this detailed description provides a number of different embodiments of the present method and apparatus . the embodiments provided herein are not intended in an exclusive or limited sense , and variations may exist in organization , dimension , hardware , software , mechanical design and configuration , and chemical aspects without departing from the claimed invention , the scope of which is provided by the attached claims and equivalents thereof . the present signal processing system is demonstrated in the following detailed description in several embodiments . some of the embodiments are demonstrated in applications involving implantable devices , such as pacemakers and cardioverter - defibrillators , however , it is understood that the present signal processing system may be used in any implantable device and may also be used by devices which are not implanted . furthermore , the concepts provided herein are not limited to sensing in the right ventricle and different electrode configurations may be used without departing from the present system . the following documents are all hereby incorporated by reference in their entirety : u . s . pat . no . 4 , 674 , 518 , issued to salo , u . s . pat . no . 4 , 686 , 987 , issued to salo and pederson , u . s . pat . no . 4 , 773 , 401 issued to citak et al ., u . s . pat . no . 5 , 036 , 849 , issued to hauck et al ., u . s . pat . no . 5 , 156 , 147 issued to warren et al ., u . s . pat . no . 5 , 235 , 976 , issued to spinelli , u . s . pat . no . 5 , 391 , 190 to pederson et al ., and u . s . pat . no . 5 , 792 , 195 , issued to carlson et al . these documents relate to a variety of systems and applications , including , but not limited to , stroke volume , minute ventilation , and maximum hemodynamic sensor rate (“ hmsr ”) systems . however , any process or apparatus which may benefit from the present system may incorporate the present system and apply the teachings provided herein . minor changes in filtering , order of processes , and signal conditioning do not necessarily depart from the present system , and the scope of the invention is determined by the attached claims and their equivalents . fig1 a is an example of an impedance measurement of a right ventricle of a heart in diastole . fig1 b is an example of an impedance measurement of a right ventricle of a heart in systole . if a current is conducted between the housing of an implantable device 12 and a tip electrode 13 on the end of a catheter 14 with the tip electrode 13 positioned in the apex of the right ventricle 15 , then the impedance observed between two electrodes , 16 and 17 , located within the right ventricle ( and before the tip electrode 13 ) will measure an increased impedance for a contracted ventricle ( systole — fig1 b ) as opposed to when the ventricle is not contracted ( diastole — fig1 a ). this is because in diastole , the ventricle is holding more blood and has more conductive volume to transfer current . in systole , the ventricle is contracted and has less blood , leaving less volume for conduction . fig2 is a block diagram showing a signal processing system according to one embodiment of the present system . device 20 may be a pacemaker , cardioverter - defibrillator , or any other implantable device . device 20 may also be located outside of the body . electrodes 21 and 22 are located in the body in one embodiment . in one application electrode 21 may be an electrode external to the heart , including , but not limited to , a mesh , a catheter electrode , a patch electrode , or a conductive portion of the housing of an implantable device . if device 20 is an implantable device , then electrode 21 may be the conductive walls of the hermetically sealed device 20 . in one embodiment , electrode 22 is located near the apex of the right ventricle . electrode 22 can be any type electrode , including , but not limited to , a tip electrode of a catheter electrode assembly . in one embodiment , electrodes 21 and 22 are capacitively coupled to a constant current pulse generator 26 . in one embodiment , the pulse generator 26 produces a number of different constant current waveforms . in one embodiment pulse generator 26 produces bursts of current pulses as shown in fig5 b . in this embodiment , these pulses are biphasic and are sent two at a time with a pulse frequency of 16 khz and a burst frequency of 73 hz . the pulses are constant current , which means that their 60 microamp peak - to - peak current value is regulated within 50 percent . in one embodiment , any current source design may be used to produce the constant current waveforms . ideal constant current supplies have an infinite output impedance . in one embodiment the constant current source has a very large output impedance compared to the impedance load between the electrodes . in one embodiment the constant current source has an output which is greater than or equal to approximately 200 kiloohms . in embodiments where the electrodes are used to measure current across the cardiac area an output impedance of approximately 200 kiloohms was demonstrated to be adequate . the waveform shown in fig5 b is useful for measurements of right ventricular function , since the burst frequency is greater than twice the right ventricular frequency range of interest , as required by the nyquist theorem . for example , the right ventricular frequency of interest lies between approximately 0 . 