A massive walkout of University of California patient care workers that began Tuesday could cost the system's medical centers up to $20 million, officials say.

The walkout is expected to last two days and involve more than 12,000 UC workers from the American Federation of State, County and Municipal Employees.

The strike, which began at 4 a.m. Tuesday, involves respiratory therapists, nursing aides, surgical technicians and other patient care workers. It is scheduled to run until 4 a.m. Thursday at five medical centers -- UCLA, UC Irvine, UC Davis, UC San Diego and UC San Francisco.

An additional 3,400 workers from the University Professional and Technical Employees union plan a one-day sympathy strike.

UC officials are preparing for the walkout by canceling elective surgeries, chemotherapy treatments and other medical procedures.

Ronald Reagan UCLA Medical Center has hired about 400 temporary workers to cover the shifts of those on the picket lines, said Tom Rosenthal, chief medical officer.

After the UC system sought to limit the number of participants, a Sacramento County Superior Court judge ruled Monday that about 450 employees cannot take part in the walkout.

The judge issued an order that the unions must maintain a minimum level of staffing among certain units, including the burn centers, the intensive-care units and the neonatal intensive-care units.

If all the respiratory therapists in the burn centers and poison-control units were to strike, the court ruled, there would be a "substantial and imminent threat to public health or safety."

UC officials said it already is affecting patient care. At UC San Francisco, for example, officials had to postpone five surgeries on children with complex heart conditions and 12 pediatric chemotherapy appointments. UC San Diego Medical Center has had to delay more than 120 surgeries, officials said.

Those participating in the strike work in  pharmacies, cancer centers, operating rooms and laboratories.

"Leaders of both unions claim their chief concern is patient care, but it's very simple: If they strike, services to patients suffer," Dwaine Duckett, vice president for systemwide human resources at the University of California, said in a statement.

Todd Stenhouse, spokesman for the AFSCME union, said UC is putting patients at risk every day by not having enough staff.

"This is not an effort to hurt UC or patients," he said. "It is an effort to save them."

The two sides have been negotiating for nearly a year over a new labor contract. The union says that the UC medical centers have unsafe staffing and that officials are more concerned about executive salaries than about frontline care providers.

UC officials have defended their safety record and said the union is resisting the pension changes that more than a dozen other bargaining units have agreed to.

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anna.gorman@latimes.com