Feasibility of Using Intracardiac Impedance Measurements for Capture Detection E. ALT, C. KRIEGLER,* P. FOTUHI. R. WILLHAUS,* W. COMBS,* M. HEINZ, and D. HAYES** From the I. Medical Clinic, Technical University of Munich, Munich, Germany, "Medtronic, Inc., Minneapolis, and the **Mayo Clinic, Rochester, Minnesota
ALT, E., ET AL.; Feasibility of Using Intracardiac Impedance Measurements for Capture Detection. Energy consumption and longevity of modern pacemakers are determined by the controJIing eJectronic circuitry and by the stimulation energy. WhiJe with technoIogicaJ progress the electronics' energy consumption has been reduced significantly, clinical practice shows that many cardiac pacemakers are programmed to suboptimal settings with regard to minimization 0/ pacing energy consumption. Several methods for optimizing pacemaker output settings have been proposed in the past. The most promising concept is an output parameter optimizing pacemaker with automatic capture detection. We examined whether it is possible to distinguish between effective and ineffective pacemaker stimulus capture by analyzing high pass filtered intracardiac impedance signals that are derived from standard bipolar pacing leads. In one series of 11 patients undergoing replacement or implantation of chronic bipolar pacemakers, four patients during electrophysiology studies, and eight volunteers undergoing invasive electrophysiology trials, we examined intracardiac impedance signals obtained with various stimulation rates and output parameter settings. Additionally we analyzed a series of five patients with implanted pacemakers that can measure and telemeter intracardiac impedance signals. Several evaluation concepts have been analyzed regarding their ability to discriminate between effective and ineffective stimuli. We developed an adequate algorithm that detects capture or loss of capture at different output parameter settings based on intracardiac impedance analysis. The sensitivity is 98.5% and specificity is 91% to loss of capture for the currently investigated algorithm and this can be used to determine the optimal setting of pulse width and amplitude with regard to energy consumption. This concept is currently under realization in the external programmer and in the future an implementation of these algorithms within the pacemaker itself is intended. (PACE, Vol. 15, November, Part JI 1992} cardiac pacing, electrodes, stimulation, energy optimization
Introduction Rationale It is well known that optimal settings of a pacemaker pacing output greatly decrease the current drain from the battery.^ This increases longevity of the pacemaker. By simply decreasing the output from 5 to 2.5 V, an increase in the life of
Address for reprints; Eckhard Alt, M.D,. I. Medical Clinic, Rechts der Isar Medical Center. Technical University of Munich. Ismaningerstr. 22, 8000 Munich 80, Germany. Fax; 49-8941805155.
PACE. Vol. 15
the pacemaker from 5 to 8-9 years will result. A prolonged battery life is beneficial for the patient since more frequent replacement surgeries puts him at a higher risk and is also costly, both for the procedure and for a new generator. Despite this fact, nnany pacemakers are not programmed to the optimal setting for the patient and may not be reprogrammed postimplant. A study of explanted generators showed that 90% were still at their nominal settings.^ Lead research is continually lowering patient thresholds and thereby increasing battery life (high impedance leads, steroid tip leads, and multiprogrammable generators).^"^ However, this
November, Part II 1992
1873
ALT, ET AL.
technological progress is not utilized if the generators are never reprogrammed to take advantage of the lower thresholds. In order for physicians to quickly and simply adjust pacing output optimal settings, automatic capture detection methods have heen explored.'''^ The majority of these have focused on detecting the myocardial depolarization that occurs when the heart is successfully stimulated [the socalled evoked response).^ The major obstacle to evoked response methods is the polarization of the lead after delivering a pacing spike.^ The delivered voltage is on the order of a few thousand millivolts. Therefore, currently used output circuitries polarize the lead during the time the evoked response occurs, which is only a few millivolts in amplitude and occurs within the first 100-msec poststimulus delivery.
h
AlAl
Another approach to discriminate hetween an effective and an ineffective stimulus involves the use of sensors that detect the mechanical contraction of the heart rather than focusing on the electrical activation. Some examples of such sensors are pressure sensors and stroke volume sensors. It is theorized that the intracardiac impedance signal is correlated to stroke volume. Since hlood is a good conductor, the impedance between the two electrodes increases as blood empties from the chamber and decreases as blood fills the chamber. An example of the ventricular intracardiac impedance signal and the corresponding ECG is given in Figure 1. Because the signal is easily measured hy a conventional bipolar electrode and requires no special sensors or pacing leads, we explored the possibility of using this impedance signal for automatic capture detection.
tt
kA
h
ECG 5000
I Raw Impedar
Intracardiac Impedance
sooo
4000
5000
Example of Ventricular Intracardiac Impedance Signal With VVI Pacing Figure 1. Example of the raw intracardiac impedance signal during W I pacing. The changes in stroke volume are easily seen as an increase in impedance with emptying of the ventricle and a decrease in impedance with filling. On this raiv signaJ, the respiratory component can also be detected.
