Computer techniques for epicardial and endocardial mapping

Computer Techniques for Epicardial and Endocardial Mapping*

William M. Smith and Raymond E. ldeker

T HE MAPPING of the electrical activation of the epicardial and endocardial surfaces

of the heart has proven to be a valuable technique both in clinical practice and in basic research. There have been striking advances in the use of mapping in the surgical treatment of serious drug-resistant arrhythmias. At the same time, mapping in the operating room and in the animal laboratory has greatly increased our knowledge of electrical activation patterns in the normal and infarcted heart during normal sinus rhythm, artificial pacing, and spontaneous arrhythmias. Traditional mapping techniques have been limited, however, by the amount of data that can be acquired, analyzed, and presented in understand- able form.

While the use of mapping was increasing, computer systems-both hardware and software-have been developing to meet the need for handling vast quantities of data from a variety of sources. The natural step was to apply the rele- vant advances to recording, manipulating, and displaying data from mapping studies.

This article is a review of the computer techniques used in epicardial, endocardial, and intramyocardial activation mapping. Methods of all phases of the mapping operation from the application of electrodes to the graphical presentation of results will be considered. Two distinct phasesaata acquisition and data analysis- will be discussed. Data acquisition can take place with or without the use of a computer; on the other hand, data analysis is difficult without digital computing techniques.

LIMITATIONS OF TRADITIONAL MAPPING METHODS

Traditionally the mapping of the electrical activation of cardiac tissue has been done pri- marily using manual processing. In some instances the data have been recorded on multichannel analog tape recorders, played back on paper, and measured manually.’ In other systems analog electronics are used to differentiate a

single data channel, derived from a hand-held probe. If the first derivative of the potential crosses a preset threshold, the maximum value of the derivative is taken as the point of local activation, and the activation time is measured with respect to a fixed reference signal.2.3 The activation times thus determined can be displayed digitally and used to construct a compos- ite activation map.

Both of these approaches to cardiac mapping have disadvantages. The manual measurement of multiple channels of electrophysiologic data is tedious. This limits the amount of data that can be processed conveniently. The analysis of more than 14-16 channels with several cycles measured in each channel is probably an unrealistic expectation. The problem can be simplified sig- nificantly by using a computer driven x-y digitizer for the measurement of cycle lengths in each channel.4 The digitizer allows the measurement of intervals by touching the points of local activation with a pen-like device. The computer can manage calibration and bookkeeping tasks. This method allows the close inspection of data and, with some difficulty, the analysis of the potentials themselves as well as of activation times. However, the basic limitation upon the number of channels and the number of cycles measured in each channel remains.

Analog analysis methods with digital readout

From the Departments of Medicine, Pathology. and Com- puter Science, Duke University Medical Center, Durham. N.C.

Supported in part by Research Grant HL-I 7670 from the National Heart, Lung, and Blood Institute and by the Joseph Barham Cardiovascular Research Fund. Dr. Ideker is the

recipient of Research Career Developmenr Award HL-00516 from ihe National Heart, Lung, and Blood Instituie.

*This paper was originally submitled in January 1981, and revised in October I98 I.

Reprint requests should be addressed to William M. Smith, Ph.D.. Box 3140, Duke University Medical Center,

Durham, NC 27710. o 1983 by Grune & Stratton, Inc. 003336620/83/2601~002$02.00/0

Progress in Cardiovascular Diseases, Vol. XXVI, No. 1 (July/August). 1983 15

16 SMITH AND IDEKER

solve many of the problems with manual measurements but still impose serious restrictions. For example it would be difficult to measure more than one channel simultaneously. In addition, isochronous maps for the display of activation sequences must be manually drawn or computer-generated from data entered through a terminal. On the other hand, the use of a hand- held probe allows the adjustment of the electrode position until data of adequate quality for analysis can be obtained. This means that measurements can be made from every point of interest, which is not always true when all electrodes are applied simultaneously.

With either manual or automated analog analysis, the lack of computer capabilities limits the ways in which data can be analyzed and presented to the user. Computers are ideally suited for the kinds of repetitive execution of well- defined algorithms required in generating activation maps of cardiac rhythms. They allow the convenient analysis of the potentials themselves, as in isopotential maps. The data can be trans- formed in many different ways for maximum effect.

In addition to these technical problems, mapping systems with a single channel or with a small number of channels have an intrinsic inad- equacy when used to map very fast or unstable cardiac arrhythmias. When only a few electrograms can be sampled simultaneously, it is necessary to sample the channels sequentially during many different beats to acquire the data from a large number of channels to construct a total activation map. This process implicitly assumes that the activation pattern of each beat during which measurements were made is the same as the activation pattern during all other beats. When this assumption is made a map can be constructed from data generated during different cardiac cycles.

