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The electrical interaction between artificial pacemakers and patients, with applications to electrocardiography
The electrical irrtermtbm lmtwem artificial pacemakers ad (wtiunts, applicatiorrs to ebctr
with Y
Stanley A. Briller, M.D.* David B. Geselowitz, Ph.D.‘* Stig D. Arlinger, M.S.E.*** Gordon K. Danielson, M.D.**** Dov Jaron, B.S.***** Clattde Ii. Joper, M.D.****** Philadelphia, Pa.
A
lthough a variety of implantable artificial pacemakers have been in common use for the past 4 years, very little is known about the physical interaction of pacemaker currents with the myocardium and other body tissues. The present studies were undertaken to gain insight into the afore-mentioned interaction and to examine in detail the relationship between a known source of current (the artificial pacemaker) in the heart and the resulting body surface potentials. The latter relationship yields significant information concerning frequency distorFrom
tion produced bv electrocardiograph& body.
the transmission currents through
of the
Method Prior to surgery, each of eleven pacemakerst was calibrated as follows: A wideband (0 to 50 megacycles per second) oscilloscopeS was used to display the opencircuit voltage and then the voltage across each of at least ten 1 per cent resistors sequentially connected to the unit. For every such load the pacemaker voltage pulse was photographed at three sweep
the Department of Medicine (Edu%rd B. Kobinette Foundation) and the Harrison Department of Surgical Re. search of the School of Medicine, and the Department of Biomedical Electronic Engineering of the Moore School of Electrical Engineering of the University of Pennsylvania. Philadelphia, Pa. Supported in part by United States Public Health Service Grants HEOB805. HE-06352, and 5TlGM-606. and by a grant from the Heart Association of Southeastern Pennsylvania. Some preliminary aspects of this study are included in a thesis entitled “Study of Potentials on the Body Surface Arising from an Implanted Cardiac Pacemaker” submitted in August, 1964. to the University of Pennsylvania by Mr. Arlinger in partial fulfillment of the requirements for the degree of Master of Science in Engineering (Biomedical Electronic Engineering). Received for publication Aug. 5, 1965. *Assistant Professor of Medicine, and Assistant Professor of Electrical Engineering. Address: Hospital of the University of Pennsylvania. 3400 Spruce St., Philadelphia, Pa., 19104. -Associate Professor of Electrical Engineering. and Assistant Professor of Electrical Engineering in Medicine. ***Johnson Wax Foundation Fellow (1963-1964). Present address: Beckombergavagen 13, Bromma, Sweden. ****Associate in Surgery. *****Biomedical Engineering Trainee. ******Associate Professor of Medicine. fTen Medtronic Model 5860 and one Model 5870, all with helical platinum-iridium leads and electrodes. :Type 547 Tektronix \vitb CA amplifier.
656
Volume Number
71 5
Electrical
interaction
RESISTIVE
Fig. 1. Pacemaker calibration and crosses indicate currents curves based on an equivalent
between arti$cial
LOAD,
Model Numbers 1llOA and per square inch for 24 hours.
1111A.
and patients
657
OHMS
showing data obtained from unit subsequently implanted at the beginning (lo) and end (IT) of pulse, respectively. circuit for the pacemaker. See text.
speeds, each chosen to best record the total pulse (0.5 msec. per centimeter), and its rising (0.5 I.tsec per centimeter) and falling (5.0 psec per centimeter) phases. The total pulse pictures show what will be referred to as the slow portion of the pulse. Voltage amplitudes were measured at the beginning and end of the pulse, and the corresponding currents, IO and IT, were calculated by dividing by the load resistance RL. The data were plotted as a function of resistive load RL. A typical graph is shown in Fig. 1. During surgery, the pacemaker current was measured at the time of implantation or replacement.’ A current probe and preamplifier* with a SO-c.p.s. to 50-megacycle bandpass was used for measurements in the pacemaker circuit now completed by the myocardium. This type of probe was utilized because it is constructed of materials which have resisted sterilization with ethylene oxide (Cry-oxcide).? Moreover, since the current probe is an inductive device which measures current in insulated wires, it presents a minimum hazard of shock to the patient. The metallic interior of the probe jaws is grounded in normal use. *Hewlett-Packard tFifteen pounds
&IN
pacemakers
in Patient 11. Triangles Solid lines are theoretical
After surgery, the total pulse and its rising and falling phases were recorded between at least one precordial point and a right arm reference connection.* Standard precordial and extremity electrodes were used. Photographs of the slow portion of the voltage pulse recorded at a sweep speed of 0.5 msec. per centimeter (Fig. 5) were optically enlarged and the amplitude of the waveform was measured every 0.25 msec., beginning with the instant of rise, and plotted on semilogarithmic paper. Results
1. Calibration. The shape of the voltage or current pulse from resistively loaded pacemakers of the type utilized is a trapezoid, an example of which appears in Fig. 2. The pulse duration ranged from 1.5 to 2.0 msec. for the units studied. Table I shows that the ratio of the magnitude of the rising portion of the pulse to the falling part (IJI T) never exceeded 1.l for the resistive loads used (200 to 25,000 ohms). These calibration data can be rationalized by equating the pacemaker with a capacitor Ci, in series with a resistor Ri, and an ideal switch (Fig. 3). The switch is closed, *Type G, “Modified” supply provided second.
Tektronix a bandwidth
preamplifier powered by #127 of 0 to 20 megacycles per
Fig. 2. Calibration records from the pacemaker implanted in I’atient 11. Fz1t.h square is a centimeter on a side. The column on the left was obtained with a SOO-ohm load (0.5 volts/cm. j whereas a lO,OOO-ohm load was used The upper row shows the total pnlse or “slow portion” (abscissa = for t:he records on the right (2.0 volts/cm.). ). the middle row shows the rising phase (abscissa = 0.5 psec,‘c-m.), and the Io~~er row shows the 0.5 Imsec./cm. falling phase (ibscissa = 5.0 psec/cm. ).
RLEEYAKLR
1
RLSSTOR
Fig. 3. Equivalent circuit of pacemaker to a resistive load used during calibration.
LOAD
connected
allowing the capacitor, which had been charged to an initial voltage of Eo volts, to discharge into an external load through Ri. After time T the switch is opened and the capacitor is recharged to Eo. The cycle is repeated at a fixed rate, typically 75 pulses per minute. The results shown in Fig. 1 are characteristic of ten of the pacemakers included
in this study. This unit had an opencircuit voltage of 8.2 volts. The solid lines are theoretical curves obtained from Equations (2) and (3) of the Appendix, using Ci = 10 pf, Ri = 1,600 ohms, and Eo= 8.2 volts. The symbols are calibration data for this unit. The maximum discrepancy shown (4 per cent) is within experimental error. One unit had a variable current output achieved with a series rheostat. Calibration data on this unit taken with the same adjustment used at surgery showed that Ri was 1,000 ohms, In summary, calibration data for the eleven pacemakers were consistent with the equivalent circuit of Fig. 3, in which (1: was 10 pf in all cases, and Iii was 1,600 ohms in ten instances and 1,000 ohms in one. The equivalent circuit discussed in the preceding paragraph is consistent with the published circuit (a 1,500-ohm current-limiting resistor or ;I rheostat, a and a transistor which lo-pf capacitor.
