Intraoperative Pacemaker Electrical Testing

Intraoperative Pacemaker Electrical Testing James W. Calvin, M.D.

lead electrodes can provide data on these two functions. Electrical testing at implantation with the achievement of ideal results can help assure long-term success of a cardiac pacemaker system. Conversely and more importantly, acceptance of compromised results can portend system failure. Yet, in clinical practice, complete electronic analysis is often neglected because of the time involved at operation and the Generator variables (output amplitude, time seeming complexity of the results. The purintervals, and inhibition capabilities) identify poses of this report are to discuss the rationale appropriate function, energy safety margin, of pacemaker electrical system analysis and and applicability of the chosen unit to indi- describe an intraoperative technique using vidual clinical situations. In a secondary opera- simplified testing equipment. tion in 45 patients, predicted generator failure was confirmed in only 34. In each of the other Materials and Methods 11 patients at least one unexpected, correctable Two hundred consecutive pacemaker operalead problem was identified. tions from October, 1972, through December, Detailed electrical testing is mandatory to ob- 1976, from one surgeon’s practice in a commutain proper function and longevity for both nity hospital were reviewed. They consisted primary and secondary pacemaker operations. of 65 endocardial ventricular, 30 epicardialThe primary function of a cardiac pacemaker myocardial ventricular, and 6 epicardialsystem is delivery of pulsed electrical energy of myocardial atrial primary implantations, 88 sufficient amplitude to cause myocardial sys- pulse generator replacements, and 11 system tolic contractions. The secondary function is de- revisions. Six of the transvenous endocardial tection of the intrinsic cardiac electrical activity ventricular leads required early repositioning. that causes the pulse generator to inhibit and Both bipolar and unipolar systems with various thereby prevent stimulation of cardiac tissue generators from several manufacturers of either when it occurs at inappropriate times. When constant-voltage or constant-current types were exposed at operation, the generator and the used. For the first 30 patients in this series, the threshold for capture was tested through the From the Departments of Surgery, Community Memorial leads using only an ordinary off-the-table exHospital and County General Hospital, Ventura, CA. Supported in part by a grant from the Medical Research ternal generator of the constant-current type Foundation of Ventura County, CA. with a fixed 1.8 or 2.0 msec pulse duration. The The following people provided valuable assistance in the intrinsic myocardial electrical potential was repreparation of this manuscript: Kenneth T. Valikai, M.S., of Medtronic, Inc, Walter Keller, M.S., of Stimulation Tech- corded only occasionally, and then with the nology, Inc, Michael Grechko, M.S., of Edwards Pacemaker cumbersome arrangement of additional offSystems, Newton 1. Friedman, M.D., of Ventura Cardiology Consultants, and Mrs. Pearl M. Knapp and Mr. Gary R. the-table cables connected to a battery-powered Luke. electrocardiograph machine for either the Q R S Presented at the Fourteenth Annual Meeting of The Society or the P wave amplitude measurement. This reof Thoracic Surgeons, Jan 23-25, 1978, Orlando, FL. quired turning off the external temporary Address reprint requests to Dr. Calvin, The Thoracic & transvenous catheter pacer system, if there was Cardiovascular Medical Group, Inc, 2793 Cabrillo Dr, Ventura, CA 93003. one, so that the direct electrograms could be ABSTRACT To determine the clinical value of intraoperative pacemaker system electrical testing, the results of 200 consecutive pacemaker procedures (95 ventricular and 6 atrial primary implantations, 88 generator replacements, and 11 system revisions) were reviewed. The rationale and technique of testing maneuvers are described, including the use of a compact pacing system analyzer wrapped and used on the sterile operating field.

165 0003-4975/78/0026-0210$01.50 @ 1978 by James W. Calvin

166 The Annals of Thoracic Surgery Vol 26 No 2 August 1978

recorded. A prototype switching box was sterilized, put on the table, and used to facilitate the procedure by turning off and on the various connecting cables. The generator to be implanted was not tested but was simply accepted from the manufacturer and presumed to meet specifications. More important, an explanted generator suspected of malfunctioning was not analyzed. In July, 1973, a pacing system analyzer (Medtronic Model 5300) of programmable width and constant-voltage configuration became available and was used for each of the subsequent 170 procedures. For the first of these procedures, the measurements from this sophisticated instrument were compared with those obtained from the older devices. When the ease of use of this new apparatus became apparent and after its accuracy and consistency were established, it was employed exclusively.

