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Implantable monitoring systems
Journal of Electrocardiology Vol. 27 Supplement
Implantable Monitoring Systems Design Considerations and Challenges
B. M. Yomtov, J. Kim, and G. M. Ayers
The use of electrocardiographic (ECG) information in the diagnosis and follow-up evaluation of cardiac patients has evolved from the simple limb lead surface ECG to more sophisticated instruments using advanced recording and diagnostic methods. Advances in electronic technology have provided continued improvement in the performance of ECG monitoring equipment such as the i2-1ead resting and stress ECG, the interpretive electrocardiograph, the signal-averaged ECG, ambulatory ECG monitors, and body surface potential mapping systems. Holter I reported in 1961 the use of a portable ECG recording device. With the introduction of this first ambulatory monitor, clinicians had the opportunity to identify the presence of various cardiac rhythms while the patient was performing his or her normal daily activities. Applications of these ambulatory monitors have included identification of arrhythmias, evaluation of patient symptoms, quantification of ischemia (for both anginal and silent episodes), and performance evaluation of implantable pacemakers and defibrillators.2 Ambulatory monitors are presently available with magnetic tape, digital random access memory, or m i n i - h a r d disk drives as their data storage medium. Some digital recorders also include real-time E C G analysis,3 which allows the ambulatory monitor to store higher-resolution ECG data during detected events. Variants of ambulatory monitors include event recorders and loop monitors. These monitors are typically used by the patients over longer periods of time, primarily for the identification of less frequent arrhythznias. However, their operation requires that any cardiac events be symptomatic to be recorded, because the patient must direct the storage of the event into memory for future retrieval and clinical review. In a similar manner, pacemaker technology has evolved from the simple nonprogrammable single-chamber pulse generators to dual-chamber rate-modulated pacemakers. The 1970s saw the introduction of the first programmable pacemakers, the use of the long-life lithium batteries, and basic memory storage with the storage of paced and sensed events. The 1980s were highlighted by the introduction
of the dual-chamber pacemaker and the rate-modulated pacemaker and by more advanced data storage including full-disclosure electrograms. Even the implantable cardioverter defibrillator (ICD) evolved from a simple nonprogrammable defibrillator to today's third-generation ICDs, which now include programmability, bradycardia pacing, antitachycardia pacing and tiered therapy, stimulation protocols, and diagnostic data storage. With the use of microprocessor technology, the advancement of these products has accelerated through the 1980s and into the 1990s by providing more versatility and functionality, along with more sophisticated data storage and communications instrumentation. Advances in electronics during the past 15 years have provided medical device companies with many new technologies to improve design performance. These include a variety of low-voltage and low-power microcomputers, increasing density of memory chips, and automated tools for the design and fabrication of custom integrated circuits. With the availability of these technologies, the incorporation of diagnostic monitoring into an implantable device can become a reality. A totally implanted cardiac monitor could provide continuous real-time analysis of electrogram data over long periods of time. An implantable monitor with its associated lead system could also improve ECG quality by minimizing lead breakage, a problem frequently associated with external surface electrode wires, and by eliminating signal waveform changes usually caused by variations in electrode position associated with the reapplication of body surface electrodes. If minimal patient-device interaction is required, compliance associated with the use of an implantable monitor should not be a problem as is seen with present-day loop and Holter monitors.
Design Philosophy An implantable monitor would have a very different design philosophy than implantable therapeutic devices, such as pacemakers and implantable defibrillators. For example, a great deal of effort in the design of pacemakers and implantable defibrillators has focused on the reduction of
From lnControl, Redmond, Washington.
Reprint requests: Barry M. Yomtov, InControl, 6675 185 Avenue NE, Redmond, WA 98052.
74
Implantable Monitoring Systems
Analysiis Algorithms Many of the signal processing techniques originally established in ambulatory monitors should be applicable to the development of an implantable monitor. Perhaps the most challenging algorithm to be developed is for discriminating between normally conducted beats from ventricular beats. Many subsequent analysis functions such as arrhythmia rhythm analysis, ST-segment measurements for ischemia episode detection, and heart rate analysis depend on the accuracy of this function. Factors that may effect performance of an implantable monitor include variations in the QRS morphology occur-
..... - . . . . . . .
Subcutaneous EC6 [ S u r f a c e ECG Difference
I
cua
.
= 0.999
I O
50 msec
A
.
.
.
.
--
# Beats = 50
. . . . . . .
"-200uU
%._od/'""./~J'"'".._./ /"- ........../".,....,,..:_...../,.......,/.,._ j/'".....