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71
Electrical
5
interaction
between art&id
acts as a switch2), provided that a resistance of 100 ohms is assigned to the transistor and its associated circuit during the time it conducts and acts as a closed switch. One unit was recalibrated with the l,SOO-ohm series resistor temporarily shorted out. The open-circuit voltage was 8.0 volts. Measured values of IO and 1~ were compared with values calculated from Equations (2) and (3) of the Appendix, taking Eo = 8.0 volts, Ci = 10 pf, and Ri = 100 ohms. Agreement within 3 per cent was achieved for all values of 10 less than 20 Ma. Hence, for all currents of interest, the transistor and associated circuit contribute 100 ohms which must be added to the limiting resistance present to obtain Ri in the pacemaker equivalent circuit of Fig. 3. When examined with a fast time base (0.5 psec per centimeter), the average rise time* of the pacemaker pulse is 0.82 psec, with a range of 0.4 to 1.15 psec (Table III). Some units display an inflection at the midpoint of the rise when resistive loads of less than about 1,000 ohms are utilized. The falling phase of the pulse consists of a rapid return to the base line (with resistive loads of less than 1,000 ohms), followed by one or more rounded “bounces” each lasting about 2 psec. 2. Measurements of current during surgery. When the pacemaker with the calibration records shown in Figs. 1 and 2 was implanted, the current pulse in Fig. 4 was obtained. The configuration of current pulses obtained at surgery in the ten other cases was similar. The response of the current probe falls off for frequencies below 50 cycles per second, and consequently the pacemaker pulse appears to droop when examined at the slower sweep speeds (0.5 msec. per centimeter) appropriate for display of the entire pulse. Accurate measurements can be made of the magnitudes of the rise (1’0) and fall (I’r) current because these portions of the stimulus are high-frequency events not subject to distortion by the low-frequency limitations of the probe. I’, ranged from 3.2 to 5.2 Ma., with an average value of 4.0 Ma. Table I shows that the ratio I’$I’r in these records averaged 1.4 (1.2 - 1.5), *The
time point.
(gsec)
from
the instant
of rise to the major
inflection
pacemakers and patients
659
Fig. 4. Records of pacemaker current obtained with a current probe. The upper three photographs are records of pacemaker current obtained from Patient 11 during implantation. Uppermost is the “slow portion” and beneath it are, respectively, the rising and falling phases of the pacemaker pulse. The lowest record is the slow portion of the current pulse delivered to an equivalent circuit of the patient load (see text) constructed with components calculated for this patient. Ordinates 2 Ms./cm.; abscissae as in Fig. 2.
significantly greater than the maximum value of 1.1 during resistive calibration. The fine structure of the rising and falling phases of the stimulus observed during surgery was virtually identical to that observed during calibration when similar peak current was present (Table II and Figs. 2 and 4). 3. Pacemaker p&es on the body surface. In all but 2 patients, both myocardial electrodes were utilized to deliver impulses to the heart (bipolar stimulation). In these 9, the peak stimulus artifact on the thorax was never more than 6 mv. When the output of the pacemaker was administered between a single myocardial electrode and an abdominal subcutaneous
660
Hriller,
Geselowhtz, Arlinger,
Table I. Rise/fall pacemaker pulses
ratios
and
Danielson,
currents
qf
/
Patient xumber
!-~--.
Surgery --.- ---__,
~ I’ IdIT i(Ml.1 /
I
1
I I
I
2.
1.1 1.1
4.0 4.0
3.0 2.9
1.3 1.4
1.5 1.4
i3 5. 6.* 7. 8. 9. 10.* 11.
1.1 1.1 1.1 1.1 1.1 1 .O 1.0 1.0 1.1
3.8 34 4.1 4.5 3.6 32 52 4.1 4.2
3 0 2.9 3 .o 3 .3 2.8 2 1 35 3.2 3.0
1.3 1.2 1.4 1.4 1.3 1.5 1 .5 1.3 1.4
1.4 1.5 1.3 1.4 1.4 1 .-I 1 ..i 1.-t
1.
*“Unipolar” stimulation. tMaxima1 values. I,: Magnitude of current at beginning (rise) of pacemaker pulse during calibration. IT: Magnitude of current at end (fall) of pacemaker pulse during calabration. ItO: Magnitude of current at beginning of pacemaker pulse after implantation at surgery. 1’1’: Magnitude of current at end of pacemaker pulse after implantation at surgery. VO: Magnitude of body surface voltage artifact at beginning of pacemaker pulse. V’T: Magnitude of body surface voltage artifact at end of pacemaker pulse.
site adjacent to the pacemaker case, the stimulus artifact attained values as high as 40 mv. The rise time of the pacemaker voltage pulse was measured in all but three instances in which low amplitude precluded triggering of the necessarily rapid sweep (0.5 psec per centimeter). It is apparent from Table II that the rise time on the body surface for a given pacemaker is of the same order noted during measurements of current at surgery, and earlier at calibration. In two instances in which rise times were somewhat slower on the body surface than elsewhere, the preamplifier utilized* had a limited high-frequency response and could not be expected to follow the swiftly rising portion of the pacemaker pulse. The “modified” type G amplifier used in all other measurements of voltage has a rise time capability of 0.25 psec and amplifies the pacemaker pulses without distortion. These data show that waveforms with rise times of the *Type
D Tektronix.
Patient number
I 1 Calibration
I. 2. .z 4. 5.
0.6 0 6 I I 1 1 0 7 1 .n 1.1 1 2 I 1 0 .8 0 5
I
.Vurgery
.5’urjace
Surfare
I’T I I’olI’T ! v,/v, (Ma.)
Jaron, and Joync~r
6.t 7.
8. 9. 1o.t 11.
0 4 0 5 1 0 09 05 0.9 0 9 1 2 1 2 0 8 n-1
0.71 0 3
1.1: 0 9 0 9 I n 1 1 04
*Time (+vec) from the instant of rise to the major inflection point. t”Unipolar” stimulation. $Preamplification with Tektronix Type 11 unit. All others utilized Modified Type G.
order of 1 psec are conducted from the myocardium to the body surface without significant distortion. The slow portion of the pulse, on the other hand, showed readily apparent differences among the pictures taken at. calibration, implantation, and on the body surface.* Photographs of the body surface voltage artifacts from Patient 11 are shown in Fig. 5. Analysis of the slow portion of pacemaker pulse. The electrical characteristics of body tissues have been shown by others4 to be substantially resistive and independent of frequency in the range of 1 to 10,000 cycles per second. These properties of body tissues, as well as the lack of distortion of portions of the pacemaker pulse with frequency content well byeond this range, suggested that the slow portion of the pulse at the body surface would follow closely the current delivered to the myocardium. As a preliminary check, the ratio of rise to fall voltage amplitudes, VO/VT, was computed for each pulse recorded *Considerable variation was noted in the shape of the voltage pulse in the case of one of two patients with a “unipolar” arrangement of stimulation electrodes. Since the shape of the current pulse at surgery was not unusual. it is likely that the variation in voltage waveform noted was due to polarization at the surface electrodes. For this reason, no further analysis of either case with “unipolar electrodes” is included.
