VECTOR CARDIOGRAPHIC ASSESSMENT OF IMPLANTED CARDIAC PACEMAKERS
J. L. BLACK, D. W. K. COLLINS , I. R. FLEMING and G. H. THOMPSON Department of Biophysics, Sir Charles Gairdner Hospital, Perth Medical Centre, Perth, Western Australia 6009 (Australia)
(Received: 20 July, 1976)
SUMMARY
The design and use of an on-line PDP 1 l/40 based vector cardiographic pacemaker assessment system is described. The system has been designed for comprehensive, automated testing of eitherjxed rate or demand implanted cardiac pacemakers, It is accurate and can be operated by a laboratory assistant without special training. Pacemaker parameters extracted are pulse height, width, rate, energy index and the frontal plane vector length and angle at maximum inspiration and during quiet breathing. In addition, a graphical representation of the patient’s ECG and the pacemaker pulse is obtained on a computer graphics terminal. Patient data are written to cartridge disk for permanent recordfollowing the test. At each subsequent test of the implanted-pacemaker, the data on disk are addended with the latest quantitative results. A separate o$-line FORTRANprogram can interrogate diskjlesfor detailed analysis of patient data and display of parameter trends since implantation.
SOMMAIRE
Cet article decrit la conception et l’utilisation dun systtme PDP 1 l/40 destine a l’tvaluation vecto-cardiographique dun pacemaker. Le systeme a Pte concu pour eflectuer des tests globaux et automatises sur des pacemakers implant& ayant une frequencefixe ou variable. Le systeme estprecis. Ilpeut Ctre maiupulepar un assistant n’ayant aucune formation particuliere. L.es parametres fournis sont l’amplitude des pulsations, leur largeur, leur frequence; l’index d’energie, la longueur et l’angle du vecteur dans replan frontal. Lors dune inspiration maximum et lots dune respiration calme. D’autrepart, un terminalgraphique donne le trace de l’e.c.g. dupatient et celui 159 Int. J. Bio-Medical Computing (8) (1977)-o Applied Science Publishers Ltd, England, 1977
Printed in Great Britain
160
J. L. BLACK, D. W. K. COLLINS, I. R. FLEMING, G. H. THOMPSON
des impulsions du pacemaker. Les donnees concernant un test sont enregistrees sur disque. Lors de chaque test, les don&es sont ajoutees auxprecedentes. Un programme FORTRAN particulier est utilise en temps di@re pour interroger les jichiers et proceder h des analyses detaillees. Ce programme permet notamment d’avoir une representation de l’evolution des dijjerents parametres depuis i’implantation du pacemaker. INTRODUCTION
Performance testing of implanted cardiac pacemakers is important in detecting battery, electronic component or pacing catheter failure and in guiding the physician in pacemaker replacement scheduling. Test procedures usually involve electronic instrumentation and the use of a storage oscilloscope. However, Black and Collins (1976) recently described a computerised system for measuring the performance of implanted and external cardiac pacemakers. The present paper describes a sophisticated on-line system for assessment of implanted pacemakers which uses some of the software routines employed in the earlier system, but provides a far more comprehensive assessment of pacemaker parameters and the ability to store patient data on computer disk. The present work utilises the concepts of Green (1975) in measuring the frontal plane vector. ECG data from four limb leads are sent to a purpose-designed amplifier which provides simultaneous Lead 1, 2 and 3 outputs. These outputs are subsequently sent to three inputs of a multiplexed analogue to digital converter interfaced to a PDP 1 l/40 computer. The computer, by recognition and analysis of the pacemaker artifact spike present in each of the three lead configurations, calculates the vector length and angle assuming the validity of Einthoven’s triangle. The program operates under a disk operating system (DOS) executive and is a FORTRAN program which calls six MACRO subroutines for data collection. A Laboratory Peripheral System analogue to digital converter (LPS), a programmable clock (LPSKW) and a Tektronix 4010 graphics terminal are required to support the program. The program runs in 19.8 K of core. A laboratory assistant can easily carry out pacemaker testing unaided. The full assessment takes approximately six minutes, meaning that a relatively large number of patients attending a busy clinic can have their pacemakers assessed comprehensively in an efficient manner. A separate off-line FORTRAN program, running in 23 K of core, is used to call down patient files, calculate means and standard deviations of pacemaker parameters and display trends in parameters over the months or years since implantation. If the results of the most recent test indicate changes in one or more of the pacemaker parameters of more than three standard deviations, the operator is warned that the pacemaker may not be functioning normally and that the parameters should be closely investigated.
CARDIAC PACEMAKERS
161
ASSESSMENT CONSIDERATIONS
The significant parameters for monitoring battery, component or lead failures are the pulse shape, the pulse width, the pulse rate and the pulse height. Long-term monitoring of these parameters can be used to predict cell end-of-life; this is particularly important for lithium iodide cell pacemakers where the width and rate parameters usually change linearly during the lifetime of the cell. The on-line system described in this paper tests all of the above functions, as well as measuring the frontal plane vector length and angle. The length of the vector is a good indicator of battery condition and the vector angle is invaluable in indicating the position of the catheter tip and catheter insulation failure (Green (1975)). The vector length and angle are measured with the patient breathing normally and under conditions of maximum inspiration. ASSESSMENT TEST PROCEDURE
Patient preparption
The patient is wired, using standard limb leads, as for an ECG. The right arm (RA), left arm (LA), left leg (LL) and right leg (RL) leads are connected directly into a purpose-designed amplifier unit (see section below headed ‘VCG amplifier’) which has three outputs-Lead 1 (RA, LA, RL common), Lead 2 (RA, LL, RL common) and Lead 3 (LA, LL, RL common). These outputs are passed directly, via co-axial cable, to three channels of the Laboratory Peripheral System (LPS) multiplexed analogue to digital converter unit, which is interfaced to the PDP 1 l/40 computer. The amplifier gain may be set to two ranges: 2.5, 5.0, 7.5 or 10 (settings usually used for testing unipolar configuration pacemakers) or 25, 50, 75 or 100 (bipolar configuration). The gain controls affect all outputs equally. At the beginning of each test the gains are set to the lowest position in the appropriate range. During the test the gain is adjusted as dictated by the acceptable input voltage range of the LPS unit, namely - 1 V to + 1 V. Program initiation
The RKOS disk, on which is stored both the computer program and all patient files, is loaded and the program (PACE) called into core by typing RU PACE on the console Decwriter. All subsequent interactive communication between the computer and its operator is via the Tektronix 4010 graphics terminal, which is placed in the clinical laboratory near the patient couch. Frames
A series of question-answer-information frames now appears on the 4010 terminal screen, with the operator being required to take the indicated action as the test proceeds. Frames are shown in Figs. 1 to 8 which should be referred to in conjunction with the following brief descriptions.
162
J. L. BLACK, D. W. K. COLLINS, I. R. FLEMING, G. H.
THOMPSON
Frame l-Opening gambit: When this frame appears the operator simply presses the RETURN key on the 4010 terminal to begin the study. The program now senses the voltages from Leads 1, 2 and 3 in turn, for approximately 50 set for each lead. The amplitude of the largest pacemaker artifact pulse detected for a given lead during the 50-set period is printed on the 4010 screen, with sign indicated. After this, the operator has a choice of repeating the opening gambit or continuing further. If the run is continued, the program inspects the results just obtained and, by comparison, chooses the lead which has shown the largest artifact pulse and prints the chosen lead on the screen. The chosen lead will be used by the program in later sections as the ‘trigger’ lead. The presence of a pulse in this lead, as determined by the voltage crossing a digital threshold, will be interpreted as indicating the presence of a pacemaker pulse. The operator is now presented with the possibility of overruling the computef s choice of trigger lead, although he will only exercise this option in rare cases for special reasons. Note that the whole opening gambit section may be bypassed if the operator so desires, in which case the operator simply enters the lead to be used for triggering.
