An implantable cardiac stimulator (10) for detecting capture or adjusting the strength or duration of pacing pulses by using an evoked response detector (41) and periodically tuning (334) the evoked response detector. When the electric evoked response detector is to be tuned, capture is verified by detecting the mechanical evoked response, as the magnitude of the stimulating pulse is adjusted to isolate the threshold as detected by the mechanical response detector (42). At the same time, the electrical evoked response is also monitored. The difference between the detected electrical signal following capture as detected by the mechanical response detector and the signal following non-capture is used to tune the electrical evoked response detection apparatus and algorithm. The energy of the pacing pulse can then be optimized (232) by adjusting both strength and duration. A very small safety margin is needed since capture is continuously monitored by the electrical evoked response apparatus and a safety pulse is immediately applied if capture is lost. The safety pulse can be applied in the same cardiac cycle so that the heart beat remains controlled.