Volume 7 1 Number
Electrical interaction
5
between artificial
pacemakers and patients
661
vation indicates that two capacitors are needed in a circuit analogue of the tissue load on bipolar pacemaker electrodes. The equivalent circuit shown in Fig. 6 has been utilized. Values for each component were determined for each case (Table III) utilizing formulae derived in the Appendix, and the following variables: (1) the rise (peak) current (1’0) measured at surgery; (2) the ratio (B/A) of the zero time intercepts of the fitted straight lines on the semilogarithmic plots; and (3) the time constants (r and T’) of the foregoing plots. r (or T’) is the time when the voltage had fallen to e-l or 36.8 per cent of its peak value. An equivalent circuit was constructed utilizing values calculated from measurements on Patient 11. The current pulse delivered to this circuit by a pacemaker of the same type actually implanted in this patient was measured by placing an oscilloscope across R. The pulse observed was Fig. 5. The upper three photographs are voltage artifacts from the precordium of Patient 11 after recovery from surgery. The record at the top is the slow portion, and beneath it are, respectively, the rising phase and the falling phase of the pacemaker pulse. Sensitivity, 2.5 mv./cm. The lowest photograph is the slow portion of the current pulse delivered to the patient’s equivalent circuit by a similar pacemaker. Oscilloscopic gain was adjusted to produce deflections comparable to those in the top photograph. Abscissae as in Fig. 2.
from marized
the
surface.
These
data
are
sum-
in Table I. In each case the surface voltage ratio obtained agreed with the implantation current ratio within experimental error. Consequently, it was tentatively assumed that the shape of the slow portion of the voltage pulse at the surface was not significantly different from the current pulse delivered to the myocardium. The semilogarithmic plot of the slow portion of the pulse was a straight line, except for the first three or four values. Differences between this line and the first three or four values were replotted and defined a second straight line on semilogarithmic paper. The slow portion of the pacemaker pulse may thus be described in terms of two exponential functions or time constants. The latter obser-
c R c’
ci
d
3 PACEMAKER
Fig. 6. Equivalent to the equivalent tient load.
:
PATIENT
LOAD
circuit of pacemaker connected circuit used to represent the pa-
Table III. Values for eatient load impedance
equivalent
circuit
Patient number 1.* 2.* 3. 4. 5.* 7. 8. 9.* 11. *Replacement
5.50 500 600 820 4.50 700 1,000 320 350 of pacemaker
6.0 9.7 4.1 5.5 5.6 5.4 2.8 10.2 5.5 unit without
290 325 9.5 115 180 270 185 115 335 thoracotomy.
1.4 1.1 2.0 4.5 1.6 1.2 1.6 1.1 0.8
of
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Briller,
Gcselowik,
Arhger,
Danielson,
virtually identical to the voltage pulse present on the patient’s thorax (Fig. 5). The current waveform in this system was determined with the current probe (Fig. 4), and it too was almost identical to the current waveform recorded at implantation. In this way, confirmation was obtained for the tentative assumption that the slon portions of the voltage pulses observed at the body surface have the same configuration as the current pulses delivered to the myocardium. Discussion
Although artificial pacemakers of one make were employed in this work, it will become evident that the current-voltage relationships derived have general significance. Two frequency-dependent electrical impedances are implicit in the foregoing studies. The first, a transfer impedance independent of both pacemaker and electrode types relates the voltage artifact at the body surface to the pacemaker current delivered to the myocardium. The second, which may be designated the patient load,3 is the total in vivo impedance placed upon the pacemaker electrodes. Patient load is a function of two classesof variables. The first class is independent of the particular electrodes utilized but includes such variables as current density, frequency spectrum of pulse, electrode separation, tissue impedance, etc. The other class is dependent upon the type of electrodes and involves variables such as the electrode material and configuration. It follows that pacemakers of any type utilizing Chardack platinum-iridium electrodes will have a load of similar characteristics placed upon them, provided that variables of the first c-lassare accounted for as in these studies. The most direct way of obtaining the patient load impedance would be to record simultaneously voltage and current at the myocardial electrodes after they are connected to the heart at surgery. However, the voltage measurements require direct myocardial connection of electronic equipment energized from power lines, with the attendant hazard of ventricular fibrillation. Current, on the other hand, was measured safely with a current probe. The necessity for voltage measurement at
.laron, cud foyrwr
the electrodes during surgery was rirculuvented by determining the equivalent circuit for the pacemaker (Fig. 3) as part of the calibration procedure. The pacemaker equivalent circuit, together with knowledge of the current. at surgery, enabled calculation of the patient load impedance and its expression as another equivalent circuit, which is shown in Fig. 6. X’alues for the four elements of the latter equivalent circuit are listed in Table I I 1. ,I consideration of the deformation of pacemaker pulses by the myocardium has led others to prepare substantially different equivalent circuits of the pacemaker load. One such circuit? was prepared for a device employing stainless steel loop electrodes. In another case the “synthetic load” failed to account for the in vivo load +vith the fidelity achieved in the present study. The slog portion of the pacemaker pulse is characterized by two time constants. The shorter is typically 0.2 msec., and the larger sometinles exceeds 10 msec. Thcrefore, important frequency components are present in the range frown a few cycles per second to several kilocycles per second. Schwan and F;a?,* showed that this frequencl’ range is one in I\-hich tissues such as muscle are well characterized 1~1,constant resistivity, and concluded that elect-rocardiogranls containing components up to a thousand cycles would not he distorted. It is reasonable then to expect that the myocardium and surrounding tissues contribute resistance alone to the patient load impedance in the frequenc)~ range encompassed bl. the slo\v portion of the pulse. This resistance, which depends on electrode separation, is represented by a fraction of the resistor K in the equivalent circuit of Fig. 6. The remainder of the equivalent circuit is attributable to polarization at the electrode-myocardial interface. Polarization is an electrochemical interface phenomenon and involves, among other things, Ihe movelllcnt of ions after t-he application oi an electrical field.” It is concluded that. electrode-luLoc~trdia1 polarization rather than tissue impedance accounts for the deformation of the slow portion of the pacemaker pulse. Measurements of (.urrclil at surger)’ were obtained either when the chest wall
Volume 71 Number 5
Electrical interaction
between artificial
was partially open after electrode implantation or after a replacement pacemaker had been inserted without a thoracotomy. The four highest values for R occur in instances in which the electrodes were newly implanted, yet the next to lowest value for R is present in a fifth such case. Since a substantial portion of the total equivalent circuit for the patient load is attributable to polarization at the myocardial-electrode interfaces, the effect of thoracotomy, if any, should be a relatively small change in the series resistor R. The equivalent circuit for the patient load has a series capacitor C (Fig. 6); an infinite impedance to direct current is therefore implied. However, this circuit was derived from a study of the distortion of the slow portion of the pacemaker pulse which has no significant components below a few cycles per second. Put in slightly different terms, the capacitor C can be shunted by high resistance (greater than 1,000 ohms) without noticeably changing I’, the current which the pacemaker would deliver to this equivalent circuit. Transfer impedance is of direct interest to electrocardiography because it relates a known myocardial current to body surface potentials in an in vivo human experiment. The rise of the pacemaker pulse contains frequency components in the frequency range of lo5 to lo6 c.p.s. In this part of the frequency spectrum the electrical properties of representative tissues enter a frequency-dependent region designated the @ dispersion> However, electrocardiographic analysis is not so much concerned with the details of the frequency dependence of the transfer impedance as with possible distortion of waveforms. Readily measured impedance variations with frequency obtained with the alternating current technique” may have only a small effect on a particular waveform. With the precision of measurement available, pacemaker voltage pulses at the body surface were observed to have the same shape as current pulses delivered to the myocardium, which suggests that distortion is small for frequency components up to lo6 cycles per second. Consequently, the transfer impedance can be approximated by a constant resistance even in this frequency range. The present experiment thus pro-
