For at least four hours Saturday night, there was not a single Ottawa ambulance available to respond to medical emergencies in the city, nearly doubling the longest period of what paramedics call "level zero" last year.

Sources confirm Ottawa was at level zero from around 9 p.m. Saturday to 1 a.m. Sunday.

Last year, the longest Ottawa was without an available ambulance was 2.5 hours, according to a CBC analysis of ambulance response times. The average level zero alert lasted 17 minutes in 2016.

It's not clear what caused the sudden demand for ambulances this weekend.

However, in an emailed statement sent on his behalf, Ottawa Paramedic Service Chief Myles Cassidy confirmed the demand was higher than normal.

When there are no ambulances available Ottawa paramedics using smaller vehicles may still respond to some calls. If the call involves cardiac arrest, police or firefighters may respond to administer CPR.

The City of Ottawa did not release official ambulance response times for the weekend, or whether there had been level zero alerts. In his statement, Cassidy said the data first needs to be validated by the provincial government.

Rural paramedics fill gap again

The city hired an additional 12 paramedics in 2016, and another 24 more finished training earlier this month. One new ambulance was added to the fleet.

But Mike Chrétien, director of emergency service for Prescott-Russell, said Ottawa is still leaning too heavily on rural ambulances to answer calls in the capital.

Mike Chrétien, director of emergency service for Prescott-Russell, said his paramedics are still responding to more calls in Ottawa than they can handle. (CBC)

He said Prescott-Russell felt the burden this weekend when his staff were called in to respond to more calls in Ottawa than usual.

"[There were] a lot more than we are able to handle," Chrétien said.

He said the busy weekend meant there were no ambulances available in Prescott-Russell either.

"It's extremely, extremely rare that we have level zero," he said.

Chrétien said ambulances from Prescott-Russell were responding to calls as far away as Nepean, a nearly one-hour drive from their service area.

Ottawa pulls out of deal

Prescott-Russell was one of three towns and counties that recently arrived at an agreement with Ottawa that would have seen them only answer life-threatening calls in the city.

But last month Ottawa pulled out of that deal without notice. The other municipalities, Prescott-Russell, Renfrew County, and Cornwall still intend to submit the agreement to the province.

"We'll be sending it to Ottawa to ask for them to sign it," Chr​étien said.