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Interference with function of unipolar pacemaker due to muscle potentials
Interference with function of unipolar pacemaker due to muscle potentials One hundred and seventy-three unipolar pulse generator systems were tested in 169 patients to determine the incidence of sensing of muscle potentials. One hundred and twenty-two implants in the pectoral region were suitable for assessment and 60 (49 per cent) demonstrated sensing. Ventricular-inhibited units sensed muscle potentials in 69 per cent (47 of 78), with 9 patients complaining of related symptoms. Ventricular-synchronous units sensed in 30 per cent (13 of 34) with no patient manifesting symptoms.
S. T. Anderson, M.B., B.S., F.R.A.C.P., Aubrey Pitt, M.D., F.R.A.C.P., J. A. Whitford, M.B., B.S., M.R.A.C.P., and B. B. Davis, M.B., B.S., F.R.A.C.S., Melbourne, Australia
Ci xtension of the functional life of pulse generators has been progressive and rewarding. The development of ventricular demand pacemakers, improved power sources, reduced current drain pulse generators, reduced size of electrode tip, and decreased pulse duration have all contributed.1"5 Interference with standby pulse generator function by external electromagnetic fields, radiofrequency fields, or leakage currents from attached electrical equipment has been documented frequently, and much design energy has been expended in reducing this problem. 6-10 Inhibition of demand pacemakers by skeletal muscle potentials has been described by Wirtzfeld, Lampadius, and Ruprecht11 and subsequently by other authors. 12-18 Following the presentation of 2 patients symptomatic from muscle inhibition of ventricular-inhibited pacemakers, all consecutive patients presenting to the Alfred Hospital Pacemaker Clinic were tested to assess the incidence of muscle sensing. Method Patients had a routine twelve-lead electrocardiogram and rhythm strip recorded with a three-channel elecFrom the Cardiovascular Diagnostic Service and C. J. Officer Brown Cardiac Surgical Unit, Alfred Hospital, Melbourne, 3181, Australia. Received for publication Sept. 10, 1975. Accepted for publication Nov. 21, 1975. Address for reprints: Dr. S. T. Anderson, Cardiovascular Diagnostic Service, Alfred Hospital, Melbourne, 3181, Australia.
698
trocardiograph. The pacemaker artifact pulse width and standard lead voltage, including effect of respiration, were recorded on Polaroid film with a Tektronics storage oscilloscope.14 For those patients whose intrinsic rate was faster than the set rate of the implanted ventricular-inhibited pacemaker, an external magnet was applied to convert the pulse generator to a fixed-rate mode. To assess sensing of muscle potentials we instructed suitable patients to perform three movements while a continuous multichannel electrocardiogram was being recorded: 1. While in the recumbent position, the patient used the arm on the implant side to rub gently the opposite shoulder. 2. The patient pushed with the outstretched arm against resistance applied by the observer. 3. The patient arose from a low chair utilizing forearm pressure on the arm rests. Following implantation of a pacemaker, patients have been evaluated 2 to 3 weeks after hospital discharge and then at regular intervals according to the type of unit and clinical condition of the patient, with a period of not greater than 6 months between visits. Patients Between Aug. 1, 1974, and June 30, 1975, 169 patients attended the clinic on one or more occasions. Four patients during the period required a change of pacemaker unit to a different model number, so that 173 pulse generators are available for assessment (Table I). Five patients did not attend during the
Volume 71 Number 5 May, 1976
699
Function of unipolar pacemakers
Table I Pulse generator
Type
Medtronic Model 5943 Medtronic Model 5945 Medtronic Model 5951 Elema 153 C.P.I. 301 V.D. Telectronics Model P10 Telectronics Model PI20 Miscellaneous
Inhibited Inhibited Inhibited Synchronous Inhibited Inhibited* Inhibited
Unsuitable for testing
No muscle sensing
Muscle sensing, no symptoms
Muscle sensing, symptoms
Totals
20 8 0 13 3 2 1 4
10 9 4 31 4 0 3 1
18 13 1 13 2 4 0 0
6 2 0 0 0 0 1 0
54 32 5 57 9 6 5 5
51
62
51
9
173
Totals
*Teletronics Model P10 pacers function as inhibited units producing a marker signal when sensing occurs.
lithium iodide units reflects our initial policy of using these in younger patients. Since 1967 our policy has been to use transvenous unipolar electrodes. Originally, we placed the pulse generator below the right clavicle, but since 1972, we have inserted it in the left pectoral region except in left-handed patients, in whom we used the right pectoral region. The small number of fixed-rate units and abdominal implants (Table II) reflects continued function of a previously satisfactory unit implanted prior to 1967.