#acemakers and patients
663
vides additional evidence that electrocardiographic signals, including details observed with wide-band amplifiers,? will not suffer frequency distortion. For a given placement of myocardial electrodes in a patient, the transfer impedance will depend on the particular pair of surface points chosen. This transfer impedance is a scalar quantity, Z, which relates the voltage V in a lead to the current I’ injected into the myocardium. Thus, V = ZI’. Since the maximum peak voltage observed was about 6.0 mv., and the average peak current was 4.0 Ma., the maximal transfer impedance is of the order of 1.5 ohms. A vectorial “transfer impedance” which is closely related but not identical to Z has been proposed by Schmitt.* He defines transfer impedance as a vector point function ?*, which relates the voltage in a lead to a current dipole moment $ in the heart region, as follows: V = 72% . ;S: If d+is a vector joining the myocardial electrode sites, and if the electrode separation is not too great, a current dipole moment, i-;‘= I’$ can be associated with the pacemaker. It follows that the scalar and vectorial transfer impedances are related by Z = 2, * 2, Since the distance between electrodes was typically 1.5 cm., and Z had a maximal value of about 1.5 ohms, vectorial transfer impedances associated with the pacemaker are of the order of 1 ohm per centimeter. Extensive measurements of vectorial transfer impedance have been made by others using torso-shaped models filled with homogeneous conducting fluids.8,g The magnitude of the impedance depends on the lead, the location of the dipole, and the resistivity of the fluid. If a value for fluid resistivity of 1,000 ohm-centimeters is used in these model studies, the vectorial transfer impedances range up to several ohms per centimeter in magnitude. It is somewhat difficult to relate our in vivo values to those obtained in model studies. The pacemaker electrodes in the epicardium constitute a dipole current source which is more ventral than the electrical center for QRS. Eccentric locations such as these should produce larger surface voltages for a given dipole moment and imply larger transfer in~pedance.g On the other hand, the current from myo-
664
Hriller,
Geselowitz, Arlinger,
Danielson,
cardial electrodes is tangent to the intracavitary mass, and, on this basis, a reduction in surface voltage and transfer impedance might be anticipated.‘” Summary Eleven artificial pacemakers were calibrated prior to surgery. The current delivered by these units to human hearts was then measured at surgery with a current probe. Body surface pacemaker voltages were recorded subsequently in nine cases. The amplitude and shape of the current pulse depends upon the patient load impedance. Patient load impedance consists of myocardial electrode polarization impedance and tissue resistance. For the pacemaker and myocardial electrodes employed, the patient load impedance is well represented by an equivalent circuit consisting of a series parallel arrangement of two capacitors and two resistors. Values of these circuit parameters were determined in nine cases. Tissue resistance is represented by a fraction of the series resistor in the equivalent circuit. The retnainder of the equivalent circuit is attributable to polarization. The voltage pulse measured at the body surface is related to the current pulse delivered to the myocardium by a transfer impedance. Since details of the current pulse, which had a rise time of less than a microsecond, were present in surface voltages, the transfer impedance is approximately resistive and independent of frequency up to a megacycle per second. It is concluded that no significant frequency distortion of electrocardiograms is caused by the electrical characteristics of body tissues. The value of the maximal scalar transfer impedance from these studies has been cast into the vectorial form employed by others in torso model studies. IJnder the conditions discussed, the vectorial transfer impedance has comparable magnitude whether obtained from model studies or from the present pacemaker study. Appendix The equivalent circuit of the pacemaker connected to a resistive load RL is shown in Fig. 3. If the switch is closed at time t = 0 with Ci charged to Eo volts, then
Jaron, and .ioyncr
The switch is reopened at t = ‘I‘ seconds, producing a current pulse of duration 1‘. During the time the switch is open, Ci is recharged to E. volts. E0 can be determined by observing the open-circuit voltage at the pacemaker terminals. For this measurement, the pacemaker is connected directly to the oscilloscope, which has an input impedance of 10 megohms. If (Ri + RL)Ci is greater than 5T (which was the case for the units studied), the exponential function of Equation (1) is almost indistinguishable from a straight line, and a trapezoidal current pulse lvill be obtained. 10 and 1~ are defined as the values of I at t = 0 and t = T, respectively. Then
-‘I‘/(R,-t
Rr,K,
IT = lee
(3)
During calibration, E, is measured and IO and IT are determined as functions of RL; R r and C i can then be calculated from Equations (2) and (3). For the pacemakers studied, Cr was 10 pf, Ri with one exception was 1,600 ohms, and E. was typically 8 volts. Now consider that the pacemaker is connected to the load shown in Fig. 6. The pacemaker current in this case will be designated I’. After the switch is closed at t= 0, l’=Ae
-t/7
-t/d +Re
(4)
where CCi CT=
RT=
(5)
Cc,
Ri+
R
(6)
Volume 71 Number
Electrical interaction
5
R’ -zz
between artificial
(9)
RT
If the subscript t= 0,
0 again
indicates
time
EO
Io=A+B=~~+~
For the pacemakers the case that
so that Equations plify to RTCT=
(8), and (9) sim7
-
(?a) R’C’
= (1 + ;)T’
R’=RT;
(1-
6)
‘1 7
By plotting I’ as a function of t on semilog paper as described in the text it is possible to determine the parameters A,B,T and T’ of Equation (4). A comparison of Equations (10) and (2) shows that R is the value of RL for which 10 = IO), and hence can be determined directly from calibration data. RT is simply R + Ri. CT is then calculated from Equation (7) or (7a), R’ from Equation (9) or (9a), and C’ from Equation (8) or (8a). Finally, C is obtained from Equation (5). CicT ‘=
ci-
CT
665
circuit, are determined from A, B, T, and r’, the parameters describing the slow portion of the current pulse delivered to the myocardium. We are grateful extensive technical
to Miss Sally assistance.
Featherstone
for
REFERENCES
used it was generally
(7),
pacemakers and patients
(12)
In this manner, R, C, R’, and C’, the parameters of the patient load equivalent
1. Briller, S. A., Geselowitz, D. B., Danielson, G. K., Joyner, C. R., and Arlinger, S. D.: Use of a current probe in the evaluation of failure of artificial pacemakers, Proc. Ann. Conference on Engineering in Med. and Biol., 6:120, 1964. W. M., Gage, A., Frederico, ,4. J., 2. Chardack, Schimert, G., and Greatbatch, W.: Clinical experience with an implantable pacemaker, Ann. New York Acad. SC. 111:1075,1964. 3. Simpson, J. A., Gibson, P., Stanford, R. W., and McLennon, D. B.: Prolonged cardiac pacemakers in Stokes-Adams disease, Lancet 2:226, 1962. 4. Schwan, H. P., and Kay, C. F.: The conductivity of living tissues, Ann. New York Acad. SC. 65:1007, 1957. D. S., and Kantrowitz, A.: Electrical 5. Feldman, characteristics of human ventricular myocardium stimulated in vivo, Clin. Res. 11:22, 1963. 6. Butler, J. A. V.: Electrical phenomena at interfaces. New York. 1951. The Macmillan Co. 7. Langner, P. H., Jr., Geselowitz, D. B., and Mansure, F. T.: High-frequency components in the electrocardiograms of normal subjects and of patients with coronary heart disease, AM. HEART J. 62:746, 1961. 8. Schmitt, 0. H.: Lead vectors and transfer ~n$ance, Ann. New York Acad. SC. 65:1092 9. Frank, E.: Determination of the electrical center of ventricular depolarization in the human heart, AM. HEART J. 49:670, 1955. 10. Brody, D. A.: A theoretical analysis of intracavitary blood mass influence on the heart-lead relationship, Circulation Res. 4:731, 1956. 11. Goldstein, H. L., Kay, C. F., and Schwan, H. P.: Phase shift in body tissues as it pertains to electrocardiography, Fed. Proc. 12:No. 1, March, 1963.
with Y
Stanley A. Briller, M.D.* David B. Geselowitz, Ph.D.‘* Stig D. Arlinger, M.S.E.*** Gordon K. Danielson, M.D.**** Dov Jaron, B.S.***** Clattde Ii. Joper, M.D.****** Philadelphia, Pa.
