- Email: [email protected]
Clinical evaluation of new pulse generator with narrow pulse width for conservation of battery energy
Clinical evaluation of new pulse generator with narrow pulse width for conservation of battery energy The prime goal in cardiac pacemaking is to extend the life of the pulse generator. The Medtronic Model 5945 unit incorporates two modifications which increase pacer longevity: a narrowed pulse width and an improved mercury-zinc cell. We implanted this unit in 21 patients. A/though the basic performance of the pacemaker was good, 30 per cent of the patients exhibited muscle inhibition under conditions of extreme stress. Consequently, the sensitivity of ten additional units was reduced to lessen this problem, and the pacemakers were implanted in 10 more patients. Seventeen of the patients from the first group (4 died of causes not related to the pacemaker) have been followed for more than 1 year, and all 10 patients from the second group have been followed for 7 months. The performance of the pulse generators is stable in all.
Nicholas P. D. Smyth, M.D.,* Clif Alferness, B.S.E.E.,** Larry Shearon, B.S.E.E.,** Ronald H. Rockland, Ph.D.,** John M. Keshishian, M.D.,* and Alvin Johnson, C.V.T.,* Washington, D. C., and Minneapolis, Minn.
The
most important goal in pacing today is the extension of the life of the pulse generator. It has been shown that energy drain on conventional batteries can be minimized by the combined use of small electrodes and reduced-output pulse generators J-:l (Fig. I). Studying the pulse wave form, whether for current or voltage, readily discloses that there is another approach to energy conservation (Fig. 2). If the current or voltage pulse is kept at conventional levels of 10 Ma. and 5 v., but the pulse width is narrowed, it is possible to achieve an even greater energy savings per pulse and therefore a greater expectation of increased pulse generator longevity";' (Fig. 3). Supported by Medtronic, Incorporated, and The Potomac Fund for Cardiovascular Research, Washington, D. C. Recieved for publication May 24, 1974, * Departments of Surgery, The Washington Hospital Center and Prince Georges General Hospital, and The George Washington University School of Medicine, Washington, D. C. * *Medtronic, Incorporated, Minneapolis, Minn. 55418.
In addition to reducing the current drain on the battery, it is possible by improving the mercury-zinc cell to increase the amount of current that can be extracted from the cell during use. A significant improvement has been achieved in the Mallory RMI cell by the addition of a second separator between the anode and cathode. As a result, internal shorts have been virtually eliminated in the improved Mallory RMI cell (Fig. 4). Comparative tests of conventional and improved cells have shown failures after 24 months in the former and none after 30 months in the latter. Furthermore, at failure the conventional cell had expended 59 per cent of its capacity, whereas after 30 months the improved cell had expended only 75 per cent of its capacity. It should be possible to extract 90 per cent of the cell capacity and thereby extend the power source life of a four-cell, ventricular inhibited pulse generator to beyond 5 years.
471
472
The Journal of Thoracic and Cardiovascular Surgery
Smyth et at.
10
75 -
Time E ~
~
2 lmos
Q.
E
.
<1 ~
"5
Time
Fig. 2. Ways of reducing pacemaker energy output. Reduction in pulse generator output per pulse, in this case measured in voltage, may be produced by narrowing the pulse width.
0
42 ma
Q.
4mm 4rnm ~--_rn2mm
o L - _--l.._ Pulsr
_
--L-_
_
.1..-_-:-'-:--_ -:' 20
25
DuratIOn In ~J h l hs ec ond'
Fig. 1. Strength-duration curves derived from chronic current thresholds measured in 76 patients with Cordis 4 mm. and 2 mm. tip electrodes. It is clear that the pulse generator output may be safely reduced to 7.5 or even 5.0 Ma. without failure to capture at a pulse width of 1.0 or 1.5 msec,
We undertook a clinical study of the Medtronic Model 5945 unit, a new unipolar pulse generator which incorporates both of these innovations. Materials and methods
The pulse generator contains four improved Mallory RMI cells. The circuit is of discrete component cordwood construction and ventricular inhibited type, potted in epoxy, and encased in a titanium shield for improved protection against electromagnetic interference (Fig. 5). The basic pacing rate is 72 beats per minute. In the interference mode, the rate is 53 beats per minute. The pulse width is
0.5 msec. and the pulse amplitude 10 Ma. and 5 v. Sensitivity is 1.5 mv. The refractory period is 300 ± 50 .msec. There is no hysteresis mode. An upper rate limit of 120 beats per minute is incorporated as a safety factor. There are two cell-depletion indicators. When voltage drops from 5.4 to 3.6 v., the rate decreases 5 to 10 beats per minute and the pulse width increases to 1.25 msec. The pulse width increase also compensates for loss of voltage to maintain constant energy delivery to the heart. The current drain is 7 ± 2 }La when the circuit is inhibited and 13 ± 2 }La when pacing into a 500 ohm load. The pulse generator was 'implanted in 21 patients selected at random. In 13 patients the unit was used for replacement, with standard adaptors when other leads were used. All were Cordis unipolar 4 mm. tip electrodes with a surface area of 0.295 sq. em. In 8 patients the unit was the first implant and was used with the Medtronic unipolar lead (Model 6907) with an electrode surface area of 0.113 sq. em. At the time of insertion or replacement of the pulse generator a unipolar, intracavitary electrogram was recorded to measure the amplitude of the R wave. A complete series of unipolar stimulating thresholds was also obtained from three variableparameter external pacemakers for comparison: The Medtronic Model 1361, the
Volume 68
New pulse generator
Number 3
473
September, 1974
6 Minimum Pulse Width 81la
i' '0 ~
(!)