In many instances these conditions are ade- quately fulfilled. For example, the use of a single channel system for mapping the sequence of conduction in the surgical treatment of Wolff- Parkinson-White syndrome has proved to be highly effective.* In this case the cycles to be mapped are either normal sinus beats, artificially paced beats, or cycles of a supraventricular ar-

rhythmia. Because of the repetitive nature of the activation sequence in each of these rhythms, activation maps can be successfully constructed from a series of individual beats. Ventricular tachycardia has also been mapped using manual methods.‘.6

On the other hand, ventricular arrhythmias can have highly unstable activation sequences from beat to beat.’ Even stable ventricular arrhythmias often are initiated by several cycles with changing activation morphology. At times it is not possible to generate a sustained ventricular tachycardia for mapping with a single channel and the total maps must be constructed from the data obtained in 5510 cycles. In ventricular arrhythmias with extremely short cycle lengths, it is possible for multiple activation fronts to coexist on the epicardium simultaneously. In each of these instances it is critical that data from all measuring points be acquired simultaneously.

Recently there has been a renewed interest in the nature of ventricular fibrillation and the relation of the electrical activation of the myocardium to the disorganized appearance of the electrocardiogram leads.x A thorough understanding of this arrhythmia clearly requires multichannel capability and computer analysis.

The use of multichannel data acquisition systems, either stand alone or computer controlled, and the use of computerized data analysis have provided a tool for the study of many of these complex arrhythmias. With these tools it is possible to map highly complex, unstable arrhythmias on a beat to beat basis. A variety of data displays can be generated to aid in the understanding of these complex events either through standard isochronous or isopotential maps, or more recently through the development of motion pic- tures that display the activation sequences dynamically.

FRONT END HARDWARE

Electrode Application

The nature of the electrodes used to measure the myocardial electrogram determines the nature and quality of the data.’ Unipolar electrodes yield information about total cardiac elec-

COMPUTER MAPPING 17

trical activity while bipolar electrodes, used properly, cancel the effects of activation of tissue far away from the electrode site.‘S’2 The electrode material can influence the amount of DC offset introduced into the recording. The method of application of the electrode influences the amount of mechanical artifact introduced into the electrogram and thus makes the data more or less difficult to analyze. The size of the electrodes, and thus the surface area in contact with the epicardium, also influences the quality of the data.

Epicardial electrodes have usually been applied by embedding the electrodes in some material that conforms to the shape of the epicar- diurn.13,14 For full coverage of the ventricular epicardial surface, electrodes have been applied using a sock made of cloth mesh or of rubber. Figure 1 shows a mesh sock pulled over a latex

Fig. 1. A nylon mesh sock on e latex model of e canine heart. Two electrodes ere embedded in each teflon button for the derivation of unipolar or bipolar electrograms. (Reproduced by permission of Futura Publishing Company. Inc. from fACE.‘Sl

model of a canine heart. The electrodes are embedded in the sock and the sock is slipped over the ventricles and anchored at the base with sutures. The electrodes can have one or two contact points for unipolar or bipolar measurements. When using a sock to apply electrodes to the epicardial surface during surgery, an estimate of the heart size should be made by ventric- ulography or echocardiography or from the body habitus to insure a good fit to the heart. A correctly sized sock allows good electrode contact while avoiding hemodynamic compromise.‘3

For coverage of a smaller area with possibly higher electrode density, a sock with the configu- ration of the electrodes modified appropriately or a patch of electrode-carrying material anchored to the epicardial surface can be used. One group of investigatorsI mounted electrodes on a piece of Mersilene mesh coated with silicone and tai- lored the mesh to cover the desired part of the epicardium. Solomon and Selvester’6 devised a clock electrode array with which intramural activation patterns could be estimated from epicardial electrograms.

Intramyocardial electrograms can be measured using a needle with wire electrodes fixed along its length (Fig. 2). In one preparation” a standard 20-gauge needle has one side filed away. The ends of 15-20 wires are placed along the open side of the needle at 1 -mm intervals and are fixed using epoxy or dental acrylic. Again, unipolar electrograms can be derived from single electrode points along the needle or the signals from two adjacent electrodes can be combined to yield a bipolar electrogram. Often the distal electrodes on the needle are in the ventricular cavity and this can be used to estimate the thickness of the wall at the point of insertion of the needle. It is possible for the needle to com- pletely transverse the cavity and for the point to be embedded in the interventricular septum, with resultant confusion in the interpretation of the electrograms. This is more likely in the right ventricle with its thin wall and small cavity than in the left.