Table II. Unsuitable for testing No. of cases
Cause
37 32 4 1 3
Fast inherent rate sinus Atrial fib. VPB Fixed rate Abdominal implant Upper limb abnormality Legend: VPB. Ventricular premature beats.
Results Table III
Type of unit
Unsuitable for testing
Muscle sensing absent
Muscle sensing present
Ventricular inhibited* Ventricular synchronous
34 13
31 31
47 13
112 57
Totals
37
62
60
169
Totals
*Includes Telectronics Model P10 units which have a marker signal when sensing occurs.
period, one who was debilitated because of advanced malignancy, one who had migrated overseas, and 3 who had transferred to interstate hospitals. Of the 169 patients, 95 were men of mean age 68.5 years (range 24 to 89) and 74 women of mean age 71 years (range 21 to 89). The mean age for the group was 70 years. Except for pulse generators with a lithium iodide power source (Cardiac Pacemakers Model 301 V.D. and Telectronics Model PI20), fourteen of which were implanted in patients of mean age 57.4 years, there was no significant difference in patient age or sex for the various units utilized. The age of the patients with
One hundred seventy-three pulse generators (Table I) were reviewed. In 51 (29.5 per cent), it was impossible to test for inhibition caused by muscle potentials via the program outlined previously (Table II): An intrinsic rate greater than that of the pulse generator was present in 37 patients, the majority being in sinus rhythm; an additional 8 patients had units implanted in the anterior abdominal wall, 3 had fixed-rate, units, and 3 had upper limb abnormalities precluding satisfactory testing (fractured forearm, rheumatoid arthritis, and amputated limb). Sensing of muscle potentials was present in 60 (49 per cent) of the 122 units suitable for testing, and 9 patients complained of symptoms attributable to suppression of pacemaker output. Pacemaker models Medtronic units. A total of 91 pulse generators manufactured by Medtronic, Inc., were implanted (Table I). Eighty-six were totally metal enclosed (Models 5943 and 5945), and five had indifferent plate electrodes (Model 5951). Twenty-eight units were unsuitable for testing; of the remaining 63, 40 showed sensing and 23 no sensing of muscle potentials.
700
The Journal of Thoracic and Cardiovascular Surgery
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Fig. 1. Simultaneous recording of Leads I, II, and III during rubbing of shoulder. Inhibition of pacemaker (Medtronic Model 5943 ventricular-inhibited unit) with an escape beat after 5.24 seconds was accompanied by mild symptoms. Thirty-eight of the 40 units showing sensing were able to be tested by all three maneuvers. In the remaining 2 cases, the patients were unable to stand from a sitting position because of general debility. Of the 38 tested by the three maneuvers eight showed sensing for one only, 11 sensing for two, and 18 sensing for all three. All Medtronic units tested were ventricular-inhibited (R-wave inhibited) unipolar implants. Consequently, when sensing of muscle potentials occurred, ventricular asystole or an escape rhythm became manifest. During testing, 8 patients complained of transient symptoms which were similar to those they had experienced in the course of normal activities since the time of implantation. Medtronic Model 5943. Twenty (37 per cent) of the 54 implants were unsuitable for assessment of the sensing of muscle potentials. Twenty-four (70 per cent) of the 34 suitable units demonstrated inhibition (Fig. 1) of the pulse generator, and 6 of the patients with these units complained of symptoms which could be attributable to muscle inhibition. One patient requested replacement of the pulse generator because of the disabling nature of symptoms and, following a change to an Elema 153 ventricular-synchronous unit, has had no further symptoms although muscle sensing still occurs. Medtronic Model 5945. Eight (25 per cent) of the 32 implants were unsuitable for testing. Fifteen (62.5 per
cent) of the 24 suitable units demonstrated muscle inhibition, with 2 patients being symptomatic. Medtronic Model 5951. Only five units have been implanted, four subcutaneously with the indifferent electrode anterior and one deep to the pectoralis major. Only the unit implanted deep to the pectoralis major exhibited muscle sensing. Elema 153 (Table I). Thirteen (23 per cent) of the 57 units were unsuitable for testing. Muscle potential sensing could be demonstrated in 13 (29 per cent) of those suitable (Fig. 2). As these are ventricularsynchronous units, producing a full impulse when sensing occurs, the sensing was accompanied by additional paced beats. Thus no patient had symptoms. Other pulse generators (Table I). Cardiac Pacemakers Model 301 V.D. A lithium iodide (Greatbach), totally metal-encapsulated unit connected to a Medtronic unipolar lead was implanted in 9 patients. Three units were unsuitable for testing, four showed no muscle sensing, and two did show muscle sensing. Telectronics Model P10. This unipolar generator was present in 6 patients and 2 units were unsuitable for testing. The remaining 4, all placed anterior to the pectoralis major, demonstrated muscle sensing. When sensing occurs, this unit emits a marking signal which has an output of approximately 10 per cent of that for a paced beat. An example of muscle sensing with this unit is presented in Fig. 3.
