An implantable medical device for stimulating a human/animal heart, comprising a housing, a processor, a memory unit, a stimulation unit configured to stimulate the His bundle, and a detection unit configured to detect an electrical signal at the His bundle. The device performs: a) stimulating the His bundle with a stimulation pulse delivered by the stimulation unit; b) measuring an electric signal at the His bundle with the detection unit upon termination of a first period of time starting upon delivering of the stimulation pulse, wherein the first period of time is from 35 ms to 500 ms; c) measuring an impedance of the same heart with the detection unit upon termination of a second period of time starting upon delivering of the stimulation pulse, wherein the second period of time is equal to or longer than the first period of time and is from 50 ms to 500 ms.