Preliminary Experience with the Use of a Programmable Pacemaker* Dryden Morse, M.D., F.C.C.P.; 00 Aaron Samuel, M.D.;t Javier Fernandez, M.D., F.C.C.P.;t Gerald Lemole, M.D., and Victor Parsonnet, M.D.11
F . C.C.P
. ;~
One hundred sixty-four patients, in whom new extemaDy programmable pacemaken bad been inserted, were studied over a two year period, beginning July, 1972. FoDowing implantation, the rate and current output of tbJs pacemaker could be changed at any time by a noninvasive technique involving electromagnetic pulse
trains emitted by an external "programmer." In 89 per· cent of the patients it was possible to reduce battery out· put by half, implying greater longevity of the pacer in these cases. In 15 percent of the patients, manipulative control of the pacemaker rate was employed and found beneficial.
since July, 1972, the authors have had the opportunity to evaluate a new series of externally programmable pacemakers implanted in 164 patients at the Deborah Heart and Lung Center and Newark Beth Israel Medical Center. All patients have been followed to date. This paper describes the functions of programmer and pacemaker and discusses the merits and results of the technique as seen in the initial one year follow-up of the patients.
Timing Cycl.e (R-wave Blocked Units)
DESCRIPTION AND FUNCTION OF THE
u NIT
The rate of stimulation is determined by the frequency of a clock oscillator in the pacer, which is programmed. All events of the pacer beat-to-beat cycle are controlled in their length by the frequency of this clock. Clock speed therefore determines both the length of the refractory period, during which the pacer does not respond to cardiac signals, and the length of the noise sampling period, fixed at 1,6 of the refractory period. During the latter interval. if incoming signals are present and meet certain conditions, the pacer goes to a fixed rate mode for one cycle until the sampling is repeated. Upon detection of an R-wave during the alert period, the pacer terminates its cycle to start a fresh one without produc-
The programmable pacemaker is available in four pacing modes: ( 1) R-wave inhibited ( Omni-Stanicor) ;§ ( 2) Rwave synchronous ( Omni-Ectocor) ;§ ( 3) atrial synchronized ventricular ( Omni-Atricor) ;§ and ( 4) fixed rate (OmniVentricor) .§ The pacemaker unit is circular in shape (Fig 1) and contains five mercuric-oxide-zinc batteries with a tangential lead connection to the cardiac electrode. Featuring digital microcircuitry, the pacer's rate and output are controllable by an external, battery-powered programmer ( Fig 2 ) which emits trains of electromagnetic pulses ( 3 msec period) that open and close a reed switch ( Fig 3) within the pacer. Because programming pulses are recognized and counted by a binary counter in the pacer, invasive penetration of the patient is not required. The pacer rate can be adjusted from 60 to 100 pulses per minute at settings of 60, 65, 70, 80, 90, or 100 ppm. Output current options are 2.3 ma, 4 ma, 6 ma, and 9 ma. This work was performed at the Deborah Heart and Lung Center, Browns Mills, New Jersey and at the Beth Israel Medical Center, Newark. ••Assistant Professor in Thoracic Surgery, Temple University Medical School, Philadelphia. tFellow in Thoracic Surgery, 1973-74, Deborah Heart and Lung Center, Browns Mills, New Jersey. tAssistant Profeasor in Thoracic Surgery, Temple University Medical School. 1!Professor in Thoracic Surgery, Temple University Medical School. llClinical Professor of Surgery, New Jersey Medical School, Newark. §Manufactured by Cordis Corporation Manuscript received June 18; revision accepted Se_ptember 16. Reprint requests: Dr. Morse, Deborah Heart and Lung Center, Browns Mills, New ]ef'sey 08015 0
544 MORSE ET AL
FIGURE 1. Photograph of obverse of pacer showing the reed switch at the top and two cans at the top containing the programming and pacing circuits. The mode of function is radiographically identifiable by the initials at the bottom ( in this case "DB"). DA=R-wave blocked; DB=R-wave triggered; DC=Atrial-Synchronous; DD=Fixed rate.