15945 Output I 131la
5
29 Microamps (Ila)
4
'ii
>
II
..J '1:1
'0
.
~
II
3
~
I-
2
M nimum Amplitude 141la
I Chronic Threshold Curve
0+-+-----+----,------,---+---,-----0.1
.5
1.0
1.5
1.7
2.0
Pulse Width - (milliseconds)
Fig. 3. Conservation of battery current drain by reduction of amplitude and pulse width. The output of the now obsolete Model 5870 pulse generator can be adjusted by putting a 10 Kohm Keith needle pot in series with the output circuit. Reduction of the output voltage of a 5 v. pulse generator with a pulse width of 1.7 msec. by this method reduces the current drain from 29 to 14 p.a and allows threshold stimulation in the patient whose strengthduration curve is shown. If the pulse width is instead narrowed to 0.1 msec., stimulation is possible with a current drain of 8 /La; this effects a 44 per cent saving in battery drain. Thus setting the Model 5945 unit at a pulse width of 0.5 msec. and an output of 5 v. results in a 13 /La current drain. The safety margin in this patient is substantial. For further details, see text.
Medtronic Model SP-1356, and a modified Cordis Synchrocor." With the latter two units, threshold readings could be taken at constant current and constant voltage at pulse durations of 0.15, 0.30, 0.50, 0.75, 1.00, and 1.50 msec. The Model 1361 pacemaker has a 10 p.f capacitive output similar to that of the implantable pulse generator. The surface area of the anode, or ground plate, was 18.0 sq. em. At the conclusion of the operative procedure, a basic (magnet) rate was recorded for each pulse generator.
Stimulating thresholds rise steeply as the pulse width narrows to less than 0.5 msec. Furthermore, the values obtained at implantation may rise as much as ten times the initial value within the first few weeks postoperatively. Usually, in our' experience, the rise is much less-about three times the initial value. However, there is at least a theoretical risk of transient loss of capture during this period if a pulse generator with a narrow pulse width is used. For this reason the patients were seen at weekly intervals during the first post-
The Journal of
4 74
Smyth et al.
Thora cic and Cardiovascular Surgery
!
-~
Pin Hole (Leakage / - , Possible Short ) 1.<:
Single-Wrap Separator
~
V'"
Double-Wrap Absorber"
~
~
r=
f-
~
i
Separator t
""r--,
~::::: ~
L
Pin Hole (Must B e Present And ~ Line Up For Leakage To Occur)
Fig. 4. Drawing shows details of improvements in the new RMI cell with double-wrap separator between electrodes (r ight), contrasted with the older unit (left).
Fig. 5. Photograph shows pulse generator (right) and titanium case (left).
operative month, then after 3, 6, and 12 months. At each visit the following studies were carried out: (1) a standard six-lead electrocardiogram; (2) a long rhythm strip including a magnet rate and a tracing taken during inhibition, with the indirect or external overdrive technique' used to demonstrate the intrinsic rhythm during pacer inhibition; (3) a check for muscle twitching caused by anodal stimulation of the adjacent pectoralis major muscle; and (4) a check for inhibition of the pulse generator by muscle potentials from the adjacent pectoralis major muscle during strong isometric contraction produced by the patient's pushing the hands together. The fourth test was included following report of the phenomenon by Wirtzfeld," Myrnin ," and their associates.
Results
No evidence of loss of capture was seen during the first or subsequent months , and there were no cases of muscle twitching. Pulse generator rates decreased 2 to 3 beats per minute during the first few months and then stabilized . Threshold measurements taken on "chronic" leads showed that no loss of capture would occur with first cell depletion , because the pulse width increased to 1.25 msec. (Fig . 6 and Table I). Seven cases of muscle inhibition were encountered, 2 in the initial implant group and 5 in the replacement group (Fig . 7). None of the patients complained of any symptoms either during the test or at any other time.