1874
November, Part 11 1992
PACE, Vol. 15
INTRACARDIAC IMPEDANCE MEASUREMENTS FOR CAPTURE DETECTION
In addition to testing the feasibility of using the signal for capture detection, we wanted to test the concept of using a continuously telemetered intracardiac impedance signal to implement an automatic capture detection scheme. This concept is currently under realization in the external programmer and in the future an implementation of these algorithms within the pacemaker itself is intended. Materials and Methods To test the feasibility of using intracardiac impedance for capture detection, intracardiac impedance along with surface ECG were collected invasively from 11 patients who were undergoing pacemaker implantation or replacement, eight volunteers with bipolar pacing leads temporarily implanted, four patients undergoing electrophysiological studies, and noninvasively from five patients with implanted research devices. For the invasive studies, the intracardiac impedance was measured with a specially designed impedance transducing device (model 2364a [Medtronic, Inc., Minneapolis, MN, USA]). A 4-kHz sinusoidal current is applied hetween the tip and ring electrode and the induced voltage is measured. The induced voltage is directly proportional to changing resistance through Ohm's law. The research devices use a slightly different approach. A 128-Hz current is used rather than a constant current source. In patients, the pacemaker was implanted and the impedance measured via real-time telemetry (Legend LZ [Medtronic, Inc.]) The impedance signal along with the surface ECG was recorded on a TEAC R-71 data recorder (TEAC Corp., Montebello, CA, USA) during several transitions between capture and loss of capture. The signals were digitized at 256 Hz for further analysis on a computer. Software was written to measure parameters from these signals and to test the sensitivity and specificity of different algorithms in distinguishing capture from loss of capture. For the noninvasive studies, a real-time system was established. This system measures the impedance signal by real-time telemetry through the programming head and the signal was recorded on a TEAC R-71 data recorder, a strip chart recorder, and digitized and stored on a computer hard disk. The computer digitized the signal
PACE, Vol. 15
and then classified beats as capture or loss of capture while we performed threshold tests using the pacemaker programmer.
Resuits Initial study of the data showed a distinct difference between the impedance signal following a pacing spike that captured and the impedance signal following a pacing spike that failed to capture. Figure 2A shows a digitized ECG and the corresponding impedance signal from an effective beat. The pacemaker spike occurred at time zero. Figure 2B shows, in the same format, an ineffective pacing spike followed after a delay by an intrinsic beat. The timing of the captured example corresponds to the changes in blood volume that occur in the ventricle. This includes an isovolumic section (about 50 msec) followed by a rapid ejection and a filling stage. In the first analysis step we explored several evaluation concepts to determine their ability to distinguish between an effective and an ineffective stimuli. It was found that several time based parameters can be used to distinguish a captured beat from one that lost capture (dZ/dT, amplitude, rise time, integral etc.]. An example of one of these parameters is shown in Figure 3. After determining the significant parameters, these were combined in different ways in search of a single reliable algorithm for all of the patients. Three algorithms that use different combinations of parameters and focus on different parts of the waveform were developed and tested. Table I summarizes the results from these three different algorithms. The first algorithm focuses on the changes that occur during the rapid ejection phase, assuming that the pacing spike was effective. If the conditions are not met, it is assumed to be an ineffective beat. The second algorithm focuses on the rapid filling stage of the waveform. It similarly looks for parameter conditions to be met, using two parameters. The third algorithm combines the two. All three of the algorithms were able to diagnose loss of capture with almost perfect accuracy (sensitivity of 98.5%]. The sensitivity was similar for all three algorithms since they all misclassified the same ineffective stimulus beat as an effective stimulus.
November, Part II 1992
1875
ALT, ET AL.