Testing Device Several manufacturers have provided pacing system analyzers. These analyzers have output functions for pacing at different amplitudes, widths, and rates, and they incorporate several input features for measuring capabilities. Together, the output and input capabilities can provide important electrical indicators of both generator and lead functions. The analyzer used in this series achieved ”peer” recognition in 1975 when it was selected for an “Award of Distinction” by the Minnesota Society of Professional Engineers because of its several capabilities. The pulse generator functions that can be measured are the output amplitude, the time intervals (pulse width and pacing interval), and sensinglinhibition capabilities. The amplitude of the pulse generator, even if it is a constant-voltage instrument, is measured by detecting its output pulse current through an arbitrary 500 ohm load (meant to be representative of a lead-electrode system in contact with myocardial tissue) and is displayed in milliamperes. While nominal values for some generators from certain manufacturers are approximately 10 ma, the obtained value should be compared with individual factory specifications. And, pulse generators from manufacturers other than that of this analyzer

may provide apparently discrepant values for output because of the particular shape of the upstroke of the pacer spike. For a cons tant-voltage permanent generator, which many are, it would be more meaningful to mentally convert the milliampere value to volts using Ohm’s law: E = IR, where E is volts, I is current in amperes, and R is resistance in ohms. Thus, the output is 5 v if the current from the generator is 10.0 ma through the 500 ohm load (5 v = 0.010 ampere X 500 ohms). Stimulating voltage thresholds of the leads, then, can be compared with the output voltage of the generator to calculate more meaningful energy safety margins [2]. For a constant-current generator, the displayed milliampere value is the more important one for this purpose. Because the analyzer used in this series has an intervalometer, the generator’s pulse width (duration) and pulse-to-pulse interval (reciprocal of its rate) are shown individually in milliseconds. Beats per minute can be calculated by dividing the pulse-to-pulse interval by 1,000 and then dividing that result into 60. Thus, if the pulse-to-pulse interval is 854 msec, 6010.854 = 70.26 beats per minute. Finally, a series of rapid impulses, meant to simulate intrinsic cardiac electrical signals of medium-low amplitude, can be delivered from the analyzer to the generator to provide indirect evidence of its proper sensingiinhibition capability or “demand function.” The first variable of lead function to be determined is the minimum or threshold pulse amplitude for consistent capture of the myocardial rhythm. An analyzer pacing rate above the existing heart rate is chosen, the voltage knob having been preset for minimal output. Then, the analyzer output voltage is gradually increased until there is consistent cardiac response. A few minutes are allowed to elapse to confirm electrode tip stability, and the voltage threshold is then redetermined. Often the value is slightly lower, self-adjustments having been made at the electrode tip-cardiac tissue interface, and this value is recorded. While threshold is expressed in both volts and in resultant milliamperes, it is in essence a voltage-determined value for all the procedures reported here. At the chosen amplitude setting,

167 Calvin: Intraoperative Pacemaker Electrical Testing

the voltage remains constant (that is, it is independent of circuit current or circuit resistance), and this value is noted as the ”stimulation threshold voltage.” While some manufacturers provide an analyzer that allows optional testing in either the constant-voltage or the constantcurrent configuration, the device used in this series was solely of the former design. The current amplitude at threshold is self-determined by the analyzer because of Ohm’s law, now transposed to: I = EIR. It can be presumed that when the electrode tip is stable against the myocardium, the electrode-tissue resistance is constant. This resultant current in milliamperes can be visualized on the digital display of the analyzer. If it does not repeat at essentially the same value, the electrode tip position may not be stable. The second lead function is impedance (electrical resistance). This cannot be directly measured with the analyzer used in this series or with other presently available analyzers because an ohmmeter has not been incorporated into the design. However, impedance can be calculated. Again, Ohm’s law is used, this time transposed to: R = EII x 1,000, where E is the selected output pulse amplitude in volts and I is the resultant current measured and displayed in milliamperes. An expedient technique to approximate the actual impedance is to adjust the voltage knob sufficiently high that the displayed resultant current is approximately 10.0 ma; then the value of the voltage knob is read and simply multiplied by 100 (ohm = vI10 x 1,000). Thus, stimulating threshold voltage in volts and resultant current in milliamperes together with calculated impedance in ohms can be recorded. Incidentally, during transvenous procedures when impedance is approximated as just described, the use of maximal output from the analyzer (10.0 v) serves the additional purpose of checking for unwanted diaphragmatic pacing. If it is found, the lead should be repositioned. The third variable of lead function to be determined is the magnitude of the intrinsic myocardial electrical potentials of the ventricle, which must be detected by the pulse generator. Because the analyzer is not designed to detect the deflection P waves, atrial pacing studies still

must depend on conventional testing equipment [3]. The instrument can, however, be adjusted to interrupt its output and enable recording of the peak-to-peak amplitude of the QRS complex. This intracavitary or myocardial R wave signal is displayed in millivolts, and the value approximates the amplitude of the electrogram, which a typical pulse generator will detect as long as the slew rate (sharpness of upstroke) [9, 131 is adequate. The device commences measuring a few milliseconds into the leading edge of the QRS just as do most, if not all, permanent pulse generators. This measurement requires approximately four or five detected R waves to reach meaningful values. After this self-adjustment by the analyzer, steady readings can be interpreted to reflect the basic underlying rhythm while interspersed unusual values, either high or low, can reflect premature ventricular contractions or other aberrant rhythms. It is important to make note of both ranges of values.