1 hz and 25 hz . any burst frequency exceeding approximately twice the upper limit satisfies the nyquist theorem . in this example , a burst frequency exceeding approximately 50 hz is adequate . additionally , the pulse frequency is much greater than the nyquist frequency , providing smaller pulses for low energy consumption . other waveforms may be generated by pulse generator 26 without departing from the present system . in one embodiment , electrodes 23 and 24 are capacitively coupled to voltage detection and processing electronics 28 . processing electronics 28 produces an output related to the relative impedances of the tissue measured . the processing of signals received by electrodes 23 and 24 is based on the constant current pulses generated by pulse generator 26 . in one embodiment , signal 29 is used to coordinate sensing events between pulse generator 26 and processing electronics 28 . in embodiments involving active pacing or defibrillation of heart tissue , the signal 29 is used to inhibit sensing by processing electronics 28 . in one embodiment , signal 29 is produced by a pacemaker , cardioverter - defibrillator , or other stimulator operating as part of or in conjunction with device 20 . in one embodiment , processing electronics 28 are blanked during excitation of the cardiac tissue . in one embodiment involving pacing , the device 20 is used in an hmsr application to pace using the relative impedance of cardiac tissue to determine a maximal pacing rate for optimal hemodynamic function . in one embodiment , the device 20 is used in a minute ventilation measurement application . electrodes 23 and 24 may be any type of electrodes , including , but not limited to , catheter electrodes mounted on a common catheter with electrode 22 being a tip electrode . thus , the constant current pulses from pulse generator 26 are transmitted between electrodes 21 and 22 , which creates a voltage gradient across electrodes 23 and 24 which is related to the impedance of the electrical pathways between electrodes 21 and 22 . as shown before , during diastole the impedance is less due to the larger conduction volume of the blood filling the ventricle than which is present in systole . fig3 relates to the block diagram of fig2 except that the heart and electrodes are modeled as impedances . impedances r 1 , r 2 , r 3 , and r 4 are the impedances of the electrodes . impedance r 5 is the time - varying impedance of the heart &# 39 ; s right ventricle . fig4 is a block diagram of voltage detection and processing electronics 28 according to one embodiment of the present system . the operation of processing electronics 28 in this embodiment is demonstrated by referring to fig6 for the signal traces at the output of each stage . for instance , pulse generator 26 ( not shown in fig4 ) provides the constant current pulses shown in fig6 a . the resulting voltage signals are received by electrodes 23 and 24 and sent to inputs c and d of processing electronics 28 and to high pass filter 40 . the high pass filter 40 has a low frequency cutoff of approximately 1000 hz in one embodiment . the high pass filter 40 has a cutoff frequency above the spectral range of the r - waves produced by the heart in a cardiac application . in the embodiment with the cutoff of approximately 1000 hz , the r - wave components are blocked by low pass filter 40 , but the 16 khz pulses are passed . amplifier 41 amplifies voltage signals from high pass filter 40 . the amplified voltage signals are then bandpass filtered by first bandpass filter 42 , as shown in fig6 c . the first bandpass filter 42 has a center frequency approximately equal to the carrier frequency of the constant current waveform , such as the pulse frequency in one embodiment of the present system . such a filter removes spectral signals outside of the bandpass which are naturally generated as part of the square wave excitation signal and extracts substantially the fundamental frequency signal components , as is known from fourier analysis of a square wave . the first bandpass filter selects a fundamental harmonic which is substantially sinusoidal and which may be processed by electronics tuned to the fundamental harmonic . in this way , the fundamental harmonic is the frequency of interest as it presents the best measure of signal - to - noise ratio . the q of the filter is adjusted to optimize this signal - to - noise ratio of the substantially fundamental harmonic components of the received signal . in one embodiment , the first bandpass filter has a center frequency of 16 khz and a q of 3 . in one embodiment , depending on the q of the bandpass filter 42 , the number of peaks of the sine wave shown in fig6 c may exceed the number of pulses per burst as shown in fig6 b . other embodiments provide different filter characteristics without departing from the present system . the first bandpass filter 42 is narrow enough to remove out of band extraneous noise which may have been amplified by amplifier 41 . the resulting first filtered signals are rectified by rectifier 43 , as shown in fig4 and the resulting signal traces are shown in fig6 d . the rectified signals are then integrated to produce a discrete integrated value for each burst of pulses . integrator 44 is shown coupled to sample - and - hold 45 in fig4 . integrator 44 is any type of known integrator or its equivalent . in one embodiment the integrated value is reset to zero after the sample - and - hold 45 acquires the signal . this relationship is shown in greater detail in fig6 e and fig6 f . after the integration of a burst of pulses is complete , sample - and - hold 45 is fired as shown in fig6 f to sample - and - hold the current value of the signal shown in fig6 e . the resulting sampled - and - held signal is shown in fig6 g . this signal is passed through the second bandpass filter 46 to produce a smoothed impedance - related signal , as shown in fig6 h . the bandpass filter output shown in fig6 h has the dc component removed , such as is provided by an embodiment incorporating a switched capacitor filter . the second bandpass filter has a bandpass in the region of frequency interest , which according to the nyquist theorem must be less than half the burst frequency . this limits the amount of high frequency noise produced by the system and reduces the burst frequency artifacts in the resulting output signal . if a first burst of pulses has greater average voltage than a second burst of pulses , then the integrated magnitude of the rectified pulses is greater for the first burst of pulses than for the second . in fig6 d the first burst of pulses on the left is exaggerated to show that it is higher in voltage than in the second burst of pulses on the right . the drawings are not to scale , and were exaggerated to demonstrate a point . therefore , the integrated magnitude “ a ” from the first burst of pulses , shown in fig6 e , is greater than the integrated magnitude “ b ” from the second burst of pulses . the sample - and - hold pulses in fig6 f then capture different values in the sampled - and - held trace of fig6 g . therefore , the magnitude “ c ” is greater than magnitude “ d ” indicating a higher average voltage in the first burst of pulses than in the second burst of pulses . this difference in voltage is related to a change in impedance of the load , since the injected current is substantially constant . in one embodiment , the sample - and - hold 45 is triggered with a slight time delay so that any phase delay from the first bandpass filter 42 is accounted for and a premature sampling of the integrated value is avoided . in such embodiments , one sequence of events includes generation of a burst of several high frequency pulses which are integrated as described , followed by a sample - and - hold at a predetermined time delay to account for phase delay in the band pass filter 42 , followed by a reset of the integrator . in one embodiment , second bandpass filter is a switched capacitor filter having a lower cutoff frequency of 0 . 