Volume 68
New pulse generator
Number 3 September, 1974
475
6.0
5.0
~
4.0
0
2:-
., C>
!! 3.0
0
>
"'"
''""
Cl.
2.0 Max. Threshold Patient (RT) Average Chronic Threshold
1.0
--"""""'4....- _. ._ _ Average Acute Threshold
o
0.15 0.30.5 0.75
1.0
1.5
2.0
Pulse Duration (ms)
Fig. 6. Unipolar voltage thresholds versus pulse duration. Acute, chronic, and maximum strength-duration curves for the first 21 patients tested. There is an adequate margin of safety in the Model 5945 output at peak voltage or with one cell depleted and with the compensatory increase in pulse width. For further data, see Table I.
Discussion In view of the findings of muscle inhibition under test conditions in 30 per cent of the patients, a separate study of the phenomenon was undertaken. Eleven patients with unipolar pacemakers were selected. In the group, there were 8 patients with Medtronic Model 5945 and 3 with Cordis Stan icor 143H7 units. Under sterile conditions with local anesthesia, sterile electromyogram electrode needles were inserted percutaneously and advanced until they made contact with the pulse generator case and anode plate. Isometric arm tensions (adduction) were applied by the patients and quantitative measurements of this tension were made by a Chatillon DPD-2 force gauge (Fig. 8). The patients were asked to exercise at two levels of tension. The first exercise involved exerting an isometric tension at 10 pounds while the second exercise involved exerting the maximum force that could be sustained for 10 seconds. The maximum force varied
Table I. Unipolar voltage thresholds versus pulse duration in 21 patients" No. of patients
13 8
Pacemaker
Cordis (4 mm. tip) Medtronic 6907
Implant data
Electrode area (sq. cm.)
Chronic
0.295
Acute
0.113
• For further information, see Fig. 6.
with each patient, ranging from 12 to 45 pounds. Skeletal muscle potentials were recorded with isolation amplifiers to ensure patient safety, and the data were recorded on a Hewlett-Packard instrumentation recorder for subsequent analysis. The results were analyzed by a Tektronix digital processing oscilloscope and a PDP 11/05 minicomputer. Five samples of skeletal muscle potentials were taken from each of the 11 patients, and an average power density spectrum was obtained from these fifty-five
The Journal of
476
Smyth et al.
Thoracic and Cardiovascu lar Surgery
Fig. 7. A . Normal pacing at a rate of 72 beats per minute in a patient with heart block . B, Several periods of complete pacer inhibition in the same patient with an isometric com-
pression force of 15 pounds in both arms.
Fig. 8. Photograph show s the Chatillon DPD-2 force gauge. The instrument was grasped in the patient's left hand while adduction pressure was applied to the plate by the palm of the right hand.
samples . This spectrum showed that the major components of the skeletal muscle potential signals were below 80 Hz, a much lower distribution of frequency components than the skeletal myoelectric energy spectra of Scott.! " The signal levels varied with each patient and with the force of isometric contractions. The maximum potential measured was 2.7 mv. peak. This is in close agreement with the skeletal muscle potential levels observed by Ohm." Obviously, skeletal muscle potentials of this amplitude can be detected
by a pulse generator with a maximum sensitivity of 2 mv. peak. (1 mv. standard square wave) . Therefore, ten additional Model 5945 units were modified to reduce this sensitivity. These modified units were implanted in 10 patients-as replacement units in 7 and as initial implants in 3 patients. The tests were repeated according to the same protocol used for the original 21 patients. None of the 10 experienced muscle inhibition . The modification was in the amplifier
Volume 68 Number 3 September, 1974
10
10 9 Medtronic
9
"0 >
7
"0
c: 0
6
0
Model 6907
'"
0. E
'" '"
"0
7
0
6
Medtronic Model 5818 0.485 cm2 Surface
Medtronic Model 6905 0.450 cm2 Surface
..,>c:
Cordis 2mm cm 2 Surface
E
5
I"
4
PATIENTS
I
..,'" ~
Q.
E
3
'"
o 175
5 4
120 PATIENTS
3
I
$.
::l
D.-
Medtronic Model 6901 o 120 cm2 Surface
2
0
"0
.2
0.295 cm 2 Surface
8
Medtroruc Model 6917 0.120 cm 2 Surface
E
17 Cordis 4mm
9
0113 cm 2 Surface
8
2
477
New pulse generator
~
D.-
2
-a-....o:::=:::::B:=== Current
OL---'---'--'--_--'-_--'-_ _----JL-_ Voltage 0.5
10
15
Pulse Durction (rns)
2
Current Voltage
OL---'--I.....---l...-_--'-_L-_ _---l...-_ _
0.5
10
15
PuIse Duration (rns)
Fig. 9. Acute thresholds versus pulse duration. Average strength-duration curves for all patients in the acute threshold group. Types of leads used are shown in inset.