Fig. 1. A photograph from the screen of the Tektronix 4010 graphics terminal for the first frame of the assessment program.
Frame 2-Gain adjustment: The next frame calls for the operator to adjust the VCG amplifier gain controls ( x 2.5, 5, 7-5 or 10 and x 10 multiplier) until a voltmeter reading, controlled by a digital to analogue converter output from the computer, reads in one of two areas coloured red-one for positive going and one for negative going pulses. The meter monitors the size of the pacemaker artifact pulse at the input to the analogue to digital converter channel chosen in the opening gambit, i.e. the
CARDIAC PACEMAKERS
163
trigger lead. The patient’s pacemaker pulse is indicated, beat by beat, by a small downward (for positive going pacemaker pulses) or upward (for negative going pacemaker pulses) flick of the meter needle from its standing value. The gain is adjusted until the standing value is in the range 0.5 to 0.7 V (positive pulses) or - 0.5 to - 0.7 V (negative pulses), indicating a satisfactory pulse size for acceptance and subsequent analysis via the LPS. After the gain is appropriately set, the operator strikes the keyboard space bar to move on to the next 4010 frame.
Fig. 2. Frames 2 and 3 of the assessment program, superimposed for presentation purposes. The ‘Y lead input on Frame 3 is reserved for future development and is not used in this program.
Frame 3-Pulse height and quiet breathingfrontalplane vector measurements: The artifact pulse heights on each of Leads 1,2 and 3 are now measured, with the chosen VCG amplifier gain, using the lead selected in the opening gambit as trigger. This is
Fig. 3. Frame 4.
164
J. L. BLACK, D. W. K. COLLINS, I. R. FLEMING, G. H. THOMPSON
repeated for eight pacemaker pulses, the procedure being carried out whilst the patient is resting quietly on the patient couch. For each pacemaker pulse, the data are combined to calculate the frontal plane vector length and angle. The program now averages the amplitude results and the vector results and prints the average amplitudes as pulse heights at the input to the three LPS channels in volts. Thevector data are not printed at this stage. Frame &Choice oJ’ lead jbr rate, width and energy measurements: Using the amplitude data just obtained, the program calculates which of the three leads is showing the largest artifact pulse (the same as that obtained in the opening gambit section, unless that section was bypassed or overruled) and prints a message to the
Fig. 4. Frame 5.
operator asking whether ECG, rate, pulse width and pulse energy measurements to follow should be carried out using the analogue data from that lead. If the operator answers Y (for yes), the next frame appears, if N (for no), he is requested to specify the lead. It is possible, for example, to measure the pulse parameters on Lead 3 (which may have a small artifact pulse for a particular patient) whereas all timing and recognition of the presence of a pacemaker pulse is through the trigger lead chosen in the opening gambit section, e.g. Lead 1. The generality thus provided has been extremely useful in clinical practice in tracking down suspected pacemaker or lead failure. Frame 5-Patient data: In this frame the operator is asked to enter the patient’s name, the patient’s disk file name, unit record number, whether or not the current test is the first test of the implanted pacemaker and whether an external magnet is being used to cause demand pacemaker operation. If the current test is not the first, the next frame then appears. If it is, further information is required by the program and it requests date of pacemaker implant, the name of the manufacturer, the pacemaker type number and whether the configuration is unipolar or bipolar. The permanent patient file is identified by a nine character name, e.g.
CARDIAC PACEMAKERS
165
JD032.PAC. The first two letters are the patient’s initials, the foliowing three numbers denote the patient number and the last three letters indicate the kind of clinical test (PACemaker). Frame 6-ECG, rate, pulse shape, width and energy: The program now proceeds automatically and measures O-7 set of ECG for the lead chosen in Frame 4. The
Fig. 5. Frame 6.
Fig. 6. Frame 7.
166
J. L. BLACK, D. W. K. COLLINS, 1. R. FLEMING, G. H. THOMPSON
0.7 set period begins on recognition of a pacemaker pulse. Following this, the pacing rate is measured to an accuracy of better than 0.02 per cent and the pulse width is measured to an accuracy of 5 ps. An index proportional to pulse energy is also calculated by numerical integration. A bell is sounded by the 4010 terminal under computer control to indicate the successful completion of each phase. No results are printed at this stage. Frame 7-Maximum inspiration instructions: This frame simply provides instructions to the operator as to the maximum inspiration and breath-holding procedure to be followed in the next part of the test-the frontal plane vector measurement at maximum inspiration. It involves no operator interaction.
Fig. 7. Frame 8.
Frame f&Maximum inspiration jiontalplane vector measurement: The procedure described in the previous frame is followed and the computer-controlled 4010 bell indicates a successful vector measurement, after which the patient relaxes. The vector measurement takes less than 2 sec. The measurement may be repeated up to a maximum of five times to ensure reproducibility. At each measurement the pulse heights on all leads, the vector angle and thevector length are tabulated on the 4010 terminal display. Following each vector measurement the operator is given the choice to finish (type F) or to repeat the vector test ‘(type R). If F is typed the operator is asked whether he/she wishes to retain, for averaging, result numbers 1,2, 3, 4 or 5 (or a lesser number if fewer measurements were taken). At least one
CARDIAC PACEMAKERS
167
measurement must be retained. This option has proved most useful in clinical practice. It may, for example, be useful to exclude a particular measurement because the operator is not satisfied that the patient held his breath satisfactorily or took a deep enough inspiration.
Fig. 8. Frame 9.
Frame 9-Final results: The final results, both quantitative and pictorial, are now displayed on the 4010 screen on one frame. This frame is photographed with a Polaroid camera and constitutes the complete report if it is the patient’s first test and part of the report if it is the second or a later test. The top picture on the frame represents 0.7 set of ECG following a pacemaker pulse and is scaled automatically to show the voltage relationship between the pacemaker artifact pulse height and the following ECG. The data for the centre picture represent the ECG following a pacemaker pulse for each of five consecutive pacemaker pulses. In this picture the actual pacemaker pulse is not displayed but a small marker represents it. This picture has been invaluable in determining if a patient’s pacemaker is ‘capturing’. If
168
J. L. BLACK, D. W. K. COLLINS, I. R. FLEMING, G. H. THOMPSON
one OF more pacemaker pulses is not followed by a clear ‘capture’ waveform, the ECG is examined in detail on a storage oscilloscope. The bottom picture shows the pacemaker artifact pulse itself on a horizontal scale of 2ms full width. Frame lO--Writing data to disk: In this frame (not shown in the Figures) the operator is given the choice to write the pacemaker data to disk. If the run has been unsuccessful for some reason, or if it has been a test run, the operator answers in the negative. If the answer is positive, the data are written to disk if it is the patient’s first test. If it is a subsequent test, the program reads the current patient file from disk into core, deletes the file from disk, addends the current data to the file just read into core, and writes the new, addended file to disk with the same disk file identification name as the original. Final printout
Following this, the Decwriter prints out, in tabular form, all of the quantitative parameters extracted for the patient’s pacemaker for all tests since implantation, including the current results. The photograph taken of Frame 9 is stapled to the printout and constitutes the patient’s final report, which is sent to the referring cardiologist with appropriate comments. An example printout is shown in Fig. 9.
TREND ANALYSIS
Program overview
The trend analysis program is a free standing FORTRAN IV program which accesses patient files from disk and analyses the data, presenting results in trend form using both tabular and graphical representation. The program consists of a mainline caliing program which calls six FORTRAN subroutines for statistical analysis including linear regression data fitting, patient and pacemaker identification details, data tabulation and graphing and comparison of current results with those obtained from previous tests. Program use
The program is designed to require the minimum of operator interaction. Initially the operator is requested to enter the patient’s file name and the patient’s name. The amount of operator/terminal interaction beyond this point depends on whether the pacemaker model is known to the program, if it uses lithium iodide cells or another type of power source, whether it is the first, second or later test and whether the pacemaker is found to be functioning normally. After the patient’s name and file name are entered, the program accesses the patient’s disk file, reading the data into core. The file data are inspected by the program to determine the type of pacemaker implanted in the patient and which test (first, second, etc.) was most recently carried out.
