Patent ID: 7027868

Claim:
An implantable cardiac stimulation device, comprising: an atrial sensing circuit; an atrial pacing circuit; and a control circuit comprising: means for determining a time interval between sensed atrial depolarizations of a first cardiac cycle; means for establishing a loss of capture window during a second cardiac cycle, the loss of capture window including a negative sensing interval within a reference A-A time interval corresponding to the time interval determined for the first cardiac cycle, wherein the negative sensing interval ends coincident with the end of the reference A-A time interval, and a positive sensing interval of approximately the same duration as the negative sensing interval, wherein the positive sensing interval begins coincident with the end of the reference A-A time interval; means for causing the atrial pacing circuit to deliver an atrial test pulse of a predetermined energy during the second cardiac cycle, the test pulse being delivered within the reference A-A time interval and prior to the negative sensing interval; means for determining whether a next sensed atrial depolarization that immediately follows the atrial test pulse occurs within the loss of capture window; and means for determining whether the atrial test pulse energy is above or below a capture threshold based upon a determination as to whether the next sensed atrial depolarization occurred within the loss of capture window.