Captured Example ECG
Intracardiac Impedance
Time After Pacing Spike (ms) Figure 2A. Example 0/ a pacing spike that successfully captured ihe heart. On the left side is the ECG signal and on the right is the intracardiac impedance signal. The pacing spike occurred at time zero. The mechanical contraction of the heart and the resulting stroke volume changes are easy to derive from the intracardiac impedance signal in this sample. The specificity to loss of capture was 81%, 91%, anci 85% for algorithms 1, 2, and 3, respectively. Algorithm 2 was the best in terras of specificity, however, we helieve that algorithm 3 will be more robust in detecting loss of capture [higher sensitivity) when more data are analyzed. This is because algorithm 3 takes into account both por-
tions of the waveform. On the other hand, there ere more conditions to be met to be called capture, therefore specificity decreases. Generally, specificity is sacrificed for a better sensitivity that is critical in capture detection algorithms. In the real-time implementation of these algorithms, a beat would be lost with each threshold
Tabie i. Resuits From Using Ventricular Intracardiac impedance For Ventricular Capture Detection Number of Beats Tested
Aigorithm (See Text)
Sensitivity % Loss of Capture Caiied Loss of Capture
Specificity % Capture Caiied Capture
Capture
LoC
1 2 3
98.6% 98.6% 98.6%
81% 91% 85%
1,068 1,068 1,088
69 69 69
The sensitivity was the same tor all three algorithms because ail misclassified the same one ioss of capture (LoC) beat as a captured beat.
1876
November, Part II 1992
PACE, V o l . 15
INTRACARDIAC IMPEDANCE MEASUREMENTS FOR CAPTURE DETECTION
Cursor 0 1
o
1
D• • • • I
X Y 0 2000 1-154.00
AX
0 4000 1 89.32
Io 2000
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243.321
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X . ^ 0 -2000 1 -70
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0 .4000
2000
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AX •510 99)
Figure 2B. Example of a pacing spike that was unsuccessfui at capturing the heart. On the left side is the ECG signal and on the right is the respective intracardiac impedance signal. The pacing spike occurred at time zero. There is no real change in Ihe signal directly/oJlowing the pacing spike. A change is not seen until the intrinsic QRS and contraction occurs.
test. This is because the impedance signal senses stroke volume changes that take approximately 200 msec fo detect. This is slightly different from evoked response methods that are able to detect and respond within 100 msec. There was a large degree of variability, in a given patient, on the captured beats, as evident in the wide distribution of captured heats in Figure 3. There are many factors contributing to the impedance signal in addition to stroke volume changes, including respiration. The respiration component modulates the amplitude by as much as half but appropriate filtering may compensate for this. The variability from heat to beat also results from asynchronous atrial contribution when recording intracardiac impedance in the ventricle. These studies were done while pacing VVI, not AV sequentially, and the contribution of the atrial contraction changes the captured impedance signal on a beat-to-beat basis. We feel that AV syn-
PACE, Vol. 15
chrony will improve specificity without compromising sensitivity.
Discussion In the 1980s, the major aim of pacemaker improvement was to create better therapies including rate adaption and dual chamber devices. In the 1990s, a major focus will he on automaticity and ease of programming in pacing.^" The method we describe provides a way to automatically store an algorithm for capture detection in an external programmer to measure pacing threshold during routine follow-ups, requiring only minimum time and effort of the physician. More important than the convenience for the physician hy such an algorithm is the greater likelihood that pacemakers would be reprogrammed to optimal settings rather than not reprogrammed at all, which unfortunately occurs frequently. Intracardiac impedance signal analysis
November, Part II 1992
1877
ALT. ET AL.
Capture Detection Patient DP Capture Detection Based on One Parameter 6-
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Figure 3. Results from one patient. One of the several parameters tried is graphed and used to distinguish capture from Joss of capture. In this patient a simple threshold detection scheme is all that is needed to distinguish the captured and the Jost beats, however, because of the Jarge variability of the captured beats, more parameters were needed in other patients to insure close to 100% sensitivity, at the expense of specificity.
within a programmer in combination with an algorithm that automatically lowers the pacing output until loss of capture could automatically calculate and choose the appropriate pacing energy for a given patient while taking into account the necessary safety margin.''•^''•'^ One major advantage of this method is that there is no need for a special sensor since it operates with a conventional hipolar electrode. Based on these feasihility studies, more analyReferences 1. Furman S. Pacemaker longevity. [From the Editor) PACE 1989; 12:1437-1438. 2. Irnich W, Kramer E, Muller. Programmierung von Herzschrittmachern—Wunsch und Wirklichkeit. Dtsch Med Wochenschr 1991; 116:601-605. 3. Mond HG. Development of low-stimulationthreshold, low-polarization electrodes. In SS Bar-
1878
sis is currently heing performed to test the algorithms during pseudofusion and fusion heats, dual chamher pacing, and atrial capture detection in addition to more real-time trials. We feel that AV synchrony will improve specificity without compromising sensitivity. Such a feature would henefit the patient by increasing the service life of the pacemaker and the physician by decreasing the time and effort needed to individually optimize the device.
