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ADAPTIVE SENSING THRESHOLD FOR
CROSS-CHAMBER REFRACTORY PERIOD
FIELD OF THE INVENTION
 This disclosure pertains to cardiac pacemakers and methods for their operation. In particular, the invention relates to sensing refractory periods and sensing thresholds.
 Implantable cardiac pacemakers are a class of cardiac rhythm management devices that provide electrical stimulation in the form of pacing pulses to selected chambers of the heart. (As the term is used herein, a pacemaker is any cardiac rhythm management device with a pacing functionality regardless of any additional functions it may perform such as cardioversion/defibrillation.) Most pacemakers are used in the treatment of bradycardia by enforcing a minimum heart rate and/or restoring atrio-ventricular conduction in order to make up for a heart's inability to pace itself at an appropriate rhythm. Also included within the concept of cardiac rhythm is the manner and degree to which the heart chambers contract during a cardiac cycle to result in the efficient pumping of blood. Pacemakers have been developed which provide electrical pacing stimulation to one or both of the atria and/or ventricles at single or multiple sites during a cardiac cycle in an attempt to improve the coordination of atrial and/or ventricular contractions, termed cardiac resynchronization therapy. Such multiple paces are usually delivered to a heart chamber during a cardiac cycle with a pacing mode similar to that used for bradycardia pacing.
 In pacemakers with sensing channels for sensing one or more heart chambers, the ventricular and/or atrial sensing channels are rendered refractory following certain events, such that certain sensed signals are ignored for the duration of the period. Sensing channels are rendered refractory in order to prevent the misinterpretation of electrogram signals resulting from a pace or a previous depolarization in the heart chamber sensed by the channel. A sensing channel may also be rendered refractory for a period of time in response to a pace or depolarization in a heart chamber different from the chamber sensed by the channel. Such refractory periods are called cross-chamber refractory periods.
SUMMARY OF THE INVENTION
 In accordance with the present invention, a crosschamber refractory period in a sensing channel for a first heart chamber which is initiated in response to a pace or sense in a second heart chamber is implemented by raising the sensing threshold of the channel above its steady-state value. The sensing threshold is raised during the crosschamber post-event refractory period to a specified maximum value, referred to as the cross-chamber maximum refractory value, which is derived from the measured peak amplitude of the far-field signal received by the sensing channel when a pace or sense occurs in the second heart chamber. Separate post-pace and post-sense cross-chamber maximum refractory values may be derived from one or more previously measured peak amplitudes of the far-field electrogram signal. The maximum refractory values in each case are derived such that the sensing channel ignores the
far-field signal from the pace or sense but still detects an actual chamber sense from an electrogram signal of higher amplitude than the far-field signal.
BRIEF DESCRIPTION OF THE DRAWINGS
 FIG. 1 is a block diagram of a multi-site pacemaker.
 FIGS. 2A and 2B illustrate the steps of an exemplary routine for implementing a cross-chamber refractory period with an adaptively adjusted sensing threshold.
 FIGS. 3A and 3B are diagrams illustrating the operation of cross-chamber refractory periods with adaptively adjusted sensing thresholds.
 The present invention may be incorporated into pacemakers having a number of different pacing configurations, including multi-site pacing configurations for delivering various types of resynchronization therapy where a pace is delivered to each of the paired atria and/or ventricles during a cardiac cycle or where multiple paces are delivered to a single chamber. For illustrative purposes, however, the invention will be described with reference to a dual-chamber pacemaker (i.e., one that senses and/or paces both the atria and ventricles) having two ventricular pacing channels for pacing both ventricles or delivering two paces to a single ventricle as shown in FIG. 1.
 a. Hardware Platform
 Pacemakers are typically implanted subcutaneously on a patient's chest and have leads threaded intravenously into the heart to connect the device to electrodes used for sensing and pacing. A programmable electronic controller causes the pacing pulses to be output in response to lapsed time intervals and sensed electrical activity (i.e., intrinsic heart beats not as a result of a pacing pulse). Pacemakers sense intrinsic cardiac electrical activity by means of internal electrodes disposed near the chamber to be sensed. A depolarization wave associated with an intrinsic contraction of the atria or ventricles that is detected by the pacemaker is referred to as an atrial sense or ventricular sense, respectively. In order to cause such a contraction in the absence of an intrinsic beat, a pacing pulse (either an atrial pace or a ventricular pace) is delivered to the chamber.