7A76
70.04 aa 92
69. 73 60 4P
2 2 69 35
1
7~' 7/76
426 1
YIQIY
4,II
ST
6. 1
4L
n
16. 2
7R 7
Y
N
u ,.
13. 3 d. N 12. B
18. e ns -.L
0.971
. -CL
0. SS6
5. 7 sa
SW
b!i
-~
.
lYtF~UF1
c
i2i. 9
::_ 4. 9 R. 557
1
SQ
7
5. 9
Q
7. 1
*
I,,
129. 5
‘a
MAX INSPIRATION ._ ._ ~ .._L _-&lju&E 139. 2 _ _ _ . ..--x _ _L. - ~~_.. 114.4 6. 3
-
~_
_~
Fig. 9. A typical printout from the console L&writer showing the pacemaker’s history since implantation.
69. 35
3
2
4) 6276
24/ 6/76
70. 58
In. m
enIF
3
2
2
1 FI1D
129
2
126. 3
334
2
s Q
8
5. 9
I
.
15/12/75 9/ L/76 2W P/76 I/ X)76 17/ 3/76
PUI CFG.
7/
24/ 6/76
4.J b.J76
YECTORS :
QUIET BREATHING --_J,EMnIN ~__.._ I)lnlf_ 6. R 15/12/7!l 137.1 9)-i?A___*2.2W 2/76 5. 7 140. 6 17.1 , q/ T/71; s3 17,’ 3/76 6. 4 135.1
_ PATIENT: D I S K F I L E : EP039. P A C URN : JOWN DOE 1s. *3 4.2-L. _ _ _ .._~. llANUFACTURER: MEDTRONIC P. _-__. ._-.-____..___ _ _. ..___ BIPOLAR LEAD 51206
_-._ .- ___ ---~.- -- _-. PACEHRKER HISTORY REPORT
__
.-_
170
J. L. BLACK, D. W. K. COLLINS, I. R. FLEMING, G. H. THOMPSON
Pacemakers known to the program with non-lithium iodide power sources
For the first test of a newly implanted pacemaker the program inspects the width and rate of the pacemaker pulses and checks if they are within the limits specified by the manufacturer. As a result of the check an appropriate message is displayed on the 4010 terminal and the operator is asked if analyses for other pacemakers are required. If the most recent test on the pacemaker was the second since implantation the manufacturer’s specification limits on width and rate are checked and a table is displayed comparing the results of the first two tests for all of the parameters measured by the assessment program. Significant discrepancies between results for parameters measured during the first two tests are brought to the attention of the operator by means of a message which appears on the 4010. terminal screen. Following this, the operator may choose to exit or to carry out analyses on other pacemaker systems.
Fig. 10. Graphics terminal tabulation of pacemaker parameters showing results of most recent test compared with means and standard deviations of earlier tests.
For third or subsequent tests the analysis is very similar, but in these cases the comparison table displays the mean value and standard deviation of all of the parameters calculated for all tests since implantation except the first test and the most recent one. The first test, usually carried out within a few days of implantation, is excluded from the statistical analysis because most pacemaker-lead systems go through a settling-in period after implantation where measured parameters may
CARDIAC PACEMAKERS
171
change significantly over a period of about three months. (The second test is usually carried out in this laboratory three months after implantation.) The results of the most recent test are tabulated against the means and standard deviations as described above. A typical 4010 terminal frame of the tabulation is shown in Fig. 10. After the display is photographed with a Polaroid camera, the analysis is continued by means of the operator pressing RETURN on the terminal. The program compares the results of the most recent test with the means and standard deviations of earlier tests and, if the data indicate change in any of the measured parameters of more than three standard deviations from the meah, a message is written on the 4010 screen to this effect.
Fig. 11.
Graphical representation of width, rate and vector data for a typical mercury cell pacemaker.
The operator now may choose to graph the pacemaker parameter trends or to move on to analysis of other pacemaker systems. If the graphing option is selected, a trend plot of the rate, width and quiet breathing and maximum inspiration vector length and angle parameters since implantation is presented. A typical plot is shown in Fig. 11. The picture shown on the top left of Fig.1 I is a graph of the pulse width in millisecondsversus time in months since implantation; a cross appears for every test,
172
J. L. BLACK, D. W. K. COLLINS, I. R. FLEMING, G. H. THOMPSON
including the first and most recent. The dotted line shows the average width, excluding the first and most recent tests. On the top right a similar graph is presented for pacing rate in beats per minute. The dotted line represents the average rate with the above exclusions. The bottom two pictures are representations of the frontal plane vector length and angle measured under conditions of quiet breathing (average of eight pulses) and maximum inspiration. The vector length scale is shown between the two pictures. The crosses represent the results from all previous tests, the asterisk represents the most recent result and this point is joined by a dashed line to the origin.
Fig. 12. Graphical representation of width, rate and vector data for a typical lithium iodide cell pacemaker.
Pacemakers known to the program with lithium iodide power sources Pacemakers powered by lithium iodide cells exhibit a linear change in rate and width of the pacemaker pulse with increasing time. The manufacturer supplies values for the start of life and end of life rate and width, and the operator is asked to enter these values immediately after the patient’s name. This enables regression lines
CARDIAC PACEMAKERS
173
to be calculated for the width and rate versus time data, from which the period to pacemaker replacement can be predicted. The program analysis then continues as for the previous case, except that the mean and standard deviation of pulse width and rate are not displayed since they are not expected to remain constant. Two independent estimates of the cell lifetime, calculated from pulse width and pulse rate data presently on file, are displayed. The final two questions asked of the operator for third and later tests are as before. Finally, the data are displayed in graphical form, a typical output being shown in Fig. 12. This display is interpreted in the same way as that of Fig. 11 except that the dotted lines on the width and rate graphs do not represent averages but the end of life characteristics for that pacemaker. The solid line is the linear regression fit for data collected from all tests except the first and the most recent. The asterisk marks the predicted time for end of life. Pacemakers not known to the program
The program has been written to cope with pacemakers which are not already known to it, but analysis in this case requires more operator interaction. Since the program does not know the pacemaker it cannot know the manufacturer’s specification for various parameters and cannot know the conditions which indicate failure of the pacemaker system. For this reason the operator is asked if the results are within specified limits and whether the pacemaker appears to be functioning normally. Pacemaker parameter trend data are tabulated as in the case of pacemakers known to the system, but the facility for graphing of data is not provided. SYSTEM DETAILS
It is not possible, because of the complexity and size of the programs described in this paper, to describe their detailed design. However, discussion on some specific items will follow. VCG ampl$er
The circuit diagram for the amplifier is shown in Fig. 13. The RA, LA and LL leads from the patient are connected to buffer amplifiers via limiting resistors and protective diodes. The outputs from the buffer amplifiers are connected to differential amplifiers which have gains of 2.5, 5,7.5 and 10 selected by switch Swl. The outputs of the differential amplifiers are applied to further amplifiers with a gain of 10. These stages have 10 K trimpots in their circuitry which are adjusted for best common mode rejection. Switch Sw2 selects either the full output from the final stages or an attenuated output (in our case, l/10). This is then passed to a voltage follower. The connection into the computer LPS analogue to digital converter is via an AC coupling circuit.
174
J. L.
BLACK, D. W. K. COLLINS, I. R. FLEMING, G. H. THOMPSON
+&I+ I
IOK
IOOK
Fig. 13. Circuit diagram of the vector cardiograph amplifier. Integrated circuits are 741’s, diodes lN914’s.
