The distance the electromagnetic field of the heart can be detected
The electomagnetic field of the heart actually changes with emotions so there is no fixed range as such (reference). It could also change with temperature and other external magnetic disturbances (reference). The heart generates the largest electromagnetic field in the body which is up to 50 mV/m, and that of the brain and other vital organs up to 5 mV/m. Here is a paper on the exposure of human beings to electromagnetic waves (reference). This paper details some of the current medical treatments focusing on the electromagnetic fields (reference).