Many implantable pacing devices rely on pacing schemes that sense atrial and/or ventricular activity. For example, a dual chamber pacing device may deliver an atrial stimulus, sense for an atrial response, and then deliver a ventricular stimulus and sense for a ventricular response. In pacing, an “evoked response” results when a delivered stimulus sufficiently “captures” or electrically activates cardiac tissue. Capture typically involves depolarization followed by contraction of one or more chambers. Of course, the ability of cardiac tissue to respond to a stimulus depends on the tissue's activity state. For instance, cardiac tissue in a refractory state typically will not respond to a stimulus. In addition, stimulus power (e.g., magnitude, duration, polarity, etc.) may determine whether a stimulus results in capture. For example, a stimulus delivered at a first power level may not depolarization sufficient tissue to lead to capture; whereas, a slightly higher power level may depolarize sufficient tissue to lead to capture. The power level at which capture occurs (e.g., in nonrefractory tissue), is typically referred to as the capture, pacing, or stimulation threshold. Various pacing schemes, generally referred to as “autocapture” schemes, aim to discern atrial and/or ventricular stimulation thresholds. Pacing at and/or near the stimulation threshold can conserve energy and, hence, increase device and/or power supply longevity.
Thus, for a variety of reasons, an implantable pacing device benefits from an ability to discern capture from noncapture. In addition, an implantable pacing device can benefit from an ability to discern native cardiac activity and/or fusion and/or pseudofusion during pacing. Fusion and/or pseudofusion may occur when native activity exists during pacing. Fusion is typically characterized by depolarization of the myocardium initiated by both a non-native stimulus and a native stimulus. Pseudofusion is typically characterized by depolarization of the myocardium initiated by a native stimulus; however, a non-native stimulus, that does not significantly contribute to depolarization, is present that distorts a sensed depolatization/repolarization wave complex. Because fusion and pseudofusion can distort sensed activity, the presence of fusion and/or pseudofusion can interfere with diagnosis of capture and/or noncapture.
Another issue typically encountered in detection of capture and/or noncapture (and/or fusion, pseudofusion, etc.) involves post-stimulus polarization of one or more sensing electrodes, which is sometimes referred to herein as “afterpotential”. Post-stimulus electrode polarization stems primarily from capacitive charging of an electrode-electrolyte interface during delivery of a pacing stimulus. Upon termination of the pacing stimulus, the post-stimulus electrode polarization decays over time, generally in an exponential fashion like a capacitor. Characteristics of post-stimulus electrode polarization generally depend on a variety of parameters, such as, electrode materials, electrode geometry, tissue characteristics, tissue contact, stimulation energy, and others, many of which vary over time. Because post-stimulus polarization may severely interfere with response sensing, detection and/or characterization, many implantable pacing devices use low polarization electrodes and/or leads, blanking periods, and/or special circuitry to minimize artifacts arising from post-stimulus electrode polarization.
Various exemplary methods and/or devices described herein enhance detection and/or characterization of cardiac activity. In particular, such methods and/or devices address signal-to-noise, electrode polarization and/or other issues related to detection and/or characterization of cardiac activity.