During Frank's trip, when he's flashing back to trying to save Rose, when he's seen sitting in the ambulance, as it drives by. All the snow is shown flying upward. When the camera closes on Frank through the ambulance window, the reflections of the surroundings are moving back to front, which means that the ambulance is driving backward rather than forward, like is shown in the street shot.
At the end of the movie we see an F.D.N.Y. Ambulance in the background. The story takes place in the early 1990s. In the early '90s, N.Y.C. E.M.S. was still under The Health and Hospitals Corporation of N.Y.C. The Fire Department did not take over E.M.S. until 1995, when Mayor Rudolph Guilliani saw that E.M.S. could supplement the income of F.D.N.Y.
Just before Marcus crashes the ambulance, Frank starts to drink from a bottle of rubbing alcohol as he sits in the right front seat. The shot immediately shifts to one through the right side passenger window. The bottle is gone, and Frank is just staring forward.
When Marcus and Frank are responding to I.B. Bangin's over-dose, they are first shown responding in a van-type ambulance, then the next shot shows them in a box-type, then back to the van-type on arrival.
When Frank and Larry are trying to resuscitate Mr. Burke, the E.K.G. shows asystole (flat line), but Larry then shocks him with a defibrillator, which would do nothing if the patient were really in asystole. In mitigation, however, what looks like asystole could really be fine ventricular fibrillation, and while the Advanced Cardiac Life Support protocols do not call for defibrillation on an asystolic patient, it is sometimes used just in case.
When Mr. Burke codes after he's brought into the E.R., the doctor orders for him to be defibrillated at 400 joules, then 500. While older defibrillators could deliver a 400 joule shock, modern defibrillators only give a maximum shock of 360 joules. Either way, Advanced Cardiac Life Support protocols at the time (before 2005) called for a starting energy of 200 joules, then 300 and 360 for subsequent shocks.
While hospitals that provide ambulance service in New York City have their own supervisors and administration, they would not be "captains" as in the character of Captain Barney; E.M.S. officers would be N.Y.C.*E.M.S. (1990s as in the movie) or F.D.N.Y. (today).
When Frank and Marcus use Narcan on I.B. Bangin, Frank injects the Narcan into one of the wrist veins. Drugs are never main-lined without a patent I.V. because possible infiltration of the drug would cause tissue damage. If an I.V. can't be established, Narcan is given either intramuscular (I.M.) or intranasal (I.N.).
When Marcus is talking on the radio, he has the microphone "keyed" so that he can transmit. The female dispatcher can be heard talking back to him while he still has the button depressed. This is impossible, since you can transmit or receive, but not at the same time, on this type of system.