Volume 71 Number 5 May, 1976
Function of unipolar pacemakers
7 01
Fig. 2. Sensing of muscle potentials in a patient with a ventricular-synchronous pacemaker (Elema 153). Sensing results in a full pacemaker output with ventricular capture.
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Fig. 3. Ventricular-inhibited unit (Telectronics Model PIO) with marker spike when sensing occurs. Apparent sinus capture after 2.5 seconds in patient with advanced degree atrioventricular block. Telectronics Model PJ20. Five of these unipolar, lithium iodide (Greatbach), totally metal-encapsulated, ventricular-inhibited units were implanted. Sensing of muscle potentials could be demonstrated in one patient who complains of minor symptoms when lifting heavy loads.
Miscellaneous. Five other units were implanted: three fixed-rate (two Telectronics Model P9 and one Elema 152), one Telectronics Model P6 in the anterior abdominal wall, and one Telectronics Model P8. Only the Telectronics Model P8 unit was suitable for testing, and sensing of muscle potentials was absent.
702
Anderson et al.
Pacemaker type (Table III) Ventricular-inhibited (R-wave inhibited) units. Sixty per cent (47 of 78) of ventricular-inhibited units suitable for testing demonstrated muscle sensing characterized by asystole and/or escape rhythm at a rate slower than the inherent rate of the pulse generator. Symptoms occurred in 9 of the 47 patients whose units manifested muscle sensing, an incidence of 19 per cent. Ventricular-synchronous units. All ventricularsynchronous units were of one manufacturer, ElemaSchonander, Model 153. Sensing occurred in 29 per cent (13 of 44) of those suitable for testing. Other factors Encapsulation. With the exception of the five Medtronic Model 5951 units, all inhibited units were totally metal enclosed. All of the ventricularsynchronous units, as with the Medtronic Model 5951, had indifferent plate electrodes, and this may account in part for the reduced incidence of sensing occurring with these units. Relationship to pectoralis muscles. The relationship to pectoralis muscles was not clear in all cases. All of the 4 Telectronics Model P10 units suitable for testing were placed anterior to the pectoralis muscles, and sensing occurred. This unit has a thin silicone rubber coating over the entire metal casing except for two 2.4 cm. circular areas, which were placed anteriorly. The only Medtronic Model 5951 unit in which sensing occurred was placed deep to the pectoralis major. Discussion Unipolar pacing represents an open electric circuit which is closed by the tissues between the tip of the electrode in the apex of the right ventricle and the indifferent electrode which commonly forms the whole or part of the metallic casing of the pulse generator. Both the stimulating and indifferent electrode can act as sensors for electrical impulses. The latter is utilized when small external stimuli are applied to assess pacemaker sensing function satisfactorily and to determine the underlying electrocardiogram when inhibited units are present. 19-21 Total metal encapsulation of pulse generators to shield against interference and to prevent tissue fluid ingress into the epoxy results in increasing the area of the indifferent electrode and hence the likelihood of sensing muscle potentials. In the current series, sensing occurred with 63 per cent (46 of 73) of the totally
The Journal of Thoracic and Cardiovascular Surgery
metal-encapsulated units but occurred in only 28.5 per cent (14 of 49) with indifferent plate electrodes. Although only five inhibited units with indifferent plate electrodes (Medtronic Model 5951) were studied, sensing was demonstrated in one unit which had to be placed deep to the pectoralis major. Ulrich and associates14 include one case of symptomatic inhibition in which satisfactory control was achieved by rotating the pulse generator so that the indifferent electrode faced anteriorly. Modification of the indifferent electrode may be of considerable value. The current Telectronics Model P10 unit has a silicone coating except for two circular areas designed to be placed anteriorly. Experience with four such units showed sensing readily in all. This may reflect the thinness of the coating or, alternatively, defects in coating at the time of implantation. Alteration of the pulse generators in terms of reduced sensitivity, change in interference-discriminating circuitry, and lengthening of sensory refractory period may all partly alleviate the problem. However, such changes may lead to problems of failure to adequately sense QRS complexes in some patients, and this is undesirable. Fixed-rate units offer reliability and the absence of sensing problems. However, this mode of pacing has to be considered undesirable, as 21.4 per cent of patients in the current series at one examination exhibited a competing rhythm. Bipolar trans venous electrodes offer an advantage, as they are less likely to sense noncardiac impulses, including muscle potentials. Although a control trial comparing unipolar with bipolar electrodes has not been undertaken, documentation of increased threshold with the latter is frequent. Brenner and associates22 in following 129 transvenous bipolar electrodes, had to convert 3.9 per cent to unipolar electrodes over a 2 year period because of elevated threshold or fracture of one of the electrodes. Ventricular-synchronous pacing has proved reliable and free from significant problems in our hands. As would be expected, there have been no symptoms caused by muscle sensing. The possible disadvantage of a shortened functional life of the pulse generator should frequent triggering of the pacemaker occur is of reduced importance with the improved power sources currently available. REFERENCES 1 Smyth, N. P. D., Keshishian, J. M., Baker, N. R., and Tarjan, P.: Physiological Rationale for Clinical Use of
Volume 71 Number 5 May, 1976
Function of unipolar pacemakers
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, B., 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, J. Surg. Res. 6: 10, 1966. 5 Smyth, N. P. D., Alferness, C , Shearon, L., Rockland, R. H., Keshishian, J. M., and Johnson, A.: Clinical Evaluation of New Pulse Generator With Narrow Pulse Width for Conservation of Battery Energy, J. THORAC. CARDIOVASC. SURG. 68: 471,
1974.