Testing Protocol At each procedure a well-functioning cardioscope attached to standard limb leads is available for visualization of the cardiac cycles. In addition, a finger plethysmographic device, a Doppler flow detector, an esophageal stethoscope, or a precordial heart tone amplifier is always used for beat-by-beat audiomonitoring of the effective circulation. Confirmation of excellent anatomical positioning of the electrode tip is of utmost importance whether the procedure is performed by the transvenous route under fluoroscopic control or is accomplished through the epicardial approach under direct visualization. Also, in either instance, a period of observation is necessary to confirm stability of position. After testing the generator, the electrode variables are measured. These are important both at the time of initial (primary) procedures and at later (secondary) operations because they help to confirm adequate position and even stability of the leads and can aid in diagnosing the cause of any later system malfunction. For convenience, the analyzer is connected to presterilized cables and is dropped into a sterile transparent wrap so that it can be brought onto

168 T h e Annals of Thoracic Surgery Vol 26 No 2 August 1978

Fig 1 . T h e pacing system analyzer i n a sterile "bowel bag." The analyzer and sterile writing equipment allow the surgeon to observe and immediately record the several variables of both generator and lead function.

the operating table (Fig 1). A gas- or heatautoclaved, plastic, surgeon's-type, felt-tipped skin-marking pen is used to record the analyzer results. The cardioscope and audio equipment are monitored to ensure cardiac function while the variables are measured and recorded. Thus, the surgeon can perform the required maneuvers and make the recordings without assistance. Generator measurements are obtained and recorded in a one or two-minute period. Lead measurements may take longer for several reasons: it is sometimes difficult to immediately confirm consistent capture with this fixed-rate device; each lead (if there are two) should be tested separately in both the unipolar and bipolar configurations; and determination of the R wave amplitude might require gradual rate decrease of a temporary pulse generator so as to avoid a prolonged period of asystole [6]. Indeed, this latter measurement should not be attempted in certain pacemaker-dependent patients in order to avoid cerebral complications.

If the initial measurement results are less than ideal, the whole procedure must be repeated after lead repositioning. Finally, observation is required of the cardiac cycle when the permanent pulse generator is connected to the leads. Both proper pacing with capture and, if there are even a few intrinsic heart beats at short enough intervals, proper sensing with inhibition should be confirmed.

Results For the last 170 procedures, each pulse generator, whether it was to be implanted or had been explanted, was tested for all its variables. At the time of initial implantation, each generator was within the specification limits as prepublished by the manufacturer. Similarly, each lead was examined. Bipolar systems were tested in each polarity and then separately in each unipolar configuration. The epicardial-myocardial leads in this series, all with the electrode tips of the corkscrew configuration and of the then standard 6 mm penetrating depth, were implanted through the transmediastinal approach exclusively into the right ventricle [4, 51. Areas of the myocardium separated from one another by only 1 to 2 mm often manifested distinctly different elec-

169 Calvin: Intraoperative Pacemaker Electrical Testing

Fig 2 . Applicator and corkscrew electrode w i t h penetrating depth of 6 m m used for epicardial approach to the thin-walled right ventricle (above) and threshold testing probe zuith the same penetrating depth used in preliminary maneuver to seek electrically responsive areas of myocardium (below).

trical measurements, sometimes in the unacceptable range. This no doubt was often due to the fact that selective areas of the heart may have had myocardial fibrosis from coronary or other degenerative disease processes. But part of the explanation is anatomical: the undulating inner surface of this thin-walled chamber would in certain areas allow total wall penetration. Because of this, a specially designed threshold testing probe (Medtronic Model 6017A) was developed and first used in March, 1973. It has the same penetrating depth as the permanent electrode (Fig 2 ) and is designed to be used in an acupuncturelike maneuver to find the highly responsive areas of myocardium before implantation. Proper pacemaker function was achieved consistently in each of the 30 epicardial-myocardial procedures in this series even in the relatively shallow right ventricular wall, but sometimes the procedure required many trial test probe insertions and permanent electrode tip implantations (Fig 3). Finally, it must be remembered when using the epicardial approach that spatial orientation of two electrodes can sometimes produce bipolar R waves

' di

SECOND ELECTRODE second implant -+' 0.8 ma for c a p t u r e

I

i

i

'

l '

-I

'

'

'1

i

SECOND ELECTRODE t h i r d implant 0.3 ma f o r c a p t u r e

Fig 3 . Results with t w o epicardial-myocardial electrodes using older (constant-current) testing equipment. Three applications w i t h the second electrode (the first implant w i t h this electrode was not measured) were necessary to achieve ideal low current threshold and high QRS amplitude variables. Even more meaningful results can now be quickly sought and obtained with the n e w constant-voltage testing equipment.

