For the most part your premise is flawed from the get go. Not trying to be a jerk and I'll explain. First, while I'm not saying it can't happen I'm yet to see pepper spray directly cause acute respiratory distress. In all of the counties I've worked in when mace is used EMS is called right away as a precaution. Usually we get there, take a quick blood pressure and pulse, wash the subject's eyes with some water and if there is no major medical history or acute emergency we go on our way and the subject goes to jail. If such an asthma attack were to be brought on by mace, by the time it worsened EMS would probably be on scene already. These precautions are in place to avoid the exact scenario you're writing. A more accurate portrayal would be the subject having an asthma attack while providers are already on scene, a quick nebulizer treatment (think of it as a fancy inhaler) and they go on their merry way, not very exciting. For the sake of suspense you could have the police and paramedics ignore the subject long enough (they could brush him off as faking, I don't know, you're the writer) and by the time they realize he's legitimately ill the subject is in full respiratory arrest (not breathing) and real heroics are needed. These measures would probably include attempting to place an endotracheal tube (just search "intubation" on youtube for a better idea of what this means) and intramuscular injections of Epinephrine (Adrenaline) and IV injection of steroids. For the most part, mace would not complicate the treatment, while it might burn like hell it's pretty benign. If your character has an underlying medical condition you could write it as the character's condition worsening from the acute stress of the interaction with the police or as random happenstance.