 The controller of the pacemaker is made up of a microprocessor 10 communicating with a memory 12, where the memory 12 may comprise a ROM (read-only memory) for program storage and a RAM (random-access memory) for data storage. A microprocessor-type controller 10 controls the overall operation of the device in accordance with programmed instructions stored in memory. The controller could be implemented by other types of logic circuitry (e.g., discrete components or programmable logic arrays) using a state machine type of design, but a microprocessor-based system is preferable. As used herein, the term "circuitry" should be taken to refer to either discrete logic circuitry or to the programming of a microprocessor. A telemetry interface 80 is provided for communicating with an external programmer 300. The external programmer is a computerized device that can interrogate the pacemaker and receive stored data as well as adjust the operating parameters of the pacemaker.
 The controller is capable of operating the pacemaker in a number of programmed modes where a programmed mode defines how pacing pulses are output in response to sensed events and expiration of time intervals. The controller causes the pacing pulses to be output in response to lapsed time intervals and sensed electrical activity (i.e., intrinsic heart beats) in accordance with a programmed mode. Most pacemakers are programmed to operate in an inhibited demand mode (a.k.a., synchronous mode), where a pacing pulse is delivered to a heart chamber during a cardiac cycle only when no intrinsic beat by the chamber is detected. An escape interval is defined for each paced chamber, which is the minimum time interval in which a beat must be detected before a pace will be delivered. The ventricular escape interval, for example, is reset by each ventricular sense or pace and thus defines the minimum rate at which the pacemaker will allow the ventricles to beat, sometimes referred to as the lower rate limit. A similar escape interval can be defined for pacing the atria. An escape interval can also be defined for pacing the ventricles in an atrial tracking or AV sequential mode, where the escape interval, referred to as the atrio-ventricular interval, is triggered by an atrial sense or pace and stopped by a ventricular sense.
 The device is equipped with multiple sensing amplifiers and pulse generators which can be configured as channels for pacing and/or sensing selected heart chambers. A MOS switching network 70 controlled by the microprocessor is used to configure a sensing or pacing channel by switching selected electrodes to the input of a sense amplifier or to the output of a pulse generator. The switching network 70 also allows the device to employ either bipolar sensing/pacing using two closely spaced electrodes of a lead or unipolar sensing/pacing using one of the electrodes of a lead and the can 60 as a reference electrode. The device shown in FIG. 1 is configured with an atrial channel for sensing or pacing an atrial site which comprises a bipolar lead with a ring electrode 33a and a tip electrode 33fc, sense amplifier 31, pulse generator 32, and an atrial channel interface 30 which communicates bidirectionally with a port of microprocessor 10. The device also has two ventricular channels for sensing or pacing ventricular sites which similarly include bipolar leads with ring electrodes 43a and 53a and tip electrodes 43b and 53fc, sense amplifiers 41 and 51, pulse generators 42 and 52, and ventricular channel interfaces 40 and 50. The channel interfaces may include comparators for comparing received electrogram signals to reference values, analog-to-digital converters for digitizing sensing signal inputs from the sensing amplifiers, registers that can be written to for adjusting the gain and threshold values of the sensing amplifiers, and registers for controlling the output of pacing pulses and/or adjusting the pacing pulse energy by changing the pulse amplitude or pulse width.