of low amplitude simply because the sensed dipole is in a peculiar electrical plane [1, 91. Similarly, the transvenous ventricular leads frequently required considerable manipulation to achieve a final electrode tip position that was

170 The Annals of Thoracic Surgery Vol 26 No 2 August 1978

satisfactory by fluoroscopic configuration and concomitantly by electrical testing results. An unstable milliampere reading on the digital display when the voltage was constant was understood to indicate instability of endocardial electrode tip position, sometimes necessitating further manipulation. Lead threshold results at primary procedures from either operative approach were virtually the same. Thresholds of 0.6 v or less with the new constant-voltage, variable-pulse-width analyzer and 0.3 ma or less with the old constant-current, fixed-pulse-width testing device were achieved in 93 (92%) of the 101 patients undergoing implantation, both ventricular and atrial. One patient with corkscrew leads and 7 with endocardial ventricular leads (8%) revealed thresholds between 0.6 and 0.9 v. It is notable that the implants in 4 of these patients failed within a year after implantation due to so-called exit block. The R wave amplitude of the intrinsic usual rhythm and of any aberrant beats ranged from 4.0 to 23.0 mv. The average value in the 95 patients undergoing primary ventricular implantation was 9.6 mv, and in 91 instances (96%) it was more than 7.0 mv. The specific variables for the primary implantation in 5 of the 6 patients with atrial pacemakers were previously reported [3]; the thresholds were from 0.5 to 0.9 v, and the P wave amplitudes, measured by special oscilloscopic equipment, ranged from 2.0 to 3.0 mv. This, of course, necessitated high-sensitivity permanent generators for these atrial systems. Six (9%) of the 65 endocardial ventricular electrode tips became displaced two days to two weeks after primary implantation. Testing at reoperation revealed in each instance a high threshold in terms of both the stimulating voltage and the resultant current, but the calculated impedance had changed only insignificantly. Repositioning under fluoroscopic control with concomitant satisfactory results on retesting restored proper measurements and proper pacemaker function in each instance. Fifty-four patients underwent delayed, secondary operations for elective pulse generator replacement in this series. There had been no evidence of impending battery fatigue or other malfunction during follow-up visits for office

pacemaker function testing. In each instance the generator and lead measurements were within predicted ranges. In only 34 (76%) of the 45 patients with delayed system malfunction, however, was the predicted diagnosis of generator failure confirmed by diminished or absent output or prolonged pulse-to-pulse interval from the explanted unit. In each of the remaining 11 patients, whose pacemakers had been implanted through a transvenous approach, unexpected, correctable lead-electrode problems were identified. In 9 patients, intraoperative testing through the leads revealed both high voltage and high resultant current thresholds and the impedance was essentially unchanged, findings virtually identical to those with the early lead displacement problem. Delayed displacement was evident in only 2 patients, however, and exit block was diagnosed for the remaining 7. Lead repositioning or conversion to a unipolar system was possible in 4 of these 9 patients. A new operation for epicardial lead implantation was performed on 4 patients. A high-output generator for replacement was used for 1 critically ill patient with exit block. The findings at secondary operation in the other 2 patients are noteworthy. A 74-year-old man experienced sudden cessation of pacemaker function nearly two years after the initial implantation. Pacemaker spikes were absent on the electrocardiogram, and intrinsic cardiac function had reverted to preimplantation bradycardia. At operation, surprisingly, output and input variables of the generator were normal, but the output required from the analyzer through the leads for even incomplete capture was nearly 10 v. The recorded resultant current was less than 1 ma. Impedance was therefore very high. This was clear evidence of an electrode fracture, and only in retrospect was the roentgenographic discontinuity of the metallic electrode within the length of the lead noted (Fig 4). Lead splicing as well as elective generator replacement restored proper pacemaker function. The other patient, a 40-year-old woman, revealed the opposite voltage-current measurements. She was scheduled for elective pulse generator replacement and had mentioned

171 Calvin: Intraoperative Pacemaker Electrical Testing

mild, intermittent tingling in the left subcostal region, which incidentally was along the course of the bipolar epicardial leads placed four years earlier. The stimulating voltage threshold in the active lead was acceptable for its chronic state (1.5 v), while the resultant current was surprisingly high (6.0 ma). The calculated impedance was less than 300 ohms. Sheath disruption allowing a short circuit and causing intermittent subcutaneous and muscle stimulation was then suspected and was confirmed by the provocative maneuver of short-lived deliberately higher voltage, which caused the patient unequivocal shocklike discomfort. The sheath fracture was exposed (Fig 5), the epicardial leads were abandoned, and a successful transvenous endocardia1 operation was carried out.

Comment

Fig 4. A complete fracture of the length of the electrode at the bend in the supraclavicular region. lf this were only partial, electrical testing would show a high stimulating zioltage threshold, yet a low resultant current, and the calculated impedance would be markedly increased. These three variables of lead function, together with recording of the R wave, can easily be determined with the analyzer used in this series.

