Image Number 6 for United States Patent #6456879.
An improved system and method for performing automatic capture/threshold detection in an implantable cardiac stimulation device or any device capable of stimulating some body organ or tissue. Prior art systems determine the cardiac tissue's stimulation threshold by detecting an evoked response to a fixed duration stimulation pulse and then increasing the stimulation pulse's amplitude by a predetermined safety margin value. Such systems are inherently based upon a belief that the chronaxie of a particular patient's strength-duration curve is essentially fixed and that the rheobase is variable. While this may be true at some times during the patient's life, e.g., during the acute phase after lead implantation absent drug effects, it is reported that drugs alone may alter the chronaxie and it is believed that other factors may also affect the chronaxie either alone or in combination with the rheobase. Accordingly, the present invention compares the present stimulation pulse to at least two pulse duration regions and, if a loss-of capture criteria is met, alters the amplitude aid/or the duration of the stimulation pulse accordingly in order to take into account a rheobase and/or chronaxie shift. The decision of whether to increase amplitude and/or duration is dependent on the current operating point. For example, in an embodiment having two pulse duration regions, the amplitude and duration of the stimulation pulse may be increased in the first duration region below the chronaxie while, in the second duration region above the chronaxie, only the amplitude may be increased.