CHEST, 67: 5, MAY, 1975
lus intensity, and the type of pacer being programmed ( Fig 2). One control switch determines the rate, which can be set at fixed rates of 60, 65, 70, and 80 ppm for all models. Another switch determines current output, or stimulus intensity, which can be varied between settings of 2.3 ma "test," 4 ma "low," 6 ma "medium" and 9 ma "high." Rates of 90 and 100 ppm can also be selected for the R-wave blocked and fixed rate models. The factors and constraints which resulted in the choice of these particular numbers are shown in Table 1. The energizing button is situated on the handle, which when depressed and released starts a train of magnetic field pulsations that reprograms the pacer. Next to this button is a small light which Hashes every time the electromagnetic coil is energized. ~· On the undersurface of the programmer is the coil of the electromagnet. This is removable and can be replaced by a plug-in st.erilizable unit which features the same coil on an eight foot cord for use in a sterile field during pacer implantation. Principle of Programmer Operation
The output of the programmer coil is a pulsating magnetic field which causes the reed switch in the pacemaker to close when the field increases above a critical value and to open when the field drops below this value. This opening and closing feeds electrical impulses first to a count.er and then to a decoder in the pacer. The magnetic field of the programmer coil pulsates once every 3 milliseconds (330/sec) until the desired number of switch closings is reached. This number is automatically determined by dialing the output rate and/or current on the programmer. Thus, the number of pulsations in each pulse train serves as a code to trigger the desired change in the rate and/or the output in the pacer. CLINICAL EXPERIENCE
F1GURE 2. The external programmer emits a train of pulses when the large dark button on the handle is released after compression. ing an output impulse (such as in the case of the R-wave blocked unit) . If, however, an R-wave does not fall in the alert period, the pacemaker completes the cycle and fires at its preset rate. Description and Function of Programmer
The programmer contains control switches for rate, stimu-
Omnicor pacemakers were inserted in 164 patients between July, 1972 and October, 1973 at Deborah Heart and Lung Center and Newark Beth Israel Medical Center. The sex distribution of the patients was almost equal; ages ranged from 19 to 87 years with an average of 64.9 years. Of the pacemakers used, 87 were ventricular inhibited, 57 ventricular synchronous, 13 fixed-rate, and 7 atrial synchroni2ed ventricular. Table I-Factor• in the Selection o/ Parameter• in the Pro.ra1nrnable Pacer Rate
FIGURE 3. Photograph of the two programming and pacing circuits enclosed in two rectangular metal "cans" with the reed switch in the upper center (two dark circles with "glass" tube between) . A dime is shown on the left for size comparison.
CHEST, 67: 5, MAY, 1975
I Desired minimum 60 ppm,• maximum 100 ppm. 2 Uniform spacing in pulse-to-pulse interval: Ranging from 600 to 1000 msec. Round numbers in ppm.
Outputt l Maximum Same as in non-programmable pacers. 2 Minimum Significantly lower than what is needed for chronic cases. Should fit the usual requirement for 3 Low chronic cases. 4 Medium As dictated by the other three settings. 5 Constraint No overlap between ranges when one cell is depleted. *ppm-pulses per Ininute. tR.efractory and alert periods are determined by the binary counter.
PROGRAMMABLE PACEMAKER: PRELIMINARY EXPERIENCE 545
Some of the venbicular synchronous units were initially used because of a manufacturing shortage of venbicular inhibited models. Fixed-rate pacers were used in patients who had gone along for years without pacemaker competition. Our recent report on stress testing in pacemaker patientsl underlines the frequency of some fonn of competition with exercise ( 85 percent) and points to the exclusive use of non-competitive models. The same findings were noted by Parsonnet and co-authors.2 The abial synchronous units were inserted in patients who already had the three ~icardial leads. No thoracotomies for pacer insertion have been done for the last three years at either of our centers. Patients in this series were followed up by telephonic means,3 at monthly intervals early after insertion and at weekly intervals after 18 months. They were also seen in a pacemaker clinic regularly, where an ECG and pacer analysis4°li were routinely performed. PROGRAMMING METHODS
The pacemaker comes from the factory set at a "medium" current output ( 6 milliamps). After an initial implant, the pacer is reprogrammed two to four months after surgery to a new setting, one setting above that which secures uninterrupted pacing. This allows a safe margin for minor changes in threshold. Similar adjustment may be made on replacements on old pacer electrodes with a stable threshold. At the time of implantation, the new pacer may be set at the next setting above the minimal one which produces pacing. In this series, 139 patients were reprogrammed to "low" setting, 15 patients were left at "medium," 3 cases converted to "high," and 7 were reprogrammed to "test" (Table 2). (If the chronic threshold at reimplantation was measured at below 1.2 ma, the setting of "test" which corresponds to 2.3 milliamps, gives a safe margin.) Programmable CapabilUy
The versatility of the pacemaker has proved to be useful in almost every patient. Fifteen percent of the patients benefited from the rate control and 92 percent from the stimulus intensity control. In 14 cases, the rate was increased above 70 ppm. In ten of these cases, the increase in rate was desirable to control venbicular multifocal premature eontractions. In three cases it was used to enhance the cardiac output, and in one case it was used to insure adequate cardiac output during incidental surgery. The rate was decreased below 70 in 11 cases. Nine patients had reverted back to normal sinus rhythm, and the reprogramming of the rate below 70 was used to encourage this sinus rhythm. In one patient, the decreased rate was used as a baseline for digitalis-controlled arrhythmias. Table 2--Repro.rammi,.. and Final Se11i,.. 0u1P'!I (164 palien18)
Output
M.A.
