in the following detailed description , reference is made to the accompanying drawings which form a part hereof , and in which is shown by way of illustration specific embodiments in which the invention may be practiced . it is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention . as discussed previously , the measurement of the respiratory component of the transthoracic impedance is complicated by the presence of the stroke volume component of the impedance signal . because the stroke volume component is present due to the filling and emptying of the heart with blood , this component is synchronized to heartbeats . implantable systems are able to sense intrinsic activity signals associated with heartbeats . the implantable systems are further able to generate fiducial markers in response to occurrences of such an activity signals . as an example , one of these activity signals is a qrs complex . a qrs complex is the activity signal associated with the process of the ventricular chambers depolarizing or contracting to empty the chambers of blood . in general , the volume of blood in the heart at an occurrence of an activity signal is fairly consistent from one occurrence of the signal to the next . thus , the stroke volume component of the transthoracic impedance will also be fairly consistent at each occurrence of the signal . if the transthoracic impedance is sampled synchronously only when the implantable system generates a specific fiducial marker , the stroke volume component will be constant during the sampling and the respiratory signal is easily extracted from the transthoracic impedance signal . examples of intrinsic heart activity signals sensed by implantable systems and useful for sampling include an onset of a p - wave , an onset of a qrs complex , an r - wave peak , or a t - wave peak . fig1 shows one embodiment of a system 100 for sampling the transthoracic impedance commenced at the occurrence of a fiducial marker . this embodiment of the system includes pulse generator 105 and endocardial lead 110 . lead 110 is shown coupled to pulse generator 105 . lead 110 is a multi - conductor lead and includes tip electrode 120 coupled to a first conductor and ring electrode 125 coupled to a second lead conductor . pulse generator 105 includes a hermetically sealed outer housing 130 . outer housing 130 ( sometimes referred to as the case or can ) is comprised of a conducting material such as titanium , and is covered by an insulating material such as silicone rubber . a hole or window in the insulating material allows a third electrode 135 to be formed from the can 130 of pulse generator 105 . pulse generator 105 also includes a header 140 for receiving the lead 110 and is formed from an insulating material such as molded plastic . header 140 also includes a fourth electrode 145 . such a four - electrode system is described in hauck et al ., u . s . pat . no . 5 , 284 , 136 “ dual indifferent electrode pacemaker ,” assigned to the assignee of the present application and which is incorporated herein by reference . other embodiments of the system include a two or three electrode system . in the embodiment shown , lead 110 is implanted in the right ventricle of a heart 115 . in this embodiment , the impedance sampling may begin , for example , at a fiducial marker indicating the onset of a qrs complex , at a fiducial marker indicating a peak of the r - wave , or at a fiducial marker indicating a peak of the t - wave . fig1 also illustrates portions of pulse generator 105 . therapy circuit 170 provides electrical pacing stimuli to the heart 115 . such pacing stimuli include providing bipolar pacing between tip electrode 120 and ring electrode 125 to initiate a contraction of the ventricles . controller 165 adjusts the rate of the pacing stimuli delivered by the therapy circuit 170 . signal processor 155 senses an intrinsic heart activity signal . when signal processor 155 senses the onset of an intrinsic heart activity signal , controller 165 initiates an impedance measurement . exciter 150 delivers an electrical excitation signal , such as a pulsed current stimulus or any other suitable measurement stimulus , to heart 115 . in one embodiment , exciter 150 delivers a predetermined current stimulus between ring electrode 125 and can electrode 135 . in other embodiments exciter 150 delivers a current stimulus between any other suitable combinations of electrodes . signal processor 155 senses the response to the excitation signal . in one embodiment , signal processor 155 senses the response between tip electrode 120 and header electrode 145 . in other embodiments , signal processor 155 senses the response between any other suitable combinations of electrodes . receiver 156 of the signal processor 155 receives a voltage through sampling element 175 in response to the onset of an intrinsic heart activity signal and the current stimulus . in the embodiment shown sampling element 175 is placed in series with header electrode 145 and the receiver 156 . in another embodiment the sampling element is placed in series with