Fig 5. lnsulating sheath disruption, allowing electrical short circuit. Testing revealed abnormally low calculated impedance, with low voltage yet high current at threshold.

There are three specific purposes for testing at initial procedures: (1)to seek areas of responsive myocardium where the threshold for capture is very low; (2) to confirm concomitantly that the same area of the heart produces R waves or P waves of sufficiently high amplitude and fast slew rate so as to be easily sensed by the generator; and (3) to indicate the choice of an appropriate generator for the individual situation. The two purposes of testing at replacement or revision operations are to define the status of the old generator and to examine the leads in terms of threshold, impedance, and adequacy of sensing. These data will usually provide the explanation for the malfunction, if any, and indicate the next s t e p w h e t h e r to replace the generator, accomplish appropriate revisions, or abandon the present system in favor of an alternative approach. Even though there has been no instance in this series of a new pulse generator with variables outside the manufacturer’s specifications, this testing should be accomplished to afford baseline information. Generator testing at a later procedure provides information that is also compared against the manufacturer’s specifications and against its own earlier baseline. A suspicious unit may prove to be functioning properly while the actual problem is in the connections, the leads, the electrode tipmyocardium interface, or the myocardium itself

172 The Annals of Thoracic Surgery Vol 26 No 2 August 1978

[2, 141. Conversely, suspicious leads with higher than usual threshold may merely have to be unipolarized or, under special circumstances, may require a generator with augmented amplitude or width. Permanently implanted leads with measured low-amplitude R waves may require a pulse generator with higher sensitivity. The electrical results of threshold at initial implantation indirectly indicate the lead position. Ideal low threshold values together with high R wave or I’ wave amplitudes must mean close anatomical electrode tip-cardiac contact. Such measurements can almost always be achieved by a persistent surgeon at either a transvenous or an epicardial operation (see Fig 3). In the latter procedure, the threshold testing probe proved to be especially useful, as has been reported by others [17]. Compromised results because of time constraints or other reasons can alter the long-term prognosis. Onthe-table electrical analysis with this new easyto-use testing apparatus facilitates proper primary lead implantation. Published results [12] show that an acceptable stimulating threshold voltage at primary operations with a 0.5 to 1.3 msec pulse width would appear to be 1.0 v or less. The results in this series, however, indicate that the threshold can always be 0.9 v or less, and that 0.6 v should be sought. Acceptable minimum R wave amplitudes have been reported to be 4.0 to 5.0 mv [2], but with persistence, a value of 7.0 mv or more can usually be obtained. Thus, the ideal electrical test results should be guided by a ”goal of sixes and sevens,” a primary threshold of less than 0.6 v and an R wave of more than 7.0 mv. It should be noted that threshold values obtained from the constant-voltage and constantcurrent analyzers are not the same. The subtle but important electrical differences between the two design configurations for permanent pacemakers as well as for pacemaker testers together with the clinical implications have been described recently [2, 141. It would seem that analyzers incorporating both design configurations should be available for complex situations since both types of generators are in wide clinical use. However, a low threshold value in voltage or current or both of these from a tester of

Clinical Interpretation of Deranged Lead Impedance, with Voltage and Current Threshold Measurements High Impedance (High v with low ma) Mechanical attenuation of electrode Frank fracture of electrode (tissue fluid being the conductor) Poor generator-electrode connection Low Impedance (Low v with high ma) Mechanical gap in insulating sheath of lead (”short circuit”) (High v with high ma) Normal Impedance Primary procedure: poor positioning Secondary procedure: displacement or ”exit block” (Unstable resultant ma or v) Varying Impedance Nonfixed electrode tip position

either configuration means excellent proximity to responsive myocardium, and by itself is meaningful information. This is the chief reason for obtaining measurements at primary operations-to provide the electrical criteria of good anatomical electrode positioning. The presently accepted range of ”normal” lead-electrode impedance is 300 to probably 800 ohms. Several factors are involved in baseline impedance, including the surface area of the electrode tip 191. Interpretation of high threshold indicators is not difficult, especially with knowledge of the impedance [2, 81. Four general statements may be made (Table). First, high voltage with low current means high impedance. This is caused by mechanical attenuation of the lead, fracture of the lead (see Fig 4)with tissue being the conductor for a short distance in an intact insulating sheath, or a poor generator-lead connection. Second, the reverse-low voltage with high current-results from low impedance and shows an additional dipole or what amounts to an electrical shunt or a short circuit. This is due to a discontinuity in the insulation (see Fig 5) that allows some of the current from the intact length of electrode to escape and be wasted. Third, high voltage together with high current (normal impedance) confirms poor proximity to viable myocardium. This fact deserves emphasis. At initial implantation it means poor positioning, while at a later procedure, it may