6 Furman, S., Parker, B., Krauthamer, M., and Escher, D. J. W.: The Influence of Electromagnetic Environment on the Performance of Artificial Pacemakers, Ann. Thorac. Surg 6: 90, 1968. 7 King, G. R., Hamburger, A. C , Forough, P., Heller, S. J., and Carleton, R. A.: Effect of Microwave Oven on Implanted Cardiac Pacemaker, J. A. M. A. 212: 1213, 1970. 8 Bilitch, M., Lau, F. Y. K., and Cosby, R. S.: "Demand" Pacemaker Inhibition by Radio Frequency Signals (Abstr.) Circulation 35: 68, 1967. (Suppl. II). 9 Pickers, B. A., and Goldberg, M. J.: Inhibition of a Demand Pacemaker and Interference With Monitoring Equipment by Radio Frequency Transmission: Br. Med. J. 2: 504, 1969. 10 Wajszczuk, W. J., Mowry, F. M., and Dugan, N. L.: Deactivation of a Demand Pacemaker by Tansurethral Electrocautery, N. Engl. J. Med. 280: 34, 1969. 11 Wirtzfeld, A., Lampadius, M., and Ruprecht, E.-O.: Unterdriickung von Demand-Schrittmachern durch Mus-
703
kelpotentiale, Dtsch. Med. Wochenschr. 97: 61, 1972. 12 Mymin, D., Cuddy, T. E., Sachehide, N. S., and Winter, D. A.: Inhibition of Demand Pacemakers by Skeletal Muscle Potentials, J. A. M. A. 223: 5, 1973. 13 Ohm, O. J., Burland, H., Pederson, O. M., and Waerness, E.: Interference Effect of Myopotentials on Function of Unipolar Demand Pacemakers, Br. Heart J. 43: 77, 1974. 14 Ulrich, B., Buchta, I., Schulte, H. D., Bircks, W., and Loogen, F.: Unterdriickung R-Zacken-Blockierter Schrittmacher durch Muskelpotentials: Thoraxchirugie 22:46, 1974. 15 Paulet, J. P., Courgeon, Y., Kerdiles, J., Pony, J. C , and Gouffault, J.: Leurre des pace-makers sentinelles par les potentials d'action musculaires; Nouv. Presse Med. 3: 1148, 1974. 16 Piller, L. W., and Kennelly, B. M.,: Myopotential Inhibition of Demand Pacemakers, Chest 66: 418, 1974. 17 Widlansky, S., and Zipes, D. P.: Suppression of a Ventricular-Inhibited Bipolar Pacemaker by Skeletal Muscle Activity, J. Electrocardiol 7: 371, 1974. 18 Wirtzfeld, A., Lampadius, M., and Ruprecht, E.-O., Die Beeinflussung von Bedarfsschrittmachern durch Muskelpotentiale, Verh. Dtsch. Ges. Inn. Med. 78: 1095, 1972. 19 Castellanos, A., Jr., and Spence, M.: Pacemaker Arrhythmias in Content, Am. J. Cardiol. 25: 372, 1970. 20 Barold, S. S., and Gaidula, J. J.: Evaluation of Normal and Abnormal Sensing Function of Demand Pacemakers, Am. J. Cardiol. 28: 201, 1971. 21 Barold, S. S., Pupillo, G. A., Gaidula, J. J., and Linhart, J. W.: Chest Wall Stimulation in Evaluation of Patients With Implanted Ventricular-Inhibited Demand Pacemakers, Br. Heart J. 32: 783, 1970. 22 Brenner, A. S., Wagner, G. S., Anderson, S. T., Rosati, R. A., and Morris, J. R.: Transvenous, Transmediastinal and Transthoracic Ventricular Pacing: A Comparison After Complete Two-Year Follow-up, Circulation 49: 407, 1974.