Test
2.3
Low
No. of Cases
of Current Percent
7
4
4
139
85
Medium
6
15
9
High
9
3
2
164
548 MORSE ET AL
1003
CLINICAL STATUS AND REsULTS
One hundred and sixty-three patients are alive and ambulatory. In this series, none had any intraoperative or postoperative complications. One patient died one month after installation, due to causes not related to the pacer. Most of the patients were already retired, although some of the younger age group are active and working. Ten pacemakers in this series were removed between July, 1972 and October, 1973, because of defective functioning. All these units were sent back to Cordis Corporation for analysis. Among these, four showed little or no output, and four suffered loss of sensing due to defective circuits. The other two appeared to be within specifications. Changes in manufacturing techniques, particularly those involving quality control in the integrated circuits, have much reduced this relatively high early failure rate. DISCUSSION
Despite improvements in techniques of insertion and technology of pacemaker manufacture, continuing questions must be answered. In the case of the programmable pacer, these include: ( 1) the reliability and longevity of the pacemaker; ( 2) the influence of electromagnetic environment on the pulse generator; ( 3) the size of the pacemaker; ( 4) the general cost to the patient; ( 5) the ease of adjustment of pacing rate and output to suit the patient's needs; and ( 6) the battery drain. Our recent reports on pacemaker longevity and reliability indicated two years as the average lifespan of previous models. 8•7 In this regard, it has been demonstrated that an asynchronous generator lasts longer than a triggered unit due to the low current drain on the battery of its simple circuitry. Also, an R-wave inhibited generator is expected to have a longer life than an R-wave synchronous generator because of occasional suppression of the pacer in the former unit and the tendency of the latter type to operate at rates over 70 ppm. These findings also apply in a lesser degree to programmable pacers. An important result of the integrated circuit construction of programmable pacemakers is the lower current drain and consequent increase in battery life compared with non-programmable pacemakers with the same output parameters. (See Figs 4 and 5. Current drain of pacers with integrated circuits is less, both in the "blocked"-or standby-and in the "firing" modes. ) When the programmable pacer is lowered in output according to the patient's threshold, it requires far less current for operation than non-programmable pacers, except those preset at the factory at a low output. The expected life of CHEST, 67: 5, MAY, 1975
14
.sE
12
0
:;
80
..
10
.E
8
Medium
·e u
.::
tn c f
... "'
FIGURE
4. The integrated programming circuit.
any battery depends upon the combination of the rate of battery self-discharge and battery drain through usage. Safety Features The pacemaker's output is directly dependent upon the product of the output current and the pulse duration. In the programmable pacer, when the battery voltage starts dropping to an equivalent of the loss of one cell, the output current drop is about 15 percent. However, due to the increase in pacer pulse width, effectiveness of the pulse remains practically unchanged at this lower level and the current is still greater than it would be at the next lower output setting with a fresh battery (Fig 6). This insures safety. Longevity Since experience with the programmable pacemaker is limited to about 2" years, it is too soon to
FIGURE
5. The integrated pacing circuit.
CHEST, 67: 5, MAY, 1975
.a"'::::s
low 6
E
·~ Cl)
4
Test
2 Programmed rate: 70 ppm Typical unit 4
5
3
Number of cells
Stimulus Strength ( ! idt] vs Battery Voltage (
6. When a decrease in current occurs after several years with i:he loss of one mercury cell to a total of four, the current time product available at each setting is still more than at the next lower setting. Even after the loss of two cells, the total stimulus strength is still more than that initially available at the next lower setting. Thus, if this pacer is turned up one setting above the lowest at which pacing occurs, capture will continue even with beginning exhaustion and rate change ( decrease) of over 10 percent. FIGURE
determine precisely the lifespan of various types of this unit. However, if longevity can be derived indirectly by comparison with non-programmable units on which significant data has accumulated (and which are equipped with battery cells similar to the programmable units), the programmable pacemaker's life can be predicted to be significantly longer, on the order of three to five years. 8 While the average self-discharge rate of the older type batteries in non-programmable pacers is estimated to be 23 microamperes, the average self-discharge rate of the newer type batteries in the programmable pacer is estimated at 6 microamperes, and the theoretical battery capacity is 1,150 milliampere hours with improved batteries. A general idea can thus be obtained of pacer PROGRAMMABLE PACEMAKER: PRELIMINARY EXPERIENCE 547
longevity from computation. At a fixed rate of 70 and at a "low" setting ( 4 ma) and operating continuously, the average pacer life of a programmable unit should be about 40 to 50 months. Thus, at this "low" setting (attained in most of our cases), the pacer is draining less than half that of previous models and should last twice as long. For comparison, we have recently studied two of the fixed rate non-programmable units from the same company that paced for 47 and 48 months and were removed after a minimal rate change detected by weekly telephone follow-up• while still functioning. If this was possible in some cases with electronics and batteries of four years ago, life of the newer programmable units may reach six years. The cost per year to the patient shows a decrease although the cost per pacemaker (programmable) is higher.