A
lthough a variety of implantable artificial pacemakers have been in common use for the past 4 years, very little is known about the physical interaction of pacemaker currents with the myocardium and other body tissues. The present studies were undertaken to gain insight into the afore-mentioned interaction and to examine in detail the relationship between a known source of current (the artificial pacemaker) in the heart and the resulting body surface potentials. The latter relationship yields significant information concerning frequency distorFrom
tion produced bv electrocardiograph& body.
the transmission currents through
of the
Method Prior to surgery, each of eleven pacemakerst was calibrated as follows: A wideband (0 to 50 megacycles per second) oscilloscopeS was used to display the opencircuit voltage and then the voltage across each of at least ten 1 per cent resistors sequentially connected to the unit. For every such load the pacemaker voltage pulse was photographed at three sweep
the Department of Medicine (Edu%rd B. Kobinette Foundation) and the Harrison Department of Surgical Re. search of the School of Medicine, and the Department of Biomedical Electronic Engineering of the Moore School of Electrical Engineering of the University of Pennsylvania. Philadelphia, Pa. Supported in part by United States Public Health Service Grants HEOB805. HE-06352, and 5TlGM-606. and by a grant from the Heart Association of Southeastern Pennsylvania. Some preliminary aspects of this study are included in a thesis entitled “Study of Potentials on the Body Surface Arising from an Implanted Cardiac Pacemaker” submitted in August, 1964. to the University of Pennsylvania by Mr. Arlinger in partial fulfillment of the requirements for the degree of Master of Science in Engineering (Biomedical Electronic Engineering). Received for publication Aug. 5, 1965. *Assistant Professor of Medicine, and Assistant Professor of Electrical Engineering. Address: Hospital of the University of Pennsylvania. 3400 Spruce St., Philadelphia, Pa., 19104. -Associate Professor of Electrical Engineering. and Assistant Professor of Electrical Engineering in Medicine. ***Johnson Wax Foundation Fellow (1963-1964). Present address: Beckombergavagen 13, Bromma, Sweden. ****Associate in Surgery. *****Biomedical Engineering Trainee. ******Associate Professor of Medicine. fTen Medtronic Model 5860 and one Model 5870, all with helical platinum-iridium leads and electrodes. :Type 547 Tektronix \vitb CA amplifier.
656
Volume Number
71 5
Electrical
interaction
RESISTIVE
Fig. 1. Pacemaker calibration and crosses indicate currents curves based on an equivalent
between arti$cial
LOAD,
Model Numbers 1llOA and per square inch for 24 hours.
1111A.
and patients
657
OHMS
showing data obtained from unit subsequently implanted at the beginning (lo) and end (IT) of pulse, respectively. circuit for the pacemaker. See text.
speeds, each chosen to best record the total pulse (0.5 msec. per centimeter), and its rising (0.5 I.tsec per centimeter) and falling (5.0 psec per centimeter) phases. The total pulse pictures show what will be referred to as the slow portion of the pulse. Voltage amplitudes were measured at the beginning and end of the pulse, and the corresponding currents, IO and IT, were calculated by dividing by the load resistance RL. The data were plotted as a function of resistive load RL. A typical graph is shown in Fig. 1. During surgery, the pacemaker current was measured at the time of implantation or replacement.’ A current probe and preamplifier* with a SO-c.p.s. to 50-megacycle bandpass was used for measurements in the pacemaker circuit now completed by the myocardium. This type of probe was utilized because it is constructed of materials which have resisted sterilization with ethylene oxide (Cry-oxcide).? Moreover, since the current probe is an inductive device which measures current in insulated wires, it presents a minimum hazard of shock to the patient. The metallic interior of the probe jaws is grounded in normal use. *Hewlett-Packard tFifteen pounds
&IN
pacemakers
in Patient 11. Triangles Solid lines are theoretical
After surgery, the total pulse and its rising and falling phases were recorded between at least one precordial point and a right arm reference connection.* Standard precordial and extremity electrodes were used. Photographs of the slow portion of the voltage pulse recorded at a sweep speed of 0.5 msec. per centimeter (Fig. 5) were optically enlarged and the amplitude of the waveform was measured every 0.25 msec., beginning with the instant of rise, and plotted on semilogarithmic paper. Results
1. Calibration. The shape of the voltage or current pulse from resistively loaded pacemakers of the type utilized is a trapezoid, an example of which appears in Fig. 2. The pulse duration ranged from 1.5 to 2.0 msec. for the units studied. Table I shows that the ratio of the magnitude of the rising portion of the pulse to the falling part (IJI T) never exceeded 1.l for the resistive loads used (200 to 25,000 ohms). These calibration data can be rationalized by equating the pacemaker with a capacitor Ci, in series with a resistor Ri, and an ideal switch (Fig. 3). The switch is closed, *Type G, “Modified” supply provided second.
Tektronix a bandwidth
preamplifier powered by #127 of 0 to 20 megacycles per
Fig. 2. Calibration records from the pacemaker implanted in I’atient 11. Fz1t.h square is a centimeter on a side. The column on the left was obtained with a SOO-ohm load (0.5 volts/cm. j whereas a lO,OOO-ohm load was used The upper row shows the total pnlse or “slow portion” (abscissa = for t:he records on the right (2.0 volts/cm.). ). the middle row shows the rising phase (abscissa = 0.5 psec,‘c-m.), and the Io~~er row shows the 0.5 Imsec./cm. falling phase (ibscissa = 5.0 psec/cm. ).
RLEEYAKLR
1
RLSSTOR
Fig. 3. Equivalent circuit of pacemaker to a resistive load used during calibration.
LOAD
connected
allowing the capacitor, which had been charged to an initial voltage of Eo volts, to discharge into an external load through Ri. After time T the switch is opened and the capacitor is recharged to Eo. The cycle is repeated at a fixed rate, typically 75 pulses per minute. The results shown in Fig. 1 are characteristic of ten of the pacemakers included
in this study. This unit had an opencircuit voltage of 8.2 volts. The solid lines are theoretical curves obtained from Equations (2) and (3) of the Appendix, using Ci = 10 pf, Ri = 1,600 ohms, and Eo= 8.2 volts. The symbols are calibration data for this unit. The maximum discrepancy shown (4 per cent) is within experimental error. One unit had a variable current output achieved with a series rheostat. Calibration data on this unit taken with the same adjustment used at surgery showed that Ri was 1,000 ohms, In summary, calibration data for the eleven pacemakers were consistent with the equivalent circuit of Fig. 3, in which (1: was 10 pf in all cases, and Iii was 1,600 ohms in ten instances and 1,000 ohms in one. The equivalent circuit discussed in the preceding paragraph is consistent with the published circuit (a 1,500-ohm current-limiting resistor or ;I rheostat, a and a transistor which lo-pf capacitor.