S. T. Anderson, M.B., B.S., F.R.A.C.P., Aubrey Pitt, M.D., F.R.A.C.P., J. A. Whitford, M.B., B.S., M.R.A.C.P., and B. B. Davis, M.B., B.S., F.R.A.C.S., Melbourne, Australia
Ci xtension of the functional life of pulse generators has been progressive and rewarding. The development of ventricular demand pacemakers, improved power sources, reduced current drain pulse generators, reduced size of electrode tip, and decreased pulse duration have all contributed.1"5 Interference with standby pulse generator function by external electromagnetic fields, radiofrequency fields, or leakage currents from attached electrical equipment has been documented frequently, and much design energy has been expended in reducing this problem. 6-10 Inhibition of demand pacemakers by skeletal muscle potentials has been described by Wirtzfeld, Lampadius, and Ruprecht11 and subsequently by other authors. 12-18 Following the presentation of 2 patients symptomatic from muscle inhibition of ventricular-inhibited pacemakers, all consecutive patients presenting to the Alfred Hospital Pacemaker Clinic were tested to assess the incidence of muscle sensing. Method Patients had a routine twelve-lead electrocardiogram and rhythm strip recorded with a three-channel elecFrom the Cardiovascular Diagnostic Service and C. J. Officer Brown Cardiac Surgical Unit, Alfred Hospital, Melbourne, 3181, Australia. Received for publication Sept. 10, 1975. Accepted for publication Nov. 21, 1975. Address for reprints: Dr. S. T. Anderson, Cardiovascular Diagnostic Service, Alfred Hospital, Melbourne, 3181, Australia.
698
trocardiograph. The pacemaker artifact pulse width and standard lead voltage, including effect of respiration, were recorded on Polaroid film with a Tektronics storage oscilloscope.14 For those patients whose intrinsic rate was faster than the set rate of the implanted ventricular-inhibited pacemaker, an external magnet was applied to convert the pulse generator to a fixed-rate mode. To assess sensing of muscle potentials we instructed suitable patients to perform three movements while a continuous multichannel electrocardiogram was being recorded: 1. While in the recumbent position, the patient used the arm on the implant side to rub gently the opposite shoulder. 2. The patient pushed with the outstretched arm against resistance applied by the observer. 3. The patient arose from a low chair utilizing forearm pressure on the arm rests. Following implantation of a pacemaker, patients have been evaluated 2 to 3 weeks after hospital discharge and then at regular intervals according to the type of unit and clinical condition of the patient, with a period of not greater than 6 months between visits. Patients Between Aug. 1, 1974, and June 30, 1975, 169 patients attended the clinic on one or more occasions. Four patients during the period required a change of pacemaker unit to a different model number, so that 173 pulse generators are available for assessment (Table I). Five patients did not attend during the
Volume 71 Number 5 May, 1976
699
Function of unipolar pacemakers
Table I Pulse generator
Type
Medtronic Model 5943 Medtronic Model 5945 Medtronic Model 5951 Elema 153 C.P.I. 301 V.D. Telectronics Model P10 Telectronics Model PI20 Miscellaneous
Inhibited Inhibited Inhibited Synchronous Inhibited Inhibited* Inhibited
Unsuitable for testing
No muscle sensing
Muscle sensing, no symptoms
Muscle sensing, symptoms
Totals
20 8 0 13 3 2 1 4
10 9 4 31 4 0 3 1
18 13 1 13 2 4 0 0
6 2 0 0 0 0 1 0
54 32 5 57 9 6 5 5
51
62
51
9
173
Totals
*Teletronics Model P10 pacers function as inhibited units producing a marker signal when sensing occurs.
lithium iodide units reflects our initial policy of using these in younger patients. Since 1967 our policy has been to use transvenous unipolar electrodes. Originally, we placed the pulse generator below the right clavicle, but since 1972, we have inserted it in the left pectoral region except in left-handed patients, in whom we used the right pectoral region. The small number of fixed-rate units and abdominal implants (Table II) reflects continued function of a previously satisfactory unit implanted prior to 1967.
Table II. Unsuitable for testing No. of cases
Cause
37 32 4 1 3
Fast inherent rate sinus Atrial fib. VPB Fixed rate Abdominal implant Upper limb abnormality Legend: VPB. Ventricular premature beats.