Protection from Interference Since a strong pulsating electromagnetic field is required for reprogramming the pacemaker, the question arises whether extraneous electromagnetic fields can influence its performance. As the microcircuitry is hermetically sealed in two steel boxes ( Fig I ) , this pacer has increased resistance to most external electromagnetic interference, and we have observed no instance of damage or malfunction in this series. Inadvertent programming of the Omnicor pacer would require an extremely strong pulsating magnetic field of at least eight consecutive pulsations every 3 milliseconds. Because no extraneous source, other than the programmer, has been found to meet these requirements, the probability of accidentally programming the pacer is very low and has not been seen in this series. Interference with this pacer from diathermy and microwave ovens, while possible, is less likely than in more conventional models. In addition, the pacer has a shunting circuit that protects it against electrical damage during defibrillation or cardioversion at up to 400 watts-seconds. The present programmable pacers are the same size as preceding models from the same company but the use of integrated circuits should allow redu~ tion in total size in the near future.
Adjustability The new programmable pacemaker offers the physician an opportunity to adjust the pacer to the •cardiac Datacorp, Inc, Philadelphia, Pa.
548 MORSE ET AL
Table
~..4d_,.,..ea
of ProSrammable Pacer•
1 Adjustments possible when patient's needs change. 2 Chronic threshold evaluation. 3 Arrhythmia control. 4 Longer service - less battery drain - less output current. 5 Less vulnerable to electromagnetic interference. 6 Smaller size possible. 7 Fewer pacers in hospital's stock. 8 Eventually other parameters can also be made adjustable.
patient's needs in regard to rate at any time following insertion. The ability to decrease the power output of each pacer to a new low level some time after implantation depending on the threshold of each individual patient, implies an additional longevity for this pacemaker. For the first time, this adjustment can be made easily and by a noninvasive technique. The substitution of integrated circuits for the soldered welds of more conventional electronic assemblies implies increased reliability, decreased power requirement, and greater longevity. This pacemaker, available in four different models, appears promising for most clinical situations involving permanent pacer implantation (Table 3). REFERENCES
1 Singer E, Gooch AS, Morse D: Exercise induced arrhythmias in patients with pacemaker. JAMA 224:15151518, 1973 2 Parsonnet V, Feldman S, Parsonnet J: Arrhythmias induced by exercise in paced patients. Am Heart J 87 :7682, 1974 3 Pennoclc RS, Dreifus LS, Morse D, et al: Long tenn monitoring of patients with implanted pacemakers. Heart &: Lung 1:227-232, 1972 4 Parsonnet V, Meyers GH, Gilbert L, et al: Prediction of impending pacemaker failure in a pacemaker clinic. Am J Cardiol, 25:311-320, 1970 5 Parsonnet V, Meyers GH, Gilbert L, et al: A regional network of clinics for analysis of implanted pacemakers. In Cardiac Arrhythmias ( Dreifus and Likoff, eds). New York, Grune&: Stratton, 1972 6 Morse D, Tesler UF, Lemole GM: The actual lifespan of pacemakers. Oiest 64:454-458, 1973 7 Parsonnet V: Types of pacemakers and their reliability. In Current Concepts of Cardiac Pacing and Cardioversion ( Meltzer and Kitchell. eds). Philadelphia, Charles Press, 1971 8 Funnan S, Escher DJW, Parker B: Pacemaker longevity. Am J Cardiol 31:111-113, 1973 9 Funnan S, Parker B, Krauthammer M, et al: The influence of electromagnetic environment on the performance of artificial cardiac pacemakers. Ann Thorac Surg 6:9095, 1968 10 Morse D, Parsonnet V, Cuddy TE, et al: External radiomagnetic control of pacer rate and power ( abst). Chest 64:403, 1973
CHEST, 67: 5, MAY, 1975
F . C.C.P
. ;~
One hundred sixty-four patients, in whom new extemaDy programmable pacemaken bad been inserted, were studied over a two year period, beginning July, 1972. FoDowing implantation, the rate and current output of tbJs pacemaker could be changed at any time by a noninvasive technique involving electromagnetic pulse
trains emitted by an external "programmer." In 89 per· cent of the patients it was possible to reduce battery out· put by half, implying greater longevity of the pacer in these cases. In 15 percent of the patients, manipulative control of the pacemaker rate was employed and found beneficial.