Volume
Number
71
Electrical
5
interaction
between art&id
acts as a switch2), provided that a resistance of 100 ohms is assigned to the transistor and its associated circuit during the time it conducts and acts as a closed switch. One unit was recalibrated with the l,SOO-ohm series resistor temporarily shorted out. The open-circuit voltage was 8.0 volts. Measured values of IO and 1~ were compared with values calculated from Equations (2) and (3) of the Appendix, taking Eo = 8.0 volts, Ci = 10 pf, and Ri = 100 ohms. Agreement within 3 per cent was achieved for all values of 10 less than 20 Ma. Hence, for all currents of interest, the transistor and associated circuit contribute 100 ohms which must be added to the limiting resistance present to obtain Ri in the pacemaker equivalent circuit of Fig. 3. When examined with a fast time base (0.5 psec per centimeter), the average rise time* of the pacemaker pulse is 0.82 psec, with a range of 0.4 to 1.15 psec (Table III). Some units display an inflection at the midpoint of the rise when resistive loads of less than about 1,000 ohms are utilized. The falling phase of the pulse consists of a rapid return to the base line (with resistive loads of less than 1,000 ohms), followed by one or more rounded “bounces” each lasting about 2 psec. 2. Measurements of current during surgery. When the pacemaker with the calibration records shown in Figs. 1 and 2 was implanted, the current pulse in Fig. 4 was obtained. The configuration of current pulses obtained at surgery in the ten other cases was similar. The response of the current probe falls off for frequencies below 50 cycles per second, and consequently the pacemaker pulse appears to droop when examined at the slower sweep speeds (0.5 msec. per centimeter) appropriate for display of the entire pulse. Accurate measurements can be made of the magnitudes of the rise (1’0) and fall (I’r) current because these portions of the stimulus are high-frequency events not subject to distortion by the low-frequency limitations of the probe. I’, ranged from 3.2 to 5.2 Ma., with an average value of 4.0 Ma. Table I shows that the ratio I’$I’r in these records averaged 1.4 (1.2 - 1.5), *The
time point.
(gsec)
from
the instant
of rise to the major
inflection
pacemakers and patients
659
Fig. 4. Records of pacemaker current obtained with a current probe. The upper three photographs are records of pacemaker current obtained from Patient 11 during implantation. Uppermost is the “slow portion” and beneath it are, respectively, the rising and falling phases of the pacemaker pulse. The lowest record is the slow portion of the current pulse delivered to an equivalent circuit of the patient load (see text) constructed with components calculated for this patient. Ordinates 2 Ms./cm.; abscissae as in Fig. 2.
significantly greater than the maximum value of 1.1 during resistive calibration. The fine structure of the rising and falling phases of the stimulus observed during surgery was virtually identical to that observed during calibration when similar peak current was present (Table II and Figs. 2 and 4). 3. Pacemaker p&es on the body surface. In all but 2 patients, both myocardial electrodes were utilized to deliver impulses to the heart (bipolar stimulation). In these 9, the peak stimulus artifact on the thorax was never more than 6 mv. When the output of the pacemaker was administered between a single myocardial electrode and an abdominal subcutaneous
660
Hriller,
Geselowhtz, Arlinger,
Table I. Rise/fall pacemaker pulses
ratios
and
Danielson,
currents
qf
/
Patient xumber
!-~--.
Surgery --.- ---__,
~ I’ IdIT i(Ml.1 /
I
1
I I
I
2.
1.1 1.1
4.0 4.0
3.0 2.9
1.3 1.4
1.5 1.4
i3 5. 6.* 7. 8. 9. 10.* 11.
1.1 1.1 1.1 1.1 1.1 1 .O 1.0 1.0 1.1
3.8 34 4.1 4.5 3.6 32 52 4.1 4.2
3 0 2.9 3 .o 3 .3 2.8 2 1 35 3.2 3.0
1.3 1.2 1.4 1.4 1.3 1.5 1 .5 1.3 1.4
1.4 1.5 1.3 1.4 1.4 1 .-I 1 ..i 1.-t
1.
*“Unipolar” stimulation. tMaxima1 values. I,: Magnitude of current at beginning (rise) of pacemaker pulse during calibration. IT: Magnitude of current at end (fall) of pacemaker pulse during calabration. ItO: Magnitude of current at beginning of pacemaker pulse after implantation at surgery. 1’1’: Magnitude of current at end of pacemaker pulse after implantation at surgery. VO: Magnitude of body surface voltage artifact at beginning of pacemaker pulse. V’T: Magnitude of body surface voltage artifact at end of pacemaker pulse.
site adjacent to the pacemaker case, the stimulus artifact attained values as high as 40 mv. The rise time of the pacemaker voltage pulse was measured in all but three instances in which low amplitude precluded triggering of the necessarily rapid sweep (0.5 psec per centimeter). It is apparent from Table II that the rise time on the body surface for a given pacemaker is of the same order noted during measurements of current at surgery, and earlier at calibration. In two instances in which rise times were somewhat slower on the body surface than elsewhere, the preamplifier utilized* had a limited high-frequency response and could not be expected to follow the swiftly rising portion of the pacemaker pulse. The “modified” type G amplifier used in all other measurements of voltage has a rise time capability of 0.25 psec and amplifies the pacemaker pulses without distortion. These data show that waveforms with rise times of the *Type
D Tektronix.
Patient number
I 1 Calibration
I. 2. .z 4. 5.
0.6 0 6 I I 1 1 0 7 1 .n 1.1 1 2 I 1 0 .8 0 5
I
.Vurgery
.5’urjace
Surfare
I’T I I’olI’T ! v,/v, (Ma.)
Jaron, and Joync~r
6.t 7.
8. 9. 1o.t 11.
0 4 0 5 1 0 09 05 0.9 0 9 1 2 1 2 0 8 n-1
0.71 0 3
1.1: 0 9 0 9 I n 1 1 04
*Time (+vec) from the instant of rise to the major inflection point. t”Unipolar” stimulation. $Preamplification with Tektronix Type 11 unit. All others utilized Modified Type G.
order of 1 psec are conducted from the myocardium to the body surface without significant distortion. The slow portion of the pulse, on the other hand, showed readily apparent differences among the pictures taken at. calibration, implantation, and on the body surface.* Photographs of the body surface voltage artifacts from Patient 11 are shown in Fig. 5. Analysis of the slow portion of pacemaker pulse. The electrical characteristics of body tissues have been shown by others4 to be substantially resistive and independent of frequency in the range of 1 to 10,000 cycles per second. These properties of body tissues, as well as the lack of distortion of portions of the pacemaker pulse with frequency content well byeond this range, suggested that the slow portion of the pulse at the body surface would follow closely the current delivered to the myocardium. As a preliminary check, the ratio of rise to fall voltage amplitudes, VO/VT, was computed for each pulse recorded *Considerable variation was noted in the shape of the voltage pulse in the case of one of two patients with a “unipolar” arrangement of stimulation electrodes. Since the shape of the current pulse at surgery was not unusual. it is likely that the variation in voltage waveform noted was due to polarization at the surface electrodes. For this reason, no further analysis of either case with “unipolar electrodes” is included.
Volume 7 1 Number
Electrical interaction
5
between artificial
pacemakers and patients
661
vation indicates that two capacitors are needed in a circuit analogue of the tissue load on bipolar pacemaker electrodes. The equivalent circuit shown in Fig. 6 has been utilized. Values for each component were determined for each case (Table III) utilizing formulae derived in the Appendix, and the following variables: (1) the rise (peak) current (1’0) measured at surgery; (2) the ratio (B/A) of the zero time intercepts of the fitted straight lines on the semilogarithmic plots; and (3) the time constants (r and T’) of the foregoing plots. r (or T’) is the time when the voltage had fallen to e-l or 36.8 per cent of its peak value. An equivalent circuit was constructed utilizing values calculated from measurements on Patient 11. The current pulse delivered to this circuit by a pacemaker of the same type actually implanted in this patient was measured by placing an oscilloscope across R. The pulse observed was Fig. 5. The upper three photographs are voltage artifacts from the precordium of Patient 11 after recovery from surgery. The record at the top is the slow portion, and beneath it are, respectively, the rising phase and the falling phase of the pacemaker pulse. Sensitivity, 2.5 mv./cm. The lowest photograph is the slow portion of the current pulse delivered to the patient’s equivalent circuit by a similar pacemaker. Oscilloscopic gain was adjusted to produce deflections comparable to those in the top photograph. Abscissae as in Fig. 2.
from marized
the
surface.