Yomtov et al.
75
ring from patient to patient, the long-term stability of the ECG waveform, and the random occurrence of noise artifact. Any or all these factors can contribute to both the sensitivity and specificity of algorithm performance. Variations in QRS morphology from patient to patient can result from differences in patient anatomy, underlying cardiac disease, or electrode placement. To manage these signal variations effectively, the decision-making process of these algorithms needs to be based on relative criteria. With this approach, each patient can act as his or her own control. Another important issue in long-term monitoring is the question of whether there will be changes over long periods of time in the QRS morphology of normal sinus beats. If so, the real-time analysis algorithms within the monitor will have to adapt to these signal changes. Baseline feature measurements or waveform templates would have to be updated at regular intervals. A continuous update of the feature parameters, acting as the control parameter, should compensate for long-term variations, which may otherwise result in false positive reporting. The trade-off for this capability will be increased microcomputer processing time and therefore reduced longevity for the implantable monitor. One of the more significant factors influencing the performance of analysis algorithms to date has been noise artifacts generated from surface electrodes and associated wires. It is assumed that invasive electrodes should be more reliable and present less noise than the body surface electrodes that are typically used with external monitoring equipment. It is unknown, however, whether invasive electrodes would replicate ECG signals as seen from the body surface. Therefore, we compared the electrogram characteristics and noise artifacts of subcutaneous elec-
stimulation threshold to increase device longevity and improve tissue interaction with the stimulating electrodes. In comparison, the design of an implantable monitor would require greater emphasis on the ease of implantation of the system (leads and monitor) to minimize risk and maximize patient acceptance. The design of software for the monitor would emphasize optimization of data storage and retrieval. This may include methods of data compression and optimizing the allocation of memory for the device. There would also be a need for efficiem data analysis algorithms to maximize device longevity. These analysis algorithms, which for ambulatolT monitors have only been required to continuously operate reliably for days, would now be expected to be versatile in providing both accuracy and efficiency continuously over periods of weeks and months of operation.
Icua=9.~
•
k
Subcutaneous ECG S u r l a c e EC6 Difference
~
~..~.,. ~.~. ......---..... .....-..-.,,..,,.,..,..._.-_.........,. .........,.............
,.......... ....,.........
50
0
msec
B
Fig. 1. (A) Beat-to-beat correlation waveform analysis. (B) Average-to-average correlation waveform analysis.
76
Journal of Electrocardiology Vol. 27 Supplement
trodes and skin surface electrodes in two separate studies. The goal of the first study was to determine the similarity of signals between body surface electrodes and subcutaneous electrodes placed in the same anatomic location. In six sheep, body surface and subcutaneous electrodes were applied to the anterior and lateral chest wall, overlying each other. The morphologies of the electrograms from both electrode sites were compared using correlation analysis methods (Fig. 1). A pointwise variance was calculated comparing the surface and the subcutaneous electrograms to calculate a correlation coefficient. The mean correlation coefficient was calculated to be 0.997 _+ 0.002. In a second study, baseline shifts and high-frequency noise were evaluated and quantified. In six dogs, 24-hour Holter recordings were made from bipolar subcutaneous catheter electrodes on the anterior chest wall. In 11 patients, similar recordings were also made from body surface electrodes in similar locations. The 24-hour recordings were analyzed for the quantity of high-frequency noise, which was defined to be greater than 50/~V root-meansquare. The recordings were also analyzed for beat-to-beat baseline shifts greater than 0.5 mV. The high-frequency noise was present in 0.04 + 0.05% of the beats from the subcutaneous recordings as compared with 0.81 _+ 1.18% of the beats from the body surface recordings (P < .001). Beat-to-beat baseline shift was found in 0.21 + 0.20% of the subcutaneous recordings and 1.59 _+ 1.28% of the body surface recordings (P < .031). We conclude from these two studies that subcutaneous electrodes do provide ECG signals similar in morphqlogy to their corresponding body surface sites. Also, the overall quality of electrogram data recorded from subcutaneous electrodes is superior to ECG recordings from body surface electrodes with respect to artifacts.