Opening gambit
During the opening gambit, 500 analogue to digital (a-d) conversions are taken at a spacing of 100 ps for a particular lead. These data are numerically differentiated and the largest positive or negative number resulting from the differentiation of the 500 samples is stored in a core location. The a-d and differentiation process is then repeated. If the largest number resulting from the differentiation of the second group of 500 samples is larger than that obtained from the first, the number replaces the one originally stored. If not, the original number is retained as the current largest derivative. This process is repeated 300 times for each lead. Thus, at the end of the
CARDIAC PACEMAKERS
175
opening gambit section, the maximum derivative, which is interpreted (using the known gain factors) as the artifact pulse height, is available for each lead. This simple technique has provedvery successful for recognition of the pacemaker artifact pulse against the ECG background. The numerical differentiation procedure is equivalent to subtraction of neighbouring a-d samples taken 100~s apart. Only the pacemaker pulse, with a rise time of substantially less than 100 ps, can result in a large derivative. Vector measurements The concept of Einthoven’s triangle is used in both quiet breathing and maximum inspiration frontal planevector measurements. Pacemaker artifact pulse heights are measured simultaneously for Leads 1,2 and 3, paying strict attention to sign (Green (1975)). The measured pulse heights are combined to calculate the vector component V,, defined as the artifact pulse height measured for Lead 1 and the vector component V,, defined as l/J?! (pulse height, Lead 2 + pulse height, Lead 3). The vector magnitude and angle are calculated from V, and V,. In the quiet breathing section, eight values of V, and V, are measured from which vector lengths and angles are calculated. These results are averaged. In the maximum inspiration test, V, and V, values are taken on the first pacemaker pulse after RETURN is pressed on the 4010 terminal. The maximum inspiration test can be repeated up to five times and the operator may choose to include any or all of the results for averaging. ECG measurement For this measurement, 175 a-d conversions are taken at a spacing of 4 ms which provides an adequate representation of the ECG waveform. The ad sequence is started on the detected presence of a pulse on the trigger lead. In the five-pulse capture waveform measurements, 0.4 set (100 points) of ECG data following a pacemaker pulse are displayed. It is especially noted that the presented ECG data are smoothed by the program using a five-point smoothing technique with coefficients of 0.0625, 0.25, 0.375, 0.25 and 0.0625. This simple technique reduces 50 Hz data contamination quite effectively. Pulse rate Analogue to digital conversions at 100 j~s spacing are taken for the lead specified in Frame 4. The MACRO subroutine counts the number of LPS programmable clock ticks between the presence of rising (for positive artifact pulses) or falling (for negative pulses) edges of neighbouring pacemaker artifact pulses, using a digital equivalent voltage threshold as reference. Logic is included in the software to deal with the situation where a demand pacemaker is firing less often than its set rate because the patient’s natural rhythm rate is similar. If a second pacemaker pulse is not
176
J. L. BLACK, D. W. K. COLLINS, I. R. FLEMING, G. H. THOMPSON
detected in a time bin of 1.2 set following detection of the first, the subroutine loops back to search for a pair satisfying the criterion. This assumes that the pacemaker rate will not be set to lower than 50 beats per minute-a good assumption in practice. If a demand pacemaker is not firing at all, orvery infrequently, the external magnet is used to initiate the unit. It is noted that with some manufacturer’s pacemakers the rate is slightly different when activated via the magnet. Pulse shape, width, height and energy
The techniques used for extraction of these parameters are very similar to those described in some detail in an earlier publication (Black and Collins (1976)). Very high speed (20&le) ad conversion in a 6 ms time window spanning the expected position of a pacemaker pulse is used for obtaining a good digital representation of the pulse. The window is opened at a program-calculated time following a pacemaker pulse, using the accurate rate data extracted in the earlier section of the program. This technique allows the possibility of obtaining the shape of the pulse prior to the digital threshold cross point, and is equivalent to a hardware linear delay technique. The pulse height is calculated at a position 80 ps in from the threshold cross point on the leading edge of the pulse and the width of the pulse is obtained using a linear interpolation and double numerical differentiation technique. This leads to the width of the pulse measured between the points of maximum slope on the front and back edges of the pulse and is highly accurate and reproducible (standard deviation 5 ps). The pulse ‘energy’ is proportional to the sum of the squares of the digital data points spanning the pacemaker artifact pulse, taking into account baseline level and all gain factors. Trend analysis program
Within the mainline program there are various main sections, the first of which reads from the patient disk file all of the patient data and the results of all of the tests to date. From these data the program is able to decide to which of the other sections it must branch. Other important information required by the program, apart from the actual pacemaker pulse parameter data, is also extracted in the first section of the program. This includes the pacemaker model, time since implant, and the number of times the pacemaker has been tested. There are separate branches for the first test, the second test and the third or later tests of a pacemaker. On entering any one of these branches the next step is to branch again according to pacemaker type. Currently there are five pacemaker types known to the system plus one branch which is designed to minimally analyse pacemakers not known to the system. It is not a difficult task to add new pacemaker models or remove obsolete ones from the program. Subroutines are used for data test comparison and parameter tabulation, standard deviation calculations and, for pacemakers with lithium iodide cells, regression fitting calculations.
CARDIAC PACEMAKERS
177
The regression analyses on pulse width and pulse rate data lead to separate estimates of the end of life of the power cells. It is expected that as the pacemaker ages and the regression fits become more meaningful, these estimates will coincide. It is noted that the correlation coefficient for the regression fit is displayed. The tabulated and graphical results are kept with the patient file and are updated after each patient visit. Graphing
All data graphing for both the assessment program and the trend analysis program is carried out using FORTRAN subroutines provided in the Tektronix terminal control system/advanced graphing package provided as support for the 4010 graphics terminals.
DISCUSSION
The experience gained using the program in the pacemaker clinic for nine months has shown that it has fulfilled its original aims, the primary ones of which were to provide extensive and accurate assessment of implanted pacemakers, to provide the opportunity for trend analysis of implanted units and the capability to recall, on demand, all data since implantation and to reduce the necessity for the presence of professional staff in routine pacemaker clinics. It is necessary to emphasise that, although the professional staff member is not required to perform the tests, he or she must always be available if the results obtained do not agree with those obtained at the previous clinic attendance. Note is made of the following two practical considerations: (i) It has been found with experience that it is often useful to photograph the maximum inspirationvector results displayed in Frame 8 of the assessment program for the patient’s first two tests following implantation and to keep these records in the patient’s file for comparison with subsequent tests. In cases of suspected failure, these data can sometimes help to elucidate the problem. (ii) With patients whose natural heart rate is faster than the set rate of a demand pacemaker, thus requiring the use of the magnet, it has sometimes been found advantageous on the patient’s first test to bypass the opening gambit and to assume a trigger lead. If the assumption proves to be a bad one, as evidenced in Frames 2 and 3, the program can be quickly restarted. It is our usual procedure to bypass the opening gambit on second and subsequent tests; the operator enters the trigger lead obtained in the opening gambit in the patient’s first test. Unless the pacemaker/lead system has failed or the pacing tip has moved, this is normally quite satisfactory. If unreliable triggering occurs, the opening gambit is run. The assessment/trend analysis system has been very successful in practice. It
178
J. L. BLACK, D. W. K. COLLINS, 1. R. FLEMING, G. H. THOMPSON
requires minimal training time in learning its use, is flexible, efficient and accurate. The full listings of all FORTRAN and MACRO modules of both programs will be made available by writing to the first named author of this paper.
ACKNOWLEDGEMENTS
We are pleased to thank Mr R. Gibson for advice in program implementation, Miss M. Anderson for enthusiastic aid in patient clinical work, Drs P. Thompson and J. Robinson for valuable discussion of the medical aspects and Mrs N. Shapter for typing the manuscript. One of us (J.L.B.) isvery pleased to thank Dr G. D. Green for valuable discussions on the use of vector cardiography techniques in the assessment of implanted pacemakers.
REFERENCES B LACK , J. L. and C OLLINS , D. W. K., Int. Journal qf Bio-Medical Computing, 7(3) (1976) pp. 163-72. G REEN , G. D., The assessment andperjbrmance @implanted cardiac pacemakers, Butterworths, 1975.