since July, 1972, the authors have had the opportunity to evaluate a new series of externally programmable pacemakers implanted in 164 patients at the Deborah Heart and Lung Center and Newark Beth Israel Medical Center. All patients have been followed to date. This paper describes the functions of programmer and pacemaker and discusses the merits and results of the technique as seen in the initial one year follow-up of the patients.
Timing Cycl.e (R-wave Blocked Units)
DESCRIPTION AND FUNCTION OF THE
u NIT
The rate of stimulation is determined by the frequency of a clock oscillator in the pacer, which is programmed. All events of the pacer beat-to-beat cycle are controlled in their length by the frequency of this clock. Clock speed therefore determines both the length of the refractory period, during which the pacer does not respond to cardiac signals, and the length of the noise sampling period, fixed at 1,6 of the refractory period. During the latter interval. if incoming signals are present and meet certain conditions, the pacer goes to a fixed rate mode for one cycle until the sampling is repeated. Upon detection of an R-wave during the alert period, the pacer terminates its cycle to start a fresh one without produc-
The programmable pacemaker is available in four pacing modes: ( 1) R-wave inhibited ( Omni-Stanicor) ;§ ( 2) Rwave synchronous ( Omni-Ectocor) ;§ ( 3) atrial synchronized ventricular ( Omni-Atricor) ;§ and ( 4) fixed rate (OmniVentricor) .§ The pacemaker unit is circular in shape (Fig 1) and contains five mercuric-oxide-zinc batteries with a tangential lead connection to the cardiac electrode. Featuring digital microcircuitry, the pacer's rate and output are controllable by an external, battery-powered programmer ( Fig 2 ) which emits trains of electromagnetic pulses ( 3 msec period) that open and close a reed switch ( Fig 3) within the pacer. Because programming pulses are recognized and counted by a binary counter in the pacer, invasive penetration of the patient is not required. The pacer rate can be adjusted from 60 to 100 pulses per minute at settings of 60, 65, 70, 80, 90, or 100 ppm. Output current options are 2.3 ma, 4 ma, 6 ma, and 9 ma. This work was performed at the Deborah Heart and Lung Center, Browns Mills, New Jersey and at the Beth Israel Medical Center, Newark. ••Assistant Professor in Thoracic Surgery, Temple University Medical School, Philadelphia. tFellow in Thoracic Surgery, 1973-74, Deborah Heart and Lung Center, Browns Mills, New Jersey. tAssistant Profeasor in Thoracic Surgery, Temple University Medical School. 1!Professor in Thoracic Surgery, Temple University Medical School. llClinical Professor of Surgery, New Jersey Medical School, Newark. §Manufactured by Cordis Corporation Manuscript received June 18; revision accepted Se_ptember 16. Reprint requests: Dr. Morse, Deborah Heart and Lung Center, Browns Mills, New ]ef'sey 08015 0
544 MORSE ET AL
FIGURE 1. Photograph of obverse of pacer showing the reed switch at the top and two cans at the top containing the programming and pacing circuits. The mode of function is radiographically identifiable by the initials at the bottom ( in this case "DB"). DA=R-wave blocked; DB=R-wave triggered; DC=Atrial-Synchronous; DD=Fixed rate.