Results Table III
Type of unit
Unsuitable for testing
Muscle sensing absent
Muscle sensing present
Ventricular inhibited* Ventricular synchronous
34 13
31 31
47 13
112 57
Totals
37
62
60
169
Totals
*Includes Telectronics Model P10 units which have a marker signal when sensing occurs.
period, one who was debilitated because of advanced malignancy, one who had migrated overseas, and 3 who had transferred to interstate hospitals. Of the 169 patients, 95 were men of mean age 68.5 years (range 24 to 89) and 74 women of mean age 71 years (range 21 to 89). The mean age for the group was 70 years. Except for pulse generators with a lithium iodide power source (Cardiac Pacemakers Model 301 V.D. and Telectronics Model PI20), fourteen of which were implanted in patients of mean age 57.4 years, there was no significant difference in patient age or sex for the various units utilized. The age of the patients with
One hundred seventy-three pulse generators (Table I) were reviewed. In 51 (29.5 per cent), it was impossible to test for inhibition caused by muscle potentials via the program outlined previously (Table II): An intrinsic rate greater than that of the pulse generator was present in 37 patients, the majority being in sinus rhythm; an additional 8 patients had units implanted in the anterior abdominal wall, 3 had fixed-rate, units, and 3 had upper limb abnormalities precluding satisfactory testing (fractured forearm, rheumatoid arthritis, and amputated limb). Sensing of muscle potentials was present in 60 (49 per cent) of the 122 units suitable for testing, and 9 patients complained of symptoms attributable to suppression of pacemaker output. Pacemaker models Medtronic units. A total of 91 pulse generators manufactured by Medtronic, Inc., were implanted (Table I). Eighty-six were totally metal enclosed (Models 5943 and 5945), and five had indifferent plate electrodes (Model 5951). Twenty-eight units were unsuitable for testing; of the remaining 63, 40 showed sensing and 23 no sensing of muscle potentials.
700
The Journal of Thoracic and Cardiovascular Surgery
Anderson et al.
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Fig. 1. Simultaneous recording of Leads I, II, and III during rubbing of shoulder. Inhibition of pacemaker (Medtronic Model 5943 ventricular-inhibited unit) with an escape beat after 5.24 seconds was accompanied by mild symptoms. Thirty-eight of the 40 units showing sensing were able to be tested by all three maneuvers. In the remaining 2 cases, the patients were unable to stand from a sitting position because of general debility. Of the 38 tested by the three maneuvers eight showed sensing for one only, 11 sensing for two, and 18 sensing for all three. All Medtronic units tested were ventricular-inhibited (R-wave inhibited) unipolar implants. Consequently, when sensing of muscle potentials occurred, ventricular asystole or an escape rhythm became manifest. During testing, 8 patients complained of transient symptoms which were similar to those they had experienced in the course of normal activities since the time of implantation. Medtronic Model 5943. Twenty (37 per cent) of the 54 implants were unsuitable for assessment of the sensing of muscle potentials. Twenty-four (70 per cent) of the 34 suitable units demonstrated inhibition (Fig. 1) of the pulse generator, and 6 of the patients with these units complained of symptoms which could be attributable to muscle inhibition. One patient requested replacement of the pulse generator because of the disabling nature of symptoms and, following a change to an Elema 153 ventricular-synchronous unit, has had no further symptoms although muscle sensing still occurs. Medtronic Model 5945. Eight (25 per cent) of the 32 implants were unsuitable for testing. Fifteen (62.5 per
cent) of the 24 suitable units demonstrated muscle inhibition, with 2 patients being symptomatic. Medtronic Model 5951. Only five units have been implanted, four subcutaneously with the indifferent electrode anterior and one deep to the pectoralis major. Only the unit implanted deep to the pectoralis major exhibited muscle sensing. Elema 153 (Table I). Thirteen (23 per cent) of the 57 units were unsuitable for testing. Muscle potential sensing could be demonstrated in 13 (29 per cent) of those suitable (Fig. 2). As these are ventricularsynchronous units, producing a full impulse when sensing occurs, the sensing was accompanied by additional paced beats. Thus no patient had symptoms. Other pulse generators (Table I). Cardiac Pacemakers Model 301 V.D. A lithium iodide (Greatbach), totally metal-encapsulated unit connected to a Medtronic unipolar lead was implanted in 9 patients. Three units were unsuitable for testing, four showed no muscle sensing, and two did show muscle sensing. Telectronics Model P10. This unipolar generator was present in 6 patients and 2 units were unsuitable for testing. The remaining 4, all placed anterior to the pectoralis major, demonstrated muscle sensing. When sensing occurs, this unit emits a marking signal which has an output of approximately 10 per cent of that for a paced beat. An example of muscle sensing with this unit is presented in Fig. 3.