The application of multiple electrodes to the endocardial surface of the heart chambers cannot be carried out as conveniently as with the epicardium. If there were a convenient method

18 SMITH AND IDEKER

Fig. 2. A plunge electrode for the measurement of intramyocardial electrograms. The electrode is submerged in an electrolyte solution and an electrical current passing through each measurement point produces the streams of bubbles. Unipolar signals can be taken from single points or two adjacent points can be used to produce a bipolar electrogram. (Reproduced by permission of the American Physiological Society in the American Journal of Physiol-

ogy.“)

for introducing a patch with multiple electrodes into the ventricular cavity, the highly trabecu- lated endocardial surface would cause great difficulty in deriving high quality electrograms without noise or mechanical artifact. An experimental approach to this problem has been the insertion of wires through the myocardial wall.‘8,‘9 The ends of the wires are hooked and inserted inside the needle into the cavity. The needle is pulled back and the wires are gently retracted until the hooks are embedded in the endocardial surface. The exact orientation and relation of the wires cannot be known in this instance and the positions of the electrodes and their relation to endocardial landmarks such as papillary muscle can only be ascertained by

postmortem dissection. Another alternative is the use of multiple electrode catheters lying against the endocardial surface with approximate positions determined by fluoroscopy but this approach limits the number of channels acquired simultaneously. During surgery it might be possible to apply a patch with electrodes to the endocardial wall through a ventriculotomy at. say, the border of an aneurysm.

In one experimental preparation” a series of electrodes was inserted into a needle curved in the approximate shape of the interventricular septum. The needle was passed longitudinally through the septum and pulled back leaving electrodes spaced along its length. These electrodes were used for pacing and recording from the septal mass. Similarly, septal electrodes have been passed through the right ventricular wall and cavity and embedded in the septal wall for pacing and recording. Other specialized electrode systems of this sort will be developed for particular experimental purposes. For clinical use the application of any electrodes must be rapid, efficient, reproducible, and safe. The development of a satisfactory system for the simultaneous measurement of many endocardial sites during surgery is one of the outstanding mapping problems.

Electrode Location

In order to produce an activation map or an isopotential map, it is necessary to know the location of each of the electrodes and to be able to transfer those locations to the graphical representation of the heart.2’m24 A precise determina- tion of the electrode position is necessary for correlating electrophysiology with anatomy. including regions of ischemia and infarction.” Figure 3 shows a schematic ventricular outline with the electrode positions indicated.

The location of epicardial electrodes can be determined in several ways. In experimental studies the location of epicardial electrodes can be marked on the excised heart with color-coded pins or sutures and precise spatial relations with respect to major landmarks can be measured.” Alternatively, the electrodes can be uniformly and reproducibly applied to the epicardium from study to study so that the approximate electrode

COMPUTER MAPPING 19

Fig. 3. A ventricular outline used to display results of mapping studies. The total epicardial surface is depicted as if the ventricles were folded out after an imaginary cut was made from the crux to the apex. The numbered circles within the border represent electrode locations during a study, and the duplicate circles outside the border are used to make the contour lines extend to the edges of the display. RV = acute margin of the right ventricle; LV = obtuse margin of the left ventricle; LAD = course of the left anterior descending coro- nary artery; PA - pulmonary arte-

ry.

ELECTRODE Bl-DEC-8B 13150121

locations are known a priori. In the surgical treatment of arrhythmias this localization may be adequate. Smaller patches of material with embedded electrodes can be applied to the epicardial surface in a known relation to major landmarks. If the positions of the individual electrodes with respect to the array are known, along with the position of the array as a whole, the location of each electrode in real space can be calculated.

The position of needle electrodes can be marked in experimental studies by pulling colored thread through the myocardial wall. The thread is attached to a small hole drilled in the distal tip of the needle and pulled through the needle track. The position of the needle at each level of myocardium can be measured accurately by slicing the heart transversely at l- to 2-mm intervals. During surgery the approximate location of the needles can be measured with respect to either anatomical landmarks or epicardial electrodes when used in the generation of graphical displays.

Again, in experimental studies, the position of endocardial electrodes can be determined after postmortem dissection of the heart, but the location of endocardial electrodes, if used during surgery, can only be approximately determined in the absence of ventriculotomy.

The exact localization of electrodes is more critical in experimental studies in which ana-

tomic correlations and precise mapping of activation pathways are desired. In the surgical treatment of arrhythmias an approximate localization of electrodes is adequate since the excision or cryosurgical ablation of tissue can be carried out with the knowledge of the areas of early myocardial activation. The times measured can be referred to the actual electrode locations without reference to a schematic display.

AmplQiers

The problems of amplification of electrograms in multichannel systems are basically the same as those in single channel systems,“6 but packaging a much greater number of components and con- trolling many input channels lead to different design considerations. Fortunately, the miniaturization and integration of electronics components and the use of microprocessors have contributed to the solution of the major problems.

Because of the high signal impedance of epicardial and endocardial electrograms, the first stage of amplification should have very high input impedance, typically on the order of 10” ohms. This input impedance is readily achievable with modern operational amplifiers. The forming of bipolar electrograms from two unipolar signals should occur after this stage. The amplifiers should have gain that is variable over a wide range because of the large variation in signal amplitudes between bipolar and unipolar electro-

20 SMITH AND IDEKER

grams. If a system is to be used in body surface mapping the range of necessary gains becomes even greater. The gain should be adjusted so that the signal uses the full range of the analog- to-digital (A/D) converters to ensure the greatest possible accuracy. Alternatively, a fixed gain can be used with A/D converters with many bits of precision.26 When a large number of channels are being recorded, it should be possible to adjust the amplifier gains automatically or semiautomatically. In some systems, each channel is sampled and the gains set according to maximum amplitudes in a separate calibration phase; the gains remain unchanged until the next calibration.14 With a more complicated system the gains in each channel could be updated continuously during the actual data acquisition. The speed necessary for these adjustments in real time for a moderate number of channels requires that the gain setting be done by hard-wired electronics instead of by digital computer. In either case it is necessary to maintain a record of gain settings for each channel at all times if variable gains are used. This gain record should be maintained in the regular data stream for availability at data analysis time.

