The Nishmat Avraham discusses two major issues in the course of his commentary on the section of Shulchan Aruch dealing with when we may violate Shabbat to save a life. The first deals with whether one is permitted or required to endanger one’s own life to save another life. This is relevant to someone who wants to be a living kidney donor, since there is some risk to the donor in the process.The second issue is related to the definition of death. The Shulchan Aruch rules that if you see someone is not breathing, you can no longer violate Shabbat to save their life. Many have taken this as an indication that halacha defines life and death by the presence or lack of breathing. The Nishmat Avraham quotes a personal conversation with Rabbi Shlomo Zalman Auerbach that this halacha is not actually relevant to the definition of death. In those days, if someone was not breathing, they had no way to save their life, or even extend their life for a little bit, so one would not be allowed to violate Shabbat to save them. Nowadays, when we have the capability to resuscitate a person who has stopped breathing, certainly the halacha would permit violating the Shabbat to save them.