old, J Mugica (eds.): New Perspectives in Cardiac Pacing. 2. Mount Kisco, NY, Futura Publishing Co., Inc., 1991, pp. 133-162. Sutton R, Guneri S. The impact of steroid eluting leads on long term pacing in the atrium and ventricle. Eur J Clin Pacing Electrophysiol 1991; 1: 10-15.
November. Part II 1992
PACE, Vol. 15
INTRACARDIAC IMPEDANCE MEASUREMENTS FOR CAPTURE DETECTION
Mond H. Stokes K. Grigg L, et al. The porous titanium steroid eluting electrode: A double blind assessing the stimulation threshold effects of steroid. PACE 1988; 11:214-219, Auerbach A. The autodiagnostic pacemaker. PACE 1979; 2:58-68. Alt E, Schrittmachertherapie des Herzens. Erlangen, perimed-Fachbuch-Verl.-Ces., 1990, pp. 26-33, 66-82. Thalen Th, Evoked Response Sensing as Automatic Control of the Pacemaker Output. Padova, Piccin Medical Books, 1982, pp. 1229-1234. Baig W, Boute W, Wilson J, et al. Use of paced
PACE, Vol. 15
10.
11.
12.
evoked response in the determination of pacing threshold. PACE 1988; 11:822. Alt E. Implantable devices—Pending issues and future trends. [Editorial) PACE 1990; 13: 1079-1081. Mugica J, Barold SS, Ripart A. The smart pacemaker. In SS Barold, J Mugica (eds.): New Perspectives in Cardiac Pacing. 2. Mount Kisco, NY, Futura Publishing Co., Inc., 1991, pp. 545-577. Preston TA, Judge RD. Alteration of pacemaker threshold by drug and physiological factors. Ann NY Acad Sci 1969; 162:686-692.
November, Part II 1992
1879
ALT, E., ET AL.; Feasibility of Using Intracardiac Impedance Measurements for Capture Detection. Energy consumption and longevity of modern pacemakers are determined by the controJIing eJectronic circuitry and by the stimulation energy. WhiJe with technoIogicaJ progress the electronics' energy consumption has been reduced significantly, clinical practice shows that many cardiac pacemakers are programmed to suboptimal settings with regard to minimization 0/ pacing energy consumption. Several methods for optimizing pacemaker output settings have been proposed in the past. The most promising concept is an output parameter optimizing pacemaker with automatic capture detection. We examined whether it is possible to distinguish between effective and ineffective pacemaker stimulus capture by analyzing high pass filtered intracardiac impedance signals that are derived from standard bipolar pacing leads. In one series of 11 patients undergoing replacement or implantation of chronic bipolar pacemakers, four patients during electrophysiology studies, and eight volunteers undergoing invasive electrophysiology trials, we examined intracardiac impedance signals obtained with various stimulation rates and output parameter settings. Additionally we analyzed a series of five patients with implanted pacemakers that can measure and telemeter intracardiac impedance signals. Several evaluation concepts have been analyzed regarding their ability to discriminate between effective and ineffective stimuli. We developed an adequate algorithm that detects capture or loss of capture at different output parameter settings based on intracardiac impedance analysis. The sensitivity is 98.5% and specificity is 91% to loss of capture for the currently investigated algorithm and this can be used to determine the optimal setting of pulse width and amplitude with regard to energy consumption. This concept is currently under realization in the external programmer and in the future an implementation of these algorithms within the pacemaker itself is intended. (PACE, Vol. 15, November, Part JI 1992} cardiac pacing, electrodes, stimulation, energy optimization
Introduction Rationale It is well known that optimal settings of a pacemaker pacing output greatly decrease the current drain from the battery.^ This increases longevity of the pacemaker. By simply decreasing the output from 5 to 2.5 V, an increase in the life of
Address for reprints; Eckhard Alt, M.D,. I. Medical Clinic, Rechts der Isar Medical Center. Technical University of Munich. Ismaningerstr. 22, 8000 Munich 80, Germany. Fax; 49-8941805155.