J. L. BLACK, D. W. K. COLLINS , I. R. FLEMING and G. H. THOMPSON Department of Biophysics, Sir Charles Gairdner Hospital, Perth Medical Centre, Perth, Western Australia 6009 (Australia)
(Received: 20 July, 1976)
SUMMARY
The design and use of an on-line PDP 1 l/40 based vector cardiographic pacemaker assessment system is described. The system has been designed for comprehensive, automated testing of eitherjxed rate or demand implanted cardiac pacemakers, It is accurate and can be operated by a laboratory assistant without special training. Pacemaker parameters extracted are pulse height, width, rate, energy index and the frontal plane vector length and angle at maximum inspiration and during quiet breathing. In addition, a graphical representation of the patient’s ECG and the pacemaker pulse is obtained on a computer graphics terminal. Patient data are written to cartridge disk for permanent recordfollowing the test. At each subsequent test of the implanted-pacemaker, the data on disk are addended with the latest quantitative results. A separate o$-line FORTRANprogram can interrogate diskjlesfor detailed analysis of patient data and display of parameter trends since implantation.
SOMMAIRE
Cet article decrit la conception et l’utilisation dun systtme PDP 1 l/40 destine a l’tvaluation vecto-cardiographique dun pacemaker. Le systeme a Pte concu pour eflectuer des tests globaux et automatises sur des pacemakers implant& ayant une frequencefixe ou variable. Le systeme estprecis. Ilpeut Ctre maiupulepar un assistant n’ayant aucune formation particuliere. L.es parametres fournis sont l’amplitude des pulsations, leur largeur, leur frequence; l’index d’energie, la longueur et l’angle du vecteur dans replan frontal. Lors dune inspiration maximum et lots dune respiration calme. D’autrepart, un terminalgraphique donne le trace de l’e.c.g. dupatient et celui 159 Int. J. Bio-Medical Computing (8) (1977)-o Applied Science Publishers Ltd, England, 1977
Printed in Great Britain
160
J. L. BLACK, D. W. K. COLLINS, I. R. FLEMING, G. H. THOMPSON
des impulsions du pacemaker. Les donnees concernant un test sont enregistrees sur disque. Lors de chaque test, les don&es sont ajoutees auxprecedentes. Un programme FORTRAN particulier est utilise en temps di@re pour interroger les jichiers et proceder h des analyses detaillees. Ce programme permet notamment d’avoir une representation de l’evolution des dijjerents parametres depuis i’implantation du pacemaker. INTRODUCTION
Performance testing of implanted cardiac pacemakers is important in detecting battery, electronic component or pacing catheter failure and in guiding the physician in pacemaker replacement scheduling. Test procedures usually involve electronic instrumentation and the use of a storage oscilloscope. However, Black and Collins (1976) recently described a computerised system for measuring the performance of implanted and external cardiac pacemakers. The present paper describes a sophisticated on-line system for assessment of implanted pacemakers which uses some of the software routines employed in the earlier system, but provides a far more comprehensive assessment of pacemaker parameters and the ability to store patient data on computer disk. The present work utilises the concepts of Green (1975) in measuring the frontal plane vector. ECG data from four limb leads are sent to a purpose-designed amplifier which provides simultaneous Lead 1, 2 and 3 outputs. These outputs are subsequently sent to three inputs of a multiplexed analogue to digital converter interfaced to a PDP 1 l/40 computer. The computer, by recognition and analysis of the pacemaker artifact spike present in each of the three lead configurations, calculates the vector length and angle assuming the validity of Einthoven’s triangle. The program operates under a disk operating system (DOS) executive and is a FORTRAN program which calls six MACRO subroutines for data collection. A Laboratory Peripheral System analogue to digital converter (LPS), a programmable clock (LPSKW) and a Tektronix 4010 graphics terminal are required to support the program. The program runs in 19.8 K of core. A laboratory assistant can easily carry out pacemaker testing unaided. The full assessment takes approximately six minutes, meaning that a relatively large number of patients attending a busy clinic can have their pacemakers assessed comprehensively in an efficient manner. A separate off-line FORTRAN program, running in 23 K of core, is used to call down patient files, calculate means and standard deviations of pacemaker parameters and display trends in parameters over the months or years since implantation. If the results of the most recent test indicate changes in one or more of the pacemaker parameters of more than three standard deviations, the operator is warned that the pacemaker may not be functioning normally and that the parameters should be closely investigated.
CARDIAC PACEMAKERS
161
ASSESSMENT CONSIDERATIONS
The significant parameters for monitoring battery, component or lead failures are the pulse shape, the pulse width, the pulse rate and the pulse height. Long-term monitoring of these parameters can be used to predict cell end-of-life; this is particularly important for lithium iodide cell pacemakers where the width and rate parameters usually change linearly during the lifetime of the cell. The on-line system described in this paper tests all of the above functions, as well as measuring the frontal plane vector length and angle. The length of the vector is a good indicator of battery condition and the vector angle is invaluable in indicating the position of the catheter tip and catheter insulation failure (Green (1975)). The vector length and angle are measured with the patient breathing normally and under conditions of maximum inspiration. ASSESSMENT TEST PROCEDURE
Patient preparption
The patient is wired, using standard limb leads, as for an ECG. The right arm (RA), left arm (LA), left leg (LL) and right leg (RL) leads are connected directly into a purpose-designed amplifier unit (see section below headed ‘VCG amplifier’) which has three outputs-Lead 1 (RA, LA, RL common), Lead 2 (RA, LL, RL common) and Lead 3 (LA, LL, RL common). These outputs are passed directly, via co-axial cable, to three channels of the Laboratory Peripheral System (LPS) multiplexed analogue to digital converter unit, which is interfaced to the PDP 1 l/40 computer. The amplifier gain may be set to two ranges: 2.5, 5.0, 7.5 or 10 (settings usually used for testing unipolar configuration pacemakers) or 25, 50, 75 or 100 (bipolar configuration). The gain controls affect all outputs equally. At the beginning of each test the gains are set to the lowest position in the appropriate range. During the test the gain is adjusted as dictated by the acceptable input voltage range of the LPS unit, namely - 1 V to + 1 V. Program initiation
The RKOS disk, on which is stored both the computer program and all patient files, is loaded and the program (PACE) called into core by typing RU PACE on the console Decwriter. All subsequent interactive communication between the computer and its operator is via the Tektronix 4010 graphics terminal, which is placed in the clinical laboratory near the patient couch. Frames
A series of question-answer-information frames now appears on the 4010 terminal screen, with the operator being required to take the indicated action as the test proceeds. Frames are shown in Figs. 1 to 8 which should be referred to in conjunction with the following brief descriptions.
162
J. L. BLACK, D. W. K. COLLINS, I. R. FLEMING, G. H.
THOMPSON
Frame l-Opening gambit: When this frame appears the operator simply presses the RETURN key on the 4010 terminal to begin the study. The program now senses the voltages from Leads 1, 2 and 3 in turn, for approximately 50 set for each lead. The amplitude of the largest pacemaker artifact pulse detected for a given lead during the 50-set period is printed on the 4010 screen, with sign indicated. After this, the operator has a choice of repeating the opening gambit or continuing further. If the run is continued, the program inspects the results just obtained and, by comparison, chooses the lead which has shown the largest artifact pulse and prints the chosen lead on the screen. The chosen lead will be used by the program in later sections as the ‘trigger’ lead. The presence of a pulse in this lead, as determined by the voltage crossing a digital threshold, will be interpreted as indicating the presence of a pacemaker pulse. The operator is now presented with the possibility of overruling the computef s choice of trigger lead, although he will only exercise this option in rare cases for special reasons. Note that the whole opening gambit section may be bypassed if the operator so desires, in which case the operator simply enters the lead to be used for triggering.
Fig. 1. A photograph from the screen of the Tektronix 4010 graphics terminal for the first frame of the assessment program.