CHEST, 67: 5, MAY, 1975
lus intensity, and the type of pacer being programmed ( Fig 2). One control switch determines the rate, which can be set at fixed rates of 60, 65, 70, and 80 ppm for all models. Another switch determines current output, or stimulus intensity, which can be varied between settings of 2.3 ma "test," 4 ma "low," 6 ma "medium" and 9 ma "high." Rates of 90 and 100 ppm can also be selected for the R-wave blocked and fixed rate models. The factors and constraints which resulted in the choice of these particular numbers are shown in Table 1. The energizing button is situated on the handle, which when depressed and released starts a train of magnetic field pulsations that reprograms the pacer. Next to this button is a small light which Hashes every time the electromagnetic coil is energized. ~· On the undersurface of the programmer is the coil of the electromagnet. This is removable and can be replaced by a plug-in st.erilizable unit which features the same coil on an eight foot cord for use in a sterile field during pacer implantation. Principle of Programmer Operation
The output of the programmer coil is a pulsating magnetic field which causes the reed switch in the pacemaker to close when the field increases above a critical value and to open when the field drops below this value. This opening and closing feeds electrical impulses first to a count.er and then to a decoder in the pacer. The magnetic field of the programmer coil pulsates once every 3 milliseconds (330/sec) until the desired number of switch closings is reached. This number is automatically determined by dialing the output rate and/or current on the programmer. Thus, the number of pulsations in each pulse train serves as a code to trigger the desired change in the rate and/or the output in the pacer. CLINICAL EXPERIENCE
F1GURE 2. The external programmer emits a train of pulses when the large dark button on the handle is released after compression. ing an output impulse (such as in the case of the R-wave blocked unit) . If, however, an R-wave does not fall in the alert period, the pacemaker completes the cycle and fires at its preset rate. Description and Function of Programmer
The programmer contains control switches for rate, stimu-
Omnicor pacemakers were inserted in 164 patients between July, 1972 and October, 1973 at Deborah Heart and Lung Center and Newark Beth Israel Medical Center. The sex distribution of the patients was almost equal; ages ranged from 19 to 87 years with an average of 64.9 years. Of the pacemakers used, 87 were ventricular inhibited, 57 ventricular synchronous, 13 fixed-rate, and 7 atrial synchroni2ed ventricular. Table I-Factor• in the Selection o/ Parameter• in the Pro.ra1nrnable Pacer Rate
FIGURE 3. Photograph of the two programming and pacing circuits enclosed in two rectangular metal "cans" with the reed switch in the upper center (two dark circles with "glass" tube between) . A dime is shown on the left for size comparison.
CHEST, 67: 5, MAY, 1975
I Desired minimum 60 ppm,• maximum 100 ppm. 2 Uniform spacing in pulse-to-pulse interval: Ranging from 600 to 1000 msec. Round numbers in ppm.
Outputt l Maximum Same as in non-programmable pacers. 2 Minimum Significantly lower than what is needed for chronic cases. Should fit the usual requirement for 3 Low chronic cases. 4 Medium As dictated by the other three settings. 5 Constraint No overlap between ranges when one cell is depleted. *ppm-pulses per Ininute. tR.efractory and alert periods are determined by the binary counter.
PROGRAMMABLE PACEMAKER: PRELIMINARY EXPERIENCE 545
Some of the venbicular synchronous units were initially used because of a manufacturing shortage of venbicular inhibited models. Fixed-rate pacers were used in patients who had gone along for years without pacemaker competition. Our recent report on stress testing in pacemaker patientsl underlines the frequency of some fonn of competition with exercise ( 85 percent) and points to the exclusive use of non-competitive models. The same findings were noted by Parsonnet and co-authors.2 The abial synchronous units were inserted in patients who already had the three ~icardial leads. No thoracotomies for pacer insertion have been done for the last three years at either of our centers. Patients in this series were followed up by telephonic means,3 at monthly intervals early after insertion and at weekly intervals after 18 months. They were also seen in a pacemaker clinic regularly, where an ECG and pacer analysis4°li were routinely performed. PROGRAMMING METHODS
The pacemaker comes from the factory set at a "medium" current output ( 6 milliamps). After an initial implant, the pacer is reprogrammed two to four months after surgery to a new setting, one setting above that which secures uninterrupted pacing. This allows a safe margin for minor changes in threshold. Similar adjustment may be made on replacements on old pacer electrodes with a stable threshold. At the time of implantation, the new pacer may be set at the next setting above the minimal one which produces pacing. In this series, 139 patients were reprogrammed to "low" setting, 15 patients were left at "medium," 3 cases converted to "high," and 7 were reprogrammed to "test" (Table 2). (If the chronic threshold at reimplantation was measured at below 1.2 ma, the setting of "test" which corresponds to 2.3 milliamps, gives a safe margin.) Programmable CapabilUy
The versatility of the pacemaker has proved to be useful in almost every patient. Fifteen percent of the patients benefited from the rate control and 92 percent from the stimulus intensity control. In 14 cases, the rate was increased above 70 ppm. In ten of these cases, the increase in rate was desirable to control venbicular multifocal premature eontractions. In three cases it was used to enhance the cardiac output, and in one case it was used to insure adequate cardiac output during incidental surgery. The rate was decreased below 70 in 11 cases. Nine patients had reverted back to normal sinus rhythm, and the reprogramming of the rate below 70 was used to encourage this sinus rhythm. In one patient, the decreased rate was used as a baseline for digitalis-controlled arrhythmias. Table 2--Repro.rammi,.. and Final Se11i,.. 0u1P'!I (164 palien18)
Output
M.A.
