Design of a centralized electrocardiographic and vectorcardiographic system∗

New Methods Design of a Centralized

Electrocardiographic

and Vectorcardiographic RALPH WALTER

S. ZITNIK, H.D.,t

JAMES R. WENNEMARK,

F. CREIGH, MICHAEL

J. CUDAHY, Chicago,

T

HE

introduction

analog

recording

has stimulated

of and

technology

high-speed, processing

System*

M.D.,$

ARTHUR

HANS U. WESSEL, M.D.,$ MILLER and ERNEST J. MOORE

Illinois

recording area by standard telephone lines, direct wires or other communication links; (3) reduce the over-all cost of electrocardiographic and vectorcardiographic recording, as well as improve the methods of recording, handling, storage and retrieval of data ; (4) provide all electrocardiographic and vectorcardiographic data in a form suitable and available for computer analysis; and (5) overcome certain technical difficulties inherent in Inultiple-lead electrocardiographic recording.

efficient of data

in the electrocardio-

field. The high cost of obtaining electrocardiograms that utilize current standard methods is due to the increasing expense of labor and material. The recording of vectorcardiograms requires more sophisticated and timeconsuming technics than does the recording of standard 12-lead electrocardio~a~. Interpretation of electrocardiograms by computer technics demands an efficient method of gathering a great number of tracings in one place and in a form suitable for analysis. A centralized electrocardiographicand vectorcardiographic system using advanced recording, communication and microfihn technics was designed for the Northwestern University Medical Center, which includes five geographically separate hospitals and a large outpatient clinic. The system was designed to (1) increase the speed of obtaining standard 1Zlead electrocardiograms by utilizing simultaneous multiple-lead pickup, transmission and recording (simuhaneous Frank system vector loops and scalar orthogonal leads also are available for each patient); (2) simplify transmission by use of either a single channel or multiple simultaneous channels to send electrocardio~aphic and vectorcardiographic data to a central graphic

OUTLINE

OF THE SYSTEM

Standard 12-lead electrocardiograms and Frank system vectorcardiograms are obtained in the patient’s room or in a centralized recording room (Fig. 1). Seventeen leads are attached to the patient at the same time. communization is established with the electrocardiographic recording center either by direct cable to proximate locations or over standard telephone lines to remote locations. The patient is identified by name, and the tracing is given a code number. From proximate areas, with direct wiring, six simuItaneous leads are transmitted in sequence: (1) I, II, III, aVR, aVL, aVF and (2) VI through Vg, as well as (3) a rhythm lead and (4) Frank system leads From remote centers, with a simulVx, V,, VP taneous three-channel telephone “Data-Phone,” the following are transmitted in sequence: (1) I, II, m; (2) a?%, aVL, aVF; (3) Vr through VS; (4) VO through Vs; (5) a rhythm lead; and (6) Frank system

* From the Department of Medicine, Northwestern University School of Medicine, Chicago, Ill., and Minnesota Mining and Manufacturing Company, St. Paul, Minn.; Marquette Electronics, Inc., Milwaukee, Wis.; Sanborn Division, Hewlett-Packard Company, Waltham, Mass. ; and the American ‘Telephone and Telegraph Company, New York, N. Y. This study was supported by a grant from the Jules J. Reingold Foundation. t Present address: Section of Medicine, Mayo Clinic, Rochester, Minn. 55901. $ Recipient of Career Development Award HE-K3-161556, U. S. Public Health Service. Present address: Department of Medicine, Section of Cardiology, University of Tennessee School of Medicine, Memphis, Tenn. $ Recipient of an Established Investigatorship, American Heart Association.

818

THE AMERICANJOURNAL OF CARDIOLOGY

Centralized

Electrocardiographic

and Vectorcardiographic PORTABLE

IL!

;

Sources _I_

CART

0,

Tronsmlsslon

Other Transmission -M

.

!I,

I

I I I

(1

1 !CENTR;L WITH

OSCILLOSCOPE ‘\ 45 set Pro_

I 1Punch

Card

‘\

‘\

,’

’

/

819

3 6 output jlmultoneous I/Slmottoneous to Leads Oscilloscope Leads or DATAPHONE Direct Wire

Single Channel ECG Data From Other

Ecci

System

I I I

/

CONSOL; MONLTOR

OSCILLOSCOPE

I

r----l

1 c~ss!ng

Output to FM Tape Recorder Or On Line Computer

FIG. 1.