 The controller uses the sensing channels in order to detect intrinsic cardiac activity in a heart chamber, referred to as a chamber sense (e.g., an atrial sense or a ventricular sense). Such detected chamber senses are then used for controlling the delivery of paces in accordance with a programmed pacing mode and/or for diagnostic purposes. In an inhibited demand pacing mode, for example, chamber senses are used to inhibit pacing, while in a triggered pacing mode chamber senses are used to trigger pacing pulses. By counting the number of senses over a defined time period, the controller is also able to measure heart rate and detect
arrhythmias using a rate-based criterion. As described above, a sensing channel includes sense amplifier circuits for amplifying and filtering the electrogram signals picked up by an electrode disposed at a cardiac site. In order to detect intrinsic cardiac activity, the signals emanating from the sense amplifier are compared with a reference potential. Only when an electrogram signal from the sense amplifier exceeds a reference potential threshold will it be treated as a chamber sense. The reference potential may thus be referred to as a sensing threshold. The sensing threshold may be implemented with analog circuitry, where the sense amplifier output is applied to one input of a comparator circuit whose other input is connected to a reference potential, or with digital circuitry operating on digitized samples of the sense amplifier output which are compared with a digitized reference value. In either case, the sensing threshold for each channel is adjustable by the controller. A clinician communicating with the controller may thus adjust the sensing threshold to give a desired degree of sensitivity and specificity so that noise from various sources is ignored while true depolarizations are detected as chamber senses. As described below, the controller may also be programmed to manipulate the sensing threshold of a sensing channel in order to implement sensing refractory periods.
 b. Sensing Refractory Periods
 A sensing refractory period for a sensing channel is a time period during which the channel is rendered relatively or absolutely insensitive to what would otherwise be interpreted as intrinsic cardiac activity for the purposes of diagnostic counting or for triggering or inhibiting paces. A sensing refractory period may be used, for example, to avoid interpreting the electrogram signal immediately following a detected intrinsic depolarization or pacing pulse as another chamber sense. In the former case, a sensing channel is rendered refractory for a post-sense refractory period once a sense is detected in that channel in order to prevent ongoing depolarization from being detected as a chamber sense. In the case of a pacing pulse, a sensing channel is rendered refractory for a post-pace refractory period after a pace is delivered though that channel in order to prevent both the pacing pulse and the depolarization evoked by the pace from being detected as a chamber sense.
 Refractory periods may be implemented in at least two different ways. One way is to disable the sensing circuitry so that no electrogram signal is received. A postpace refractory period, for example, may be at least partly implemented with a so-called blanking interval in which a sensing amplifier of the sensing/pacing channel is blanked (i.e., disabled) immediately upon delivery of a pacing pulse. One way of implementing blanking is to disconnect the sense amplifier from the sensing electrodes via the MOS switching network 70. Blanking the sensing amplifier both prevents the pacing pulse from re-entering the amplifier's input where it would cause saturation of the amplifier and avoids amplifying the large amplitude after-potentials generated by polarization of the electrodes after the pace. A blanking interval is an example of an absolute refractory period during which no chamber sense will be detected regardless of the magnitude of the electrogram signal. Another way to render a sensing channel refractory is to raise the sensing threshold above its normal or steady-state value. Raising the sensing threshold may either implement an absolute refractory period (i.e., by increasing the threshold to infinity or some maximum allowable value) or a relative refractory period where a larger magnitude electrogram signal is required for detection of a chamber sense than is the case during normal operation. Implementing a refractory period by raising the sensing threshold may be referred to as soft-blanking as opposed to normal blanking where the sensing amplifier is disabled. With soft-blanking, although the sensing channel is rendered absolutely or relatively insensitive with respect to chamber sense detection, the electrogram signal is still received by the sensing circuitry and may be measured or used for other purposes.
 As aforesaid, one of the reasons for initiating a refractory period in a sensing channel is to prevent the detection of a chamber sense shortly after detection of a sense or delivery of a pace. If the channel were not made refractory, the electrogram signal due to areas of the myocardium continuing to depolarize could be interpreted as another chamber sense when it is actually part of the same contraction. An electrogram signal that would be wrongly interpreted as a chamber sense by a sensing channel under normal sensing conditions is referred to herein as an artifact. Interpreting an artifact as a chamber sense, or oversensing, can result in, among other things, inappropriate pacing rates, undesirable inhibition of pacing pulses, and/or misdiagnoses of arrhythmias. This problem can be avoided after a sense or pace by blinding the sensing channel to chamber senses during a period of time when it is considered highly improbable that another contraction of the sensed chamber would occur. A post-sense absolute refractory period would thus begin immediately after detection of sense, while a postpace absolute refractory period would begin upon delivery of a pace after the end of any blanking interval.