The amplifier should also have variable frequency cutoffs at both low and high ends of the frequency spectrum. The low frequency limit should be set high enough to eliminate DC baseline drift with high gains, as in bipolar recording. When lower gains are used, the low frequency cutoff can be lower. If ST segment analysis is carried out the amplifier should be direct coupled so that a true baseline can be determined.

The upper frequency cutoff is related to the rate at which the data are sampled and the amount of high frequency noise contaminating the signal. According to the Sampling Theo- rem,27 an analog waveform can be precisely reconstructed from samples of the waveform if the sampling rate is at least twice the highest frequency component of the signal. Strict conditions must be imposed for this theorem to hold and, in practice, the sampling rate should be 5 or 6 times the upper frequency cutoff. Barr and Spach28 have shown that a sampling rate of 1000 samples per second is sufficient to reproduce

epicardial electrograms with reasonable accuracy. When samples of many signals are taken at a single instant of time to construct an isochronous map, these considerations are less critical. The filters can then be set to eliminate any high frequency noise that confuse the interpretation of the data.

Recorders

After the signals have been amplified, most mapping systems have some kind of device for recording the data before computer processing. The data are recorded continuously in real time and can be played back from the recording device into the host computer. The computer should also be able to acquire the data directly in real time with or without recording. At least one mapping system2’ has no real-time recording device, but allows the recording of selected 2-set intervals of data on high speed digital magnetic tape. Using an off-line recorder has several advantages. First, off-line recording provides continuous data acquisition capability. The amount of data gath- ered at one time is not limited by the size of the computer memory. Second, if the data acquisition device is independent of the host computer, it is possible to acquire and analyze data simultaneously. Thus, during analysis of one arrhythmia, an experiment or clinical study can proceed, recording data for later analysis. This means, however, that the recording system cannot be controlled by the same computer used for data analysis. Finally, continuous recording of the data provides archival records of an experiment or study. This can furnish a valuable data base for future research efforts.

There are many different recording schemes in use that yield satisfactory results. In the simplest of these each channel of data is recorded on a multichannel analog frequency-modulated tape recorder.30 This method is very expensive and is practically limited to no more than 32 channels of data. It is possible to increase the number of channels by multiplexing several waveforms onto each tape channel.26 When time division multiplexing is used, each data channel is sampled for a short period of time and the sampled analog value is recorded on the appropriate tape channel. The sampling rates required are determined

COMPUTER MAPPING 21

by the Sampling Theorem. For analysis the samples for each channel must be demultiplexed and converted to digital form for computer input. In frequency division multiplexing each signal to be multiplexed modulates a carrier of a different frequency. The signals are mixed and then recorded on the appropriate channel. The proper filters must be used on playback to demultiplex the data and reconstruct the original signals. Again, the signals must be converted to digital form for analysis purposes. With either multiplexing scheme the band width requirements of the tape recorder are substantially increased so that for significant multiplexing capabilities it is necessary to use recorders with high band pass heads and to operate the recorders at a rate of at least 30 in/sec.3’ The high tape speeds limit the amount of time during which data can be recorded continuously. For example, a 3600-foot tape recorded at 30 in/set is filled in 24 min.

Another analog recording scheme32*33 uses inexpensive video cassette recorders to record I92 channels of data. The electrograms are sampled and the analog values are recorded on the single video tape channel, using time division multiplexing. This scheme has the advantages of using standard low cost video tape recorders and of allowing continuous recording of data for 60 min.

Analog recording systems in general are limited by tape noise and the accuracy with which they can reproduce the original signal. A typical signal to noise ratio for optimum signals is on the order of 40 DB. This means that a digitization accuracy of more than 8 bits or 1 part in 256 is superfluous.

An alternative to recording the data in analog form and then digitizing the data for input to the host computer is recording the data in digital form.34 The major advantage of this technique is the improvement in signal-to-noise ratio over analog recording. One technique for digital recording utilizes standard instrumentation tape recorders. The data are digitized and coded and then are recorded either as a modulation of an FM carrier or directly on the analog tape. A single datum can be recorded in parallel, 1 bit per channel, or can be serialized and multiplexed onto a single channel. A multichannel recorder

can then be regarded conceptually as several single channel machines. As in the analog case, instrumentation tape recorders used in this fash- ion must be modified with high band width recording heads to provide the bandwidth necessary for the high speed digital signals.