Volume 71 Number 5 May, 1976
Function of unipolar pacemakers
7 01
Fig. 2. Sensing of muscle potentials in a patient with a ventricular-synchronous pacemaker (Elema 153). Sensing results in a full pacemaker output with ventricular capture.
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Fig. 3. Ventricular-inhibited unit (Telectronics Model PIO) with marker spike when sensing occurs. Apparent sinus capture after 2.5 seconds in patient with advanced degree atrioventricular block. Telectronics Model PJ20. Five of these unipolar, lithium iodide (Greatbach), totally metal-encapsulated, ventricular-inhibited units were implanted. Sensing of muscle potentials could be demonstrated in one patient who complains of minor symptoms when lifting heavy loads.
Miscellaneous. Five other units were implanted: three fixed-rate (two Telectronics Model P9 and one Elema 152), one Telectronics Model P6 in the anterior abdominal wall, and one Telectronics Model P8. Only the Telectronics Model P8 unit was suitable for testing, and sensing of muscle potentials was absent.
702
Anderson et al.
Pacemaker type (Table III) Ventricular-inhibited (R-wave inhibited) units. Sixty per cent (47 of 78) of ventricular-inhibited units suitable for testing demonstrated muscle sensing characterized by asystole and/or escape rhythm at a rate slower than the inherent rate of the pulse generator. Symptoms occurred in 9 of the 47 patients whose units manifested muscle sensing, an incidence of 19 per cent. Ventricular-synchronous units. All ventricularsynchronous units were of one manufacturer, ElemaSchonander, Model 153. Sensing occurred in 29 per cent (13 of 44) of those suitable for testing. Other factors Encapsulation. With the exception of the five Medtronic Model 5951 units, all inhibited units were totally metal enclosed. All of the ventricularsynchronous units, as with the Medtronic Model 5951, had indifferent plate electrodes, and this may account in part for the reduced incidence of sensing occurring with these units. Relationship to pectoralis muscles. The relationship to pectoralis muscles was not clear in all cases. All of the 4 Telectronics Model P10 units suitable for testing were placed anterior to the pectoralis muscles, and sensing occurred. This unit has a thin silicone rubber coating over the entire metal casing except for two 2.4 cm. circular areas, which were placed anteriorly. The only Medtronic Model 5951 unit in which sensing occurred was placed deep to the pectoralis major. Discussion Unipolar pacing represents an open electric circuit which is closed by the tissues between the tip of the electrode in the apex of the right ventricle and the indifferent electrode which commonly forms the whole or part of the metallic casing of the pulse generator. Both the stimulating and indifferent electrode can act as sensors for electrical impulses. The latter is utilized when small external stimuli are applied to assess pacemaker sensing function satisfactorily and to determine the underlying electrocardiogram when inhibited units are present. 19-21 Total metal encapsulation of pulse generators to shield against interference and to prevent tissue fluid ingress into the epoxy results in increasing the area of the indifferent electrode and hence the likelihood of sensing muscle potentials. In the current series, sensing occurred with 63 per cent (46 of 73) of the totally
The Journal of Thoracic and Cardiovascular Surgery
metal-encapsulated units but occurred in only 28.5 per cent (14 of 49) with indifferent plate electrodes. Although only five inhibited units with indifferent plate electrodes (Medtronic Model 5951) were studied, sensing was demonstrated in one unit which had to be placed deep to the pectoralis major. Ulrich and associates14 include one case of symptomatic inhibition in which satisfactory control was achieved by rotating the pulse generator so that the indifferent electrode faced anteriorly. Modification of the indifferent electrode may be of considerable value. The current Telectronics Model P10 unit has a silicone coating except for two circular areas designed to be placed anteriorly. Experience with four such units showed sensing readily in all. This may reflect the thinness of the coating or, alternatively, defects in coating at the time of implantation. Alteration of the pulse generators in terms of reduced sensitivity, change in interference-discriminating circuitry, and lengthening of sensory refractory period may all partly alleviate the problem. However, such changes may lead to problems of failure to adequately sense QRS complexes in some patients, and this is undesirable. Fixed-rate units offer reliability and the absence of sensing problems. However, this mode of pacing has to be considered undesirable, as 21.4 per cent of patients in the current series at one examination exhibited a competing rhythm. Bipolar trans venous electrodes offer an advantage, as they are less likely to sense noncardiac impulses, including muscle potentials. Although a control trial comparing unipolar with bipolar electrodes has not been undertaken, documentation of increased threshold with the latter is frequent. Brenner and associates22 in following 129 transvenous bipolar electrodes, had to convert 3.9 per cent to unipolar electrodes over a 2 year period because of elevated threshold or fracture of one of the electrodes. Ventricular-synchronous pacing has proved reliable and free from significant problems in our hands. As would be expected, there have been no symptoms caused by muscle sensing. The possible disadvantage of a shortened functional life of the pulse generator should frequent triggering of the pacemaker occur is of reduced importance with the improved power sources currently available. REFERENCES 1 Smyth, N. P. D., Keshishian, J. M., Baker, N. R., and Tarjan, P.: Physiological Rationale for Clinical Use of
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Function of unipolar pacemakers
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, B., 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, J. Surg. Res. 6: 10, 1966. 5 Smyth, N. P. D., Alferness, C , Shearon, L., Rockland, R. H., Keshishian, J. M., and Johnson, A.: Clinical Evaluation of New Pulse Generator With Narrow Pulse Width for Conservation of Battery Energy, J. THORAC. CARDIOVASC. SURG. 68: 471,
1974.