Fig. 10. Chronic thresholds versus pulse duration. Average strength-duration curves for all patients in the chronic threshold group. Types of leads used are shown in inset.
frequency response and consisted in reducing the maximum sensitivity from 1 to 2 mv. and reducing the 3 db frequency response from 120 to 100 Hz. Analysis of the frequency content of muscle artifacts indicated that the frequency spectrum of cardiac muscle potentials and skeletal muscle potentials overlap to such a degree that one cannot completely eliminate the effects of muscle artifacts by reducing the band pass of the amplifier alone. It is also necessary to reduce the sensitivity of the amplifier across the cardiac frequency spectrum. This makes the pulse generator not only less sensitive to skeletal muscle potentials, but also less sensitive to cardiac muscle potentials. In fact, in 1 of the patients in whom a modified (reduced sensitivity) unit had been implanted, there was a brief period of intermittent lack of sensing during the first 2 postoperative weeks. The problem resolved itself spontaneously, and the unit has now been sensing properly for 6 months. This incident underscores the marginal nature of the trade off involving reduced sensitivity units. The exact balance between re-
duced sensitivity to muscle artifacts and the risk of increased sensing problems has yet to be resolved. At the present time the investigation is continuing, and modified units are available upon special request for those physicians who view muscle inhibition as a significant problem in general or in a particular patient. We have not found it to be a problem and continue to use the standard unmodified Model 5945 pulse generator. At the conclusion of the second study with the Model 5945 generator, the threshold values obtained in both patient groups were combined and averaged. The thresholds values obtained at the time of initial pacemaker insertion are shown in Fig. 9 and those obtained at the time of pulse generator replacement are shown in Fig. 10. It is clear that there is a substantial safety margin between the chronic thresholds and the pulse generator output at 0.5 msec., even with an electrode with a larger surface area. This fact is more clearly shown in Fig. 6: The average acute, average chronic, and highest chronic thresholds obtained in
The Journal of
478
Smyth et al.
Thoracic and Cardiovascular Surgery
the first patient group are plotted with the output of the pulse generator shown at peak voltage and with one cell depleted, with the pulse width increased to 1.25 msec. There is still a substantial safety margin for all the patients in either group. Conclusion
The original group of 21 patients has now been followed for more than 1 year. Four of them have died of causes unrelated to the pacemaker. One of them was shot to death as an innocent bystander in a service station holdup. In the remaining 17 patients, pacemaker function is stable. The 10 patients with less sensitive units have now been followed for 7 months, and, all units are pacing normally. One patient has a minor problem with intermittent twitching of the pectoralis major muscle. Discrimination between muscle and cardiac potentials in a ventricular inhibited, demand pacemaker requires further study. It has not been a clinical problem in our experience. The principle of narrowing the pulse width to conserve energy seems sound, and a study of 31 patients shows no problems related to the narrow pulse width. All patients will be followed to see if the expected increase in pulse generator longevity is realized. REFERENCES Smyth, N. P. D., Keshishian, J. M., Baker, N. R., and Tarjan, P.: Physiological Rationale
for the Clinical Use of Low Output Pacemakers, Med. Ann. D. C. 43: 257, 1974. 2 Center, S., and Tarjan, P.: The Clinical Application of Low-Output Pacemakers: J. THORAC. CARDIOVASC. SURG. 64: 6, 1972. 3 Furman, S., Parker, Escher, D. J. W., and Solomon, N.: Endocardial Threshold of Cardiac Response as a Function of Electrode Surface Area, J. Surg. Res. 8: 161, 1968. 4 Furman, S., Denize, A., Escher, D. J. W., and Schwedel, J. B.: Energy Consumption for Cardiac Stimulation as a Function of Pulse Duration, 1. Surg. Res. 6: 10, 1966. 5 Chardack, W. M., Baken, E. E., Bolduc, L., Giori, F. A., and Gage, A. A.: Magnetically Actuated Pulse Width Control for Implantable Pacemakers, Ann. Cardio!. et Angeiol. 20: 4, 1972. 6 Smyth, N. P. D., Bacos, J. M., Boivin, M., and Keller, J. W.: A Modified Variable Parameter Cardiac Pacer, Chest 57: 3, 1970. 7 Smyth, N. P. D., Bacos, J. M., and Keller, J. W.: Experimental and Clinical Use of a Variable Parameter Cardiac Pacer, Chest 53: 93, 1968. 8 Wirtzfeld, A., Lampadius, M., and Ruprecht, E.-O.: U nterdruckung von Demand- Schrittmachern durch Muskelpotentiale, Dtsch. Med. Wochenschr. 97: 61, 1972. 9 Mymin, D., Cuddy, T. E., Sachehide, N. S., and Winter, D. A.: Inhibition of Demand Pacemakers by Skeletal Muscle Potentials, 1. A. M. A. 223: 5, 1973. 10 Scott, R. N.: Myo-electric Energy Spectra, Med. BioI. Eng. 5: 303, 1967. 11 Ohm, O. 1., Bruland, H., Pederson, O. M., and Waerness, E.: Interference Effect of Myopotentials On Function Of Unipolar Demand Pacemakers, Br. Heart J. 43: 77, 1974.