Optimal Data Storage The storage of data within an implanted device is always an important design feature. The Holter monitor has been used clinically with the assumption that all the ECG is available for trained personnel to verify and edit the performance of the analysis algorithms. The typical two-channel digital monitor requires from four to eight megabytes of memory for every 24 hours of operation, given that data reduction and data compression methods are used. Without data reduction and compression, the memory requirements for 24 hours would be excess of 44 megabytes. Present memory technology is inadequate for complete electrogram storage for long periods of time. Therefore, it is imperative that an implanted monitor have not only accurate detection algorithms to reduce the need for continuous electrogram storage but also use efficient memory management schemes. Also, the memory management of an implantable monitor must be able to provide the ability to preselect a data storage management scheme based on the individual patient's conditions or the physician's preference similar to how a pacemaker is programmed to different operating modes.
Even with present memory chip technology, a significant amount of data can still be stored with an implanted monitor, which leads to yet another design challenge (ie, the ability to retrieve a large amount of data from an implanted device within a reasonable amount of time). For example, 1 megabyte of stored data could require data transmission rates in excess of 500 kbits/s to retrieve the data in less than 20 seconds from the device to the programmer.
Device Design for Low-power Consumption The factors that significantly affect the longevity of an implantable monitor will be those functions that affect the steady-state power consumption the most. This includes the data acquisition of the ECG signals and the efficiency of ongoing real-time analysis algorithms such as the beat discrimination. Power consumption from data acquisition may be influenced by bandpass requirements, amplifier designs, digitization sample rate, and signal filtering requirements. Algorithms running in real time would be affected by the efficiency of the microcomputer's instruction set and the software requirements of the signal processing methods. To the designer, this will be a constant trade-off of performance versus longevity.
Patient Alarms Another potential feature is the incorporation- of an alarm into the monitor. This alarm could be activated whenever the monitor detects a significant cardiac event, thus providing direct feedback to the patient. From the design implementation point of view, one basic question is how to communicate to the patient. Should there be a piezoelectric audible alarm (as used in the early ICDs) or should there be some form of electrical stimulation to provide the patient with feedback? Although a patient alarm in theory can be valuable to both the physician and patient, in practice, it could create many logistic problems. The first concern may be patient awareness and compliance to an alarm. The question is whether patients would be capable of perceiving an alarm if it were to occur during sleeping, or even during waking, hours. If the patient can perceive the alarm, how would he or she react to it? Would there be anticipation or anxiety by the patient, thus introducing chances of false perception of the alarm? Other considerations may also include the effect of this feature on the design of the clinical validation study and on the regulatory approval process.
Justification for an Implantable Monitor As mentioned earlier, the concept of implanting a nontherapeutic device in a patient would have a greater acceptance if the implantation procedure were to be simple and the hardware designed for a small size. This would provide
Implantable Monitoring Systems a product that would minimize risk to patients and optimize their acceptance. So far, we have reviewed many of the technical challenges that may be encountered in developing an implantable cardiac monitor. Perhaps a greater challenge this product faces may be the issue of whether cardiologists would implant a monitor, and if so, who would pay for both the equipment and the associated implantation costs. Most vital to establishing a diagnostic role for a long-term implantable monitor will be to keep the overall costs low, This includes the equipment, the implantation procedure, and patient follow-up evaluation. Historically, implantable electronic devices have provided some therapeutic benefit directly to the patient, whereas an implantable monitor may only provide the clinician with long-term diagnostic information with possible indirect patient benefit. If it can be shown that this new information were to assist the physician in providing earlier medical intervention of a patient, then perhaps more aggressive diagnostic procedures could potentially be reduced and result in a long-term cost benefit. It is also ,difficult to predict whether future healthcare reform will be either a benefit or a barrier to the development of a totally implantable monitor.
Conclusion As we haw~ presented here, a number of new issues may be considered in developing a totally implantable monitor:
•
Yomtov et al.
77
However, these issues should be no more challenging than those that have been encountered with other implantable products. The greater challenge may be the acceptance of such a device and the ability to pay for this new product within the environment of healthcare reform. We have also shown that subcutaneous electrodes may provide electrograms virtually identical in morphology and with fewer artifacts than surface electrodes. The design and development of a totally implantable cardiac monitor therefore provides new challenges for the application of implantable device technology and for the implantable device industry.
