Cardiac rhythm management devices are devices that either pace or otherwise excite selected chambers of the heart by electrical stimulation. A pacemaker, for example, is a cardiac rhythm management device that enforces a particular heart rate by delivering pacing pulses to the heart in response to lapsed time intervals and sensed cardiac electrical activity (i.e., intrinsic heart beats). Cardiac rhythm management devices, either in addition to or instead of pacing the heart, may also deliver stimulation pulses in order to excite a particular chamber or region of a chamber in synchrony with sensed intrinsic cardiac activity occurring elsewhere. Such devices are typically implanted subcutaneously on a patient's chest and have leads threaded intravenously into the heart to connect the device to the electrodes used for sensing and stimulation.
In order to cause a contraction in the absence of intrinsic activity (referred to herein as excitation), the stimulus pulses delivered to the heart by the device must achieve “capture,” which refers to causing sufficient depolarization of the myocardium that a propagating wave of excitation and contraction result (i.e., a heart beat). A stimulus pulse that does not capture the heart is thus an ineffective pulse. This not only wastes energy from the battery of the device, but can have deleterious physiological effects as well. For example, a pacemaker that is not achieving capture is not performing its function in enforcing a minimum heart rate. For a particular implanted lead, a capture threshold exists which is the lowest energy pulse that will achieve capture when a stimulus pulse is output through the lead. (The energy of a stimulus pulse is determined by the voltage and duration of the pulse.) A common technique used to determine if capture is present during a given cardiac cycle is to look for an “evoked response” immediately following a stimulus pulse. The evoked response is the wave of depolarization that results from the stimulus pulse and evidences that the stimulated chamber has responded appropriately and contracted. In the case of ventricular stimulation, by detecting the evoked R-wave, the device is able to detect whether the stimulus pulse was effective in capturing the heart, that is, causing a ventricular contraction. Capture verification can be performed by a clinician using an external programmer to adjust operating parameters and monitor sensing data to ensure that the heart is reliably paced and/or otherwise excited. Alternatively, a cardiac rhythm management device may be programmed to perform capture verification tests automatically, either at periodic intervals or upon a command from an external programmer.
Stimulation/sensing leads are generally of two types, bipolar and unipolar. A unipolar lead has one electrode at its distal end connected to a conductor within the lead which is covered by insulation. A bipolar lead has two conductors which are respectively connected to a distal and proximal electrode. The distal electrode is usually a tip electrode which may be fixed into the myocardium, and the proximal electrode is a ring electrode around the body of the lead. In bipolar stimulation, a stimulus pulse is output between two electrodes, and in bipolar sensing, the voltage difference between the two electrodes is sensed. A bipolar stimulus pulse may be output with a polarity such that one electrode is the cathode (electrically negative) and the other electrode is the anode (electrically positive). In unipolar sensing/stimulation, the electrode at the end of the lead is used as either a cathode or anode, and the housing of the device or an electrode on another lead is used as an electrode of the opposite polarity, referred to as a reference. Cathodal stimulation of the myocardium is generally preferred because it has a lower and more stable capture threshold. Cardiac rhythm management devices with bipolar leads may typically be programmed to deliver either bipolar or unipolar stimulus pulses through a lead. Bipolar stimulation is preferred by some clinicians, however, because it more precisely delivers energy to the heart with less risk of causing contractions of overlying skin or skeletal muscle.
Once a cardiac rhythm management device has been implanted, a clinician typically sets up the configuration of the device using an external programmer. Among the parameters that can be adjusted during this process, is the stimulus pulse energy for a given lead, which is normally set to a value corresponding to the capture threshold determined for that lead. For example, the stimulus pulse energy may be set to the value of the capture threshold plus some specified safety margin. The present invention is directed toward an improved method and apparatus for adjusting the stimulation parameters of a cardiac rhythm management device employing bipolar stimulation.