You may want to try contacting SamaritansPurse.org
Do a GOOGLE search for mission hospitla, el salvador or missions project, church, and you will find churches around the country who raise money and send missionaries including doctors to help the poor.
Here is an example of a website I found for a church in NC that helps a medical clinic in El Salvador. You could call the church and ask if they could put you in contact with someone there: Â*Salvadoran Mission Projects - Home
Also, do a search for medical volunteer, El Salvador and there are a lot of leads that you can follow from those websites.
The United Methodist Missions is well respected for world missions projects. You might want to contact them as well.