PACE. Vol. 15
the pacemaker from 5 to 8-9 years will result. A prolonged battery life is beneficial for the patient since more frequent replacement surgeries puts him at a higher risk and is also costly, both for the procedure and for a new generator. Despite this fact, nnany pacemakers are not programmed to the optimal setting for the patient and may not be reprogrammed postimplant. A study of explanted generators showed that 90% were still at their nominal settings.^ Lead research is continually lowering patient thresholds and thereby increasing battery life (high impedance leads, steroid tip leads, and multiprogrammable generators).^"^ However, this
November, Part II 1992
1873
ALT, ET AL.
technological progress is not utilized if the generators are never reprogrammed to take advantage of the lower thresholds. In order for physicians to quickly and simply adjust pacing output optimal settings, automatic capture detection methods have heen explored.'''^ The majority of these have focused on detecting the myocardial depolarization that occurs when the heart is successfully stimulated [the socalled evoked response).^ The major obstacle to evoked response methods is the polarization of the lead after delivering a pacing spike.^ The delivered voltage is on the order of a few thousand millivolts. Therefore, currently used output circuitries polarize the lead during the time the evoked response occurs, which is only a few millivolts in amplitude and occurs within the first 100-msec poststimulus delivery.
h
AlAl
Another approach to discriminate hetween an effective and an ineffective stimulus involves the use of sensors that detect the mechanical contraction of the heart rather than focusing on the electrical activation. Some examples of such sensors are pressure sensors and stroke volume sensors. It is theorized that the intracardiac impedance signal is correlated to stroke volume. Since hlood is a good conductor, the impedance between the two electrodes increases as blood empties from the chamber and decreases as blood fills the chamber. An example of the ventricular intracardiac impedance signal and the corresponding ECG is given in Figure 1. Because the signal is easily measured hy a conventional bipolar electrode and requires no special sensors or pacing leads, we explored the possibility of using this impedance signal for automatic capture detection.
tt
kA
h
ECG 5000
I Raw Impedar
Intracardiac Impedance
sooo
4000
5000
Example of Ventricular Intracardiac Impedance Signal With VVI Pacing Figure 1. Example of the raw intracardiac impedance signal during W I pacing. The changes in stroke volume are easily seen as an increase in impedance with emptying of the ventricle and a decrease in impedance with filling. On this raiv signaJ, the respiratory component can also be detected.
1874
November, Part 11 1992
PACE, Vol. 15
INTRACARDIAC IMPEDANCE MEASUREMENTS FOR CAPTURE DETECTION
In addition to testing the feasibility of using the signal for capture detection, we wanted to test the concept of using a continuously telemetered intracardiac impedance signal to implement an automatic capture detection scheme. This concept is currently under realization in the external programmer and in the future an implementation of these algorithms within the pacemaker itself is intended. Materials and Methods To test the feasibility of using intracardiac impedance for capture detection, intracardiac impedance along with surface ECG were collected invasively from 11 patients who were undergoing pacemaker implantation or replacement, eight volunteers with bipolar pacing leads temporarily implanted, four patients undergoing electrophysiological studies, and noninvasively from five patients with implanted research devices. For the invasive studies, the intracardiac impedance was measured with a specially designed impedance transducing device (model 2364a [Medtronic, Inc., Minneapolis, MN, USA]). A 4-kHz sinusoidal current is applied hetween the tip and ring electrode and the induced voltage is measured. The induced voltage is directly proportional to changing resistance through Ohm's law. The research devices use a slightly different approach. A 128-Hz current is used rather than a constant current source. In patients, the pacemaker was implanted and the impedance measured via real-time telemetry (Legend LZ [Medtronic, Inc.]) The impedance signal along with the surface ECG was recorded on a TEAC R-71 data recorder (TEAC Corp., Montebello, CA, USA) during several transitions between capture and loss of capture. The signals were digitized at 256 Hz for further analysis on a computer. Software was written to measure parameters from these signals and to test the sensitivity and specificity of different algorithms in distinguishing capture from loss of capture. For the noninvasive studies, a real-time system was established. This system measures the impedance signal by real-time telemetry through the programming head and the signal was recorded on a TEAC R-71 data recorder, a strip chart recorder, and digitized and stored on a computer hard disk. The computer digitized the signal
PACE, Vol. 15
and then classified beats as capture or loss of capture while we performed threshold tests using the pacemaker programmer.
