Calibration of impedance monitoring of respiratory volumes using thoracic D.C. impedance

Abstract

A system includes an implantable medical device that includes a trans-thoracic impedance measurement circuit providing a trans-thoracic impedance signal of a subject. A controller is coupled to the trans-thoracic impedance circuit. The controller extracts a respiration signal from the trans-thoracic impedance signal, measures a breathing volume of the subject using the amplitude of the respiration signal and a breathing volume calibration factor, computes an adjusted breathing volume calibration factor using a reference baseline value of the trans-thoracic impedance and a measured baseline value of the trans-thoracic impedance, and computes a calibrated breathing volume using the adjusted breathing volume calibration factor.

Claims

1. A system comprising: a medical device, including: an implantable trans-thoracic impedance measurement circuit to provide a trans-thoracic impedance signal of a subject; and a controller coupled to the trans-thoracic impedance circuit, the controller configured to: store a reference baseline value of a DC component of a trans-thoracic impedance; measure a more recent value of the DC component of a trans-thoracic impedance different from the reference baseline value; adjust a breathing volume calibration factor using the reference baseline value and the more recent measured value of a DC component of a trans-thoracic impedance, wherein the breathing volume calibration factor translates a measure of impedance to a measure of breathing volume, and wherein the adjusted breathing volume calibration factor includes a ratio of a reference DC trans-thoracic impedance to a more recent measured value of the DC trans-thoracic impedance; extract a respiration signal from the trans-thoracic impedance signal, the respiration signal including a respiration amplitude; and compute a breathing volume of the subject using the respiration amplitude scaled by the adjusted breathing volume calibration factor. 2. The system of claim 1 , wherein the medical device is an implantable medical device and further includes a posture detecting circuit coupled to the controller, and wherein the controller is further operable to detect a first patient posture and to adjust a breathing volume calibration factor if the more recent value of a DC component of a trans-thoracic impedance is measured while a patient is in the first posture. 3. The system of claim 1 , wherein the medical device is an implantable medical device and further includes a posture detecting circuit coupled to the controller, and wherein the controller is further operable to store a first reference baseline impedance value associated with a first posture and at least one other reference baseline impedance value associated with another posture and to adjust the calibration factor when at least one of: (a) a measured change from the first reference baseline value is detected while a patient is in the first posture; (b) a measured change from the at least one other reference baseline value is detected while the patient is in the other posture. 4. The system of claim 3 , wherein the controller is further operable to compute the breathing volume using a first calibration factor associated with a first posture and at least one other calibration factor associated with another posture, and to adjust the first calibration factor when a measured change from the first reference baseline value is detected while a patient is in the first posture and to adjust the other calibration factor when a measured change from the other reference baseline value is detected while the patient is in the other posture. 5. The system of claim 1 , wherein the medical device includes a storage circuit coupled to the controller, the storage circuit for storing at least one trend in measured DC trans-thoracic impedance values. 6. The system of claim 1 , wherein the medical device includes a storage circuit coupled to the controller, the storage circuit for storing at least one trend in breathing volumes. 7. The system of claim 1 , wherein a breathing volume calibration factor is adjusted using the measured reference baseline value of a DC component of a trans-thoracic impedance and the measured more recent value of the DC trans-thoracic impedance and further using a measure of a change in hematocrit in blood of the subject. 8. The system of claim 1 , wherein the system further includes a second device operable to communicate with the medical device, wherein the second device includes a display, and wherein the second device is further operable to display breathing volume information obtained from the trans-thoracic impedance signal. 9. The system of claim 8 , wherein the second device is further operable to display a histogram of DC trans-thoracic impedance of the patient. 10. The system of claim 8 , wherein the second device is further operable to display trending information of DC trans-thoracic impedance. 11. The system of claim 8 , wherein the second device is further operable to display an indication of fluid accumulation in a patient's lungs. 12. The system of claim 1 , wherein the system further includes a second device operable to communicate with the medical device, wherein the second device is in communication with a computer network. 