6 Furman, S., Parker, B., Krauthamer, M., and Escher, D. J. W.: The Influence of Electromagnetic Environment on the Performance of Artificial Pacemakers, Ann. Thorac. Surg 6: 90, 1968. 7 King, G. R., Hamburger, A. C , Forough, P., Heller, S. J., and Carleton, R. A.: Effect of Microwave Oven on Implanted Cardiac Pacemaker, J. A. M. A. 212: 1213, 1970. 8 Bilitch, M., Lau, F. Y. K., and Cosby, R. S.: "Demand" Pacemaker Inhibition by Radio Frequency Signals (Abstr.) Circulation 35: 68, 1967. (Suppl. II). 9 Pickers, B. A., and Goldberg, M. J.: Inhibition of a Demand Pacemaker and Interference With Monitoring Equipment by Radio Frequency Transmission: Br. Med. J. 2: 504, 1969. 10 Wajszczuk, W. J., Mowry, F. M., and Dugan, N. L.: Deactivation of a Demand Pacemaker by Tansurethral Electrocautery, N. Engl. J. Med. 280: 34, 1969. 11 Wirtzfeld, A., Lampadius, M., and Ruprecht, E.-O.: Unterdriickung von Demand-Schrittmachern durch Mus-
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kelpotentiale, Dtsch. Med. Wochenschr. 97: 61, 1972. 12 Mymin, D., Cuddy, T. E., Sachehide, N. S., and Winter, D. A.: Inhibition of Demand Pacemakers by Skeletal Muscle Potentials, J. A. M. A. 223: 5, 1973. 13 Ohm, O. J., Burland, H., Pederson, O. M., and Waerness, E.: Interference Effect of Myopotentials on Function of Unipolar Demand Pacemakers, Br. Heart J. 43: 77, 1974. 14 Ulrich, B., Buchta, I., Schulte, H. D., Bircks, W., and Loogen, F.: Unterdriickung R-Zacken-Blockierter Schrittmacher durch Muskelpotentials: Thoraxchirugie 22:46, 1974. 15 Paulet, J. P., Courgeon, Y., Kerdiles, J., Pony, J. C , and Gouffault, J.: Leurre des pace-makers sentinelles par les potentials d'action musculaires; Nouv. Presse Med. 3: 1148, 1974. 16 Piller, L. W., and Kennelly, B. M.,: Myopotential Inhibition of Demand Pacemakers, Chest 66: 418, 1974. 17 Widlansky, S., and Zipes, D. P.: Suppression of a Ventricular-Inhibited Bipolar Pacemaker by Skeletal Muscle Activity, J. Electrocardiol 7: 371, 1974. 18 Wirtzfeld, A., Lampadius, M., and Ruprecht, E.-O., Die Beeinflussung von Bedarfsschrittmachern durch Muskelpotentiale, Verh. Dtsch. Ges. Inn. Med. 78: 1095, 1972. 19 Castellanos, A., Jr., and Spence, M.: Pacemaker Arrhythmias in Content, Am. J. Cardiol. 25: 372, 1970. 20 Barold, S. S., and Gaidula, J. J.: Evaluation of Normal and Abnormal Sensing Function of Demand Pacemakers, Am. J. Cardiol. 28: 201, 1971. 21 Barold, S. S., Pupillo, G. A., Gaidula, J. J., and Linhart, J. W.: Chest Wall Stimulation in Evaluation of Patients With Implanted Ventricular-Inhibited Demand Pacemakers, Br. Heart J. 32: 783, 1970. 22 Brenner, A. S., Wagner, G. S., Anderson, S. T., Rosati, R. A., and Morris, J. R.: Transvenous, Transmediastinal and Transthoracic Ventricular Pacing: A Comparison After Complete Two-Year Follow-up, Circulation 49: 407, 1974.