Another approach is the use of industry standard 9 track digital tape recorders to record the data as they are digitized.3s A standard 2400- foot tape recorded at 15 in/set can record data continuously for about 6.5 min.

Recently it has been shown36 that inexpensive video casette recorders can be used to record high speed digital data successfully. These recorders have bandwidths of 3-4 MHz. Optimally, 2 bits/set can be recorded for every hertz of bandwidth of the instrument3’ Therefore, video tape recorders have a theoretical capability of recording data from several hundred channels digitized with lo-12 bits of accuracy. Again, these recorders have the advantages of low cost and an hour or more of record time. The major problems with these devices are the synchronization problems due to inconstant tape speeds and the loss of data during the switch from one recording head to the other.

Digital recording systems can be either totally stand alone or can use a microprocessor to control the digitization, serialization, and encoding processes. In accordance with the above considerations, however, the computer that controls the recording process should not be the same one that is used for data analysis.

COMPUTER ANALYSIS SYSTEMS

The necessary characteristics for the computer system used in the analysis of data derived from cardiac mapping are determined by the intended applications. There are basically three recog- nized classes of computers, all of which have been applied to mapping studies.

In a strictly experimental environment in which the experiments are done and the data are analyzed off-line, it would be possible to use a large, multiuser, time sharing computer in which turn-around time is measured in hours or even days. This is probably the most cost-effective way to analyze electrophysiologic data. Many university computing systems have available var-

22 SMITH AND IDEKER

ious graphics devices for displaying the results, including CRT terminals and plotters for hard copy. If experimental results are not needed in “real time,” this method of analysis is adequate.

If the conduct of an experiment is to be modified according to the results of the mapping studies and during mapping for the surgical treatment of arrhythmias, faster turn-around is necessary. In this case it would probably be necessary to have a dedicated computer ana- lyzing the data. The processors called minicom- puters are quite versatile and are of sufficiently low cost to be dedicated to one laboratory. These computers can be committed to the analysis of data during experiments or during clinical studies and can be used as general purpose multiuser machines when the time constraints are less critical. With these machines results can be produced in a time frame consistent with surgical intervention for the interruption of reentrant pathways in either supraventricular or ventricular tachycardia. These computers have also been used to control laboratory parameters during on-line measurements?

During the last several years the third class of processor. microprocessors, has become increas- ingly important in areas of process control and on-line data acquisition where dedicated computing capabilities are necessary. When microprocessors are packaged with memory and input/output devices they are called microcomputers. Microprocessors are useful as controllers of the data acquisition process. They can be used to set gains and other amplifier parameters, control on-line displays. and drive the analog to digital conversion process. Microcomputers have been used to control cardiac stimulators39 and can be used as portable mapping systems.” Typi- cally one gives up the multiuser capability and some speed in exchange for a low cost mapping system based on a microcomputer. A microcomputer with hardware or microcoded floating point processor is adequate for most mapping applications.

When an activation map is needed in quasi- real time the computer used for analysis should have a high speed random access buffer device. This device can be used for temporary storage of the data during the analysis process. Typically

these devices have been disc drives. Alternate devices, as they become available. will be mass memory, bubble memory, or charge coupled devices. The rate at which technology in these areas is progressing promises suitable buffer devices at reasonable cost.

The choice of a graphics device for the display of the data and results is important in determining the capabilities of a computer mapping system. For speed, resolution, and interactive capabilities a graphics terminal is necessary. There is a wide variety of technologies from which to choose a terminal. The storage tube terminals yield high resolution at moderate cost. The high resolution is necessary when displaying fast signals sampled at a high rate. Storage tubes limit interaction, since portions of the display cannot be updated selectively; to change part of the display it is necessary to clear the screen and redraw the entire image. Random vector displays provide high resolution and real-time interaction. but at a higher cost. For the display of maps and other results, the resolution of video terminals is usually adequate, and the cost is quite reasonable. These television-like devices also can provide color images and shades of gray that allow the generation of realistic images, at the cost of substantial computer time. Hard copy devices that make paper or film copies of the graphics display are now available for any of these devices.

The choice of computer software is highly dependent on the type of computer used in the analysis stage. Off-line analysis of data will typically be done on a large multipurpose machine using a multiuser operating system. Dedicated computers should have operating systems with real time data acquisition capability even though basically multiuser. The data acquisition phase of the process must have priority over other tasks in the system in order to insure the integrity of the sampling rate. The data retrieval and analysis programs impose much less restrictive demands on the system. These programs can run as a single task in a multi-task or multiuser system. Most programs in a mapping system should be written in a high level computing language such as Fortran. Only the most time critical routines and those with a need

COMPUTER MAPPING 23

to interact with the operating system must be written in machine or assembler language. The use of high level languages will decrease development time and increase reliability and maintain- ability of the software used.4’ Typical computer programs that will be necessary are a program to transfer data from the continuous recording device into the computer’s memory and onto the high speed buffer device, programs to display the data or selected portions of it, programs to choose local activation times and allow modification of them by the operator, and programs to generate the graphics displays that will convey the results to the user, such as isochronous or isopotential maps.

