The response time for Boston’s emergency medical personnel to arrive at the scene of heart attacks, shootings, and other serious incidents has slowed in recent years, largely because of a little-understood surge in calls for help, city officials say.
The median response time for the most serious, or Priority 1, cases has trended upward since 2009, when it was 5.5 minutes. During 2014, it climbed to 6.4 minutes, above the agency’s own goal of 6 minutes, according to data provided to the Globe by Boston Emergency Medical Services.
This year, through the end of October, the median response time was 6.8 minutes for Priority 1 cases.
“Obviously, I worry that the times have gone up,” Boston EMS chief James Hooley said. “We really have to pay close attention to it.”
However, he said, “We haven’t seen that there has been any negative impact on our patients and their outcomes, and that’s the ultimate thing we have to look for.”
The slower response times in Boston are similar to those in other cities, including San Jose, Calif., and Washington.
Boston EMS said it has seen a 26 percent rise in calls over the past decade, from about 95,500 in 2005 to about 120,000 in 2014. The agency said the increase in calls has driven the rise in response times.
“The call volume is way up. It’s a very, very busy system,” said James Orsino, union president of the EMS division of the Boston Police Patrolmen’s Association.
Priority 1 calls include heart attacks, uncontrollable arterial bleeding, and unconsciousness. Similar increases in response times have been seen in recent years for less-serious Priority 2 and Priority 3 calls.
Boston EMS officials said they have taken a number of steps in recent years to speed up response times.
The agency, which has 350 full-time staff, expanded its uniformed personnel by about 14 percent and added ambulances between 2008 and 2010. It also has staff work overtime hours when needed.
The agency opened a new EMS station, the 16th in the city, in downtown Boston several years ago. The agency is also working with city officials to find a home for a new EMS station in the burgeoning Innovation District.
And the agency just opened a new, larger EMS garage in Mattapan that it expects will improve response times in the surrounding neighborhood.
Agency officials also noted that they must balance speed against the risks that come with having ambulances career through busy streets.
While emphasizing the steps they have taken to address response-time issues, EMS officials also point to research in recent years suggesting that faster response times aren’t necessarily linked to better outcomes for patients.
The research found that only certain types of emergencies, such as heart attacks, serious blood loss, and airway obstructions, are time-sensitive.
Hooley acknowledged that patients and their families want a speedy response. “To the person who is calling, from their perspective, yes, it is important how quick you get there,” he said.
Hooley said Boston EMS continues to closely monitors response times. But it also measures its performance by other metrics, including the track record of EMTs to quickly and accurately recognize signs of certain medical conditions, including heart attacks, strokes, and drug overdoses.
“A lot of the focus is on what we do once we get there,” he said.
He said the agency seeks to provide “better patient care to make sure taxpayers are getting more for their dollar out of us, and that patients are getting better outcomes.”
Mayor Martin J. Walsh expressed confidence in Boston EMS, saying it does a “tremendous job.”
“While we will continue to monitor response rates closely to ensure that we are best serving everyone who needs these services, I know that EMS is meeting the health care needs in our city skillfully and compassionately,” Walsh said in a statement.
Some of the increases in other cities have also been attributed, at least in part, to an increase in call volume.
“From talking to some other providers around the country, I think most systems are seeing an increased demand for EMS services,” said Hooley, adding that he had heard of other call volume spikes in New York, Philadelphia, and Denver.
What exactly is driving the increase in the number of calls? The reason is not fully understood.
The Globe spoke to more than two dozen experts, all of whom said they did not know of any definitive research that had pinned down why the calls had risen.
“From everyone I’ve talked to, it’s a bit of a mystery, and that’s definitely concerning to us,” said James L. Robinson, president of the International Association of EMS Chiefs.
The experts suggested that a number of potential factors could be involved, probably in combination with one another.
They almost unanimously agreed that growth in the overall population, and increases in how long people live, account for some of the increased demand.
Many experts also said shrinking access to basic medical services could be behind increased demand for EMS. Some patients find themselves suddenly in need of immediate care because — sometimes related to factors beyond their control — they have not made more routine doctor’s visits.
Some suggested that with more people now covered because of health insurance reform several years ago in Massachusetts and more recently under the Affordable Care Act, there might be fewer people who are scared off by the cost of an ambulance ride.
Other possible causes of rising ambulance use cited by the experts included shifting cultural attitudes. The experts said people might be calling 911 because they’re being extra cautious, because they want to skip emergency department waiting rooms, because it’s easier with the rise of cellphones, or because of the increasingly on-demand culture.
Experts said they did not believe the recent opioid crisis is a significant factor.
They said causes of call volume increases may vary from one location to another.
Dr. Howard Mell, a spokesman for the American College of Emergency Physicians, said he believes larger EMS departments in cities such as Boston are best equipped to make changes necessary to keep up with call volume and to keep response times down.
“They’ll adapt,” he said. “Boston is a fantastic system. . . . They’re extremely well-respected with some top-flight folks.”Matt Rocheleau can be reached at firstname.lastname@example.org. Follow him on Twitter @mrochele.