Resuits Initial study of the data showed a distinct difference between the impedance signal following a pacing spike that captured and the impedance signal following a pacing spike that failed to capture. Figure 2A shows a digitized ECG and the corresponding impedance signal from an effective beat. The pacemaker spike occurred at time zero. Figure 2B shows, in the same format, an ineffective pacing spike followed after a delay by an intrinsic beat. The timing of the captured example corresponds to the changes in blood volume that occur in the ventricle. This includes an isovolumic section (about 50 msec) followed by a rapid ejection and a filling stage. In the first analysis step we explored several evaluation concepts to determine their ability to distinguish between an effective and an ineffective stimuli. It was found that several time based parameters can be used to distinguish a captured beat from one that lost capture (dZ/dT, amplitude, rise time, integral etc.]. An example of one of these parameters is shown in Figure 3. After determining the significant parameters, these were combined in different ways in search of a single reliable algorithm for all of the patients. Three algorithms that use different combinations of parameters and focus on different parts of the waveform were developed and tested. Table I summarizes the results from these three different algorithms. The first algorithm focuses on the changes that occur during the rapid ejection phase, assuming that the pacing spike was effective. If the conditions are not met, it is assumed to be an ineffective beat. The second algorithm focuses on the rapid filling stage of the waveform. It similarly looks for parameter conditions to be met, using two parameters. The third algorithm combines the two. All three of the algorithms were able to diagnose loss of capture with almost perfect accuracy (sensitivity of 98.5%]. The sensitivity was similar for all three algorithms since they all misclassified the same ineffective stimulus beat as an effective stimulus.
November, Part II 1992
1875
ALT, ET AL.
Captured Example ECG
Intracardiac Impedance
Time After Pacing Spike (ms) Figure 2A. Example 0/ a pacing spike that successfully captured ihe heart. On the left side is the ECG signal and on the right is the intracardiac impedance signal. The pacing spike occurred at time zero. The mechanical contraction of the heart and the resulting stroke volume changes are easy to derive from the intracardiac impedance signal in this sample. The specificity to loss of capture was 81%, 91%, anci 85% for algorithms 1, 2, and 3, respectively. Algorithm 2 was the best in terras of specificity, however, we helieve that algorithm 3 will be more robust in detecting loss of capture [higher sensitivity) when more data are analyzed. This is because algorithm 3 takes into account both por-
tions of the waveform. On the other hand, there ere more conditions to be met to be called capture, therefore specificity decreases. Generally, specificity is sacrificed for a better sensitivity that is critical in capture detection algorithms. In the real-time implementation of these algorithms, a beat would be lost with each threshold
Tabie i. Resuits From Using Ventricular Intracardiac impedance For Ventricular Capture Detection Number of Beats Tested
Aigorithm (See Text)
Sensitivity % Loss of Capture Caiied Loss of Capture
Specificity % Capture Caiied Capture
Capture
LoC
1 2 3
98.6% 98.6% 98.6%
81% 91% 85%
1,068 1,068 1,088
69 69 69
The sensitivity was the same tor all three algorithms because ail misclassified the same one ioss of capture (LoC) beat as a captured beat.
1876
November, Part II 1992
PACE, V o l . 15
INTRACARDIAC IMPEDANCE MEASUREMENTS FOR CAPTURE DETECTION
Cursor 0 1
o
1
D• • • • I
X Y 0 2000 1-154.00
AX
0 4000 1 89.32
Io 2000
AY
243.321
Cur3or 0
O
1
D
••AM
X . ^ 0 -2000 1 -70
AX
0 .4000
2000
"1 I-S81 54|
AX •510 99)
Figure 2B. Example of a pacing spike that was unsuccessfui at capturing the heart. On the left side is the ECG signal and on the right is the respective intracardiac impedance signal. The pacing spike occurred at time zero. There is no real change in Ihe signal directly/oJlowing the pacing spike. A change is not seen until the intrinsic QRS and contraction occurs.
test. This is because the impedance signal senses stroke volume changes that take approximately 200 msec fo detect. This is slightly different from evoked response methods that are able to detect and respond within 100 msec. There was a large degree of variability, in a given patient, on the captured beats, as evident in the wide distribution of captured heats in Figure 3. There are many factors contributing to the impedance signal in addition to stroke volume changes, including respiration. The respiration component modulates the amplitude by as much as half but appropriate filtering may compensate for this. The variability from heat to beat also results from asynchronous atrial contribution when recording intracardiac impedance in the ventricle. These studies were done while pacing VVI, not AV sequentially, and the contribution of the atrial contraction changes the captured impedance signal on a beat-to-beat basis. We feel that AV syn-
PACE, Vol. 15
chrony will improve specificity without compromising sensitivity.
