Alison Baulos says her 73-year-old father was about to head to a Kentucky hospital for open-heart surgery when it was abruptly canceled early Friday morning. His was one of the many operations and medical treatments halted across the country because of a global technology outage.

“It does really make you just realize how much we rely on technology and how scary it is,” Baulos said from her home in Chicago.

The major internet outage disrupted flights, banks and businesses, as well as medical centers, around the world. The outage was caused by a faulty software update issued by a cybersecurity firm that affected its customers running Microsoft Windows.

The American Hospital Association said the impact varied widely: Some hospitals were not affected while others had to delay, divert or cancel care.

Baulos said her father, Gary Baulos, was told Wednesday that some routine tests showed that he had eight blockages and an aneurysm, and needed surgery. He prepped for the surgery Thursday and got a hotel near Baptist Hospital in Paducah, Kentucky. He was about to head to the hospital at about 4 a.m. Friday when he received a call that the operation had to be postponed because of the outage. Phone messages left with the spokesperson at Baptist Hospital seeking comment were not immediately returned.

At the Guthrie Clinic in Ithaca, New York, the emergency departments were open but outpatient lab tests and routine imaging appointments were canceled. All elective surgeries were postponed and clinics were operating on paper Friday morning, according to information posted on the clinic’s website.

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Sahana Singh arrived at the clinic at 9 a.m. to learn her heart test would have to be rescheduled in two weeks.

“We look at technology as helping us to be more efficient,” the 56-year-old author said. “We don’t expect just one little software update to paralyze the whole system, globally.”

The Boston-based health system Mass General Brigham said on its website that it was canceling all non-urgent visits due to the outage, but its emergency rooms remained open. The health system said it couldn’t access patient health records and schedules.

Harris Health System, which runs public hospitals and clinics in the Houston area, said early Friday it had to suspend hospital visits “until further notice.” Elective hospital procedures were canceled and rescheduled. Clinic appointments were temporarily halted but later resumed, according to a post on X.

The outage affected records systems for Providence, a health system with 51 hospitals in Alaska, California, Montana, Oregon and Washington state. Access to patient records had been restored but workstations were still down, according to a statement Friday from the Renton, Washington-based health system.

Kaleida Health Network posted messages on websites for several Buffalo, New York, hospitals that said procedures may be delayed as it dealt with the outage. But it also encouraged patients and employees to report as scheduled.

“We appreciate your patience while we work to restore full functionality,” the statement said.

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Associated Press reporters Mark Thiessen in Anchorage, Alaska, and Tom Murphy in Indianapolis contributed to this report.