Nicholas P. D. Smyth, M.D.,* Clif Alferness, B.S.E.E.,** Larry Shearon, B.S.E.E.,** Ronald H. Rockland, Ph.D.,** John M. Keshishian, M.D.,* and Alvin Johnson, C.V.T.,* Washington, D. C., and Minneapolis, Minn.
The
most important goal in pacing today is the extension of the life of the pulse generator. It has been shown that energy drain on conventional batteries can be minimized by the combined use of small electrodes and reduced-output pulse generators J-:l (Fig. I). Studying the pulse wave form, whether for current or voltage, readily discloses that there is another approach to energy conservation (Fig. 2). If the current or voltage pulse is kept at conventional levels of 10 Ma. and 5 v., but the pulse width is narrowed, it is possible to achieve an even greater energy savings per pulse and therefore a greater expectation of increased pulse generator longevity";' (Fig. 3). Supported by Medtronic, Incorporated, and The Potomac Fund for Cardiovascular Research, Washington, D. C. Recieved for publication May 24, 1974, * Departments of Surgery, The Washington Hospital Center and Prince Georges General Hospital, and The George Washington University School of Medicine, Washington, D. C. * *Medtronic, Incorporated, Minneapolis, Minn. 55418.
In addition to reducing the current drain on the battery, it is possible by improving the mercury-zinc cell to increase the amount of current that can be extracted from the cell during use. A significant improvement has been achieved in the Mallory RMI cell by the addition of a second separator between the anode and cathode. As a result, internal shorts have been virtually eliminated in the improved Mallory RMI cell (Fig. 4). Comparative tests of conventional and improved cells have shown failures after 24 months in the former and none after 30 months in the latter. Furthermore, at failure the conventional cell had expended 59 per cent of its capacity, whereas after 30 months the improved cell had expended only 75 per cent of its capacity. It should be possible to extract 90 per cent of the cell capacity and thereby extend the power source life of a four-cell, ventricular inhibited pulse generator to beyond 5 years.
471
472
The Journal of Thoracic and Cardiovascular Surgery
Smyth et at.
10
75 -
Time E ~
~
2 lmos
Q.
E
.
<1 ~
"5
Time
Fig. 2. Ways of reducing pacemaker energy output. Reduction in pulse generator output per pulse, in this case measured in voltage, may be produced by narrowing the pulse width.
0
42 ma
Q.
4mm 4rnm ~--_rn2mm
o L - _--l.._ Pulsr
_
--L-_
_
.1..-_-:-'-:--_ -:' 20
25
DuratIOn In ~J h l hs ec ond'
Fig. 1. Strength-duration curves derived from chronic current thresholds measured in 76 patients with Cordis 4 mm. and 2 mm. tip electrodes. It is clear that the pulse generator output may be safely reduced to 7.5 or even 5.0 Ma. without failure to capture at a pulse width of 1.0 or 1.5 msec,
We undertook a clinical study of the Medtronic Model 5945 unit, a new unipolar pulse generator which incorporates both of these innovations. Materials and methods
The pulse generator contains four improved Mallory RMI cells. The circuit is of discrete component cordwood construction and ventricular inhibited type, potted in epoxy, and encased in a titanium shield for improved protection against electromagnetic interference (Fig. 5). The basic pacing rate is 72 beats per minute. In the interference mode, the rate is 53 beats per minute. The pulse width is
0.5 msec. and the pulse amplitude 10 Ma. and 5 v. Sensitivity is 1.5 mv. The refractory period is 300 ± 50 .msec. There is no hysteresis mode. An upper rate limit of 120 beats per minute is incorporated as a safety factor. There are two cell-depletion indicators. When voltage drops from 5.4 to 3.6 v., the rate decreases 5 to 10 beats per minute and the pulse width increases to 1.25 msec. The pulse width increase also compensates for loss of voltage to maintain constant energy delivery to the heart. The current drain is 7 ± 2 }La when the circuit is inhibited and 13 ± 2 }La when pacing into a 500 ohm load. The pulse generator was 'implanted in 21 patients selected at random. In 13 patients the unit was used for replacement, with standard adaptors when other leads were used. All were Cordis unipolar 4 mm. tip electrodes with a surface area of 0.295 sq. em. In 8 patients the unit was the first implant and was used with the Medtronic unipolar lead (Model 6907) with an electrode surface area of 0.113 sq. em. At the time of insertion or replacement of the pulse generator a unipolar, intracavitary electrogram was recorded to measure the amplitude of the R wave. A complete series of unipolar stimulating thresholds was also obtained from three variableparameter external pacemakers for comparison: The Medtronic Model 1361, the
Volume 68
New pulse generator
Number 3
473
September, 1974
6 Minimum Pulse Width 81la
i' '0 ~
(!)