Frame 2-Gain adjustment: The next frame calls for the operator to adjust the VCG amplifier gain controls ( x 2.5, 5, 7-5 or 10 and x 10 multiplier) until a voltmeter reading, controlled by a digital to analogue converter output from the computer, reads in one of two areas coloured red-one for positive going and one for negative going pulses. The meter monitors the size of the pacemaker artifact pulse at the input to the analogue to digital converter channel chosen in the opening gambit, i.e. the
CARDIAC PACEMAKERS
163
trigger lead. The patient’s pacemaker pulse is indicated, beat by beat, by a small downward (for positive going pacemaker pulses) or upward (for negative going pacemaker pulses) flick of the meter needle from its standing value. The gain is adjusted until the standing value is in the range 0.5 to 0.7 V (positive pulses) or - 0.5 to - 0.7 V (negative pulses), indicating a satisfactory pulse size for acceptance and subsequent analysis via the LPS. After the gain is appropriately set, the operator strikes the keyboard space bar to move on to the next 4010 frame.
Fig. 2. Frames 2 and 3 of the assessment program, superimposed for presentation purposes. The ‘Y lead input on Frame 3 is reserved for future development and is not used in this program.
Frame 3-Pulse height and quiet breathingfrontalplane vector measurements: The artifact pulse heights on each of Leads 1,2 and 3 are now measured, with the chosen VCG amplifier gain, using the lead selected in the opening gambit as trigger. This is
Fig. 3. Frame 4.
164
J. L. BLACK, D. W. K. COLLINS, I. R. FLEMING, G. H. THOMPSON
repeated for eight pacemaker pulses, the procedure being carried out whilst the patient is resting quietly on the patient couch. For each pacemaker pulse, the data are combined to calculate the frontal plane vector length and angle. The program now averages the amplitude results and the vector results and prints the average amplitudes as pulse heights at the input to the three LPS channels in volts. Thevector data are not printed at this stage. Frame &Choice oJ’ lead jbr rate, width and energy measurements: Using the amplitude data just obtained, the program calculates which of the three leads is showing the largest artifact pulse (the same as that obtained in the opening gambit section, unless that section was bypassed or overruled) and prints a message to the
Fig. 4. Frame 5.
operator asking whether ECG, rate, pulse width and pulse energy measurements to follow should be carried out using the analogue data from that lead. If the operator answers Y (for yes), the next frame appears, if N (for no), he is requested to specify the lead. It is possible, for example, to measure the pulse parameters on Lead 3 (which may have a small artifact pulse for a particular patient) whereas all timing and recognition of the presence of a pacemaker pulse is through the trigger lead chosen in the opening gambit section, e.g. Lead 1. The generality thus provided has been extremely useful in clinical practice in tracking down suspected pacemaker or lead failure. Frame 5-Patient data: In this frame the operator is asked to enter the patient’s name, the patient’s disk file name, unit record number, whether or not the current test is the first test of the implanted pacemaker and whether an external magnet is being used to cause demand pacemaker operation. If the current test is not the first, the next frame then appears. If it is, further information is required by the program and it requests date of pacemaker implant, the name of the manufacturer, the pacemaker type number and whether the configuration is unipolar or bipolar. The permanent patient file is identified by a nine character name, e.g.
CARDIAC PACEMAKERS
165
JD032.PAC. The first two letters are the patient’s initials, the foliowing three numbers denote the patient number and the last three letters indicate the kind of clinical test (PACemaker). Frame 6-ECG, rate, pulse shape, width and energy: The program now proceeds automatically and measures O-7 set of ECG for the lead chosen in Frame 4. The
Fig. 5. Frame 6.
Fig. 6. Frame 7.
166
J. L. BLACK, D. W. K. COLLINS, 1. R. FLEMING, G. H. THOMPSON
0.7 set period begins on recognition of a pacemaker pulse. Following this, the pacing rate is measured to an accuracy of better than 0.02 per cent and the pulse width is measured to an accuracy of 5 ps. An index proportional to pulse energy is also calculated by numerical integration. A bell is sounded by the 4010 terminal under computer control to indicate the successful completion of each phase. No results are printed at this stage. Frame 7-Maximum inspiration instructions: This frame simply provides instructions to the operator as to the maximum inspiration and breath-holding procedure to be followed in the next part of the test-the frontal plane vector measurement at maximum inspiration. It involves no operator interaction.
Fig. 7. Frame 8.
Frame f&Maximum inspiration jiontalplane vector measurement: The procedure described in the previous frame is followed and the computer-controlled 4010 bell indicates a successful vector measurement, after which the patient relaxes. The vector measurement takes less than 2 sec. The measurement may be repeated up to a maximum of five times to ensure reproducibility. At each measurement the pulse heights on all leads, the vector angle and thevector length are tabulated on the 4010 terminal display. Following each vector measurement the operator is given the choice to finish (type F) or to repeat the vector test ‘(type R). If F is typed the operator is asked whether he/she wishes to retain, for averaging, result numbers 1,2, 3, 4 or 5 (or a lesser number if fewer measurements were taken). At least one
CARDIAC PACEMAKERS
167
measurement must be retained. This option has proved most useful in clinical practice. It may, for example, be useful to exclude a particular measurement because the operator is not satisfied that the patient held his breath satisfactorily or took a deep enough inspiration.
Fig. 8. Frame 9.
Frame 9-Final results: The final results, both quantitative and pictorial, are now displayed on the 4010 screen on one frame. This frame is photographed with a Polaroid camera and constitutes the complete report if it is the patient’s first test and part of the report if it is the second or a later test. The top picture on the frame represents 0.7 set of ECG following a pacemaker pulse and is scaled automatically to show the voltage relationship between the pacemaker artifact pulse height and the following ECG. The data for the centre picture represent the ECG following a pacemaker pulse for each of five consecutive pacemaker pulses. In this picture the actual pacemaker pulse is not displayed but a small marker represents it. This picture has been invaluable in determining if a patient’s pacemaker is ‘capturing’. If
168
J. L. BLACK, D. W. K. COLLINS, I. R. FLEMING, G. H. THOMPSON
one OF more pacemaker pulses is not followed by a clear ‘capture’ waveform, the ECG is examined in detail on a storage oscilloscope. The bottom picture shows the pacemaker artifact pulse itself on a horizontal scale of 2ms full width. Frame lO--Writing data to disk: In this frame (not shown in the Figures) the operator is given the choice to write the pacemaker data to disk. If the run has been unsuccessful for some reason, or if it has been a test run, the operator answers in the negative. If the answer is positive, the data are written to disk if it is the patient’s first test. If it is a subsequent test, the program reads the current patient file from disk into core, deletes the file from disk, addends the current data to the file just read into core, and writes the new, addended file to disk with the same disk file identification name as the original. Final printout
Following this, the Decwriter prints out, in tabular form, all of the quantitative parameters extracted for the patient’s pacemaker for all tests since implantation, including the current results. The photograph taken of Frame 9 is stapled to the printout and constitutes the patient’s final report, which is sent to the referring cardiologist with appropriate comments. An example printout is shown in Fig. 9.
TREND ANALYSIS
Program overview
The trend analysis program is a free standing FORTRAN IV program which accesses patient files from disk and analyses the data, presenting results in trend form using both tabular and graphical representation. The program consists of a mainline caliing program which calls six FORTRAN subroutines for statistical analysis including linear regression data fitting, patient and pacemaker identification details, data tabulation and graphing and comparison of current results with those obtained from previous tests. Program use
The program is designed to require the minimum of operator interaction. Initially the operator is requested to enter the patient’s file name and the patient’s name. The amount of operator/terminal interaction beyond this point depends on whether the pacemaker model is known to the program, if it uses lithium iodide cells or another type of power source, whether it is the first, second or later test and whether the pacemaker is found to be functioning normally. After the patient’s name and file name are entered, the program accesses the patient’s disk file, reading the data into core. The file data are inspected by the program to determine the type of pacemaker implanted in the patient and which test (first, second, etc.) was most recently carried out.