DESCRIPTION OF MAPPING SYTEM

A block diagram of a system used at Duke University42.43 in cardiac mapping studies is shown in Fig. 4. This system allows the simultaneous acquisition of up to 32 channels of data and subsequent computer analysis. The data are

DATA ACQUISITIDN DATA ANALYSIS

Fig. 4. A block diagram of a computerized data acquisition system. The terms are explained in the text. (Repro- duced by permission of Academic Press, Inc. from Com- puters and Biomedical Research.“)

amplified by buffer amplifiers and variable gain amplifiers and are recorded on a 32-channel analog tape recorder. Signals are taken from the analog tape either as they are recorded or played back at a later time for analysis. The output 01 the tape recorder is presented to a 32-channel analog-to-digital (A/D) converter interfaced to a Digital Equipment Corporation PDPl l/34. The computer is equipped with 96 K words of core memory, 2 disc drives with combined capac- ity of approximately 2.4 million words, a line printer, and a Tektronics 4014 graphics terminal with hard copy unit. One of the disc drives is used as a circular buffer to store the data as they are converted to digital form. When the disc drive is filled the oldest data are overwritten. In this way the disc always contains the latest 38.4 set of data. The A/D conversion process, controlled by the program ADCPUT, and storage of data on disc are started and proceed continuously until the operator intervenes when, for example, an interesting event occurs. When the desired data have been captured on the disc, data analysis can begin. The first program in the analysis system is named VT]. Using VTl, the operator chooses two channels for display; all 38.4 set of the 2 chosen channels can be displayed or the user can select a portion of data at the beginning or at the end of the disc buffer. Figure 5 is an example of the image generated by this program. The data at this point are displayed in a highly filtered form; every eighth digitized point is displayed to decrease the display time as much as possible. When the desired two channels have been displayed on the graphics terminal, the graphics cross-hair cursor is used to select a window within which the data will be further analyzed. The data from each channel within the chosen window are displayed, five channels at a time, on the graphics screen by the program VT2, which uses previously computed calibration factors (Fig. 6). A simple algorithm chooses the estimated time of activation in each beat in each channel. After all automatically chosen activations have been displayed, the operator is allowed to delete or alter the selected activations or add activations that were missed by the program. This is similar to a system described by Harumi et al.44 Activation times are measured with

SMITH AND IDEKER 24

l ICK Y!NDW WCSCT . . . .

27

30

Fig. 5. The output of the program VT1 for the onset of ventricular fibrillation induced by programmed premature

stimulation. The lines in the rows marked “s” show the points at which stimuli are detected. The data from channel 27. an epicardial electrode. and channel 30, a limb lead, are shown. The long vertical lines in the second row of data indicate the

window chosen for further analysis. The small diamonds represent one second intervals. (Reproduced by permission of Academic Press, Inc. from Compurers and Biomedical Re~eerch.~)

I ‘I-.-l-

,,-

Fig. 6. The data from 5 epicardial electrodes in the window chosen in Fig. 5. This is the output of the program VT2. The vertical bars in the potentials are the points et which the program has chosen local activation times. Ten milli- volt calibration marks are displayed at the beginning of each electrogram. Each screen can display 960 msec of data; if the window is longer, multiple screens are used. (Reproduced by permission of Academic Press, Inc. from Computers and Biomedical Research.“)

COMPUTER MAPPING 25

Fig. 7. Two methods used by VT3 for displaying the first five activations chosen in the display of Fig. 6, for all electrodes. The numbers in the vertical array at the left of the display ara electrode numbers end the long vertical dot- dashed lines represent the

stimuli. (A) Each activation is represented by a beat number. The spaces between beat numbers are proportional to the corresponding R-R intervals. The asterisk for beat 1 in channel 25 indicates that this beat was blocked at that electrode. IB) A display of the activation times themselves. The spacing of the vertical hash marks is proportional to the times displayed. The times are measured with respect to the onset of the window chosen in VTl. (Figure 7A reproduced by permission of Academic Press. Inc. from Computers and Biomedi-

cal Researcb.” 1

A

B

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respect to the onset of the window chosen for analysis. The activation times are stores in a matrix on disc with the rows being the electrode numbers and the columns corresponding to beats.

The program VT3 controls the displays of the measured activations. The activations can be displayed by beat number (Fig. 7A) or with the

actual activation time given for each beat (Fig., 7B). After inspection of the activation matrix it is possible to indicate where a beat has been blocked in a channel and to delete single activa-, tions or entire channels. In a rapid complex arrhythmia it is sometimes necessary to study the patterns very carefully to determine which activations correspond to which heart cycle.