the lead electrodes 120 , 125 and the receiver 156 . the signal processor 155 then measures the voltage by any method known in the art such as by an analog to digital converter . transthoracic impedance is obtained from the predetermined current stimulus and the measured voltage . the transthoracic impedance may then be used to determine respiratory information . fig2 illustrates the system 100 implanted in the thorax region of a patient . it can be seen from the positioning of pulse generator 105 and lead electrodes 120 and 125 that the system 100 measures the impedance across a substantial portion of the patient &# 39 ; s thorax . in one embodiment of the system 100 , a time index is stored along with the impedance value obtained . the time index and impedance value are then used to derive a lung tidal volume . as discussed in the hartley patent , lung tidal volume is obtained by taking the difference between the maximum and minimum impedance values stored for the patient &# 39 ; s previous breath . a larger tidal volume value indicates a deeper breath for the patient than a smaller tidal volume value . in another embodiment , respiratory rate is derived from the impedance signal . one method to obtain respiratory rate would be to determine the time interval between maximum impedance values over a period of time and convert the data to breaths per minute . based on information from the lung tidal volume and respiratory rate , controller 165 adjusts the rate of the delivery of therapy to the heart 115 . a further embodiment of the system 100 is a combination of cardiac rhythm management and treatment for sleep apnea . in this embodiment , the system 100 determines if the respiratory activity falls below a predetermined level . if the respiratory activity falls below the predetermined level , the system provides therapy to treat the sleep apnea such as diaphragmatic pacing . an apparatus for diaphragmatic pacing to treat sleep apnea is described in scheiner et al ., u . s . pat . no . 6 , 415 , 183 “ a method and apparatus for diaphragmatic pacing ,” assigned to the assignee of the present application and which is incorporated herein by reference . fig3 shows an embodiment of the system 100 that uses multiple endocardial leads 100 , 111 . leads 110 , 111 are multi - conductor leads and include tip electrodes 120 , 121 coupled to a first conductor and ring electrodes 125 , 126 coupled to a second lead conductor within their respective lead . in the embodiment shown , lead 110 is implanted in the right ventricle of a heart 115 and lead 111 is implanted in the right atrium of the heart . if lead 111 is used to measure the impedance , the impedance sampling may begin , for example , at a fiducial marker indicating the onset of the p - wave rather than the qrs complex or the peak of an r - wave . this embodiment of the system 100 further shows a pulse generator 105 that includes selector 180 . selector 180 is able to change the electrode combination providing the stimulus from a combination including ring electrode 125 to a combination including ring electrode 126 . selector 180 also changes the electrode combination measuring the stimulus response from a combination including tip electrode 120 to a combination including tip electrode 121 . this ability to change the electrode combination is useful if , for example , measuring the sensed response using tip electrode 120 proves to be difficult due to signal noise , and use of another combination of electrodes provides a better measurement . it should be noted that other embodiments of the system 100 deliver the current stimulus or measure the response between any other suitable combinations of electrodes . fig4 illustrates a multiple lead embodiment of the system 100 implanted in the thorax region of a patient . it can be seen from the positioning of pulse generator 105 and lead electrodes 120 , 121 and 125 , 126 that the system 100 measures the impedance across a substantial portion of the patient &# 39 ; s thorax . it can also be seen that selecting different combinations of electrodes will result in an impedance measurement taken across different vectors of the thorax . for example , using tip and ring electrodes 121 , 126 and header and can electrodes 145 , 135 will measure impedance across a vector originating from the atrium , while using tip and ring electrodes 120 , 125 and can electrodes 145 , 135 will measure impedance across a vector originating from the ventricle . thus , it is beneficial for the system 100 to have flexibility in its measurement configuration to take full advantage of its positioning . fig5 is a graphical illustration 500 of sampling the transthoracic impedance synchronously to fiducial markers that indicate r - wave peaks . qrs complexes from heart activity are shown in graph 510 . in graph 520 , a transthoracic impedance signal obtained by sampling every 50 milliseconds is shown . the variation of the impedance signal with cardiac stroke volume can be seen . it can also be seen that the higher frequency stroke volume component is superimposed onto a