13. A system comprising: a medical device, including: an implantable trans-thoracic impedance measurement circuit to provide a trans-thoracic impedance signal of a subject, the measurement circuit including an adjustable test current circuit; and a controller coupled to the trans-thoracic impedance circuit, the controller operable to: store a reference baseline value of a DC component of a trans-thoracic impedance; measure a more recent value of the DC component of a trans-thoracic impedance different from the reference baseline value; adjust a test current value until the measured value of the DC component of a trans-thoracic impedance is substantially the same as the reference baseline DC voltage value; extract a respiration signal from the trans-thoracic impedance signal, the respiration signal including a respiration amplitude; and compute a breathing volume of the subject. 14. The system of claim 13 , wherein the controller is further operable to adjust the test current value by the ratio of the reference baseline DC trans-thoracic impedance value to the more recent measured value of DC voltage value. 15. The system of claim 13 , wherein the medical device includes a storage circuit coupled to the controller, the storage circuit for storing at least one trend in measured DC trans-thoracic impedance values. 16. The system of claim 13 , wherein the medical device is an implantable medical device and includes a posture detecting circuit coupled to the controller, and wherein the controller is configured to detect a first patient posture and to adjust a breathing volume calibration factor if the more recent value of a DC component of a trans-thoracic impedance is measured while a patient is in the first posture. 17. The system of claim 13 , including a filtering circuit configured to filter the trans-thoracic impedance signal to attenuate frequencies higher than 0.5 Hz. 18. The system of claim 13 , including a therapy circuit configured to provide a pacing therapy to the subject, and wherein the controller circuit is configured to change a pacing rate in response to the measured breathing volume. 19. The system of claim 13 , including a clock circuit communicatively coupled to the controller, wherein the controller is configured to initiate a measurement of trans-thoracic impedance according to a circadian cycle.
CROSS-REFERENCE TO RELATED APPLICATION This application is related to co-pending, commonly assigned U.S. patent application Ser. No. 10/921,503, entitled “THORACIC IMPEDANCE DETECTION WITH BLOOD RESISTIVITY COMPENSATION,” filed on Aug. 19, 2004 and is incorporated herein by reference. TECHNICAL FIELD The field generally relates to implantable medical devices and, in particular, but not by way of limitation, to systems and methods for monitoring respiratory volumes using electrical measurements of trans-thoracic impedance. BACKGROUND Implantable medical devices (IMDs) are devices designed to be implanted into a patient. Some examples of these devices include cardiac rhythm management (CRM) devices such as implantable pacemakers, implantable cardioverter defibrillators (ICDs), and devices that include a combination of pacing and defibrillation. The devices are typically used to treat patients using electrical therapy and to aid a physician or caregiver in patient diagnosis through internal monitoring of a patient's condition. The devices may include electrical leads in communication with sense amplifiers to monitor electrical heart activity within a patient, and often include sensors to monitor other internal patient parameters. Other examples of implantable medical devices include implantable insulin pumps or devices implanted to administer drugs to a patient. Congestive heart failure (CHF) is a disease that reduces the ability of the ventricles of the heart to contract or relax. This results in an inadequate amount of blood being pumped into circulation. The reduced ability to move blood from veins to arteries may increase volume and pressure within the heart leading to heart failure progression. If left unchecked, CHF can lead to death. Respiratory function of a patient changes as the disease progresses. Monitoring aspects of respiration, such as the lung tidal volume, gives an indication of the progression of the disease. SUMMARY The present inventors have recognized a need for improved monitoring of lung tidal volumes, or breathing volumes, using medical devices. This document discusses, among other things, systems and methods for monitoring lung tidal volume. A system embodiment comprises an implantable medical device that includes a trans-thoracic impedance measurement circuit. The trans-thoracic impedance measurement circuit provides a trans-thoracic impedance signal of a subject. A controller is coupled to the trans-thoracic impedance circuit. The controller extracts a respiration signal from the trans-thoracic impedance signal, measures a breathing volume of the subject using the amplitude of the respiration signal and a breathing volume calibration factor, computes an adjusted breathing volume calibration factor, and computes a calibrated breathing volume using the adjusted breathing volume calibration factor. The adjusted breathing volume calibration factor is computed using a reference baseline value of the trans-thoracic impedance and a measured change from the reference baseline value of the trans-thoracic impedance. A method embodiment comprises measuring breathing volume of a patient using an implantable medical device, detecting a change in DC trans-thoracic impedance, and adjusting a breathing volume calibration factor used by the implantable medical device in measuring the breathing volume in accordance with the change in DC trans-thoracic impedance. This summary is intended to provide an overview of the subject matter of the present patent application. It is not intended to provide an exclusive or exhaustive explanation of the invention. The detailed description is included to provide further information about the subject matter of the present patent application. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates an embodiment of portions of a system that uses an implantable medical device. FIGS. 2A and 2B show graphical representations of trans-thoracic impedance waveforms. FIG. 3 is a block diagram of portions of an embodiment of a system for monitoring lung tidal volume. FIG. 4 is an illustration of portions of an embodiment of a system for monitoring lung tidal volume that includes a cardiac rhythm management device. FIG. 5 is a block diagram of an embodiment of a method for monitoring lung tidal volume. FIG. 6 is a block diagram of another embodiment of a method for monitoring lung tidal volume. DETAILED DESCRIPTION In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and specific embodiments in which the invention may be practiced are shown by way of illustration. It is to be understood that other embodiments may be used and structural or logical changes may be made without departing from the scope of the present invention. The present application discusses, among other things, systems and methods for monitoring lung tidal volume, or breathing volume. Illustrative examples of methods of monitoring lung tidal volume by measuring trans-thoracic impedance are described in Hartley et al. U.S. Pat. No. 6,076,015 entitled “RATE ADAPTIVE CARDIAC RHYTHM MANAGEMENT DEVICE USING TRANSTHORACIC IMPEDANCE,” which is incorporated herein by reference. As discussed above, congestive heart failure (CHF) involves the reduced ability to move blood from veins to arteries. Insufficient flow from veins to arteries allows blood to build up in the pulmonary veins. This leads to pulmonary edema, that is, fluid accumulation in the lungs. As the fluid accumulates, the increased fluid may cause the impedance of current pathways through the lungs to be reduced. This in turn causes the amplitude of an impedance signal to be reduced for a given respiration level. The present inventors have recognized that the effect of pulmonary edema needs to be considered when calculating tidal volume-based performance measures of a patient's pulmonary function. FIG. 1 illustrates an embodiment of portions of a system 100 that uses an implantable medical device (IMD) 110 . As one example, the system 100 shown is used to treat a cardiac arrhythmia. A pulse generator (PG) or other IMD 110 is coupled by a cardiac lead 108 , or additional leads, to a heart 105 of a patient 102 . Examples of IMD 110 include, without limitation, a pacer, a defibrillator, a cardiac resynchronization therapy (CRT) device, or a combination of such devices. System 100 also typically includes an IMD programmer or other external device 170 that communicates wireless signals 160 with the IMD 110 , such as by using radio frequency (RF) or other telemetry signals. Cardiac lead 108 includes a proximal end that is coupled to IMD 110 and a distal end, coupled by an electrode or electrodes to one or more portions of a heart 105 . The electrodes typically deliver cardioversion defibrillation, pacing, resynchronization therapy, or combinations thereof to at least one chamber of the heart 105 . The IMD 110 typically includes components that are enclosed in a hermetically-sealed canister or “can.” Additional electrodes may be located on the can, or on an insulating header extending from the can, or on other portions of IMD 110 , such as for providing unipolar pacing energy, defibrillation energy, or both in conjunction with the electrodes disposed on or around heart 105 . The lead 108 or leads and electrodes are also typically used for sensing electrical activity of a heart 105 . Hartley et al., describes obtaining a trans-thoracic impedance signal of a patient using an IMD. A test current is applied between electrodes placed across a thorax region of a patient. This test current typically uses a sufficiently small magnitude, or a sufficiently high frequency, or both, to avoid interference with cardiac activity. Measuring the resulting voltage between the electrodes provides a trans-thoracic impedance signal for the patient. Signal processing circuitry extracts lung tidal ventilation information by filtering the impedance signal. FIG. 2A shows a graphical representation 200 of a trans-thoracic impedance vs. time waveform 205 of the lung tidal ventilation information for a normal lung. The peaks and valleys in the impedance signal correspond to breathing cycles of the patient. Z DC 215 is the DC artifact of the trans-thoracic impedance signal with variations at frequencies lower than the respiration component of the trans-thoracic impedance signal. In this document, the term “DC” component of the trans-thoracic