15945 Output I 131la
5
29 Microamps (Ila)
4
'ii
>
II
..J '1:1
'0
.
~
II
3
~
I-
2
M nimum Amplitude 141la
I Chronic Threshold Curve
0+-+-----+----,------,---+---,-----0.1
.5
1.0
1.5
1.7
2.0
Pulse Width - (milliseconds)
Fig. 3. Conservation of battery current drain by reduction of amplitude and pulse width. The output of the now obsolete Model 5870 pulse generator can be adjusted by putting a 10 Kohm Keith needle pot in series with the output circuit. Reduction of the output voltage of a 5 v. pulse generator with a pulse width of 1.7 msec. by this method reduces the current drain from 29 to 14 p.a and allows threshold stimulation in the patient whose strengthduration curve is shown. If the pulse width is instead narrowed to 0.1 msec., stimulation is possible with a current drain of 8 /La; this effects a 44 per cent saving in battery drain. Thus setting the Model 5945 unit at a pulse width of 0.5 msec. and an output of 5 v. results in a 13 /La current drain. The safety margin in this patient is substantial. For further details, see text.
Medtronic Model SP-1356, and a modified Cordis Synchrocor." With the latter two units, threshold readings could be taken at constant current and constant voltage at pulse durations of 0.15, 0.30, 0.50, 0.75, 1.00, and 1.50 msec. The Model 1361 pacemaker has a 10 p.f capacitive output similar to that of the implantable pulse generator. The surface area of the anode, or ground plate, was 18.0 sq. em. At the conclusion of the operative procedure, a basic (magnet) rate was recorded for each pulse generator.
Stimulating thresholds rise steeply as the pulse width narrows to less than 0.5 msec. Furthermore, the values obtained at implantation may rise as much as ten times the initial value within the first few weeks postoperatively. Usually, in our' experience, the rise is much less-about three times the initial value. However, there is at least a theoretical risk of transient loss of capture during this period if a pulse generator with a narrow pulse width is used. For this reason the patients were seen at weekly intervals during the first post-
The Journal of
4 74
Smyth et al.
Thora cic and Cardiovascular Surgery
!
-~
Pin Hole (Leakage / - , Possible Short ) 1.<:
Single-Wrap Separator
~
V'"
Double-Wrap Absorber"
~
~
r=
f-
~
i
Separator t
""r--,
~::::: ~
L
Pin Hole (Must B e Present And ~ Line Up For Leakage To Occur)
Fig. 4. Drawing shows details of improvements in the new RMI cell with double-wrap separator between electrodes (r ight), contrasted with the older unit (left).
Fig. 5. Photograph shows pulse generator (right) and titanium case (left).
operative month, then after 3, 6, and 12 months. At each visit the following studies were carried out: (1) a standard six-lead electrocardiogram; (2) a long rhythm strip including a magnet rate and a tracing taken during inhibition, with the indirect or external overdrive technique' used to demonstrate the intrinsic rhythm during pacer inhibition; (3) a check for muscle twitching caused by anodal stimulation of the adjacent pectoralis major muscle; and (4) a check for inhibition of the pulse generator by muscle potentials from the adjacent pectoralis major muscle during strong isometric contraction produced by the patient's pushing the hands together. The fourth test was included following report of the phenomenon by Wirtzfeld," Myrnin ," and their associates.
Results
No evidence of loss of capture was seen during the first or subsequent months , and there were no cases of muscle twitching. Pulse generator rates decreased 2 to 3 beats per minute during the first few months and then stabilized . Threshold measurements taken on "chronic" leads showed that no loss of capture would occur with first cell depletion , because the pulse width increased to 1.25 msec. (Fig . 6 and Table I). Seven cases of muscle inhibition were encountered, 2 in the initial implant group and 5 in the replacement group (Fig . 7). None of the patients complained of any symptoms either during the test or at any other time.
Volume 68
New pulse generator
Number 3 September, 1974
475
6.0
5.0
~
4.0
0
2:-
., C>
!! 3.0
0
>
"'"
''""
Cl.
2.0 Max. Threshold Patient (RT) Average Chronic Threshold
1.0
--"""""'4....- _. ._ _ Average Acute Threshold
o
0.15 0.30.5 0.75
1.0
1.5
2.0
Pulse Duration (ms)
Fig. 6. Unipolar voltage thresholds versus pulse duration. Acute, chronic, and maximum strength-duration curves for the first 21 patients tested. There is an adequate margin of safety in the Model 5945 output at peak voltage or with one cell depleted and with the compensatory increase in pulse width. For further data, see Table I.