References 1. Holter N J: New methods for heart studies: continuous electrocardiography of active subjects over long periods of time is now practical. Science 134:1214, 1961 2. Bigger JT, Reiffel JA, Coromilas J e t ah Ambulatory electrocardiography, p. 36. In Management of cardiac arrhythmias: The non pharmacological approach. JB Lippincott, Philadelphia, 1987 3. Mark RG, Ripley KL: Ambulatory monitoring: realtime analysis versus tape scanning systems, p. 323. In Proceedings from a workshop in Pisa. Martinus-Nijhoff, Boston, 1983
Implantable Monitoring Systems Design Considerations and Challenges
B. M. Yomtov, J. Kim, and G. M. Ayers
The use of electrocardiographic (ECG) information in the diagnosis and follow-up evaluation of cardiac patients has evolved from the simple limb lead surface ECG to more sophisticated instruments using advanced recording and diagnostic methods. Advances in electronic technology have provided continued improvement in the performance of ECG monitoring equipment such as the i2-1ead resting and stress ECG, the interpretive electrocardiograph, the signal-averaged ECG, ambulatory ECG monitors, and body surface potential mapping systems. Holter I reported in 1961 the use of a portable ECG recording device. With the introduction of this first ambulatory monitor, clinicians had the opportunity to identify the presence of various cardiac rhythms while the patient was performing his or her normal daily activities. Applications of these ambulatory monitors have included identification of arrhythmias, evaluation of patient symptoms, quantification of ischemia (for both anginal and silent episodes), and performance evaluation of implantable pacemakers and defibrillators.2 Ambulatory monitors are presently available with magnetic tape, digital random access memory, or m i n i - h a r d disk drives as their data storage medium. Some digital recorders also include real-time E C G analysis,3 which allows the ambulatory monitor to store higher-resolution ECG data during detected events. Variants of ambulatory monitors include event recorders and loop monitors. These monitors are typically used by the patients over longer periods of time, primarily for the identification of less frequent arrhythznias. However, their operation requires that any cardiac events be symptomatic to be recorded, because the patient must direct the storage of the event into memory for future retrieval and clinical review. In a similar manner, pacemaker technology has evolved from the simple nonprogrammable single-chamber pulse generators to dual-chamber rate-modulated pacemakers. The 1970s saw the introduction of the first programmable pacemakers, the use of the long-life lithium batteries, and basic memory storage with the storage of paced and sensed events. The 1980s were highlighted by the introduction
of the dual-chamber pacemaker and the rate-modulated pacemaker and by more advanced data storage including full-disclosure electrograms. Even the implantable cardioverter defibrillator (ICD) evolved from a simple nonprogrammable defibrillator to today's third-generation ICDs, which now include programmability, bradycardia pacing, antitachycardia pacing and tiered therapy, stimulation protocols, and diagnostic data storage. With the use of microprocessor technology, the advancement of these products has accelerated through the 1980s and into the 1990s by providing more versatility and functionality, along with more sophisticated data storage and communications instrumentation. Advances in electronics during the past 15 years have provided medical device companies with many new technologies to improve design performance. These include a variety of low-voltage and low-power microcomputers, increasing density of memory chips, and automated tools for the design and fabrication of custom integrated circuits. With the availability of these technologies, the incorporation of diagnostic monitoring into an implantable device can become a reality. A totally implanted cardiac monitor could provide continuous real-time analysis of electrogram data over long periods of time. An implantable monitor with its associated lead system could also improve ECG quality by minimizing lead breakage, a problem frequently associated with external surface electrode wires, and by eliminating signal waveform changes usually caused by variations in electrode position associated with the reapplication of body surface electrodes. If minimal patient-device interaction is required, compliance associated with the use of an implantable monitor should not be a problem as is seen with present-day loop and Holter monitors.
Design Philosophy An implantable monitor would have a very different design philosophy than implantable therapeutic devices, such as pacemakers and implantable defibrillators. For example, a great deal of effort in the design of pacemakers and implantable defibrillators has focused on the reduction of
From lnControl, Redmond, Washington.
Reprint requests: Barry M. Yomtov, InControl, 6675 185 Avenue NE, Redmond, WA 98052.
74
Implantable Monitoring Systems
Analysiis Algorithms Many of the signal processing techniques originally established in ambulatory monitors should be applicable to the development of an implantable monitor. Perhaps the most challenging algorithm to be developed is for discriminating between normally conducted beats from ventricular beats. Many subsequent analysis functions such as arrhythmia rhythm analysis, ST-segment measurements for ischemia episode detection, and heart rate analysis depend on the accuracy of this function. Factors that may effect performance of an implantable monitor include variations in the QRS morphology occur-
..... - . . . . . . .
Subcutaneous EC6 [ S u r f a c e ECG Difference
I
cua
.
= 0.999
I O
50 msec
A
.
.
.
.
--
# Beats = 50
. . . . . . .
"-200uU
%._od/'""./~J'"'".._./ /"- ........../".,....,,..:_...../,.......,/.,._ j/'".....