lower frequency respiratory component . the downwardly pointing arrows shown in graph 520 correspond to the occurrence of r - waves in graph 510 . graph 530 shows the impedance signal obtained when the impedance is sampled synchronously to the r - waves . graph 530 shows that the lower frequency respiratory signal is extracted from the higher frequency stroke volume component . fig6 is a graphical illustration 600 of sampling the transthoracic impedance during deep and slow breathing . graph 610 shows the r - waves from heart activity . graph 620 shows the transthoracic impedance signal obtained by sampling at a frequency high enough to obtain both the stroke volume and the respiratory component . graph 630 shows the respiratory component obtained through sampling and filtering with a 4 th - order elliptic low pass filter with the filter pole at 0 . 2 hertz ( hz ). in implantable devices , such filters are generally implemented with active circuits . while these circuits can be designed to operate at low power , even low power circuits have an appreciable effect on battery life when the implanted period is on the order of five years . graph 640 shows the respiratory component obtained with r - wave synchronous sampling . graphs 630 and 640 show that similar results are obtained concerning phase , amplitude and frequency using the low pass filtering method and the r - wave sampling method . thus , similar results for impedance measurements at slow and deep breathing can be attained while conserving the power required by an active low pass filtering circuit . fig7 is a graphical illustration 700 of sampling the transthoracic impedance during fast and shallow breathing . graph 710 shows the r - waves of heart activity . graph 720 shows the transthoracic impedance signal obtained by sampling at a frequency high enough to obtain both the stroke volume and the respiratory component . graph 730 shows the respiratory component obtained with r - wave synchronous sampling . graph 740 shows the respiratory component obtained from the high frequency sampling filtering with a 4 th - order elliptic low pass filter with the filter pole at 0 . 2 hertz ( hz ). graph 750 shows the actual measured air volume passing through the lungs of the patient . a comparison of graphs 740 and 750 shows that in attempting to capture the transthoracic impedance during fast and shallow breathing , a 0 . 2 hz pole can mask some of the impedance information as the frequency of the respiratory activity approaches the frequency of the stroke volume . graph 730 shows that r - wave synchronous sampling has some advantage in reproducing amplitude information . this is because the sampling increases with the heart rate of the patient . thus , somewhat improved results for impedance measurements at fast and shallow breathing can be attained while conserving the power required by an active low pass filtering circuit . fig8 is a flow chart illustrating a method 800 of measuring transthoracic impedance . at 810 , a predetermined pulsed current stimulus is applied across a thorax region of a patient in synchrony with intrinsic heart activity signals . at 820 , a voltage across the thorax region is sampled when applying the predetermined pulsed current stimulus . at 830 , impedance is calculated from the measured voltage and the predetermined pulsed current stimulus . fig9 is a flow chart illustrating a method of monitoring lung ventilation . at 910 , a predetermined pulsed current stimulus is applied across a thorax region of a patient in synchrony with intrinsic heart activity signals . at 920 , a voltage across the thorax region is sampled when applying the predetermined current stimulus . at 930 , impedance is calculated from the sampled voltage and the predetermined current stimulus . at 940 , respiratory activity is determined from the calculated impedance . although specific examples have been illustrated and described herein , it will be appreciated by those of ordinary skill in the art that any arrangement calculated to achieve the same purpose could be substituted for the specific example shown . this application is intended to cover any adaptations or variations of the present invention . therefore , it is intended that this invention be limited only by the claims and the equivalents shown .
a system comprising implantable device , the implantable medical device including an intrinsic cardiac signal sensor , an impedance measurement circuit configured to apply a specified current to a transthoracic region of a subject and to sample a transthoracic voltage resulting from the specified current , and a processor coupled to the intrinsic cardiac signal sensor and the impedance measurement circuit . the processor is configured to initiate sampling of a transthoracic voltage signal in a specified time relation to a fiducial marker in a sensed intrinsic cardiac signal , wherein the sampling attenuates or removes variation with cardiac stroke volume from the transthoracic voltage signal , and determine lung respiration using the sampled transthoracic voltage signal .