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The newly completed ACAD Centre in Park Royal, north-west London, is the nearest thing to a production-line hospital in the UK. Patients walk in off the street at one end, and, without retracing their steps, emerge a few hours later from the other end with their hernias, breast lumps or cataracts removed.

Although the compact 8000 m² building may look like a glorified health centre, it is in fact a virtually self-contained hospital, with four operating theatres and a full battery of high-tech X-ray scanners and endoscopy machines. It is one of the most advanced of the new generation of day hospitals, its key innovation being a sophisticated computer scheduling system that streamlines patient diagnoses and treatments.

The beauty of the building, as well as the service it offers, is that most patients will be completely oblivious to the mechanistic production-line treatment they are subjected to. This is partly because patients are propelled through the building not by a conveyor belt but by their own two feet. The ACAD Centre's full name, the Ambulatory Care and Diagnostic Centre, translates into layman's language as a walk-through hospital.

More than that, the NHS building itself is as welcoming, gracious and clearly laid out as the most expensive private clinic. Designed by Avanti of London, it features a wide, double-height central mall awash with daylight and sunlight, landscaped courtyards and ample windows and skylights.

How, then, has the architect managed to reconcile the clinical production-line with the self-evident humanity of the architecture? Avanti director John Cooper says: "At first, it seems like a paradox. But if you think about it, the two requirements are not in conflict. The production line moves people quickly through the building, which cuts down on back-up services, including long-stay wards. It also cuts down on the extent of circulation spaces, so these can be tall, lofty, light and wide while still providing an efficient ratio of net-to-gross floor areas."

As for the decor of the building, Avanti has steered clear of any hint of domestic styling and homely furnishings with which many health building designers reassure and patronise patients. Materials are plain, high quality and crisply detailed, with the mall featuring white-painted rendered walls, judicious splashes of colour, terrazzo flooring and cylindrical columns in fairface concrete. Off the mall, patients are greeted by luxurious deep-piled carpets and maple fittings and furniture.

"Given the amount of daylight and sunlight in the building, if there are sufficiently simple and strong forms, these do the work for you," says Cooper.

For patients, the real benefit of the new walk-through hospital is that it offers the latest in keyhole surgery, allowing them to return home in one day. It does so by providing, in the words of the centre manager Jean Silcoff, a "seamless" service. "The building brings together six separate services all under one roof, and this has never been done before. The interplay between diagnostics and treatment is what makes us different."

This integrated service means that patients who have been referred by their general practitioner need visit only once or, at the most, twice, to receive full diagnosis and treatment. During the first visit, the patient undergoes all scans and testing, the results of which can be accessed digitally on the spot by the consultant, who can then prescribe treatment. On the second visit, the patient undergoes the physical operation prescribed. No longer are patients forced to make separate visits for tests, consultant's diagnoses and treatment, each a prolonged foray into an unsettling labyrinth of departments and bleak waiting rooms.

Close co-ordination of the precise equipment, operating theatre, staff and specialist consultants needed for each patient is central to the streamlined production system. The co-ordination is down to a scheduling software package that was specifically developed for the project by IQ Systems Services.

The smooth flow of diagnosis and treatment is also helped by the fact that the hospital does not deal with emergency cases; these would inevitably disrupt the programming of the rest of its caseload. In fact, the hospital admits only patients waiting for elective surgery – procedures such as breast cancer surgery, blood transfusions and the removal of cataracts. As Silcoff points out, there is a reciprocal benefit, as the ACAD Centre takes the pressure off the emergency and trauma treatment facilities located in the main departments of the centre's parent institution, the Central Middlesex Hospital.

Other vital strands of the ACAD Centre's seamless service are multiskilled nursing staff and the eradication of boundaries between hospital departments. This latter policy has liberated the architect from designing a series of "territories" and allowed it to focus on creating a fully integrated "facility" instead. This has allowed Avanti to radically simplify the building into two clear-cut primary zones, plus two subsidiary ones.

A high-tech zone containing the four operating theatres and all the imaging equipment takes the form of a 15 m wide, three-storey strip. The top floor is devoted entirely to plant that feeds air-conditioning to the levels below. A patient zone – 10 consulting rooms, reception and recovery areas – has an expansive cruciform arrangement on two floors, with large windows facing the hospital campus and two landscaped courtyards.

The double-storey height central mall runs between, and gives access to, the high-tech and the patient zones. The mall forms part of the public zone of the building, which also contains a double-height entrance hall – likened by Silcoff to a market square – and a café serving patients, staff and visitors. A staff zone is located at the rear end of the building.

Patients' progress down the walk-through production line is linear and largely self-evident, thanks to the clarity of the layout. Patients begin by checking in at the main reception desk in the entrance hall. If they are scheduled for a test, scan or dental care, they are directed along the mall to one of five doors on the ground floor of the high-tech zone. If they are to have invasive surgery or endoscopy, they are directed up the prominent open staircase within the mall to a pre-intervention room on the first floor. Here they are met by a multiskilled nurse and led over one of the three bridges across the mall to the upper floor of the high-tech wing.

After treatment, patients are led to one of two recovery suites back in the patient zone, where they recover on beds for up to four hours or overnight. Once they have recuperated, they walk a few steps further to a pre-discharge lounge and finally to a public lounge, where they can be met by relatives before leaving through the rear entrance. The linear sequence of spaces avoids subjecting incoming patients to the unnerving experience of passing newly treated patients who may be leaving in a groggy state, or wearing bandages.

The clinical areas culminate in the recovery suite, which is suffused in daylight through double clerestory windows. In the bays, four beds are arranged around a large floor-to-ceiling bay window that offers generous views of the hospital gardens.

The building is wedge-shaped to fit into the site at the edge of the existing hospital. The patient zone faces the landscaped spaces of the hospital, whereas the high-tech zone forms a hard edge on the public street facing a future industrial development area.

Like the interiors, the building exterior is modern in style, crisply detailed and uses high-quality materials such as reconstituted stone blockwork, charcoal-grey cladding, and, next to the main entrance, a wall of travertine marble. Unfortunately, Avanti has chosen to articulate the various zones of the buildings with differing materials and treatments, ranging from the rounded ends in 1930s modern style to projecting services risers clad in 1990s cedar boarding. A less frenetic architectural composition may have been more appropriate to the hospital's seamless service.

Aside from the overactive exterior, the ACAD Centre building looks like providing a perfect fit for the seamless, one-stop hospital service provided inside. It also shows that a health service driven by administrative efficiency need not be incompatible with gracious architecture that puts patients at ease.