Yomtov et al.
75
ring from patient to patient, the long-term stability of the ECG waveform, and the random occurrence of noise artifact. Any or all these factors can contribute to both the sensitivity and specificity of algorithm performance. Variations in QRS morphology from patient to patient can result from differences in patient anatomy, underlying cardiac disease, or electrode placement. To manage these signal variations effectively, the decision-making process of these algorithms needs to be based on relative criteria. With this approach, each patient can act as his or her own control. Another important issue in long-term monitoring is the question of whether there will be changes over long periods of time in the QRS morphology of normal sinus beats. If so, the real-time analysis algorithms within the monitor will have to adapt to these signal changes. Baseline feature measurements or waveform templates would have to be updated at regular intervals. A continuous update of the feature parameters, acting as the control parameter, should compensate for long-term variations, which may otherwise result in false positive reporting. The trade-off for this capability will be increased microcomputer processing time and therefore reduced longevity for the implantable monitor. One of the more significant factors influencing the performance of analysis algorithms to date has been noise artifacts generated from surface electrodes and associated wires. It is assumed that invasive electrodes should be more reliable and present less noise than the body surface electrodes that are typically used with external monitoring equipment. It is unknown, however, whether invasive electrodes would replicate ECG signals as seen from the body surface. Therefore, we compared the electrogram characteristics and noise artifacts of subcutaneous elec-
stimulation threshold to increase device longevity and improve tissue interaction with the stimulating electrodes. In comparison, the design of an implantable monitor would require greater emphasis on the ease of implantation of the system (leads and monitor) to minimize risk and maximize patient acceptance. The design of software for the monitor would emphasize optimization of data storage and retrieval. This may include methods of data compression and optimizing the allocation of memory for the device. There would also be a need for efficiem data analysis algorithms to maximize device longevity. These analysis algorithms, which for ambulatolT monitors have only been required to continuously operate reliably for days, would now be expected to be versatile in providing both accuracy and efficiency continuously over periods of weeks and months of operation.
Icua=9.~
•
k
Subcutaneous ECG S u r l a c e EC6 Difference
~
~..~.,. ~.~. ......---..... .....-..-.,,..,,.,..,..._.-_.........,. .........,.............
,.......... ....,.........
50
0
msec
B
Fig. 1. (A) Beat-to-beat correlation waveform analysis. (B) Average-to-average correlation waveform analysis.
76
Journal of Electrocardiology Vol. 27 Supplement
trodes and skin surface electrodes in two separate studies. The goal of the first study was to determine the similarity of signals between body surface electrodes and subcutaneous electrodes placed in the same anatomic location. In six sheep, body surface and subcutaneous electrodes were applied to the anterior and lateral chest wall, overlying each other. The morphologies of the electrograms from both electrode sites were compared using correlation analysis methods (Fig. 1). A pointwise variance was calculated comparing the surface and the subcutaneous electrograms to calculate a correlation coefficient. The mean correlation coefficient was calculated to be 0.997 _+ 0.002. In a second study, baseline shifts and high-frequency noise were evaluated and quantified. In six dogs, 24-hour Holter recordings were made from bipolar subcutaneous catheter electrodes on the anterior chest wall. In 11 patients, similar recordings were also made from body surface electrodes in similar locations. The 24-hour recordings were analyzed for the quantity of high-frequency noise, which was defined to be greater than 50/~V root-meansquare. The recordings were also analyzed for beat-to-beat baseline shifts greater than 0.5 mV. The high-frequency noise was present in 0.04 + 0.05% of the beats from the subcutaneous recordings as compared with 0.81 _+ 1.18% of the beats from the body surface recordings (P < .001). Beat-to-beat baseline shift was found in 0.21 + 0.20% of the subcutaneous recordings and 1.59 _+ 1.28% of the body surface recordings (P < .031). We conclude from these two studies that subcutaneous electrodes do provide ECG signals similar in morphqlogy to their corresponding body surface sites. Also, the overall quality of electrogram data recorded from subcutaneous electrodes is superior to ECG recordings from body surface electrodes with respect to artifacts.