26

When the activation matrices have been satis- factorily determined, it is possible to draw isochronous or isopotential maps of the data, using electrode positions previously entered using an x-y digitizer. If isochronous maps are desired, the user is asked to enter the beats for which maps are to be generated and the maps are generated and displayed with or without activation times displayed for each of the beats requested. The user selects the intervals to be left between isochronous lines and the time from which activations are to be measured. Activation times can be measured with respect to the onset of the data window as mentioned above, from the time of activation of the earliest electrode, as in Fig. 8, from a stimulus time, or from the onset of a QRS complex. Maps are generated by the algorithm of Davidow and Brown.”

If isopotential maps are desired, the interval during which the isopotentials are to be drawn is selected by the graphics cursor of the terminal. The baseline is adjusted to eliminate DC shift of the potentials, and isopotentials are generated at the requested intervals in the selected window.

These programs are the standard programs used in the analysis of data in this laboratory, but a series of special purpose programs for ana- lyzing the data in different ways have been developed. For example, animated color movies

SMITH AND IDEKER

of the spread of activation superimposed on isopotential maps have been generated with a special purpose program.

Other mapping systems in use have different capabilities that reflect the varied uses to which they are put. The system’J reported by Wit et al. records 192 channels of data on an instrumentation tape recorder using multiplexing. The gains are set using hard-wired circuitry in a special run. The data are analyzed using a PDPI l/34 computer, and other hardware similar to that described above.

Fontaine et al.“” use a general purpose mini- computer to generate epicardial maps from the data obtained in the studies of ventricular activation during surgery for ventricular tachycardia.

In the mapping system described by de Bakker et al.” and by Janse et al.Jh the data are not recorded at all in real-time. Sixty-four channels of data are digitized directly and stored in a fast memory buffer that can contain I set of data at a sampling rate of 250 samples per second. Alter- natively, 16 channels can be sampled at 1000 samples/set per channel. When an event of interest is observed, the operator can cause the data in the buffer to be written to magnetic tape.

Wyatt has reported a system” that uses ;I dedicated front end for digitizing and recording

RUX

APEX

PROCEDURE 45 BEAT NlMBER 1

.B 1 ~TIUATION TIM IW

TIME ZERO IS E(IRLIEST LAT IN BEClT ISOCHRONE INTERUClL = 10 HSEC

Fig. 8. An isochronous map of

beat 1 of Fig. 7. The numbers on

the map are the activation times,

with respect to the earliest mea-

sured activation time, at the elec-

trode positions shown in Fig. 3.

The isochronous lines are esti-

mated lines of equal activation

times. The blocked beat of channel 25 is shown as a row of asterisks.

The interval between isochronous

lines is selected by the user. (Ae-

produced by permission of Aca-

demic Press, Inc. from Computers

and Biomedical Research.“)

COMPUTER MAPPING 27

the data on industry standard 9-track tape. The data are then analyzed off-line using a time- shared computer or on a dedicated processor.

Watanabe et al. have described a flexible microcomputer system for generating body surface, precordial, and epicardial maps.47 For epicardial maps, 48 channels of data are acquired and analyzed.

RESULTS

Many results have been obtained using multichannel computerized epicardial, intramyocardial, and endocardial mapping systems that would have been impractical or impossible to achieve with single channel systems or with a smaller numer of channels with manual data processing.

Ideker et a1.8.48 have mapped the beats in the transition from normal sinus rhythm or ventricular tachycardia to ventricular fibrillation. Their data show that as cycle lengths decrease and activation times increase a point is reached at which multiple activation fronts coexist on the epicardium (Fig. 9). Even though the activation fronts are organized and repeatable, the changing phase between cycles can lead to a chaotic limb lead as in ventricular fibrillation.

Klein et al.’ have investigated the nature of activation during the onset of ventricular tachycardia induced by programmed premature stimulation. In even sustained repeatable ventricular tachycardia, in which the spread of activation is consistent from beat to beat, the first few cycles can vary widely in activation morphology. The activation patterns in some of these beats suggest that participation of the specialized conduction system is important in sustaining or initiating the arrhythmia. In other instances the first several beats display a pattern in which the wavefront continues to arise from near the stimulus site (Fig. 10).

In systems where only activation time is measured for the production of isochronous maps, all morphological information is lost for analysis. In computer mapping systems it is possible to study the correlation between isopotential maps and isochronous maps and the correlations of both isopotentials and isochrones with the underlying anatomic morphology in infarcted tissue. Other

characteristics of the waveform morphology can be studied as well, such as areas under selected portions of the curve. Some investigators49 have reported significance in the isoarea mapping of the QRS complex.

The kinds of studies mentioned here would not have been possible without the simultaneous acquisition of data from many channels on the epicardium. The analysis of data from many channels would be practically impossible if man-. ual methods were used.

FRONTIERS

Even though computers have made a signifi-, cant impact on the conduct of epicardial, intra-, myocardial, and endocardial mapping, there are many technical problems to be addressed in the: application of these techniques to the study 01 activation sequences experimentally and during the surgical treatment of arrhythmias. The tech-, nical problems cannot be solved independently of new electrophysiologic knowledge. The results of biologic experiments direct the nature of the technical innovations and suggest new paths to follow, while new technology broadens the areas of experimental interests that can be addressed.

