the basic function of a modern pacemaker is to deliver pacing pulses to the heart in accordance with sensed cardiac signals . programmability of the pacemaker allows it to operate in a number of different pacing modes . the electronic circuitry for controlling the pacemaker can be either in the form of custom integrated circuits or a microprocessor based design with application specific software . because of its inherent flexibility and the ease with which additional functionality can be added , a microprocessor - based designs is the most widely used . in the description that follows , a microprocessor - based pacemaker will be referred to as incorporating the system and method which is the present invention . it should be appreciated , however , the invention could also be incorporated into a pacemaker controlled by custom logic circuitry either in addition to or instead of a programmed microprocessor . the term “ circuitry ” as used herein should therefore be taken to mean either custom circuitry or a microprocessor executing programmed instructions contained in a processor - readable storage medium along with associated circuit elements . fig1 shows a system diagram of a microprocessor - based dual - chamber pacemaker . a microprocessor 10 communicates with a memory 12 via a bidirectional data bus . the memory 12 typically comprises a rom for program storage and a ram for data storage . the microprocessor also has a port for communicating with the telemetry interface 40 which in turn receives programming data from and transmits telemetry data to an external programmer by a radio link . the pacemaker also has atrial sensing and pacing channels comprising electrode 34 , lead 33 , sensing amplifier 31 , pulse generator 32 , and an atrial channel interface 30 which communicates bidirectionally with a port of microprocessor 10 . the ventricular sensing and pacing channels similarly comprise electrode 24 , lead 23 , sensing amplifier 21 , pulse generator 22 , and a ventricular channel interface 20 . for each channel , the same lead and electrode are used for both sensing and pacing . the channel interfaces 20 and 30 include analog - to - digital converters for digitizing sensing signal inputs from the sensing amplifiers and registers which can be written to by the microprocessor in order to output pacing pulses , change the pacing pulse amplitude , and adjust the gain and threshold values for the sensing amplifiers . a cardiac pacemaker is basically a device that responds to sensed cardiac events and elapsed time intervals by changing its functional states so as to deliver pacing pulses to the heart at appropriate times and properly interpret sensed data . as such , the essential operation of a pacemaker can be usefully represented from a state machine perspective . a state machine is a computational device which transitions from one state to another based on a defined set of rules unique to each state regarding inputs and timer events . in the following description , the operation of a pacemaker incorporating the present invention is described as such a state machine . it should be realized , however , that this description refers to the external behavior of the device and not to any specific implementation by either the programming of the microprocessor or discrete logic circuitry . fig2 shows a state machine representation of a simplified pacemaker operating in ddd mode which has five basic states : state s 0 where interval timers are initialized , state s 1 indicating a ventricular sense , state s 2 indicating an atrial sense , state s 3 to pace the ventricle , and state s 4 to pace the atrium . the system moves from state to state based upon sensed events and the outputs of timers for two intervals : aei for atrial escape interval which defines the minimum rate at which the atria will be paced in the absence of spontaneous activity , and avi for atrioventricular interval which defines a minimum time interval in which a spontaneous ventricular depolarization must occur following an atrial sense or pace . if the avi expires following an atrial sense or pace , a ventricular pace is delivered . not shown are other time intervals used to define refractory periods for the sensing channels during which time the channels are closed so that inputs are ignored . ( the term “ refractory ” means a no sensing condition when applied to a sensing channel , to be distinguished from the physiological refractory period of excitable tissue .) sensing channels are rendered refractory both in order to prevent reentry into the system of an output pacing pulse ( in which case the sensing amplifiers are blanked ) and to prevent the misinterpretation of input data by the sensing of afterpotentials or by crosstalk between sensing channels . an actual ddd pacemaker may even use additional algorithms for interpreting input data , but it must still exhibit the functionality defined by fig1 if it is desired to perform passive testing in the device of fig2 in order to store a record of sense signal amplitudes and lead impedances , test states can be added to the operation of the pacemaker which allows the testing and recording to be done while the pacemaker is operating in its programmed mode . the non - test states of the device will be referred to a programmed states . in order to test a programmed sensing or pacing channel passively , there must be a corresponding test state for each such channel to be tested . for example , one test state is an amplitude storing state for storing signal amplitudes which is entered upon detection that a corresponding programmed sensing state has been entered due to receipt of a signal from a sensing channel which is not in its refractory period . another test state is a lead impedance measuring state which is entered after detecting that the pacemaker has entered a corresponding pacing state due to the occurrence of an event which triggers output of a pacing pulse according to the programmed mode . the lead impedance measuring state measures an impedance value of the lead through which the pacing pulse is delivered during output of the pacing pulse and stores the value in a memory . a test state has the same transition rules as its corresponding programmed state so that the programmed operation of the device is undisturbed by the passive testing . fig3 shows a state machine diagram of a pacemaker operating in vvi mode which has three basic states : state s 0 for initialization , state s 1 indicating a ventricular sense , and state s 3 for outputting a pacing pulse to the ventricle . the system moves from state to state based upon sensed ventricular events ( i . e ., r waves ) and the output of a lower rate interval ( lri ) timer that defines a minimum rate at which the ventricles will be paced in the absence of spontaneous activity . the ventricular sense signal amplitude and ventricular lead impedance can be measured