Devices and methods to heat and cool human beings, including inducing and maintaining hypothermia in human patients. Methods include inducing hypothermia to treat ischemic events, including heart attack and stroke, to limit damage caused by the ischemic event. Methods can include: using the lungs for heat exchange; using cooled gases for ventilation; using helium in the ventilation gas mixture, using medications to control reflex heat production; and injecting a perfluorocarbon mist into the gas stream to increase the cooling rate. The high thermal conductivity and diffusivity of helium results in greater inspired gas temperature equalization toward body temperature. Due to the latent heat of vaporization, addition of even small quantity of phase-change perfluorocarbon dramatically increases the heat carrying capacity of the respiratory gases. Hypothermia may be terminated by discontinuing the medications and warming the patient using a warmed helium-oxygen mixture.