7A76
70.04 aa 92
69. 73 60 4P
2 2 69 35
1
7~' 7/76
426 1
YIQIY
4,II
ST
6. 1
4L
n
16. 2
7R 7
Y
N
u ,.
13. 3 d. N 12. B
18. e ns -.L
0.971
. -CL
0. SS6
5. 7 sa
SW
b!i
-~
.
lYtF~UF1
c
i2i. 9
::_ 4. 9 R. 557
1
SQ
7
5. 9
Q
7. 1
*
I,,
129. 5
‘a
MAX INSPIRATION ._ ._ ~ .._L _-&lju&E 139. 2 _ _ _ . ..--x _ _L. - ~~_.. 114.4 6. 3
-
~_
_~
Fig. 9. A typical printout from the console L&writer showing the pacemaker’s history since implantation.
69. 35
3
2
4) 6276
24/ 6/76
70. 58
In. m
enIF
3
2
2
1 FI1D
129
2
126. 3
334
2
s Q
8
5. 9
I
.
15/12/75 9/ L/76 2W P/76 I/ X)76 17/ 3/76
PUI CFG.
7/
24/ 6/76
4.J b.J76
YECTORS :
QUIET BREATHING --_J,EMnIN ~__.._ I)lnlf_ 6. R 15/12/7!l 137.1 9)-i?A___*2.2W 2/76 5. 7 140. 6 17.1 , q/ T/71; s3 17,’ 3/76 6. 4 135.1
_ PATIENT: D I S K F I L E : EP039. P A C URN : JOWN DOE 1s. *3 4.2-L. _ _ _ .._~. llANUFACTURER: MEDTRONIC P. _-__. ._-.-____..___ _ _. ..___ BIPOLAR LEAD 51206
_-._ .- ___ ---~.- -- _-. PACEHRKER HISTORY REPORT
__
.-_
170
J. L. BLACK, D. W. K. COLLINS, I. R. FLEMING, G. H. THOMPSON
Pacemakers known to the program with non-lithium iodide power sources
For the first test of a newly implanted pacemaker the program inspects the width and rate of the pacemaker pulses and checks if they are within the limits specified by the manufacturer. As a result of the check an appropriate message is displayed on the 4010 terminal and the operator is asked if analyses for other pacemakers are required. If the most recent test on the pacemaker was the second since implantation the manufacturer’s specification limits on width and rate are checked and a table is displayed comparing the results of the first two tests for all of the parameters measured by the assessment program. Significant discrepancies between results for parameters measured during the first two tests are brought to the attention of the operator by means of a message which appears on the 4010. terminal screen. Following this, the operator may choose to exit or to carry out analyses on other pacemaker systems.
Fig. 10. Graphics terminal tabulation of pacemaker parameters showing results of most recent test compared with means and standard deviations of earlier tests.
For third or subsequent tests the analysis is very similar, but in these cases the comparison table displays the mean value and standard deviation of all of the parameters calculated for all tests since implantation except the first test and the most recent one. The first test, usually carried out within a few days of implantation, is excluded from the statistical analysis because most pacemaker-lead systems go through a settling-in period after implantation where measured parameters may
CARDIAC PACEMAKERS
171
change significantly over a period of about three months. (The second test is usually carried out in this laboratory three months after implantation.) The results of the most recent test are tabulated against the means and standard deviations as described above. A typical 4010 terminal frame of the tabulation is shown in Fig. 10. After the display is photographed with a Polaroid camera, the analysis is continued by means of the operator pressing RETURN on the terminal. The program compares the results of the most recent test with the means and standard deviations of earlier tests and, if the data indicate change in any of the measured parameters of more than three standard deviations from the meah, a message is written on the 4010 screen to this effect.
Fig. 11.
Graphical representation of width, rate and vector data for a typical mercury cell pacemaker.
The operator now may choose to graph the pacemaker parameter trends or to move on to analysis of other pacemaker systems. If the graphing option is selected, a trend plot of the rate, width and quiet breathing and maximum inspiration vector length and angle parameters since implantation is presented. A typical plot is shown in Fig. 11. The picture shown on the top left of Fig.1 I is a graph of the pulse width in millisecondsversus time in months since implantation; a cross appears for every test,
172
J. L. BLACK, D. W. K. COLLINS, I. R. FLEMING, G. H. THOMPSON
including the first and most recent. The dotted line shows the average width, excluding the first and most recent tests. On the top right a similar graph is presented for pacing rate in beats per minute. The dotted line represents the average rate with the above exclusions. The bottom two pictures are representations of the frontal plane vector length and angle measured under conditions of quiet breathing (average of eight pulses) and maximum inspiration. The vector length scale is shown between the two pictures. The crosses represent the results from all previous tests, the asterisk represents the most recent result and this point is joined by a dashed line to the origin.
Fig. 12. Graphical representation of width, rate and vector data for a typical lithium iodide cell pacemaker.
Pacemakers known to the program with lithium iodide power sources Pacemakers powered by lithium iodide cells exhibit a linear change in rate and width of the pacemaker pulse with increasing time. The manufacturer supplies values for the start of life and end of life rate and width, and the operator is asked to enter these values immediately after the patient’s name. This enables regression lines
CARDIAC PACEMAKERS
173
to be calculated for the width and rate versus time data, from which the period to pacemaker replacement can be predicted. The program analysis then continues as for the previous case, except that the mean and standard deviation of pulse width and rate are not displayed since they are not expected to remain constant. Two independent estimates of the cell lifetime, calculated from pulse width and pulse rate data presently on file, are displayed. The final two questions asked of the operator for third and later tests are as before. Finally, the data are displayed in graphical form, a typical output being shown in Fig. 12. This display is interpreted in the same way as that of Fig. 11 except that the dotted lines on the width and rate graphs do not represent averages but the end of life characteristics for that pacemaker. The solid line is the linear regression fit for data collected from all tests except the first and the most recent. The asterisk marks the predicted time for end of life. Pacemakers not known to the program
The program has been written to cope with pacemakers which are not already known to it, but analysis in this case requires more operator interaction. Since the program does not know the pacemaker it cannot know the manufacturer’s specification for various parameters and cannot know the conditions which indicate failure of the pacemaker system. For this reason the operator is asked if the results are within specified limits and whether the pacemaker appears to be functioning normally. Pacemaker parameter trend data are tabulated as in the case of pacemakers known to the system, but the facility for graphing of data is not provided. SYSTEM DETAILS
It is not possible, because of the complexity and size of the programs described in this paper, to describe their detailed design. However, discussion on some specific items will follow. VCG ampl$er
The circuit diagram for the amplifier is shown in Fig. 13. The RA, LA and LL leads from the patient are connected to buffer amplifiers via limiting resistors and protective diodes. The outputs from the buffer amplifiers are connected to differential amplifiers which have gains of 2.5, 5,7.5 and 10 selected by switch Swl. The outputs of the differential amplifiers are applied to further amplifiers with a gain of 10. These stages have 10 K trimpots in their circuitry which are adjusted for best common mode rejection. Switch Sw2 selects either the full output from the final stages or an attenuated output (in our case, l/10). This is then passed to a voltage follower. The connection into the computer LPS analogue to digital converter is via an AC coupling circuit.
174
J. L.
BLACK, D. W. K. COLLINS, I. R. FLEMING, G. H. THOMPSON
+&I+ I
IOK
IOOK
Fig. 13. Circuit diagram of the vector cardiograph amplifier. Integrated circuits are 741’s, diodes lN914’s.