Test
2.3
Low
No. of Cases
of Current Percent
7
4
4
139
85
Medium
6
15
9
High
9
3
2
164
548 MORSE ET AL
1003
CLINICAL STATUS AND REsULTS
One hundred and sixty-three patients are alive and ambulatory. In this series, none had any intraoperative or postoperative complications. One patient died one month after installation, due to causes not related to the pacer. Most of the patients were already retired, although some of the younger age group are active and working. Ten pacemakers in this series were removed between July, 1972 and October, 1973, because of defective functioning. All these units were sent back to Cordis Corporation for analysis. Among these, four showed little or no output, and four suffered loss of sensing due to defective circuits. The other two appeared to be within specifications. Changes in manufacturing techniques, particularly those involving quality control in the integrated circuits, have much reduced this relatively high early failure rate. DISCUSSION
Despite improvements in techniques of insertion and technology of pacemaker manufacture, continuing questions must be answered. In the case of the programmable pacer, these include: ( 1) the reliability and longevity of the pacemaker; ( 2) the influence of electromagnetic environment on the pulse generator; ( 3) the size of the pacemaker; ( 4) the general cost to the patient; ( 5) the ease of adjustment of pacing rate and output to suit the patient's needs; and ( 6) the battery drain. Our recent reports on pacemaker longevity and reliability indicated two years as the average lifespan of previous models. 8•7 In this regard, it has been demonstrated that an asynchronous generator lasts longer than a triggered unit due to the low current drain on the battery of its simple circuitry. Also, an R-wave inhibited generator is expected to have a longer life than an R-wave synchronous generator because of occasional suppression of the pacer in the former unit and the tendency of the latter type to operate at rates over 70 ppm. These findings also apply in a lesser degree to programmable pacers. An important result of the integrated circuit construction of programmable pacemakers is the lower current drain and consequent increase in battery life compared with non-programmable pacemakers with the same output parameters. (See Figs 4 and 5. Current drain of pacers with integrated circuits is less, both in the "blocked"-or standby-and in the "firing" modes. ) When the programmable pacer is lowered in output according to the patient's threshold, it requires far less current for operation than non-programmable pacers, except those preset at the factory at a low output. The expected life of CHEST, 67: 5, MAY, 1975
14
.sE
12
0
:;
80
..
10
.E
8
Medium
·e u
.::
tn c f
... "'
FIGURE
4. The integrated programming circuit.
any battery depends upon the combination of the rate of battery self-discharge and battery drain through usage. Safety Features The pacemaker's output is directly dependent upon the product of the output current and the pulse duration. In the programmable pacer, when the battery voltage starts dropping to an equivalent of the loss of one cell, the output current drop is about 15 percent. However, due to the increase in pacer pulse width, effectiveness of the pulse remains practically unchanged at this lower level and the current is still greater than it would be at the next lower output setting with a fresh battery (Fig 6). This insures safety. Longevity Since experience with the programmable pacemaker is limited to about 2" years, it is too soon to
FIGURE
5. The integrated pacing circuit.
CHEST, 67: 5, MAY, 1975
.a"'::::s
low 6
E
·~ Cl)
4
Test
2 Programmed rate: 70 ppm Typical unit 4
5
3
Number of cells
Stimulus Strength ( ! idt] vs Battery Voltage (
6. When a decrease in current occurs after several years with i:he loss of one mercury cell to a total of four, the current time product available at each setting is still more than at the next lower setting. Even after the loss of two cells, the total stimulus strength is still more than that initially available at the next lower setting. Thus, if this pacer is turned up one setting above the lowest at which pacing occurs, capture will continue even with beginning exhaustion and rate change ( decrease) of over 10 percent. FIGURE
determine precisely the lifespan of various types of this unit. However, if longevity can be derived indirectly by comparison with non-programmable units on which significant data has accumulated (and which are equipped with battery cells similar to the programmable units), the programmable pacemaker's life can be predicted to be significantly longer, on the order of three to five years. 8 While the average self-discharge rate of the older type batteries in non-programmable pacers is estimated to be 23 microamperes, the average self-discharge rate of the newer type batteries in the programmable pacer is estimated at 6 microamperes, and the theoretical battery capacity is 1,150 milliampere hours with improved batteries. A general idea can thus be obtained of pacer PROGRAMMABLE PACEMAKER: PRELIMINARY EXPERIENCE 547
longevity from computation. At a fixed rate of 70 and at a "low" setting ( 4 ma) and operating continuously, the average pacer life of a programmable unit should be about 40 to 50 months. Thus, at this "low" setting (attained in most of our cases), the pacer is draining less than half that of previous models and should last twice as long. For comparison, we have recently studied two of the fixed rate non-programmable units from the same company that paced for 47 and 48 months and were removed after a minimal rate change detected by weekly telephone follow-up• while still functioning. If this was possible in some cases with electronics and batteries of four years ago, life of the newer programmable units may reach six years. The cost per year to the patient shows a decrease although the cost per pacemaker (programmable) is higher.
