The extended shutdown of a Canadian nuclear reactor is being felt in Ventura County, where some healthcare providers are dealing with a shortage of medical isotopes used for some medical tests.

At Community Memorial Hospital, some patients must wait a little longer to get cardiac studies using the radioactive isotopes, said Dr. John Seder, a nuclear medicine specialist at the hospital in Ventura.

"We've had some shortages, but it's not insurmountable," Seder said. "It's been more of a nuisance than anything."

The reactor is North America's biggest source of the radioactive isotope that makes technetium-99, used in myocardial perfusion studies to examine the blood flow to the heart and look for artery blockages, said Seder, who also is a member of the Society of Nuclear Medicine.

The shortage arose after Atomic Energy of Canada in mid-November shut down its National Research Universal reactor in Chalk River, Ontario, for routine maintenance. The company has since extended the shutdown for several weeks to install additional equipment, leaving medical suppliers short of the necessary isotopes.

Technetium-99 has a very short, six-hour half-life, so it cannot be stockpiled for later use, Seder said. The shortage of the isotope has meant fewer deliveries from Community Memorial's supplier, and sometimes an overnight delay for patients who need the cardiac testing, Seder said. The substance is also used for some lung studies, which can be conducted using other types of tests, he said.

The changes are not affecting the quality of patient care, he said. "If we can't do it that day, we do it the next morning," Seder said.

Ventura County Medical Center uses a different isotope, thallium, for its cardiac testing, and has not been affected by the shortage, said Dr. Robert Gonzalez, medical director for the Ventura County Health Care Agency.

Los Robles Hospital & Medical Center has had to change its ordering practices — placing orders each day rather than relying on a standing bulk order — but otherwise has not been affected, said spokeswoman Kris Carraway-Bowman.

Simi Valley Hospital and St. John's Regional Medical Center in Oxnard reported no problems because of the shortage so far. Not all of the hospitals use the same supplier, which could account for the range of effects.

The reactor's closure is now expected to continue into January. In a news release issued earlier this week, Atomic Energy officials said they "continue to work around the clock" in an effort to return the reactor to service as quickly as possible.

— The Associated Press contributed to this report.