Optimal Data Storage The storage of data within an implanted device is always an important design feature. The Holter monitor has been used clinically with the assumption that all the ECG is available for trained personnel to verify and edit the performance of the analysis algorithms. The typical two-channel digital monitor requires from four to eight megabytes of memory for every 24 hours of operation, given that data reduction and data compression methods are used. Without data reduction and compression, the memory requirements for 24 hours would be excess of 44 megabytes. Present memory technology is inadequate for complete electrogram storage for long periods of time. Therefore, it is imperative that an implanted monitor have not only accurate detection algorithms to reduce the need for continuous electrogram storage but also use efficient memory management schemes. Also, the memory management of an implantable monitor must be able to provide the ability to preselect a data storage management scheme based on the individual patient's conditions or the physician's preference similar to how a pacemaker is programmed to different operating modes.
Even with present memory chip technology, a significant amount of data can still be stored with an implanted monitor, which leads to yet another design challenge (ie, the ability to retrieve a large amount of data from an implanted device within a reasonable amount of time). For example, 1 megabyte of stored data could require data transmission rates in excess of 500 kbits/s to retrieve the data in less than 20 seconds from the device to the programmer.
Device Design for Low-power Consumption The factors that significantly affect the longevity of an implantable monitor will be those functions that affect the steady-state power consumption the most. This includes the data acquisition of the ECG signals and the efficiency of ongoing real-time analysis algorithms such as the beat discrimination. Power consumption from data acquisition may be influenced by bandpass requirements, amplifier designs, digitization sample rate, and signal filtering requirements. Algorithms running in real time would be affected by the efficiency of the microcomputer's instruction set and the software requirements of the signal processing methods. To the designer, this will be a constant trade-off of performance versus longevity.
Patient Alarms Another potential feature is the incorporation- of an alarm into the monitor. This alarm could be activated whenever the monitor detects a significant cardiac event, thus providing direct feedback to the patient. From the design implementation point of view, one basic question is how to communicate to the patient. Should there be a piezoelectric audible alarm (as used in the early ICDs) or should there be some form of electrical stimulation to provide the patient with feedback? Although a patient alarm in theory can be valuable to both the physician and patient, in practice, it could create many logistic problems. The first concern may be patient awareness and compliance to an alarm. The question is whether patients would be capable of perceiving an alarm if it were to occur during sleeping, or even during waking, hours. If the patient can perceive the alarm, how would he or she react to it? Would there be anticipation or anxiety by the patient, thus introducing chances of false perception of the alarm? Other considerations may also include the effect of this feature on the design of the clinical validation study and on the regulatory approval process.
Justification for an Implantable Monitor As mentioned earlier, the concept of implanting a nontherapeutic device in a patient would have a greater acceptance if the implantation procedure were to be simple and the hardware designed for a small size. This would provide
Implantable Monitoring Systems a product that would minimize risk to patients and optimize their acceptance. So far, we have reviewed many of the technical challenges that may be encountered in developing an implantable cardiac monitor. Perhaps a greater challenge this product faces may be the issue of whether cardiologists would implant a monitor, and if so, who would pay for both the equipment and the associated implantation costs. Most vital to establishing a diagnostic role for a long-term implantable monitor will be to keep the overall costs low, This includes the equipment, the implantation procedure, and patient follow-up evaluation. Historically, implantable electronic devices have provided some therapeutic benefit directly to the patient, whereas an implantable monitor may only provide the clinician with long-term diagnostic information with possible indirect patient benefit. If it can be shown that this new information were to assist the physician in providing earlier medical intervention of a patient, then perhaps more aggressive diagnostic procedures could potentially be reduced and result in a long-term cost benefit. It is also ,difficult to predict whether future healthcare reform will be either a benefit or a barrier to the development of a totally implantable monitor.
Conclusion As we haw~ presented here, a number of new issues may be considered in developing a totally implantable monitor:
•
Yomtov et al.
77
However, these issues should be no more challenging than those that have been encountered with other implantable products. The greater challenge may be the acceptance of such a device and the ability to pay for this new product within the environment of healthcare reform. We have also shown that subcutaneous electrodes may provide electrograms virtually identical in morphology and with fewer artifacts than surface electrodes. The design and development of a totally implantable cardiac monitor therefore provides new challenges for the application of implantable device technology and for the implantable device industry.
References 1. Holter N J: New methods for heart studies: continuous electrocardiography of active subjects over long periods of time is now practical. Science 134:1214, 1961 2. Bigger JT, Reiffel JA, Coromilas J e t ah Ambulatory electrocardiography, p. 36. In Management of cardiac arrhythmias: The non pharmacological approach. JB Lippincott, Philadelphia, 1987 3. Mark RG, Ripley KL: Ambulatory monitoring: realtime analysis versus tape scanning systems, p. 323. In Proceedings from a workshop in Pisa. Martinus-Nijhoff, Boston, 1983