Protection from Interference Since a strong pulsating electromagnetic field is required for reprogramming the pacemaker, the question arises whether extraneous electromagnetic fields can influence its performance. As the microcircuitry is hermetically sealed in two steel boxes ( Fig I ) , this pacer has increased resistance to most external electromagnetic interference, and we have observed no instance of damage or malfunction in this series. Inadvertent programming of the Omnicor pacer would require an extremely strong pulsating magnetic field of at least eight consecutive pulsations every 3 milliseconds. Because no extraneous source, other than the programmer, has been found to meet these requirements, the probability of accidentally programming the pacer is very low and has not been seen in this series. Interference with this pacer from diathermy and microwave ovens, while possible, is less likely than in more conventional models. In addition, the pacer has a shunting circuit that protects it against electrical damage during defibrillation or cardioversion at up to 400 watts-seconds. The present programmable pacers are the same size as preceding models from the same company but the use of integrated circuits should allow redu~ tion in total size in the near future.
Adjustability The new programmable pacemaker offers the physician an opportunity to adjust the pacer to the •cardiac Datacorp, Inc, Philadelphia, Pa.
548 MORSE ET AL
Table
~..4d_,.,..ea
of ProSrammable Pacer•
1 Adjustments possible when patient's needs change. 2 Chronic threshold evaluation. 3 Arrhythmia control. 4 Longer service - less battery drain - less output current. 5 Less vulnerable to electromagnetic interference. 6 Smaller size possible. 7 Fewer pacers in hospital's stock. 8 Eventually other parameters can also be made adjustable.
patient's needs in regard to rate at any time following insertion. The ability to decrease the power output of each pacer to a new low level some time after implantation depending on the threshold of each individual patient, implies an additional longevity for this pacemaker. For the first time, this adjustment can be made easily and by a noninvasive technique. The substitution of integrated circuits for the soldered welds of more conventional electronic assemblies implies increased reliability, decreased power requirement, and greater longevity. This pacemaker, available in four different models, appears promising for most clinical situations involving permanent pacer implantation (Table 3). REFERENCES
1 Singer E, Gooch AS, Morse D: Exercise induced arrhythmias in patients with pacemaker. JAMA 224:15151518, 1973 2 Parsonnet V, Feldman S, Parsonnet J: Arrhythmias induced by exercise in paced patients. Am Heart J 87 :7682, 1974 3 Pennoclc RS, Dreifus LS, Morse D, et al: Long tenn monitoring of patients with implanted pacemakers. Heart &: Lung 1:227-232, 1972 4 Parsonnet V, Meyers GH, Gilbert L, et al: Prediction of impending pacemaker failure in a pacemaker clinic. Am J Cardiol, 25:311-320, 1970 5 Parsonnet V, Meyers GH, Gilbert L, et al: A regional network of clinics for analysis of implanted pacemakers. In Cardiac Arrhythmias ( Dreifus and Likoff, eds). New York, Grune&: Stratton, 1972 6 Morse D, Tesler UF, Lemole GM: The actual lifespan of pacemakers. Oiest 64:454-458, 1973 7 Parsonnet V: Types of pacemakers and their reliability. In Current Concepts of Cardiac Pacing and Cardioversion ( Meltzer and Kitchell. eds). Philadelphia, Charles Press, 1971 8 Funnan S, Escher DJW, Parker B: Pacemaker longevity. Am J Cardiol 31:111-113, 1973 9 Funnan S, Parker B, Krauthammer M, et al: The influence of electromagnetic environment on the performance of artificial cardiac pacemakers. Ann Thorac Surg 6:9095, 1968 10 Morse D, Parsonnet V, Cuddy TE, et al: External radiomagnetic control of pacer rate and power ( abst). Chest 64:403, 1973
CHEST, 67: 5, MAY, 1975