passively with testing states as described above . if the vvi pacemaker is a dual - chamber one having the capability of operating in ddd mode and physically configured with an atrial lead and sensing channel , it would be desirable to be able to passively measure atrial signal amplitude as well . simply turning on the atrial sense amplifier ( which may be turned off when the pacemaker is in vvi mode to conserve power ) with an amplitude storing state is not enough because the operation of the pacemaker does not include an atrial sensing state in its current programming . this means that there are no transition rules that define how an atrial sensing state should be entered and exited . nor does the pacemaker have an atrial channel refractory period timer which is needed in order not to misinterpret a ventricular depolarization or a pacing pulse as an atrial sense signal . fig4 shows a state transition diagram of a pacemaker incorporating a testing system in accordance with the present invention . a virtual sensing test state vs corresponding to the currently unprogrammed sensing channel ( i . e ., the atrial channel in this case ) is added to the operation of the pacemaker where the pacemaker then operates as if programmed in a mode including that sensing state . the virtual sensing test state includes a refractory period timer in its transition logic in order to properly interpret sensing signals . the virtual testing state is thus transitioned to upon receipt of a sensing signal in the corresponding sensing channel , and an amplitude value corresponding to the sense signal is stored in memory . the term “ virtual ” in this context means that transition to the virtual sensing state results in no triggering or inhibition of pacing pulses as would be the case were the pacemaker simply programmed to operate with the atrial sensing channel active . the transition rule of the virtual sensing test state is to return to the state from which the virtual state was entered in order to leave the programmed operation of the device undisturbed . the testing states as described may be part of a passive measurement testing mode which when activated causes the testing states to persist until signal amplitudes and lead impedances for all available channels are stored . the microprocessor is thus programmed to maintain the testing mode until all testing has been performed and then deactivate the testing mode . an amplitude test interval timer can be used to set a limit on the time for which the system will wait for a sensing signal to store . upon expiration of the interval , the system aborts the test sequence . similarly , a lead test interval timer can be used to limit the time the system waits for a pacing pulse to occur through a lead in order to measure its impedance . upon expiration of that interval , however , instead of aborting the lead impedance test , a virtual pacing test state may be employed where “ virtual pacing ” in this context means the outputting of a pacing pulse to the chamber to which the lead is connected occurs shortly after a depolarization during the chamber &# 39 ; s refractory period . the impedance of the lead can then be measured and stored . similarly , a virtual pacing state may be used to measure the impedance of a shock lead in the case of an implantable cardioverter - defibrillator . the passive measurement testing mode as described can be initiated either by a telemetry command from an external programmer or by a timer which causes periodic testing at regular intervals in order to store a record of historical values . for example , the passive measurement testing mode can be set to activate once per day and store a week &# 39 ; s worth of signal amplitude and lead impedance values . an average taken over a selected number of most recent values can also be calculated and stored for later retrieval . the system and method as described above may be used in situations where it is desired to evaluate operating parameters of an implanted pacemaker which are not currently derivable in its current operating mode . such nonprogrammed parameters may be useful in planning a switch of operating modes . for example , atrial triggering modes such as ddd or vdd are desirable in chronotropically competent patients in need of ventricular pacing because they allow the pacing to track the physiologically normal atrial rhythm . atrial triggering modes are contraindicated , however , in patients prone to atrial fibrillation or flutter or in whom a reliable atrial sense cannot be obtained . in the former case , the ventricles will be paced at too high a rate . failing to sense an atrial p wave , on the other hand , results in a loss of atrial tracking which can lead to negative hemodynamic effects . this is because the pacemaker then reverts to its minimum ventricular pacing rate after failing to sense the p wave , and the next p waves may occur so early that they fall within the post - ventricular refractory period of the atrial channel . the minimum rate pacing may thus continue for several cardiac cycles until a p wave is again sensed . for such patients , a synchronous non - atrial triggered mode such as vvi or dvi set at an appropriate minimum rate is usually best . it may be the case , however , that with proper adjustment of parameters , such as amplifier gain and signal threshold , reliable atrial sensing could actually be accomplished . if it can be determined that such parameter adjustments are successful , the patient can be switched to the more desirable atrial triggered mode . the present invention allows periodic samples of atrial sensing signals to be taken and stored over time even while the pacemaker is operating in vvi , dvi , or any other non - atrial sensing mode . the atrial sensing data can then be used to decide if the atrial sensing is reliable enough to allow switching the patient to an atrial triggered mode . although the invention has been described in conjunction with the foregoing specific embodiment , many alternatives , variations , and modifications will be apparent to those of ordinary skill in the art . such alternatives , variations , and modifications are intended to fall within the scope of the following appended claims .
a system and method for passively testing a cardiac pacemaker in which sensing signal amplitudes and lead impedance values are measured and stored while the pacemaker is functioning in its programmed mode . the amplitude and impedance data may be gotten and stored periodically at regular intervals to generate a historical record for diagnostic purposes . sensing signal amplitudes may also be measured and stored from a sensing channel which is currently not programmed to be active as long as the pacemaker is physically configured to support the sensing channel . such data can be useful in evaluating whether a switch in the pacemaker &# 39 ; s operating mode is desirable .