118 is the number for all emergency services (police, firemen, ambulance). The chances are that the operators only speak Spanish, and the ambulance service is notorious for not being available when you need it.
You can also call 101 to talk to someone in English if you have a Claro mobile phone.
The lack of a reliable ambulance service means that in the event of an emergency you should either call a taxi or contact a friend that has a car. We’ve said elsewhere that most expats don’t have cars in Nicaragua, so your best bet is probably to have a reliable taxi service on speed dial.
If an ambulance does respond, individuals are taken to the nearest hospital - usually a public hospital that accepts everyone. If you request a private hospital, you will be taken to one. Payment is usually on a cash basis, however private hospitals will accept most credit cards as well.