TUESDAY afternoon would've been a bad time to suffer a heart attack or other medical emergency in Philadelphia.

The city didn't have any ambulances available. Zero.

Peter Crespo, the Fire Department's executive chief, confirmed yesterday that about 12:30 p.m. Tuesday, the city's ambulance corps was completely tapped out for about 11 minutes.

"It was just with the temperature, and a large influx of calls that came in," Crespo said. "For some reason, around that time frame there was just a large spike."

About five hours later, an apparent leak from a 100-pound propane tank caused the La Parrillada Chapina food truck to erupt in an enormous fireball in Feltonville, sending 13 people to the hospital. Five of them remained in critical condition yesterday.

Fire communications radio traffic reported shortly after the explosion that they were out of available ambulances, Crespo said.

The problem reared its head again about 9:30 p.m., said Tim McShea, the vice president of the International Association of Fire Fighters Local 22.

In any other big city, a shortage of medic units - however brief - might qualify as unsettling news.

But in Philly, it's just fodder for a never-ending battle between fire-union officials and the Nutter administration.

Both sides have bitterly disagreed for years over bread-and-butter issues like staffing numbers and whether it takes the Fire Department too long to respond to emergency calls.

"Look, we run out of medics almost every day, especially during the summer on a hot day," McShea said.

"Our response times have been up for years," he added. "We just don't have enough medic units to cover the city."

Crespo said that the city has about 50 medic units available on a daily basis, but that he would welcome more.

He said the system had recovered by the time the food truck exploded on Wyoming Avenue near 3rd Street on Tuesday, and that there was no shortage then. But five ambulances were dispatched immediately in response.

"Something of that magnitude is obviously going to shock the system," Crespo said.

The Inquirer reported that the shortage of ambulances caused delays in treating some of the less seriously injured, according to a Fire Department source. Crespo disputed that yesterday.

McShea said the city is gambling - dangerously - that the medic-unit shortages won't end in a tragedy.

"If you're lucky enough to call 9-1-1 in an area where a medic is available, you could be looking at a 5- to 7-minute wait," he said.

"But most of the time they're scattered around the city, and you could be waiting upwards of a half-hour to 45 minutes."

The Daily News previously reported that 84 percent of the nearly 300,000 emergencies that the Fire Department responded to in 2012 were medical emergencies. The department has 1,912 firefighters, but only 248 medics.

In March, the Nutter administration sought approval from the Civil Service Commission to begin sending one paramedic and one EMT out on advanced life-support calls, based on recommendations from outside advisers.

The city had been sending two paramedics, based on national standards.

The administration argued that the change would lead to more ambulances being available; the union countered that the move was about hiring more EMTs, who make less money.

City Controller Alan Butkovitz criticized the Fire Department in reports in 2007 and 2011 for taking too long to respond to emergency calls.

McShea said the Nutter administration "just disregards" these concerns.

"They figure it'll be news today, but it'll be forgotten in another week," he said. "I'll tell you this: We'll definitely run out of medic units on the Fourth of July."

Nutter spokesman Mark McDonald declined to comment yesterday.