Are you referring to emergency transport specifically? Usually emergency transport aircraft are covered by a nurse and a paramedic (depending on the state, the two licenses complement each other well with each one capable of doing things the other cannot), but some medical transport services use a doctor. See this one:
Air Ambulance Services | Medical Capabilities - AirMed International
I don't know about PA/NP. My sense is that these services are generally premium and therefore are less likely to go for midlevel providers, but I don't know enough about the topic to say for sure.