impedance signal refers not just to zero frequency, but also includes signal frequencies below a cutoff frequency having a value of about 0.1 Hz, such as at signal frequencies between 0 Hz and 0.05 Hz, because the cardiac stroke and respiration components of the trans-thoracic impedance signal lie at higher frequencies. The rate at which the impedance waveform 205 crosses the DC level 215 in either the positive or negative direction corresponds to the rate of breathing cycles of the patient measured in breaths per unit of time, such as breaths/minute for example. This is often referred to as the ventilation rate (VR), respiration rate, or breathing rate. Z TV 210 is an amplitude of the respiration modulation component of the trans-thoracic impedance signal. It indicates the cyclic change in impedance resulting from a patient's breathing. The signal represents the impedance variation of the patient measured in ohms/breath. A calibration factor (CF) is used to translate the impedance information from ohms/breath to a measure of volume per breath. For example, if the calibration factor is a measure of liters-of-air per ohm, then multiplying the impedance measure of ohms/breath by the calibration factor of liters/ohm results in a measure of liters/breath. The measure of lung volume per breath is often referred to as lung tidal volume (TV). Thus, TV (volume/breath)= Z TV (ohms/breath)* CF (volume/ohms).  Equ. 1. Lung tidal volume is one measure of metabolic need of a patient. For example, increased lung volumes may indicate that a patient requires an increased pacing rate because of an increased level of physical activity. A calibration factor for the impedance measurements can be set after implant of the IMD. For example, a clinician can run tests to measure the volume of a deep breath of a patient. This volume is then divided by the maximum peak-to-valley value of impedance to determine a calibration factor, which is then programmed into the IMD. Hartley et al. further describes how lung tidal volume is useful in determining minute ventilation (MV), where MV is function of tidal volume (TV) and ventilation rate (VR). MV measures a patient's air flow rate (e.g. liters/minute), and has been shown to correlate well with a patient's metabolic need for a change in heart rate. Hartley et al. describes the usefulness of MV in cardiac rhythm management devices (CRMs) such as rate adaptive pacemakers. However, the sensitivity of impedance measurements to respiratory volumes may change with pulmonary edema. As fluid in trans-thoracic tissue builds up as heart disease progresses, the impedance of current pathways through the lungs may be reduced. This reduces the respiration amplitude of an impedance signal for a given respiration level. The impedance may be reduced because the fluid that builds up contains electrolytes that act to “short circuit” the current pathways. This phenomenon is shown by a comparison of FIG. 2A with FIG. 2B . FIG. 2B shows a graphical representation 220 of a trans-thoracic impedance waveform 225 corresponding to a “wet” lung with fluid build-up. Even though the patient may be breathing just as deeply as in FIG. 2A , the lower amplitude makes it appear that the patient is not breathing as deeply. The graph 220 illustrates that both Z TV 230 and Z DC 235 are reduced from the graph 200 in FIG. 2A . This reduction in amplitude of the impedance signal could be mistaken for a change in depth of respiration even though it in fact results from a change in impedance due to fluid accumulation. Therefore, it could result in misdiagnosis of a patient. This problem is alleviated or avoided by adjusting the calibration factor. In one example, an adjusted calibration factor is determined using a reference baseline value of trans-thoracic impedance and a measured change from the reference baseline value. In some embodiments, the calibration factor is adjusted as a function that includes a reference DC impedance value and a measured DC impedance value. The reference DC impedance, Z DC,ref , is a baseline DC impedance such as Z DC in FIG. 2A . The measured DC impedance refers to a change in Z DC from the baseline, such as the change resulting in the value of Z DC in FIG. 2B . A tidal volume (TV) measurement is then re-calibrated using the adjusted calibration factor. If TV RAW is an unadjusted (or raw) value of TV calculated as in Equation 1 above, then a re-calibrated tidal volume TV CAL can be determined as a function that includes the raw TV, the reference DC impedance, and the measured DC impedance, or TV CAL =f{TV RAW ,Z DC ,Z DC,ref }.  Equ. 2 In some of the embodiments, the calibration factor is adjusted by the ratio of the reference DC impedance to the measured DC impedance, or Z DC,ref /Z DC , i.e., TV CAL =Z*CF *( Z DC,ref /Z DC ).  