 Completely blinding a sensing channel with an absolute refractory period, however, presents a risk of undersensing. Undersensing occurs when the channel fails to detect a chamber sense when an actual contraction occurs and can result in inappropriate pacing rates, failure to detect arrhythmias, and/or asynchronous pacing. In order to lessen the risk of undersensing, a relative refractory period can be utilized instead of an absolute refractory period. In a softblanked relative refractory period, the sensing threshold is raised above the normal steady-state value in order to not detect an artifact, but an electrogram signal of sufficient amplitude will still be detected as a chamber sense. The two approaches can be combined by initiating a post-pace or post-sense absolute refractory period after the pace or sense which is immediately followed by a relative refractory period. The total post-sense or post-pace refractory period is then made up of an absolute refractory period (including any initial blanking interval in the case of a pace) during which no chamber senses are permitted and a relative refractory period during which the sensing threshold is raised to a supranormal value. (The refractory period can also include other intervals such as a retriggerable noise interval.) When the refractory period starts, the sensing threshold is raised to a specified value selected to be above the channel's steadystate value. After a specified time period, the sensing threshold is then abruptly returned to the steady-state value or may decay to the steady-state value in a gradual (continuous or step-wise) fashion. The refractory sensing threshold value to which the sensing threshold is raised during a post-pace or post-sense relative refractory period can either be a programmable fixed value or may be adaptively determined by the implantable device. One scheme for adaptively adjusting
the refractory sensing threshold value involves measurement of the peak amplitude of the electrogram signal during the soft-blanked absolute refractory period immediately following a chamber sense or a pace and then setting the maximum refractory sensing threshold value to a specified fraction (e.g., 75%) of the measured peak amplitude (or of an average of previously measured peak amplitudes).
 As described above, a post-pace or post-pace refractory period can be implemented in the same channel in which a pace or sense occurs, referred to as a same-chamber refractory period, in order to prevent interpreting any continuing depolarization as another chamber sense. Another problem that arises when sensing depolarizations at a particular local cardiac site with a sensing channel dedicated to that site, however, is the additional sensing of electrogram signals that originate from another cardiac site or a pace delivered to that site, referred to as a far-field signal. When sensing channels are provided for multiple chambers (e.g., an atrium and a ventricle, both atria, and/or both ventricles), sensing by a sensing channel dedicated to one chamber of far-field signals produced by another chamber or a pace to that chamber can produce a form of cross-talk between the sensing channels. Cross-talk between sensing channels occurs when a sensing channel senses a far-field artifact and interprets it as a chamber sense. In order to prevent crosstalk between channels, a cross-chamber refractory period can be implemented in a channel so that it is rendered less sensitive upon detection of a depolarization in another chamber or when a pace is delivered to that chamber, referred to as a cross-chamber refractory period. A wellknown example of a cross-chamber refractory period is the post-ventricular atrial refractory period, or PVARP, used to prevent an atrial sensing channel from interpreting ventricular depolarizations as atrial senses which causes a pacemaker-mediated tachycardia in an atrial-tracking pacing mode.
 Soft-blanking, as described above with respect to same-chamber refractory periods, can be used to implement both post-pace cross-chamber and post-sense refractory periods. (See U.S. Pat. No. 6,169,918 assigned to Cardiac Pacemakers, Inc. and hereby incorporated by reference in its entirety.) One way of implementing a post-sense crosschamber or post-pace refractory period is to raise the sensing threshold of a first chamber sensing channel above its steady-state value upon detection of a depolarization in a second chamber sensing channel or upon delivery of a pace to the second chamber, where the sensing threshold is raised to a value during the cross-chamber refractory period which is equal to the channel's same-chamber refractory sensing threshold value or some specified fraction thereof. As the same-chamber refractory sensing threshold value of the channel adapts based upon the measured peak amplitude of electrogram signals following chamber senses or paces, therefore, so too does the cross-chamber refractory sensing threshold value. This technique may not result in an optimum level of sensitivity and specificity for the channel during the cross-chamber refractory period, however, because the cross-chamber refractory sensing threshold value is determined without regard to the amplitude of the actual far-field signals received by it.