Block diagram

leads V,, V,, V,.

of the

Single-channel

centralized clectrocardiopzphic

transmission

is pos-

sible. The orthogonal leads are recombined into the frontal, horizontal and sagittal loops at the recording console (Fig. 1). Incoming tracings are displayed on a monitor When oscilloscope screen at the recording center. the signal is seen to be free of artifacts, the sweep of a slave oscilloscope is activated, with automatic positioning of the beams. The shutter of a camera opens and closes synchronously with each sweep of the slave oscilloscope. A 35 mm. film, premounted on a standard camera card, is developed and delivered 45 seconds after the last sweep of the oscilloscope. Simultaneous output to multichannel tape or an on-line computer facility is provided. Standard-sized copies of the electrocardiogram and vectorcardiogram are reproduced directly from the camera card. The data can be read directly from a standard microfilm viewer. Alphameric key-punched coding of the cards allows efficient handling, storage and retrieval of data. Electrocardiograms and vectorcardiograms recorded at the center may be immediately viewed at patient areas by the closedcircuit television (Fig. 1).

COMPONENTS OF ELECTROCARDIOGRAPHIC SYSTEM ELECTROCARDIOGRAPHIC SYSTEM PREAMPLIFIERS The preamplifier portion of the centralized electrocardiographic system comprises all the signal condiVOLUME19, JUNE 1967

and uectorcardiqprafhic

systems.

See

text for details.

tioning, lead selection and amplification apparatus required to provide signals of several volts for transmission to remote recording stations and delivers up to six channels of data simultaneously (Sanborn Division, Hewlett-Packard Co., Waltham, Mass.). A master lead-selection switch allocates three groups of cardiographic leads to transmission channels (Table I). The V,, V, and V, signals as listed here refer to the X, Y and Z components of vectorcardiographic voltages established by the Frank resistance mixing network. A block diagram of the over-all preamplifier portion of the electrocardiographic system is shown in Figure 2. A 77-wire patient cable connects with the 17 electrodes that are applied to the patient’s body. Of these, the one on the right leg is permanently connected to a right-leg driving amplifier. The other three limb electrodes and one chest electrode also are TABLE I Three Groups of Cardiographic Leads Available for Transmission Channels Channel No. 1 2 3 4 5

6

1 I I1 III aVR aVL aVF

Groups------2 VI VZ VJ V4 VS V6

3 VX VY VZ

820

Zitnik

et al.

LA

LL

Frc. 2.

Block diagram of the buffer amplifier system. Note that buffer amplifiers are interposed between the patient and main amplifiers, thereby negating differences in resistance at the electrodes on the standard electrocardiographic and vectorcardiographic networks which follow the main amplifiers.

“1 I

“2 E

“3 M

“4

+-a

CeO

“5

7

-

H+-O

8 w

---lzz= permanently connected to four buffer amplifier inputs, while the remaining five buffer amplifiers are connected to either of two groups of five electrodes, depending on whether one is recording conventional limb leads, unipolar chest leads (Group 1 and Group 2 of Table I) or the vector leads (Group 3). Regardless of the setting of the lead selector switch, any electrode that is in use is loaded only by the input impedance of a buffer amplifier. Since this impedance is of the order of 40 to 50 megohms, the potential at each electrode site is delivered without loss to the amplifier system even if the electrode contact impedance is high. The unipolar limb and chest leads and the orthogonal V,, V, and V. leads all require mixtures of the potentials obtained from the electrodes that are involved. These mixtures, in turn, are derived from appropriately proportioned resistor networks. If such networks are connected directly to the electrodes, the contact impedances at the electrodes have to be maintained at very low values in order to prevent serious loading of the electrode circuits by the resistor networks.’ The interposition of the buffer amplifiers obviates

6 MAIN AMPLIFIERS

this difficulty by recreating each electrode potential at each buffer-amplifier output and by making this potential appear to come from a source of about 100 ohms, regardless of the actual contact impedance at the electrode. The buffer amplifiers are permanently connected to all the resistance networks required by the various leads. The resistance level of these networks is sufficiently high when compared with 100 ohms that it does not disturb the voltage levels at the buffer-amplifier outputs. Lead selection becomes a matter of selecting the appropriate terminals of the buffer-amplifier output or the appropriate junctions of the resistor network, or both, to deliver the chosen signals to the six amplifiers that follow the selector switch. While this portion of the selection process is going on, other poles on the same switch determine which of two groups of electrodes are connected to five of the buffer-amplifier inputs. The buffer amplifiers are normally operated with a voltage gain of two, but in the event that large amplitude complexes are encountered, as in the V leads, the gain of all buffer amplifiers can be reduced to unity, thus cutting the amplitude in half. THE