Equ. 3, The tidal volume measurement is thus re-calibrated to take into account a new level of fluid build-up. Thus, if the impedance is reduced due to buildup of fluid in the trans-thoracic tissue, Z DC is less than Z DC,ref and the calibration factor is increased to correct for the reduction. If the measurement is not automatically recalibrated as fluid builds-up, recalibration will not occur or at best will be postponed until a patient's next visit to a clinician. This could result in a long period of time where the impedance measurements do not accurately reflect a patient's true respiration. In one example, if the medical device uses a measure of trans-thoracic impedance in deciding whether to automatically adjust a pacing rate, an increased metabolic need of the patient may result in an inadequate increase in the pacing rate. Thus, the medical device would not be performing optimally. Similarly, patient diagnostic measures relying on impedance-derived estimates of tidal volume may inaccurately report shallow breathing, even though the breathing is normal. According to some embodiments, the problem of the reduction in amplitude of the impedance signal is alleviated or avoided by adjusting the test current applied between the electrodes placed across a thorax region of a patient. If fluid builds up in the trans-thoracic tissue, the measured voltage, V, due to a given current will be reduced resulting in a reduced measured DC impedance value Z DC . This reduction can be corrected by increasing the test current until the DC voltage, V DC , equals V DC,ref or by increasing the current by the ratio of Z DC,ref /Z DC . The adjusted impedance measurements are used to calculate tidal volume, TV, in Equation 1. Conversely, if the DC impedance value increases, the test current is decreased until V DC equals V DC,ref or by decreasing the current by the ratio of Z DC,ref /Z DC . While these embodiments are more complex to implement than recalibrating CF, the embodiments could lower the average energy used by the impedance sensor if lower test current is used, and they also have the advantage of raising the measurable voltage if the current is being shunted by fluid buildup. FIG. 3 is a block diagram of portions of a system 300 for monitoring lung tidal volume. In this example, the system 300 comprises an implantable medical device (IMD) 305 that includes a trans-thoracic impedance measurement circuit 310 and a controller 315 coupled to the trans-thoracic impedance circuit 310 . The trans-thoracic impedance measurement circuit 310 provides a trans-thoracic impedance signal of a subject. The trans-thoracic impedance signal typically contains other physiological artifacts in addition to respiration, such as artifacts from heart contractions. The controller 315 extracts a respiration signal 320 from the trans-thoracic impedance signal, such as by frequency filtering of the trans-thoracic impedance signal. In an embodiment, extracting the respiration signal 320 includes attenuating frequencies of the impedance signal that are higher than 0.5 Hz. In some embodiments, the frequency cutoff is a variable that is based, at least in part, on a patient's heart rate. In some embodiments, the respiration signal 320 is filtered using dedicated hardware filtering circuits, and in some embodiments the respiration signal 320 is filtered using digital signal processing by the controller 315 or elsewhere. The controller 315 is typically operable by executing or interpreting instructions in software, firmware, hardware, or any combination of software, firmware, or hardware. The controller 315 also measures a breathing volume of the subject using the amplitude of the respiration signal 320 and a breathing volume calibration factor as discussed previously. The breathing volume calibration factor is computed using a reference baseline value of the DC trans-thoracic impedance. If the controller 315 measures a change from the reference baseline value of the DC trans-thoracic impedance, the controller 315 computes an adjusted breathing volume calibration factor 325 using the measured change. The controller 315 then computes a calibrated breathing volume 330 using the adjusted breathing volume calibration factor 325 . In some of the embodiments, the adjusted breathing volume calibration factor includes a ratio of a reference DC trans-thoracic impedance to a measured change in DC trans-thoracic impedance. According to some embodiments, the IMD 305 of the system 300 further includes a storage circuit coupled to the controller 315 to store the reference baseline impedance value of the DC trans-thoracic impedance. The controller 315 adjusts a breathing volume calibration factor in response to a measured change in impedance value from the reference baseline impedance value. In some further embodiments, in which measurements of the trans-thoracic impedance include a reference DC trans-thoracic impedance, the IMD 305 includes storage for storing trends in measured DC trans-thoracic impedance values. In some embodiments, trends in breathing volumes are stored. According to some embodiments, the IMD 305 includes a posture detecting circuit coupled to the controller 315 . One example, of a posture detecting circuit is an accelerometer. An amount of fluid buildup in a patient's lungs may change with a patient's posture. For example, a patient who is lying down may have more fluid buildup than when the patient is upright. In some embodiments, the controller adjusts the breathing volume calibration without regard to changes in posture of a patient. In some embodiments, the controller 315 detects when a patient is in one posture and only adjusts a breathing volume calibration factor based on measurements while the patient is in that posture. This is to avoid recalibrating the breathing volume calibration factor when the patient changes posture. Limiting the recalibration to only