1 hz and an upper cutoff frequency of 30 hz . in the embodiment where signals are measured in the cardiac tissue , the resulting trace in fig6 h is related to the systolic and diastolic cycles , and may be used to calculate various cardiac performance parameters or to control the device 20 . in one embodiment , all of the connections between the various stages are fully differential for enhanced noise immunity of the circuit . in one embodiment , the output signal is fed to an analog - to - digital convertor for further digital domain processing of the output signal . in one embodiment , the analog - to - digital convertor is a 12 - bit design . the present integrating system produces an output which is directed related to the energy of the rectified signal . this system also has considerable noise immunity and a relatively straightforward system for sampling - and - holding the integrated signal . thus the need for complicated timing of the integrated signal is eliminated . the output signal can be related to impedance due to the constant current nature injection of the excitation source . in one embodiment , the signal is a relative signal and not absolute due to the processing involved in converting a voltage signal into a rectified signal and integrated signal and sampled and held in band - pass filter . different electrode placements may result in different output signals . one benefit of the present system is that the carrier frequency of the modulated constant current signal ( the pulse frequency in one example ) is set at a frequency which is high compared to intra corporeal electrical noise in one embodiment . the large separation of frequency allows the processing electronics 28 to easily separate the impedance signal from electrical noise . for example , a pulse frequency of approximately 16 khz is much greater than that of intra corporeal noise , such as the 0 . 1 to 100 hz for intra cardiac ecm . such a design provides good isolation between the impedance signal and any r - waves . the design also provides high impedance signal bandwidth for applications such as hmsr . another advantage of the new system is that the current level is minimized so as to reduce or completely eliminate impact to a surface ecg electrogram . in low constant current designs the need to perform cardiac sense amplifier blanking is reduced or completely eliminated . a micro - power embodiment is provided by incorporating the teachings of the present system . fig5 a shows a frequency chart according to one application and one embodiment demonstrating how the right ventricular frequency range of interest , 0 . 1 hz to 25 hz , is processed with a burst frequency of 73 hz , which exceeds the calculated 50 hz nyquist frequency . the trace of fig5 b is one of many such current pulse waveforms which may be used to provide the required information for monitoring of the right ventricle . other current pulse waveforms may be used without departing from the present system . one way to conserve energy is to switch the 16 khz pulse generator off when a burst is not being transmitted . the analog impedance signal processing circuits can also be switched off when not in use ; they can be switched in synchrony with the burst of pulses to conserve energy . the present signal processing system may be incorporated or used in combination with a variety of devices and applications , including , but not limited to , the devices and applications described in detail by the documents incorporated by reference in this patent application . other devices and applications incorporating the present teachings will be readily apparent to those skilled in the art upon reading and understanding the present detailed description .
a low power processing system for processing bursted amplitude modulated signals performing impedance - related measurements across a load including injecting current pulses of constant amplitude across the load using at least a first electrode and a second electrode , the current pulses including bursts of a plurality of pulses at a pulse frequency at which the current pulses are repeated , the bursts transmitted at a burst frequency ; detecting voltages across at least a third electrode and a fourth electrode ; high pass filtering the voltages to produce filtered voltages ; amplifying the filtered voltages to produce amplified voltage signals ; bandpass filtering the amplified voltage signals with a bandpass filter with a center frequency equal to approximately the pulse frequency to generate first filtered signals ; rectifying the first filtered signals to produce rectified signals ; integrating the rectified signals to produce integrated signals ; sampling - and - holding the integrated signals after each burst to capture an integrated pulse value for each burst , creating a plurality of discrete integrated pulse values ; and bandpass filtering the plurality of discrete integrated pulse values using a filter including an upper cutoff frequency less than the burst frequency to produce the output related to the time - varying impedance of the load . in one application , the first electrode is positioned near to an apex of a right ventricle , the second electrode is outside of the right ventricle , the third electrode and fourth electrode are located within the right ventricle and wherein the output is related to the time - varying impedance of the right ventricle during systole and diastole .