The standard of emergency medicine and ambulance services has rapidly improved over the last few years - at least in Lima, cities in the provinces unfortunately lag behind. Most private ambulance services have modern vehicles, are prepared for many emergencies and equipped to international standards including ventilators and defibrillators.
Private ambulance services usually offer different plans including services for all sorts of emergencies and medical care, but also phone /online or at home consultations, a patient transfer service and more. However, if you aren't a member of any private ambulance service or haven't signed up to one of their plans, you can still call them and you are charged for their service or visit.
Be aware that next to below recommended private ambulance services, numerous private hospitals and health insurance companies operate their own ambulance or emergency vehicles, but exclusively for those insured with them.
If you are looking for a more economical option, check with the municipality you are staying or living in. Some as well have their own ambulance vehicles which at least in the better districts in Peruvian cities are well equipped and often free of charge. And last but not least, an excellent service with very well equipped units - at least in Lima - provides the voluntary fire brigade (Bomberos Voluntarios del Peru). In case of an accident or any other medical emergency, they respond quickly and provide good first aid. Their service is free of charge, however a donation is appreciated and always welcome.