one posture removes this variation due to posture. For example, assume the controller 315 is programmed to only detect changes from the reference baseline value of a DC component of a trans-thoracic impedance while the patient is in an upright position. If the controller 315 detects a change from the reference baseline value, the controller then determines if the patient is upright and can use the computed change. If the patient is upright, the controller 315 uses the computed change to adjust the breathing volume calibration factor. If the patient is lying down, the controller 315 does not adjust the breathing volume calibration factor. In another example, the controller 315 is enabled to detect changes from the reference baseline value only while the patient is in one posture. In some embodiments, the controller 315 tracks changes in DC trans-thoracic impedance for a plurality of postures. As an example, consider the relatively simple case of tracking changes for two postures. In the example, the controller 315 stores a first reference baseline impedance value associated with a first posture, such as an upright posture, and a second reference baseline impedance value associated with a second posture, such as lying down. The controller 315 adjusts an upright calibration factor 325 when a measured change from the first reference baseline value is detected while a patient is upright. The controller 315 also adjusts a lying down calibration factor 325 when a measured change from the second reference baseline value is detected while the patient is lying down. In this way, the controller 315 provides a calibrated measurement of tidal volume while the patient is upright or lying down. In some embodiments the controller 315 uses hysteresis before adjusting the calibration factor 325 . This is useful to account for changes in a patient's posture that are only temporary. Some embodiments of the trans-thoracic impedance measurement circuit 310 include a test current circuit 335 coupled to the controller 315 , and a voltage measurement circuit 340 coupled to the controller 315 . A plurality of implantable electrodes is coupled to the test current circuit 335 and the voltage measurement circuit 340 . The test current circuit 335 delivers a test current using the implantable electrodes, and the voltage measurement circuit 340 measures a voltage resulting from the test current. The resulting voltage may be measured using electrodes that are the same or different from the electrodes used to deliver the test current. FIG. 4 is an illustration of portions of a system 400 for monitoring lung tidal volume. In this example, the system 400 includes an IMD 405 that is a cardiac rhythm management device. The IMD 405 is coupled to heart 415 by a cardiac lead 410 that includes lead tip and ring electrodes 420 , 425 . The IMD 405 includes components that are enclosed in a hermetically-sealed canister or “can” 430 . Additional electrodes include an electrode 435 located on the can 430 and a header electrode 445 located on the insulating header 440 . A therapy circuit 470 is used to provide pacing and/or defibrillation energy in conjunction with the electrodes 420 , 425 disposed on or around heart 415 . The lead 410 and lead electrodes 420 , 425 are used in conjunction with sense amplifiers 475 for sensing electrical activity of a heart 415 . The IMD 405 includes a trans-thoracic impedance measurement circuit 495 coupled to a controller 465 . The impedance measurement circuit 495 includes a test current circuit 450 and a voltage measurement circuit 455 . In the embodiment shown, the test current is delivered using the ring electrode 425 and the can electrode 435 . The voltage is measured using tip electrode 420 and the header electrode 445 . Embodiments can use any combination of electrodes to deliver the current and measure the voltage. For example, the electrodes used in FIG. 4 to deliver the test current and those to measure the test voltage could be swapped. Or, some or all of the electrodes used to deliver the current could be the same as the electrodes to measure the voltage. For example, the test current could be delivered from the ring electrode 425 to the can electrode 435 and the voltage could be measured from the tip electrode 420 to the can electrode 435 , or the test current could be delivered between the tip electrode 420 and the header electrode 445 , and the voltage could measured using the same two electrodes 420 , 445 . According to some embodiments, the system 400 further includes an external device 490 operable to communicate with the IMD 405 using the communication circuit 485 . The communication is through wireless signals such as telemetry signals or RF signals. In an embodiment, the IMD 405 communicates information related to breathing volume to the external device 490 . This information may include a calibrated breathing volume, or it may only include the impedance information that is related to breathing and the external device 490 computes the calibrated breathing volume. In one example, the external device 490 includes a display, and the external device 490 displays breathing volume information obtained from the trans-thoracic impedance signal. In some embodiments, displayed breathing volume information includes a histogram of DC trans-thoracic impedance of the patient. In another