173 Calvin: Intraoperative Pacemaker Electrical Testing

mean lead displacement or exit block due either to an excessive fibrotic capsule surrounding the electrode tip [lo] or to subjacent intramyocardial degenerative changes. Fourth, a varying current threshold when the voltage remains constant means instability of electrode position. In the latter two circumstances both the length of the electrode and its sheath may be in every way intact. More sophisticated methods of threshold determination (”energy” expressed in microjoules or “charge” expressed in microcoulombs) have broader meaning, especially when one is calculating safety margins between the permanent pacemaker output and the determined threshold requirement [2,8], and might become clinically applicable in the future. The determination of the several rheobase and chronaxie values, or the strength-duration curve, plotted at each pacemaker procedure have been advocated [15, 161 and have provided useful overall information. However, the extra time required and the fact that a generator with a known pulse width has often been preselected would seem to argue against routine measurement of the several thresholds. Usually, a single pulse width setting comparable to that of the proposed pulse generator would seem to be all that is necessary in a routine clinical setting. Arguments are made for wave-form analysis of the pacer impulse going to the heart or of the intrinsic electrogram coming from the heart. The shape of the generator’s pacer spike can provide additional useful information about impending malfunction not detectable by other means, but this requires high-fidelity oscilloscopes that are not generally available. Slew rate in the electrogram has been demonstrated to be important for proper generator sensing/ inhibition [9,13], but this measurement is similarly difficult to obtain with easily available testing equipment. Neither observation was made for the patients in this series. A stronger argument can be made for the use of a battery-powered or otherwise properly grounded electrocardiograph [7] for determination of the current of injury and of the vector of the QRS complex or the P wave, such information providing confirmation that the tip of an endocardia1 electrode is in proper position

abutting against the endocardium and has not perforated [9, 11, 161. This maneuver requires off-the-table cables, a properly calibrated recording device, and coordination with extra technical personnel, and it would seem that the correct conclusions can almost always be drawn from the roentgenographic or direct visual appearance together with the electrical variables from the analyzer. Finally, it should be remembered that a recorded low R wave or low P wave may simply be due to an orthogonal problem, the vector of the electrogram as “seen” through the permanent leads. This could result in apparent attenuation of amplitude [l, 91. Repositioning or, in a bipolar system, conversion to a unipolar configuration might be required merely to achieve a new spatial orientation. It is concluded that the concept of electrical testing of the total pacemaker system at the operating table has important clinical applicability. This analysis, easy to obtain with the pacing system analyzer, provides information concerning both generator and lead integrity as well as myocardial responses. At primary operations, variables can be obtained confirming sufficient generator output energy for cardiac rhythm capture, ideal positioning of the electrode tip, and adequate detectable intrinsic cardiac energy for appropriate generator inhibition. At secondary operations, meaningful conclusions from electrical pacemaker system analysis can be drawn, identifying generator malfunction, electrode tip displacement, electrode fracture, insulating lead sheath disruption, exit block, or combinations of these problems. Such testing at pacemaker operations would seem mandatory to obtain proper system function and longevity.

References 1. Barold SS, Keller JW: Sensing problems with car-

diac pacemakers, in Cardiac Pacing. Edited by P Samet. New York, Grune & Stratton, 1973, p 403 2. Barold SS, Winner ]A: Cardiovascular instrumentation: techniques and significance of threshold measurement for cardiac pacing. Chest 70:760, 1976 3. Calvin JW: Permanent atrial pacing. Epicardial approach-”pinch-on” electrodes. Arch Surg 111:712, 1976

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4. Calvin JW, Stemmer EA, Steedman RA, et al: Clinical application of parasternal mediastinotomy. Arch Surg 102:322, 1971 5 . Calvin JW, Zuber WF, Bilitch M, et al: Parasternal mediastinal approach for permanent cardiac pacemaker implantation. Chest 68:405, 1975 6. Escher DJW: Historical aspects of pacing, in Cardiac Pacing. Edited by P Samet. New York, Grune & Stratton, 1973, p 6 7. Evans GL, Glasser SP: Intracavitary electrocardiography as a guide to pacemaker positioning. JAMA 216:484, 1971 8. Furman S: Cardiac pacing and pacemakers VI; analysis of pacemaker malfunction. Am Heart J 94:378, 1977 9. Furman S, Hurzeler P, DeCaprio V: Cardiac pacing and pacemakers Ill; sensing the cardiac electrogram. Am Heart J 93:794, 1977 10. Furman S, Hurzeler P, Parker B: Clinical thresholds of endocardial cardiac stimulation: a long-term study. J Surg Res 9:149, 1975 11. Gulotta SJ: Transvenous cardiac pacing: techniques for optimal electrode positioning and prevention of coronary sinus placement. Circulation 62:701, 1970 12. Huffman JK, Goodman GR: Considerations for intraoperative pacemaker and lead system analysis. Cardiovasc Dis: Bull Texas Heart Inst 3:236, 1976 13. Hurzeler P, DeCaprio V, Furman S: Endocardia1 electrograms and pacemaker sensing. Med Instrum 10:178, 1976 14. Lulu DJ, Buysman JR: A threshold ambiguity problem involving a cardiac pacemaker. Am Surg 41:413, 1975 15. Smyth NPD, Keshishian JM, Baker NR, et al: Physiological rationale for the clinical use of low output pacemakers. Med Ann D C 43:257, 1974 16. Smyth NPD, Tarjan PP, Chemoff E, et al: The significance of surface area and stimulation thresholds in permanent cardiac pacing. J Thorac Cardiovasc Surg 71:559, 1976 17. Varriale P, Naclerio EA, Niznik J: Selection of site for permanent epicardial pacing using a myocardial testing electrode (abstract). Chest 72:414, 1977