Opening gambit
During the opening gambit, 500 analogue to digital (a-d) conversions are taken at a spacing of 100 ps for a particular lead. These data are numerically differentiated and the largest positive or negative number resulting from the differentiation of the 500 samples is stored in a core location. The a-d and differentiation process is then repeated. If the largest number resulting from the differentiation of the second group of 500 samples is larger than that obtained from the first, the number replaces the one originally stored. If not, the original number is retained as the current largest derivative. This process is repeated 300 times for each lead. Thus, at the end of the
CARDIAC PACEMAKERS
175
opening gambit section, the maximum derivative, which is interpreted (using the known gain factors) as the artifact pulse height, is available for each lead. This simple technique has provedvery successful for recognition of the pacemaker artifact pulse against the ECG background. The numerical differentiation procedure is equivalent to subtraction of neighbouring a-d samples taken 100~s apart. Only the pacemaker pulse, with a rise time of substantially less than 100 ps, can result in a large derivative. Vector measurements The concept of Einthoven’s triangle is used in both quiet breathing and maximum inspiration frontal planevector measurements. Pacemaker artifact pulse heights are measured simultaneously for Leads 1,2 and 3, paying strict attention to sign (Green (1975)). The measured pulse heights are combined to calculate the vector component V,, defined as the artifact pulse height measured for Lead 1 and the vector component V,, defined as l/J?! (pulse height, Lead 2 + pulse height, Lead 3). The vector magnitude and angle are calculated from V, and V,. In the quiet breathing section, eight values of V, and V, are measured from which vector lengths and angles are calculated. These results are averaged. In the maximum inspiration test, V, and V, values are taken on the first pacemaker pulse after RETURN is pressed on the 4010 terminal. The maximum inspiration test can be repeated up to five times and the operator may choose to include any or all of the results for averaging. ECG measurement For this measurement, 175 a-d conversions are taken at a spacing of 4 ms which provides an adequate representation of the ECG waveform. The ad sequence is started on the detected presence of a pulse on the trigger lead. In the five-pulse capture waveform measurements, 0.4 set (100 points) of ECG data following a pacemaker pulse are displayed. It is especially noted that the presented ECG data are smoothed by the program using a five-point smoothing technique with coefficients of 0.0625, 0.25, 0.375, 0.25 and 0.0625. This simple technique reduces 50 Hz data contamination quite effectively. Pulse rate Analogue to digital conversions at 100 j~s spacing are taken for the lead specified in Frame 4. The MACRO subroutine counts the number of LPS programmable clock ticks between the presence of rising (for positive artifact pulses) or falling (for negative pulses) edges of neighbouring pacemaker artifact pulses, using a digital equivalent voltage threshold as reference. Logic is included in the software to deal with the situation where a demand pacemaker is firing less often than its set rate because the patient’s natural rhythm rate is similar. If a second pacemaker pulse is not
176
J. L. BLACK, D. W. K. COLLINS, I. R. FLEMING, G. H. THOMPSON
detected in a time bin of 1.2 set following detection of the first, the subroutine loops back to search for a pair satisfying the criterion. This assumes that the pacemaker rate will not be set to lower than 50 beats per minute-a good assumption in practice. If a demand pacemaker is not firing at all, orvery infrequently, the external magnet is used to initiate the unit. It is noted that with some manufacturer’s pacemakers the rate is slightly different when activated via the magnet. Pulse shape, width, height and energy
The techniques used for extraction of these parameters are very similar to those described in some detail in an earlier publication (Black and Collins (1976)). Very high speed (20&le) ad conversion in a 6 ms time window spanning the expected position of a pacemaker pulse is used for obtaining a good digital representation of the pulse. The window is opened at a program-calculated time following a pacemaker pulse, using the accurate rate data extracted in the earlier section of the program. This technique allows the possibility of obtaining the shape of the pulse prior to the digital threshold cross point, and is equivalent to a hardware linear delay technique. The pulse height is calculated at a position 80 ps in from the threshold cross point on the leading edge of the pulse and the width of the pulse is obtained using a linear interpolation and double numerical differentiation technique. This leads to the width of the pulse measured between the points of maximum slope on the front and back edges of the pulse and is highly accurate and reproducible (standard deviation 5 ps). The pulse ‘energy’ is proportional to the sum of the squares of the digital data points spanning the pacemaker artifact pulse, taking into account baseline level and all gain factors. Trend analysis program
Within the mainline program there are various main sections, the first of which reads from the patient disk file all of the patient data and the results of all of the tests to date. From these data the program is able to decide to which of the other sections it must branch. Other important information required by the program, apart from the actual pacemaker pulse parameter data, is also extracted in the first section of the program. This includes the pacemaker model, time since implant, and the number of times the pacemaker has been tested. There are separate branches for the first test, the second test and the third or later tests of a pacemaker. On entering any one of these branches the next step is to branch again according to pacemaker type. Currently there are five pacemaker types known to the system plus one branch which is designed to minimally analyse pacemakers not known to the system. It is not a difficult task to add new pacemaker models or remove obsolete ones from the program. Subroutines are used for data test comparison and parameter tabulation, standard deviation calculations and, for pacemakers with lithium iodide cells, regression fitting calculations.
CARDIAC PACEMAKERS
177
The regression analyses on pulse width and pulse rate data lead to separate estimates of the end of life of the power cells. It is expected that as the pacemaker ages and the regression fits become more meaningful, these estimates will coincide. It is noted that the correlation coefficient for the regression fit is displayed. The tabulated and graphical results are kept with the patient file and are updated after each patient visit. Graphing
All data graphing for both the assessment program and the trend analysis program is carried out using FORTRAN subroutines provided in the Tektronix terminal control system/advanced graphing package provided as support for the 4010 graphics terminals.
DISCUSSION
The experience gained using the program in the pacemaker clinic for nine months has shown that it has fulfilled its original aims, the primary ones of which were to provide extensive and accurate assessment of implanted pacemakers, to provide the opportunity for trend analysis of implanted units and the capability to recall, on demand, all data since implantation and to reduce the necessity for the presence of professional staff in routine pacemaker clinics. It is necessary to emphasise that, although the professional staff member is not required to perform the tests, he or she must always be available if the results obtained do not agree with those obtained at the previous clinic attendance. Note is made of the following two practical considerations: (i) It has been found with experience that it is often useful to photograph the maximum inspirationvector results displayed in Frame 8 of the assessment program for the patient’s first two tests following implantation and to keep these records in the patient’s file for comparison with subsequent tests. In cases of suspected failure, these data can sometimes help to elucidate the problem. (ii) With patients whose natural heart rate is faster than the set rate of a demand pacemaker, thus requiring the use of the magnet, it has sometimes been found advantageous on the patient’s first test to bypass the opening gambit and to assume a trigger lead. If the assumption proves to be a bad one, as evidenced in Frames 2 and 3, the program can be quickly restarted. It is our usual procedure to bypass the opening gambit on second and subsequent tests; the operator enters the trigger lead obtained in the opening gambit in the patient’s first test. Unless the pacemaker/lead system has failed or the pacing tip has moved, this is normally quite satisfactory. If unreliable triggering occurs, the opening gambit is run. The assessment/trend analysis system has been very successful in practice. It
178
J. L. BLACK, D. W. K. COLLINS, 1. R. FLEMING, G. H. THOMPSON
requires minimal training time in learning its use, is flexible, efficient and accurate. The full listings of all FORTRAN and MACRO modules of both programs will be made available by writing to the first named author of this paper.
ACKNOWLEDGEMENTS
We are pleased to thank Mr R. Gibson for advice in program implementation, Miss M. Anderson for enthusiastic aid in patient clinical work, Drs P. Thompson and J. Robinson for valuable discussion of the medical aspects and Mrs N. Shapter for typing the manuscript. One of us (J.L.B.) isvery pleased to thank Dr G. D. Green for valuable discussions on the use of vector cardiography techniques in the assessment of implanted pacemakers.
REFERENCES B LACK , J. L. and C OLLINS , D. W. K., Int. Journal qf Bio-Medical Computing, 7(3) (1976) pp. 163-72. G REEN , G. D., The assessment andperjbrmance @implanted cardiac pacemakers, Butterworths, 1975.