These
data
are
sum-
in Table I. In each case the surface voltage ratio obtained agreed with the implantation current ratio within experimental error. Consequently, it was tentatively assumed that the shape of the slow portion of the voltage pulse at the surface was not significantly different from the current pulse delivered to the myocardium. The semilogarithmic plot of the slow portion of the pulse was a straight line, except for the first three or four values. Differences between this line and the first three or four values were replotted and defined a second straight line on semilogarithmic paper. The slow portion of the pacemaker pulse may thus be described in terms of two exponential functions or time constants. The latter obser-
c R c’
ci
d
3 PACEMAKER
Fig. 6. Equivalent to the equivalent tient load.
:
PATIENT
LOAD
circuit of pacemaker connected circuit used to represent the pa-
Table III. Values for eatient load impedance
equivalent
circuit
Patient number 1.* 2.* 3. 4. 5.* 7. 8. 9.* 11. *Replacement
5.50 500 600 820 4.50 700 1,000 320 350 of pacemaker
6.0 9.7 4.1 5.5 5.6 5.4 2.8 10.2 5.5 unit without
290 325 9.5 115 180 270 185 115 335 thoracotomy.
1.4 1.1 2.0 4.5 1.6 1.2 1.6 1.1 0.8
of
662
Briller,
Gcselowik,
Arhger,
Danielson,
virtually identical to the voltage pulse present on the patient’s thorax (Fig. 5). The current waveform in this system was determined with the current probe (Fig. 4), and it too was almost identical to the current waveform recorded at implantation. In this way, confirmation was obtained for the tentative assumption that the slon portions of the voltage pulses observed at the body surface have the same configuration as the current pulses delivered to the myocardium. Discussion
Although artificial pacemakers of one make were employed in this work, it will become evident that the current-voltage relationships derived have general significance. Two frequency-dependent electrical impedances are implicit in the foregoing studies. The first, a transfer impedance independent of both pacemaker and electrode types relates the voltage artifact at the body surface to the pacemaker current delivered to the myocardium. The second, which may be designated the patient load,3 is the total in vivo impedance placed upon the pacemaker electrodes. Patient load is a function of two classesof variables. The first class is independent of the particular electrodes utilized but includes such variables as current density, frequency spectrum of pulse, electrode separation, tissue impedance, etc. The other class is dependent upon the type of electrodes and involves variables such as the electrode material and configuration. It follows that pacemakers of any type utilizing Chardack platinum-iridium electrodes will have a load of similar characteristics placed upon them, provided that variables of the first c-lassare accounted for as in these studies. The most direct way of obtaining the patient load impedance would be to record simultaneously voltage and current at the myocardial electrodes after they are connected to the heart at surgery. However, the voltage measurements require direct myocardial connection of electronic equipment energized from power lines, with the attendant hazard of ventricular fibrillation. Current, on the other hand, was measured safely with a current probe. The necessity for voltage measurement at
.laron, cud foyrwr
the electrodes during surgery was rirculuvented by determining the equivalent circuit for the pacemaker (Fig. 3) as part of the calibration procedure. The pacemaker equivalent circuit, together with knowledge of the current. at surgery, enabled calculation of the patient load impedance and its expression as another equivalent circuit, which is shown in Fig. 6. X’alues for the four elements of the latter equivalent circuit are listed in Table I I 1. ,I consideration of the deformation of pacemaker pulses by the myocardium has led others to prepare substantially different equivalent circuits of the pacemaker load. One such circuit? was prepared for a device employing stainless steel loop electrodes. In another case the “synthetic load” failed to account for the in vivo load +vith the fidelity achieved in the present study. The slog portion of the pacemaker pulse is characterized by two time constants. The shorter is typically 0.2 msec., and the larger sometinles exceeds 10 msec. Thcrefore, important frequency components are present in the range frown a few cycles per second to several kilocycles per second. Schwan and F;a?,* showed that this frequencl’ range is one in I\-hich tissues such as muscle are well characterized 1~1,constant resistivity, and concluded that elect-rocardiogranls containing components up to a thousand cycles would not he distorted. It is reasonable then to expect that the myocardium and surrounding tissues contribute resistance alone to the patient load impedance in the frequenc)~ range encompassed bl. the slo\v portion of the pulse. This resistance, which depends on electrode separation, is represented by a fraction of the resistor K in the equivalent circuit of Fig. 6. The remainder of the equivalent circuit is attributable to polarization at the electrode-myocardial interface. Polarization is an electrochemical interface phenomenon and involves, among other things, Ihe movelllcnt of ions after t-he application oi an electrical field.” It is concluded that. electrode-luLoc~trdia1 polarization rather than tissue impedance accounts for the deformation of the slow portion of the pacemaker pulse. Measurements of (.urrclil at surger)’ were obtained either when the chest wall
Volume 71 Number 5
Electrical interaction
between artificial
was partially open after electrode implantation or after a replacement pacemaker had been inserted without a thoracotomy. The four highest values for R occur in instances in which the electrodes were newly implanted, yet the next to lowest value for R is present in a fifth such case. Since a substantial portion of the total equivalent circuit for the patient load is attributable to polarization at the myocardial-electrode interfaces, the effect of thoracotomy, if any, should be a relatively small change in the series resistor R. The equivalent circuit for the patient load has a series capacitor C (Fig. 6); an infinite impedance to direct current is therefore implied. However, this circuit was derived from a study of the distortion of the slow portion of the pacemaker pulse which has no significant components below a few cycles per second. Put in slightly different terms, the capacitor C can be shunted by high resistance (greater than 1,000 ohms) without noticeably changing I’, the current which the pacemaker would deliver to this equivalent circuit. Transfer impedance is of direct interest to electrocardiography because it relates a known myocardial current to body surface potentials in an in vivo human experiment. The rise of the pacemaker pulse contains frequency components in the frequency range of lo5 to lo6 c.p.s. In this part of the frequency spectrum the electrical properties of representative tissues enter a frequency-dependent region designated the @ dispersion> However, electrocardiographic analysis is not so much concerned with the details of the frequency dependence of the transfer impedance as with possible distortion of waveforms. Readily measured impedance variations with frequency obtained with the alternating current technique” may have only a small effect on a particular waveform. With the precision of measurement available, pacemaker voltage pulses at the body surface were observed to have the same shape as current pulses delivered to the myocardium, which suggests that distortion is small for frequency components up to lo6 cycles per second. Consequently, the transfer impedance can be approximated by a constant resistance even in this frequency range. The present experiment thus pro-
#acemakers and patients
663
