical drama in the '90s what Hill Street Blues had done for cop shows in the '80s. The doctors and nurses of County General replaced the superhuman ciphers of Doctor Kildare and Marcus Welby, MD. In the halls of ER's inner-city hospital, its viewers saw an everyday heroism that was absent from the more rarefied environs of the Bartlett White House, or Jack Bauer's counter-terrorism unit. With its characters striving to save lives in an underfunded, under-resourced system, it anticipated The Wire. And with its workplace details and specialist jargon it paved the way for The West Wing. It shared all of that show's idealism, but little of its optimism.

"It was one of the first shows that made doctors fallible," says Stuart Levine, assistant managing editor at Variety magazine. "Before then, doctors on television were godlike, they did everything right, and every week they'd see a patient and fix them. Here you had doctors who made mistakes. They were doing 17-hour shifts and their home lives were a mess. No one had seen that before."

By the end of the pilot episode, viewers had also met John Carter (Noah Wyle), the hapless young doctor who would become ER's most enduring character. Carter was the son of privilege who found that he was less skilled at surgery than at interacting with patients – until he was stabbed by one in season six, leaving him physically and psychologically scarred.

As Carter helped to deliver a baby in episode one, nearby head nurse Carol Hathaway lay close to death after a suicide attempt. Hathaway, played by Julianna Margulies, was originally slated to shuffle off in that pilot episode. Audiences warmed to her so much, however, that she was allowed to live, and immediately added to the permanent cast. Her relationship with (Dr) Ross (played by George Clooney) became a televisual romance to rival Ross (Geller) and Rachel – and one of the joys of the final series has been learning that the pair stayed together since leaving Chicago nine years ago.

Ross was ER's loveable rogue, a philandering drunk redeemed by his magnificent bedside manner and skill as a paediatrician. In season two's 'Hell and High Water' episode, he saved a young boy from a storm drain in dramatic scenes designed to tug open the tear ducts of anxious viewers everywhere. Greene, meanwhile, was the gentle everyman whose first marriage was failing, and who later learned to love again in the arms of Dr Elizabeth Corday (Alex Kingston).

Ross left County General for Seattle after five seasons; Greene died from a brain tumour after eight. Carter, who had entered the emergency room a clumsy medical student, finally left it as an experienced senior doctor after 11 years. Each of their departures seemed to mark a gradual diminution of the show's quality. For many, the low point came in episode 209, when the eminently dislikeable Dr Robert Romano – whose arm had already been shorn off in one helicopter accident – was killed by a second chopper falling on his head.

The televisual term for this sort of thing is 'jumping the shark' – named after the notorious episode of Happy Days in which Fonzie jumped over a shark while waterskiing. It means the moment when a show goes beyond its own carefully-built bounds of plausibility: when Monica and Chandler hooked up in Vegas, for example, or when Pam Ewing woke to find Bobby in the shower, and realised that the whole thing had been a dream. ER slowly but surely edged closer to soap opera as the seasons progressed, with shoot-outs and a tank assault among the other less plausible plotlines.

Yet it still proved itself vital: by putting Parminder Nagra, a British Asian, in the female lead; or by having doctors Carter, Kovac and Pratt travel to Congo and Darfur with Médecins Sans Frontières, even as the rest of the US media ignored the conflicts in Africa.

"I was very proud of what we did with the John Carter character in his first trips to Africa," ER's producer John Wells recently told a US interviewer. "They were not particularly well received by the audience. A lot of people didn't watch them or tuned them out because the subject matter is very difficult. But I felt it was very important... to be able to dramatise some of the things that were happening in Congo and Sudan, on a broad-based network entertainment show – I was proud of and remain very proud."

ER's final episodes, which feature Carter's return to County General, have long been in the minds of the creatives involved. Indeed, Wells says he discussed the plot of the finale with Noah Wyle as long ago as season eight, following Greene's death. But a new cadre of characters – in particular the pairing of Maura Tierney and Goran Visnjic as troubled lovers Nurse Abby Lockhart and Dr Luka Kovac – rejuvenated the show and gave it life after Carter.

ER has its origins in the experiences of the late author Michael Crichton, himself a former doctor in a Boston hospital, who wrote a screenplay on the subject in 1974. But it was only 20 years later, after working with Steven Spielberg on the film adaptation of his novel Jurassic Park, that the idea finally made it into production as a two-hour TV pilot with Wells, of Spielberg's Amblin Entertainment, in the producer's chair.

Crichton's 1974 script remained remarkably unchanged, despite the time-lag, though Dr Lewis became a woman, and Dr Benton an African-American.

Like many hugely successful ensemble shows, ER has launched few big Hollywood careers. Clooney, its one bona fide superstar, was also the first actor to be cast in the show, demanding the ensemble role of Dr Ross over a lead he'd been offered, in a pilot for a long-forgotten legal drama. He returned to ER for one of the final episodes with little fanfare: Warner Brothers kept his involvement secret from NBC until the last possible moment, preventing the network from using his appearance to bump up its ad sales.

In his book about the positive effects of popular culture, Everything Bad is Good for You, Steven Johnson cites ER as one of the shows that made multi-threaded narrative acceptable to television audiences. Unlike the popular television dramas of the 1970s, which followed a single plot strand in each self-contained episode, ER – along with the likes of Thirtysomething, Twin Peaks, NYPD Blue and LA Law – followed up to 10 threads per episode, many of which remained unresolved for entire seasons. As such, it prepared the ground for later acclaimed dramas like The West Wing, The Sopranos, The Wire and Mad Men.

"But what really made ER unique was the steadicam," says Levine. "They would do long takes, or 'oners' without any editing, which moved all the way through the hospital."

These long shots allowed the scripts' multiple narratives to overlap physically, their plot-paths crossing one another at speed in the hallways of County General.

ER's signature steadicam is now a staple of television drama, heralding the famous single-take 'walk-and-talk' scenes in The West Wing; though in ER's case, it's more like walk-and-yell-and-perform-open-heart-surgery, a clattering chase through the corridors, doors swinging, gurneys clattering, doctors and nurses yelling clinical acronyms.

ER may have become too expensive for NBC to maintain, but it has, at least, been able to choose the manner of its passing. Not many shows have that privilege, especially in the US, where they can be cancelled mid-season if the ratings slip. Schlamme's last series, Studio 60 on the Sunset Strip, a big-budget NBC drama written by West Wing creator Aaron Sorkin, was cut short after a single season. Deadwood, the HBO anti-western which earned rave reviews from critics and its few viewers, was not afforded the luxury of tying up its loose ends after the end of its third.

So, how much do you want to know about the end of ER? Wells has written the screenplay for the feature-length finale, which he describes as a "homage" to the show's very first episode. Rod Holcomb, who directed that original pilot, has taken the helm one last time. And it can be revealed that County General breathes its last accompanied by some good memories and a few familiar faces. It's had a good run, but sadly there's no saving it; this time, it's terminal. Time of death: 10.30pm, 14 June 2009, RTE One.