Discussion DR. PHILIP VARRIALE (New York, NY): The theme of Dr. Calvin’s presentation is most relevant and cannot be emphasized too strongly. Electrical testing with a pacing system analyzer permits a comprehensive and meaningful approach in establishing a more secure pacing system during pacemaker implantation and in the diagnosis and management of pacemaker complications. Since 1973 we have extended the application of electrophysiological techniques with a hand-held

myocardial testing electrode (Medtronic 6017A) especially designed for selecting the best pacing site in either ventricle. This probe is easily inserted into various preselected sites on the exposed ventricle during operation to provide preliminary stimulation thresholds and sensing measurements prior to permanent epicardial lead installation. In our studies we have found relatively disparate stimulation properties in each ventricle, including sites that were considered suboptimal (> 1.5 volts) for pacing. The optimal average thresholds were most frequently identified in the lower part of the middle to anterior wall of the right ventricle and the lower anterolateral wall of the left ventricle. The optimal stimulatory measurements (after selection with the test probe) were approximately the same for the right and left ventricles. A myocardial electrogram is also obtained with a high-speed physiological recorder through the test probe for a better analysis of the signal available for sensing. The sensing measurements of the left ventricle were invariably two to three times greater than those of the right ventricle. This finding suggests a distinct advantage for left ventricular epicardial pacing. We have found the myocardial testing electrode invaluable in selecting the best available pacing site. It also contributes to the integrity of long-term cardiac pacing by eliminating indiscriminate insertion of the electrode. DR. KAMAL MANSOUR (Atlanta, GA): I cannot agree more with Dr. Calvin that detailed electrical testing is essential to obtain proper electrode function and pulse generator longevity. Although controversy remains regarding epicardial versus endocardial pacing and left ventricular versus right ventricular implantation, there should be no doubt about the importance of intraoperative electrical testing. Late failures to pace and sense properly are frequently the results of a poor implantation site. We are proficient in implantation techniques, be they endocardial or epicardial, sutured or sutureless. Now, more time should be devoted to careful intraoperative evaluation of the implant site. At my institution, we also use the Medtronic Model 5300, a constant-voltage, variable pulse width pacing system analyzer, during acute pacemaker implantations and pulse generator replacements or revisions. Before it was available, only stimulating current in milliamperes was measured; we had no idea of the required output voltage or electrode impedance. 1 compliment Dr. Calvin for bringing such a timely discussion to the attention of The Society. DR. GERALD M . LAWRIE (Houston, TX): We also have used the same Medtronic analyzer since its introduction, and our experience has been quite similar to that of Dr. Calvin. Of greatest interest to me in this

175 Calvin: Intraoperative Pacemaker Electrical Testing

study was the contrast between the fate of the epicardially paced group and the transvenously paced group. Although both began with comparable good lead function, the group with transvenous pacers ultimately had a late failure rate of 21% compared with a late failure rate of 0 in the epicardial group. I think when someone like Dr. Calvin obtains these sorts of results, it is time to really consider the efficacy of transvenous pacing, particularly since the transvenous lead failure rate he has experienced is similar to that at many other centers. At our institution we have steadily favored the epicardial approach, and since the introduction of the left subcostal approach, which I described in 1976 at this meeting, we now have operated on 160 patients using this procedure. It is extremely safe. We have been able to place sutureless screw electrodes into either the left ventricle or the septa1 area of the right ventricle. The thresholds obtained have been so consistently low that we have not found a need for any kind of testing device such as that shown by Dr. Naclerio. As a result of the Medtronic recall, we have been able to accumulate long-term data on some 25 of our 160 patients. In each, the lead function has been entirely satisfactory after a mean follow-up of more than two years. We have been very enthusiastic about epicardial pacing through a limited approach. To date, we have had no deaths in elective procedures, and, in a review of the literature, we found several other large series with zero mortality. It is possible that the trend toward more widespread transvenous pacing is due to the greater reliability of the epicardial approach. DR. G . F. 0.TYERS (Galveston, TX): I enjoyed Dr. Calvin's presentation and the privilege of previewing his paper. He has made a clear and concise presentation of essentially optimal methods for intraoperative testing of pacemaker systems, methods that in the long run conserve the surgeon's time and increase patient safety. Although I had not seen it previously described, we also use the Doppler flow detector to speed intraoperative testing. A radial Doppler is attached before operation and, as threshold is reached, the audible signal instantly changes to the intrinsic rhythm. The operator rarely has to look up from the field, and there can be no question about whether or not a given pacer spike is associated with capture, an occasional problem when electrocardiographic techniques are used alone. Essential threshold, impedance, and R wave levels can be determined readily in two or three minutes. Because impedance is directly available with a Telectronics analyzer, a small advantage, we use it rather than a Medtronic analyzer. Cordis and Biotronik also manufacture implant analyzers, and we are currently doing comparative studies. We have someone at the table record the