Discussion In the 1980s, the major aim of pacemaker improvement was to create better therapies including rate adaption and dual chamber devices. In the 1990s, a major focus will he on automaticity and ease of programming in pacing.^" The method we describe provides a way to automatically store an algorithm for capture detection in an external programmer to measure pacing threshold during routine follow-ups, requiring only minimum time and effort of the physician. More important than the convenience for the physician hy such an algorithm is the greater likelihood that pacemakers would be reprogrammed to optimal settings rather than not reprogrammed at all, which unfortunately occurs frequently. Intracardiac impedance signal analysis
November, Part II 1992
1877
ALT. ET AL.
Capture Detection Patient DP Capture Detection Based on One Parameter 6-
'
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Figure 3. Results from one patient. One of the several parameters tried is graphed and used to distinguish capture from Joss of capture. In this patient a simple threshold detection scheme is all that is needed to distinguish the captured and the Jost beats, however, because of the Jarge variability of the captured beats, more parameters were needed in other patients to insure close to 100% sensitivity, at the expense of specificity.
within a programmer in combination with an algorithm that automatically lowers the pacing output until loss of capture could automatically calculate and choose the appropriate pacing energy for a given patient while taking into account the necessary safety margin.''•^''•'^ One major advantage of this method is that there is no need for a special sensor since it operates with a conventional hipolar electrode. Based on these feasihility studies, more analyReferences 1. Furman S. Pacemaker longevity. [From the Editor) PACE 1989; 12:1437-1438. 2. Irnich W, Kramer E, Muller. Programmierung von Herzschrittmachern—Wunsch und Wirklichkeit. Dtsch Med Wochenschr 1991; 116:601-605. 3. Mond HG. Development of low-stimulationthreshold, low-polarization electrodes. In SS Bar-
1878
sis is currently heing performed to test the algorithms during pseudofusion and fusion heats, dual chamher pacing, and atrial capture detection in addition to more real-time trials. We feel that AV synchrony will improve specificity without compromising sensitivity. Such a feature would henefit the patient by increasing the service life of the pacemaker and the physician by decreasing the time and effort needed to individually optimize the device.
old, J Mugica (eds.): New Perspectives in Cardiac Pacing. 2. Mount Kisco, NY, Futura Publishing Co., Inc., 1991, pp. 133-162. Sutton R, Guneri S. The impact of steroid eluting leads on long term pacing in the atrium and ventricle. Eur J Clin Pacing Electrophysiol 1991; 1: 10-15.
November. Part II 1992
PACE, Vol. 15
INTRACARDIAC IMPEDANCE MEASUREMENTS FOR CAPTURE DETECTION
Mond H. Stokes K. Grigg L, et al. The porous titanium steroid eluting electrode: A double blind assessing the stimulation threshold effects of steroid. PACE 1988; 11:214-219, Auerbach A. The autodiagnostic pacemaker. PACE 1979; 2:58-68. Alt E, Schrittmachertherapie des Herzens. Erlangen, perimed-Fachbuch-Verl.-Ces., 1990, pp. 26-33, 66-82. Thalen Th, Evoked Response Sensing as Automatic Control of the Pacemaker Output. Padova, Piccin Medical Books, 1982, pp. 1229-1234. Baig W, Boute W, Wilson J, et al. Use of paced
PACE, Vol. 15
10.
11.
12.
evoked response in the determination of pacing threshold. PACE 1988; 11:822. Alt E. Implantable devices—Pending issues and future trends. [Editorial) PACE 1990; 13: 1079-1081. Mugica J, Barold SS, Ripart A. The smart pacemaker. In SS Barold, J Mugica (eds.): New Perspectives in Cardiac Pacing. 2. Mount Kisco, NY, Futura Publishing Co., Inc., 1991, pp. 545-577. Preston TA, Judge RD. Alteration of pacemaker threshold by drug and physiological factors. Ann NY Acad Sci 1969; 162:686-692.
November, Part II 1992
1879