embodiment, the displayed breathing volume information includes trending information of DC trans-thoracic impedance. In another embodiment, the displayed breathing volume information includes an indication of fluid accumulation in a patient's lungs. According to some embodiments, the external device 490 is part of, or in communication with, a computer network such as a hospital computer network or the internet. FIG. 5 is a block diagram of an embodiment of a method 500 for monitoring lung tidal volume. At 510 , a breathing volume of a patient is measured using an IMD. The IMD uses a breathing volume calibration factor in measuring the breathing volume. In an embodiment, the IMD is a cardiac rhythm management device (CRM). In one example, a therapy delivered by the CRM device, such as a pacing rate, can be changed in accordance with the measured breathing volume. In another embodiment, the breathing volume is measured using trans-thoracic impedance. In some embodiments, measuring trans-thoracic impedance includes delivering a test current using first and second implantable electrodes, and measuring a voltage resulting from the test current using third and fourth implantable electrodes. In some of the embodiments, at least one of the first and second implantable electrodes is the same as one of the third and fourth implantable electrodes. Thus, embodiments include measuring the impedance using four, three, or two electrodes. In some embodiments, using a breathing volume calibration factor includes using the breathing volume calibration factor to associate a measurement of ohms-per-breath with respiratory volume-per-breath. For example, extraction of a respiration signal waveform as in FIGS. 2A , 2 B above results in a measurement of ohms/breath. Multiplying ohms/breath by a calibration factor with units of liters/ohm results in a measurement of liters/breath. At 520 , a change is detected in DC trans-thoracic impedance. At 530 , the breathing volume calibration factor is adjusted in accordance with the change in DC trans-thoracic impedance. In some embodiments, the breathing volume calibration factor is adjusted using a function of a reference DC impedance and a measured DC impedance, and measuring breathing volume includes calibrating a raw measurement of breathing volume using the adjusted breathing volume calibration factor. In one embodiment, the function of a reference DC impedance and a measured DC impedance includes a ratio of the reference DC impedance to the measured DC impedance (Z DC,ref /Z DC ). According to some embodiments, detecting a change in DC trans-thoracic impedance includes monitoring DC trans-thoracic impedance over time. This includes monitoring impedance with a sampling period of seconds, minutes, hours, or days. It also includes monitoring impedance for an interval of time or monitoring constantly. In some of the embodiments, impedance is measured under the same conditions with respect to posture to reduce the effect of changes in posture on the measurement. In an example, time of day is used to measure impedance according to a circadian cycle when a patient is most likely to be lying down. In some embodiments, detecting a change in DC trans-thoracic impedance includes determining a baseline DC trans-thoracic impedance and detecting changes in impedance from the baseline DC trans-thoracic impedance. In some embodiments, the measurements are made under the same posture conditions and detecting a change in DC trans-thoracic impedance includes determining a first baseline DC trans-thoracic impedance for a first patient posture, such as an upright posture, and a second DC trans-thoracic impedance for a second patient posture, such as lying down. Changes in impedance from the first baseline impedance are monitored when the patient is in the first posture. Changes in impedance from the second baseline impedance are monitored when the patient is in the second posture. Posture can be determined from a posture sensor, such as a two-axis accelerometer, or posture can be deduced, such as from time of day according to a circadian cycle. A clock circuit is used to determine time of day. According to some embodiments, the method further includes storing trending information related to DC trans-thoracic impedance. In some of the embodiments, the method further includes communicating the trending information to an external device for display. In another embodiment, the method includes communicating DC trans-thoracic impedance information to an external device connected to a computer network such as a hospital computer network or the internet. FIG. 6 is a block diagram of another embodiment of a method 600 for monitoring lung tidal volume. At 610 , a trans-thoracic impedance signal of a subject is measured using an implantable medical device (IMD), such as by using any of the systems or methods described previously. At 620 , a respiration signal is extracted from the trans-thoracic impedance signal. The respiration signal includes a component of the trans-thoracic impedance that is modulated by a breathing of the subject. The trans-thoracic impedance signal includes components from other sources such as heart contractions. Signal processing methods are used to extract the respiration signal. In some embodiments, the respiration signal is extracted by filtering the impedance