Discussion In view of the findings of muscle inhibition under test conditions in 30 per cent of the patients, a separate study of the phenomenon was undertaken. Eleven patients with unipolar pacemakers were selected. In the group, there were 8 patients with Medtronic Model 5945 and 3 with Cordis Stan icor 143H7 units. Under sterile conditions with local anesthesia, sterile electromyogram electrode needles were inserted percutaneously and advanced until they made contact with the pulse generator case and anode plate. Isometric arm tensions (adduction) were applied by the patients and quantitative measurements of this tension were made by a Chatillon DPD-2 force gauge (Fig. 8). The patients were asked to exercise at two levels of tension. The first exercise involved exerting an isometric tension at 10 pounds while the second exercise involved exerting the maximum force that could be sustained for 10 seconds. The maximum force varied
Table I. Unipolar voltage thresholds versus pulse duration in 21 patients" No. of patients
13 8
Pacemaker
Cordis (4 mm. tip) Medtronic 6907
Implant data
Electrode area (sq. cm.)
Chronic
0.295
Acute
0.113
• For further information, see Fig. 6.
with each patient, ranging from 12 to 45 pounds. Skeletal muscle potentials were recorded with isolation amplifiers to ensure patient safety, and the data were recorded on a Hewlett-Packard instrumentation recorder for subsequent analysis. The results were analyzed by a Tektronix digital processing oscilloscope and a PDP 11/05 minicomputer. Five samples of skeletal muscle potentials were taken from each of the 11 patients, and an average power density spectrum was obtained from these fifty-five
The Journal of
476
Smyth et al.
Thoracic and Cardiovascu lar Surgery
Fig. 7. A . Normal pacing at a rate of 72 beats per minute in a patient with heart block . B, Several periods of complete pacer inhibition in the same patient with an isometric com-
pression force of 15 pounds in both arms.
Fig. 8. Photograph show s the Chatillon DPD-2 force gauge. The instrument was grasped in the patient's left hand while adduction pressure was applied to the plate by the palm of the right hand.
samples . This spectrum showed that the major components of the skeletal muscle potential signals were below 80 Hz, a much lower distribution of frequency components than the skeletal myoelectric energy spectra of Scott.! " The signal levels varied with each patient and with the force of isometric contractions. The maximum potential measured was 2.7 mv. peak. This is in close agreement with the skeletal muscle potential levels observed by Ohm." Obviously, skeletal muscle potentials of this amplitude can be detected
by a pulse generator with a maximum sensitivity of 2 mv. peak. (1 mv. standard square wave) . Therefore, ten additional Model 5945 units were modified to reduce this sensitivity. These modified units were implanted in 10 patients-as replacement units in 7 and as initial implants in 3 patients. The tests were repeated according to the same protocol used for the original 21 patients. None of the 10 experienced muscle inhibition . The modification was in the amplifier
Volume 68 Number 3 September, 1974
10
10 9 Medtronic
9
"0 >
7
"0
c: 0
6
0
Model 6907
'"
0. E
'" '"
"0
7
0
6
Medtronic Model 5818 0.485 cm2 Surface
Medtronic Model 6905 0.450 cm2 Surface
..,>c:
Cordis 2mm cm 2 Surface
E
5
I"
4
PATIENTS
I
..,'" ~
Q.
E
3
'"
o 175
5 4
120 PATIENTS
3
I
$.
::l
D.-
Medtronic Model 6901 o 120 cm2 Surface
2
0
"0
.2
0.295 cm 2 Surface
8
Medtroruc Model 6917 0.120 cm 2 Surface
E
17 Cordis 4mm
9
0113 cm 2 Surface
8
2
477
New pulse generator
~
D.-
2
-a-....o:::=:::::B:=== Current
OL---'---'--'--_--'-_--'-_ _----JL-_ Voltage 0.5
10
15
Pulse Durction (rns)
2
Current Voltage
OL---'--I.....---l...-_--'-_L-_ _---l...-_ _
0.5
10
15
PuIse Duration (rns)
Fig. 9. Acute thresholds versus pulse duration. Average strength-duration curves for all patients in the acute threshold group. Types of leads used are shown in inset.
