Lower birth rates, increased competition from regional hospitals, and the departure of a pediatric cardiac surgeon have contributed to a drop in the number of heart surgeries performed during the past year at the Children’s Heart Center at Holtz Children’s Hospital, threatening the program’s status as an approved healthcare provider for the most severely ill children in the state and the hospital’s reputation as one of the most prestigious speciality medical institutions in South Florida.

In April, administrators for Holtz’s pediatric cardiovascular program, which is staffed by University of Miami Miller School of Medicine physicians, voluntarily accepted probation from the Florida Department of Health’s Division of Children’s Medical Services, which coordinates healthcare for children with intense medical needs, including congenital heart disease and arrhythmia.

Probation will not affect day-to-day operations at the Children’s Heart Center, including physicians’ ability to treat patients, said Dr. William Harmon, a UM physician and the center’s chief of pediatric cardiology.

But the designation indicates that state regulators are concerned about the declining volume of heart surgeries, and unless the number increases to required levels over the next two years the center could be disapproved as a CMS facility.

Like each of the eight approved CMS cardiac facilities in Florida — including Miami Children’s Hospital, and Joe DiMaggio Children’s Hospital in Hollywood — Holtz’s pediatric cardiovascular program is required to perform at least 150 heart surgeries each year, according to state standards, in order to ensure that surgeons remain proficient.

But last year, the Children’s Heart Center fell “just shy” of the requirement, Harmon said.

Physicians must have performed at least 100 heart surgeries in the past year, though, or else state regulators would have been required under CMS standards to provide a notice of intent to disapprove the facility.

Instead, state regulators placed the center on probation, which means the annual number of surgeries fell below the required 150 but above 100. Probation may be extended one more year if the center shows a “positive trend” in meeting the required volume, according to state standards. Failure to meet the standard at the end of a second year of probation, though, would lead to formal disapproval, a process that requires a notice of intent from the state and a 90-day transition period.

Harmon, who noted that UM physicians helped write the CMS standards for pediatric cardiovascular programs, said the state standards help justify the expense of such programs.

“You want to make sure there’s an expertise in the institution, and you want to make sure that there’s an adequate volume to support that expertise,’’ he said. “It would be inappropriate to have a program that did 15 cases a year. That would be inordinately expensive. If you have extreme changes [in surgical volumes], you could have quality concerns.’’

Steven Burghart, chief administrative officer for Holtz, said there are many reasons for the drop in children’s heart surgeries at Holtz. But he singled out lower birth rates as “the most important driver.”