signal. In an embodiment, the respiration signal is extracted by attenuating frequencies higher than 0.5 Hz in the impedance signal. In some embodiments, the frequency cutoff is a heart rate dependent variable. At 630 , a breathing volume of the subject is measured using the amplitude of the respiration signal and a breathing volume calibration factor. Some embodiments include measuring a breathing volume of the subject using the peak-to-peak amplitude of the respiration signal. The breathing volume calibration factor is used to convert an impedance-per-breath measurement of the respiration signal into a volume-per-breath measurement. In some embodiments, the breathing volume is measured by combining the measurement from the respiration signal with other measurements. Some of the embodiments include using a measure of hematocrit in blood of the subject. Some of the embodiments include using a measure of resistivity of blood within a heart. Systems, methods, and devices that detect changes in blood resistivity are described in co-pending, commonly assigned U.S. patent application Ser. No. 10/921,503, entitled “THORACIC IMPEDANCE DETECTION WITH BLOOD RESISTIVITY COMPENSATION,” filed on Aug. 19, 2004 which is incorporated herein by reference. At 640 , the breathing volume calibration factor is adjusted using a reference baseline value of the DC trans-thoracic impedance and a measured change from this reference baseline value. At 650 , a calibrated breathing volume of the subject is computed by applying the adjusted breathing volume calibration factor to the measured breathing volume of the subject. In some embodiments, the calibration factor is adjusted according to a ratio of a reference baseline DC trans-thoracic impedance to a measured DC trans-thoracic impedance, and wherein the breathing volume is calibrated by multiplying the measured breathing volume by the adjusted calibration factor. In some embodiments, the breathing volume calibration factor is adjusted by combining the measurement from the respiration signal with other measurements. Some of the embodiments include adjusting the calibration factor using a measure of a change in hematocrit in blood of the subject. Some of the embodiments include adjusting the calibration factor using a measure of a change in resistivity of blood within a heart. Some of the embodiments include adjusting the calibration factor using a measure of a change in posture of a patient. In some of the embodiments, the breathing volume calibration factor is adjusted by combining the measurement from the respiration signal with any combination that includes a measured change in hematocrit, blood resistivity, or patient posture. By adjusting a calibration for respiratory sensing, a patient's respiration can be monitored more accurately over varying degrees of pulmonary edema. Accurate monitoring of respiration of a patient experiencing pulmonary edema leads to improved patient management and improved patient care. Although trans-thoracic impedance has been described as a measurement between an implanted device and a cardiac lead, other methods of trans-thoracic impedance measurement may be utilized without departing from the scope of this invention. For example, measurements between any two or more electrodes may be used. Further, measurements within any portion of the thoracic cavity are within the scope of this invention. The accompanying drawings that form a part hereof, show by way of illustration, and not of limitation, specific embodiments in which the subject matter may be practiced. The embodiments illustrated are described in sufficient detail to enable those skilled in the art to practice the teachings disclosed herein. Other embodiments may be utilized and derived therefrom, such that structural and logical substitutions and changes may be made without departing from the scope of this disclosure. This Detailed Description, therefore, is not to be taken in a limiting sense, and the scope of various embodiments is defined only by the appended claims, along with the full range of equivalents to which such claims are entitled. Such embodiments of the inventive subject matter may be referred to herein, individually and/or collectively, by the term “invention” merely for convenience and without intending to voluntarily limit the scope of this application to any single invention or inventive concept if more than one is in fact disclosed. Thus, although specific embodiments have been illustrated and described herein, it should be appreciated that any arrangement calculated to achieve the same purpose may be substituted for the specific embodiments shown. This disclosure is intended to cover any and all adaptations, or variations, or combinations of various embodiments. Combinations of the above embodiments, and other embodiments not specifically described herein, will be apparent to those of skill in the art upon reviewing the above description. The Abstract of the Disclosure is provided to comply with 37 C.F.R. §1.72(b), requiring an abstract that will allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. In addition, in the foregoing Detailed Description, it can be seen that various features are grouped together in a single embodiment for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own.

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