In strictly computing terms the areas of interest for cardiac mapping include analog-to-digital conversion techniques, data transmission and. recording, data compression algorithms, pattern recognition, numerical analysis, and computer graphics. Application of new developments in any of these areas will increase the usefulness 01 computers when applied to electrophysiologic: studies.

One area where progress in several of these fields could be applied is the presentation of results of mapping studies to the user. First, the: approximate localization of the electrodes used in the measurements is necessary. In animal studies this localization can be carried out in the postmortem heart. For clinical studies it will be necessary to devise methods for estimating electrode locations in the alive, beating heart. The localization of electrodes is necessary for pre- senting the results of study in the form of isocontour maps of various kinds.

For the purpose of generating maps it will be necessary to explore ways of representing the

28 SMITH AND IDEKER

Fig. 9. Simultaneous exis-

tence of several activation

fronts on the epicardium during

the transition to ventricular

fibrillation after reperfusion of

a canine heart. Cycle 12. as

labeled, moves across the epi-

cardial surface and the activation fronts of cycles 11, 13, 14

are also present at various times. Panels A-H show the

locations of the activation

fronts at 24-msec intervals.

Limb lead II is at the bottom of

each panel. The long vertical

line superimposed on it shows

the point in time represented

by the panel. The short hash

marks below the limb lead are

100 msec apart. The stippled

area in each panel is an area of

ischemia followed by reperfu-

sion as determined by postmor-

tem dye injection. (Reproduced

by permission of the American

Heart Association from Circu-

/8tiO”.‘)

ventricles in three-dimensional form on a computer graphics terminal. A realistic representation of the ventricles would greatly facilitate the interpretation of results. Most systems now use a projection of the ventricular surface that causes unavoidable distortions. Techniques for the rep-

resentation of three-dimensional objects are widely used in other areas.5oS5’ In addition it would be useful to improve the algorithms by which isocontour lines are generated. Typically isochronous or isopotential lines are estimated using linear interpolation between actual mea-

COMPUTER MAPPING 29

surements at the electrode location.2’~24 This means, for example, that the point of earliest epicardial breakthrough can only be measured in an electrode site, which is an artificial limitation on the electrophysiology. Algorithms that generate nonlinear interpolated isocontours would remove that constraint and potentially make the displays conform more closely to reality. How-

Fig. 10. lsochronous maps of epicardial activation for a short run of ventricular tachycardia induced by programmed premature stimulation in a canine heart. Limb lead II is shown below the maps with an arrow pointing to the beat being mapped. RSl, RSZ, and RS3 are responses to stimuli. Vl -V5 are the spontaneous beats of the arrhythmia, showing breakthrough to the epicardium near the pacing electrode. lsochromes are at lo-msac intervals in all maps. The stippled area represents the extent of experimental myocardial infarct. (Reproduced by permission of Martinus-Nijhoff from Sudden Death.- I

ever, it is also possible to distort the maps by introducing overly complex contour generation techniques.

One approach to the problem has been out- lined by Bartram et aL5’ This technique uses a. parametric representation of the ventricular surface and B-splines to generate a three-dimensional activation map. Other investigators’4’5’

G

30

simply light up the points on a cathode ray tube that correspond to electrode positions at times proportional to activation times.

There is also a great deal of work needed in the area of digital signal processing for making the electrograms more useful. For example, the application of data compression algorithms to the cardiac electrograms would allow the long- term storage of a reasonable representation of the original data. This would allow more detail than the simple storage of activation times but could avoid the massive storage requirements of the original data themselves. Algorithms like those that have been used for some time in electrocardiographic monitoring’4,5’ could be applied to the wave forms arising from the cardiac tissues.

At the same time a deeper understanding of the nature of the signals that are recorded would allow the more rational application of data compression techniques and decrease the amount of operator intervention necessary when activation times are incorrectly chosen due to artifact or noise. Better algorithms for the selection of the moment of local activation underneath or close to an electrode would make the computer programs more reliable and would lessen the need for human review of the data. This would require a

SMITH AND IDEKEA

full exploration of the correlation between unipolar and bipolar electrograms in all kinds of cardiac tissue and between those extracellular signals and the underlying electrophysiology.” ” This would also require a correlation between the electrophysiology and the viability of the underlying or surrounding cardiac tissue in areas of ischemia or infarct.

Another area that promises to greatly influence the nature of cardiac mapping systems is the continuing miniaturization of electronic components, both analog and digital, as well as the decrease in cost per functional capability. Portable mappers are already in use in body surface mapping and could be used in the operating room or in experimental studies as well. The increased quality and decreased size of the components used in mapping systems mean that these systems can be used more convenientI> and. more importantly, with much higher reliability.

Even though computers are contributing sig- nificantly to the efficacy of cardiac mapping studies both in the animal laboratory and in clinical applications. further research and development is critical to reaching the full potential of these systems in electrophysiologic studies.

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Documents

Computer techniques for epicardial and endocardial mapping