data on a fill-in-the-blank sheet, but in many institutions where technicians are not always on call, Dr. Calvin's "sterile notebook" technique is the only way to assure accurate data. Our results and interpretations are very similar to Dr. Calvin's. We also have not found impedance useful in separating lead displacement from exit block, but impedance studies have been very valuable in determining whether or not there is an electrode break (high impedance) or insulation break (low impedance). Quite often with a lead fracture, there is no surface ECG spike from the pacemaker, and on 2 occasions we have incorrectly diagnosed pacemaker failure preoperatively, as occurred in 1 of Dr. Calvin's patients. Even with no obvious roentgenographic demonstration, electrode impedance testing will confirm that a break is present. While at Penn State, Mr. Robert R. Brownlee and I developed and tested in patients a telemetry system for noninvasive transtelephonic monitoring of implanted pacers and electrodes, which permits postimplant determination of stimulation impedance and other critical implant status factors. Intermedic pacers with this system have been implanted at several centers. In contrast to the R wave, current, voltage, and impedance studies reported by Dr. Calvin, which are very valuable to the busy practitioner, wave form analysis is probably better reserved for those interested in research in this area. DR. EMIL NACLERIO (New York, NY): At the American College of Surgeons' meeting in Dallas, Dr. Varriale and I had an exhibit demonstrating use of a myocardial testing electrode for selection of the optimal site for permanent epicardial pacing. We learned then that a number of surgeons had performed two, three, and even four insertions of sutureless screw-in electrodes before finding a suitable site for permanent implantation. The indiscriminate multiple installation of leads could have been avoided if previous myocardial mapping with a testing electrode had been done. A colleague of ours recently implanted a "twist-in" double-barbed sutureless electrode (of a type that is infrequently used) in the wall of the left ventricle for recurrent syncope caused by heart block. Immediately following implantation, he obtained no sensing measurements and consequently, he elected to remove the electrode. During removal, cardiac arrhythmias and major hemorrhage occurred and proved fatal. If the exploring electrode had been employed before electrode insertion, death would have been averted. Rarely, if ever, would Dr. Varriale and I consider implanting a pacing electrode in the myocardium without first performing electrophysiological tests with a myocardial testing electrode. Such easy-toperform studies help select a myocardial site optimum for permanent electrode implantation.

176 The Annals of Thoracic Surgery Vol 26

No 2 August 1978

DR. CALVIN: Dr. Varriale, I agree with you that some of these patients listed as having ”exit block” might really have had internal displacement or, alternatively, either partial or through-and-through wall penetration by the electrode tip. Exit block in the broad sense did seem to occur more frequently in the transvenous pacer group. I am an enthusiast for the epicardial approach to pacing, but that is not the thrust of this paper. I agree with you about the importance of optimal electrode placement. Dr. Mansour, I appreciate your kind remarks, and I endorse your emphasis on the implantation site. Similarly, Dr. Lawrie, your comments are important, but the definitive discussion of epicardial versus endocardial pacing really should be for a different paper. Dr. Naclerio, we have all enjoyed your enthusiasm. Dr. Tyers, your remarks about constant audiomonitoring of the circulatory status are most appropriate. We are looking forward to those new generators with their capacity for telemetry of multiple indicators of pacemaker function. I am pleased that these will include impedance. This paper emphasized detailed electrical testing at the operating table; soon you will be able to provide us the oppor-

tunity for detailed electrical testing noninvasively after operation for both short- and long-term follow-up. One additional comment: at first it was nights or weekends in particular, when the cardiologist or hospital technician (or, worse, the manufacturer’s representative) was not available for attendance at an emergency operation, that I found it most convenient to be able to do my own testing and recording of results on the sterile field. Now it is routine for all procedures. I suggest that new generations of analyzers be designed that are able to measure both generator output by selection in either volts or milliamperes and lead threshold by selection of either a constantvoltage or a constant-current configuration, have a demand mode, and measure impedance directly with an ohmmeter. Also, they should have the sophistication to detect the P wave (and measure it down to a millivolt), be able to measure the QRS vector and detect a current of injury, and allow the permanent generator to pace the leads through the analyzer with an arrangement for incremental voltage or current attenuation. Perhaps they also should record in rate (rather than interval), have an easier battery check, and be still smaller and lighter.