An ordinary day for the London Ambulance Service means dozens of car crashes, overdoses, suicide attempts and - statistically - 28 cardiac arrests, where every second can make the difference between life and death.By 11am, seven cardiac arrests have come in, and now there are two more people fighting for their lives. As one crew battles through London traffic, knowing seconds and minutes can affect the outcome of a dad of two's heart attack, across the city another crew face a difficult decision - whether to stop resuscitation of their patient knowing full-well the impact it will have on the family.The brain of the ambulance service is the control room, and when a number of stabbings, suicide attempts and a double shooting flood the 999 phone lines simultaneously, they threaten to overwhelm the service. A highly-skilled Advanced Paramedic has barely finished with a double shooting before he's called to a man who has fallen from a sky scraper. In the control room they're running short of ambulances and they know the next call could be a major incident needing dozens of crews.When there's an unexpected spike in 999 calls far outstripping the number of ambulances available, drastic action has to be taken, and some emergency callers have to be told the ambulance can't come to them while the service prioritise reaching those in most urgent need who maybe fighting for their lives. Multiple calls about an explosion in a flat come in, heralding their worst fear, and the whole of the London Ambulance Service now has to step up.