Listen to last night's class, "Returning From a 'Medical Mission' on Shabbat," here.
Follow along with the sources here.
For further research:
1) Read R. J. David Bleich's article, "Returning from Missions of Mercy on Shabbat," here.
2) Look at the source sheet on this topic (pg. 1-12) from the Olamot website, here.