Fig. 10. Chronic thresholds versus pulse duration. Average strength-duration curves for all patients in the chronic threshold group. Types of leads used are shown in inset.
frequency response and consisted in reducing the maximum sensitivity from 1 to 2 mv. and reducing the 3 db frequency response from 120 to 100 Hz. Analysis of the frequency content of muscle artifacts indicated that the frequency spectrum of cardiac muscle potentials and skeletal muscle potentials overlap to such a degree that one cannot completely eliminate the effects of muscle artifacts by reducing the band pass of the amplifier alone. It is also necessary to reduce the sensitivity of the amplifier across the cardiac frequency spectrum. This makes the pulse generator not only less sensitive to skeletal muscle potentials, but also less sensitive to cardiac muscle potentials. In fact, in 1 of the patients in whom a modified (reduced sensitivity) unit had been implanted, there was a brief period of intermittent lack of sensing during the first 2 postoperative weeks. The problem resolved itself spontaneously, and the unit has now been sensing properly for 6 months. This incident underscores the marginal nature of the trade off involving reduced sensitivity units. The exact balance between re-
duced sensitivity to muscle artifacts and the risk of increased sensing problems has yet to be resolved. At the present time the investigation is continuing, and modified units are available upon special request for those physicians who view muscle inhibition as a significant problem in general or in a particular patient. We have not found it to be a problem and continue to use the standard unmodified Model 5945 pulse generator. At the conclusion of the second study with the Model 5945 generator, the threshold values obtained in both patient groups were combined and averaged. The thresholds values obtained at the time of initial pacemaker insertion are shown in Fig. 9 and those obtained at the time of pulse generator replacement are shown in Fig. 10. It is clear that there is a substantial safety margin between the chronic thresholds and the pulse generator output at 0.5 msec., even with an electrode with a larger surface area. This fact is more clearly shown in Fig. 6: The average acute, average chronic, and highest chronic thresholds obtained in
The Journal of
478
Smyth et al.
Thoracic and Cardiovascular Surgery
the first patient group are plotted with the output of the pulse generator shown at peak voltage and with one cell depleted, with the pulse width increased to 1.25 msec. There is still a substantial safety margin for all the patients in either group. Conclusion
The original group of 21 patients has now been followed for more than 1 year. Four of them have died of causes unrelated to the pacemaker. One of them was shot to death as an innocent bystander in a service station holdup. In the remaining 17 patients, pacemaker function is stable. The 10 patients with less sensitive units have now been followed for 7 months, and, all units are pacing normally. One patient has a minor problem with intermittent twitching of the pectoralis major muscle. Discrimination between muscle and cardiac potentials in a ventricular inhibited, demand pacemaker requires further study. It has not been a clinical problem in our experience. The principle of narrowing the pulse width to conserve energy seems sound, and a study of 31 patients shows no problems related to the narrow pulse width. All patients will be followed to see if the expected increase in pulse generator longevity is realized. REFERENCES Smyth, N. P. D., Keshishian, J. M., Baker, N. R., and Tarjan, P.: Physiological Rationale
for the Clinical Use of Low Output Pacemakers, Med. Ann. D. C. 43: 257, 1974. 2 Center, S., and Tarjan, P.: The Clinical Application of Low-Output Pacemakers: J. THORAC. CARDIOVASC. SURG. 64: 6, 1972. 3 Furman, S., Parker, Escher, D. J. W., and Solomon, N.: Endocardial Threshold of Cardiac Response as a Function of Electrode Surface Area, J. Surg. Res. 8: 161, 1968. 4 Furman, S., Denize, A., Escher, D. J. W., and Schwedel, J. B.: Energy Consumption for Cardiac Stimulation as a Function of Pulse Duration, 1. Surg. Res. 6: 10, 1966. 5 Chardack, W. M., Baken, E. E., Bolduc, L., Giori, F. A., and Gage, A. A.: Magnetically Actuated Pulse Width Control for Implantable Pacemakers, Ann. Cardio!. et Angeiol. 20: 4, 1972. 6 Smyth, N. P. D., Bacos, J. M., Boivin, M., and Keller, J. W.: A Modified Variable Parameter Cardiac Pacer, Chest 57: 3, 1970. 7 Smyth, N. P. D., Bacos, J. M., and Keller, J. W.: Experimental and Clinical Use of a Variable Parameter Cardiac Pacer, Chest 53: 93, 1968. 8 Wirtzfeld, A., Lampadius, M., and Ruprecht, E.-O.: U nterdruckung von Demand- Schrittmachern durch Muskelpotentiale, Dtsch. Med. Wochenschr. 97: 61, 1972. 9 Mymin, D., Cuddy, T. E., Sachehide, N. S., and Winter, D. A.: Inhibition of Demand Pacemakers by Skeletal Muscle Potentials, 1. A. M. A. 223: 5, 1973. 10 Scott, R. N.: Myo-electric Energy Spectra, Med. BioI. Eng. 5: 303, 1967. 11 Ohm, O. 1., Bruland, H., Pederson, O. M., and Waerness, E.: Interference Effect of Myopotentials On Function Of Unipolar Demand Pacemakers, Br. Heart J. 43: 77, 1974.