vides additional evidence that electrocardiographic signals, including details observed with wide-band amplifiers,? will not suffer frequency distortion. For a given placement of myocardial electrodes in a patient, the transfer impedance will depend on the particular pair of surface points chosen. This transfer impedance is a scalar quantity, Z, which relates the voltage V in a lead to the current I’ injected into the myocardium. Thus, V = ZI’. Since the maximum peak voltage observed was about 6.0 mv., and the average peak current was 4.0 Ma., the maximal transfer impedance is of the order of 1.5 ohms. A vectorial “transfer impedance” which is closely related but not identical to Z has been proposed by Schmitt.* He defines transfer impedance as a vector point function ?*, which relates the voltage in a lead to a current dipole moment $ in the heart region, as follows: V = 72% . ;S: If d+is a vector joining the myocardial electrode sites, and if the electrode separation is not too great, a current dipole moment, i-;‘= I’$ can be associated with the pacemaker. It follows that the scalar and vectorial transfer impedances are related by Z = 2, * 2, Since the distance between electrodes was typically 1.5 cm., and Z had a maximal value of about 1.5 ohms, vectorial transfer impedances associated with the pacemaker are of the order of 1 ohm per centimeter. Extensive measurements of vectorial transfer impedance have been made by others using torso-shaped models filled with homogeneous conducting fluids.8,g The magnitude of the impedance depends on the lead, the location of the dipole, and the resistivity of the fluid. If a value for fluid resistivity of 1,000 ohm-centimeters is used in these model studies, the vectorial transfer impedances range up to several ohms per centimeter in magnitude. It is somewhat difficult to relate our in vivo values to those obtained in model studies. The pacemaker electrodes in the epicardium constitute a dipole current source which is more ventral than the electrical center for QRS. Eccentric locations such as these should produce larger surface voltages for a given dipole moment and imply larger transfer in~pedance.g On the other hand, the current from myo-
664
Hriller,
Geselowitz, Arlinger,
Danielson,
cardial electrodes is tangent to the intracavitary mass, and, on this basis, a reduction in surface voltage and transfer impedance might be anticipated.‘” Summary Eleven artificial pacemakers were calibrated prior to surgery. The current delivered by these units to human hearts was then measured at surgery with a current probe. Body surface pacemaker voltages were recorded subsequently in nine cases. The amplitude and shape of the current pulse depends upon the patient load impedance. Patient load impedance consists of myocardial electrode polarization impedance and tissue resistance. For the pacemaker and myocardial electrodes employed, the patient load impedance is well represented by an equivalent circuit consisting of a series parallel arrangement of two capacitors and two resistors. Values of these circuit parameters were determined in nine cases. Tissue resistance is represented by a fraction of the series resistor in the equivalent circuit. The retnainder of the equivalent circuit is attributable to polarization. The voltage pulse measured at the body surface is related to the current pulse delivered to the myocardium by a transfer impedance. Since details of the current pulse, which had a rise time of less than a microsecond, were present in surface voltages, the transfer impedance is approximately resistive and independent of frequency up to a megacycle per second. It is concluded that no significant frequency distortion of electrocardiograms is caused by the electrical characteristics of body tissues. The value of the maximal scalar transfer impedance from these studies has been cast into the vectorial form employed by others in torso model studies. IJnder the conditions discussed, the vectorial transfer impedance has comparable magnitude whether obtained from model studies or from the present pacemaker study. Appendix The equivalent circuit of the pacemaker connected to a resistive load RL is shown in Fig. 3. If the switch is closed at time t = 0 with Ci charged to Eo volts, then
Jaron, and .ioyncr
The switch is reopened at t = ‘I‘ seconds, producing a current pulse of duration 1‘. During the time the switch is open, Ci is recharged to E. volts. E0 can be determined by observing the open-circuit voltage at the pacemaker terminals. For this measurement, the pacemaker is connected directly to the oscilloscope, which has an input impedance of 10 megohms. If (Ri + RL)Ci is greater than 5T (which was the case for the units studied), the exponential function of Equation (1) is almost indistinguishable from a straight line, and a trapezoidal current pulse lvill be obtained. 10 and 1~ are defined as the values of I at t = 0 and t = T, respectively. Then
-‘I‘/(R,-t
Rr,K,
IT = lee
(3)
During calibration, E, is measured and IO and IT are determined as functions of RL; R r and C i can then be calculated from Equations (2) and (3). For the pacemakers studied, Cr was 10 pf, Ri with one exception was 1,600 ohms, and E. was typically 8 volts. Now consider that the pacemaker is connected to the load shown in Fig. 6. The pacemaker current in this case will be designated I’. After the switch is closed at t= 0, l’=Ae
-t/7
-t/d +Re
(4)
where CCi CT=
RT=
(5)
Cc,
Ri+
R
(6)
Volume 71 Number
Electrical interaction
5
R’ -zz
between artificial
(9)
RT
If the subscript t= 0,
0 again
indicates
time
EO
Io=A+B=~~+~
For the pacemakers the case that
so that Equations plify to RTCT=
(8), and (9) sim7
-
(?a) R’C’
= (1 + ;)T’
R’=RT;
(1-
6)
‘1 7
By plotting I’ as a function of t on semilog paper as described in the text it is possible to determine the parameters A,B,T and T’ of Equation (4). A comparison of Equations (10) and (2) shows that R is the value of RL for which 10 = IO), and hence can be determined directly from calibration data. RT is simply R + Ri. CT is then calculated from Equation (7) or (7a), R’ from Equation (9) or (9a), and C’ from Equation (8) or (8a). Finally, C is obtained from Equation (5). CicT ‘=
ci-
CT
665
circuit, are determined from A, B, T, and r’, the parameters describing the slow portion of the current pulse delivered to the myocardium. We are grateful extensive technical
to Miss Sally assistance.
Featherstone
for
REFERENCES
used it was generally
(7),
pacemakers and patients
(12)
In this manner, R, C, R’, and C’, the parameters of the patient load equivalent
1. Briller, S. A., Geselowitz, D. B., Danielson, G. K., Joyner, C. R., and Arlinger, S. D.: Use of a current probe in the evaluation of failure of artificial pacemakers, Proc. Ann. Conference on Engineering in Med. and Biol., 6:120, 1964. W. M., Gage, A., Frederico, ,4. J., 2. Chardack, Schimert, G., and Greatbatch, W.: Clinical experience with an implantable pacemaker, Ann. New York Acad. SC. 111:1075,1964. 3. Simpson, J. A., Gibson, P., Stanford, R. W., and McLennon, D. B.: Prolonged cardiac pacemakers in Stokes-Adams disease, Lancet 2:226, 1962. 4. Schwan, H. P., and Kay, C. F.: The conductivity of living tissues, Ann. New York Acad. SC. 65:1007, 1957. D. S., and Kantrowitz, A.: Electrical 5. Feldman, characteristics of human ventricular myocardium stimulated in vivo, Clin. Res. 11:22, 1963. 6. Butler, J. A. V.: Electrical phenomena at interfaces. New York. 1951. The Macmillan Co. 7. Langner, P. H., Jr., Geselowitz, D. B., and Mansure, F. T.: High-frequency components in the electrocardiograms of normal subjects and of patients with coronary heart disease, AM. HEART J. 62:746, 1961. 8. Schmitt, 0. H.: Lead vectors and transfer ~n$ance, Ann. New York Acad. SC. 65:1092 9. Frank, E.: Determination of the electrical center of ventricular depolarization in the human heart, AM. HEART J. 49:670, 1955. 10. Brody, D. A.: A theoretical analysis of intracavitary blood mass influence on the heart-lead relationship, Circulation Res. 4:731, 1956. 11. Goldstein, H. L., Kay, C. F., and Schwan, H. P.: Phase shift in body tissues as it pertains to electrocardiography, Fed. Proc. 12:No. 1, March, 1963.