AMERICAN

JOURNAL

OF

CARDIOLOGY

Centralized

Electrocardiographic

The right-leg driviq ampl$er circuits are shown in Figure 3, in which the Ql and Q2 transistor pair constitutes a buffer amplifier, with feedback from the second-stage collector circuit to the first-stage emitter. When the switch SlA is closed, all the available output is fed back and results in a gain very close to unity. When SlA is open, half the output is fed back, giving a gain of approximately two. The transistor pair 43 and 44 constitutes the right-leg driving amplifier. 44 is connected as an emitter follower, so that there is phase inversion between the base of 43 and the emitter of 44, and a large volt. . age amphficatron IS obtained between these two points. The emitter of 44, which is the output terminal of the right-leg driving amplifier, is connected to the right leg of the patient, and through the patient’s other electrodes this output feeds back through Ql and 45 and resistors RA and RB to the input of the right-leg amplifier. Because of the phase inversion around this circuit, and the high loop gain, the direct current level to which the emitter of 44 will set itself is a value that will allow the average of the voltages at points A and B to be correct for normal operation of Q3. By setting the emitter of 43 at 1.1 volts, we obtain a level of about -0.55 at A and B and near 0.0 at the bases of Ql and Q5. The signals chosen by the selector switch feed a bank of six all-transistor amplifiers. These are differential input, single-ended output devices. The input impedance exceeds 2 megohms, so that they have a negligible loading effect on the mixing networks. The amplifier gain is normally set at 200, which, in conjunction with the buffer amplifier gain of 2, gives an over-all gain of 400. The output capability is a swing of f 1 volt into a load as low as 700 ohms. The physical arrangement is shown in Figure 4. DATA

The

SET SECTION

data

transmitter

(American

FIG. 4. Centralized electrocardiographic and uectorcardiographic systems. Portable electrocardiographic and vectorcardiographic preamplifier systems on top of the console; Data-Phone on left of console; central console; and processor camera and oscilloscope (enclosed) on far right.

VOLUME 19, JUNE 1967

Telephone

and

and Vectorcardiographic

FIG. 3.

Circuitry

System

of two buffer ampl$iers.

821

See text for

details. Telegraph Co., New York, “Data-Phone” sets x 604A [transmitter] and x 604B [receiver]) is equipped with a standard telephone jack (Fig. 4). Transmission is accomplished by placing a telephone call with the use of the data set. Once contact is established, a

822

Zitnik et al. Balanced ECG

Input

System

From

/ Channel

From

Line

Telephone Line Control

\ A

Telephone

Preamps B

C

Differential Am pl ifier

Signal Amplifier

Voltage Cont rot led Oscillator

To Telephone FIG. 5. Block diagram See text for details.

Line

ofmulttchannel Data-Phone transmitter.

output Channel

A

/

\ push button connects the data set to the telephone line. Signals from the electrocardiographic system preamplifiers are delivered to three differential amplifiers (Fig. 5). Differential amplifiers were chosen for the data set input to avoid longitudinal hum and noise pickup. The balanced input impedance of these amplifiers is about 1000 ohms, and the response The output of each extends down to direct current. amplifier serves as the controls for a voltage-controlled oscillator. Each oscillator output frequency varies linearly with the differential amplifier input voltage. At the receiving location each of the three line signals is amplified and demodulated in a zero-crossing detector (Fig. 6). The design of the system calls for a gain of 1.0 (0 db.) from the input of the transmitter to the output of the receiver, within limits set by manufacturing tolerances, line voltage and temperature. The three on-line signals are in the middle of the audio range, between 1,200 and 2,300 c.p.s. (Hz). Each has a 200 c.p.s. (Hz) peak-to-peak deviation corresponding to the =tl volt (2 v. peak-to-peak) modulating signal. CONSOLE

The console in Figure 4 (Marquette Electronics, Milwaukee, Wis.) receives electrocardiographic signals of relatively high level (1 to 2 v. peak-to-peak for 30 mm. deflection) from telephone lines, radio signal, direct wiring, or any other communication system and presents these signals to a high-resolution, 17 inch, large-screen monitor oscilloscope (International Telephone and Telegraph Co. Scope type 702, New

To, Con sole 6. Block diagram of multichannel Data-Phone See text for details.

FIG.

receiver.

York) and simultaneously on a similar slave oscilloscope for photography. The 12 standard electrocardiographic signals are displayed either in a single-channel format or in three or six leads simultaneously. A time duration of 3 sec./lead (75 mm.) is allowed. Later expansion of the system provides for simultaneous vectorcardiographic loops and X, Y and Z orthogonal leads of the Frank system. All central switching (with the exception of input signal relays) has been accomplished by the use of bistable semiconductor circuits in order to provide silent operation, maximal reliability and ease of remote control. Three- or six-channel multiple-trace transmission, reception and photography utilize a high-speed “free-running” transistor ring which switches and blanks the trace at a rate of approximately 80 msec./sample. Trace positions, in any mode, are selected by push buttons. A rhythm strip can be selected from any lead and displayed on a separate channel, with a time duration of 6 seconds (150 mm.). The slave oscilloscope has a high-precision, etchedglass graticule containing small divisions of 1.4 mm. lengths and large divisions of 7.0 mm. lengths in both axes. When reproduced after photographic reduction and enlarging, these divisions yield standard fine lines of 1 mm. length and bold lines of 5 mm. length. The graticule etchings are filled with phosTHE

AMERICAN

JOURNAL

OF

CARDIOLOGY

Centralized

Electrocardiographic

and Vectorcardiographic

System

FIG. 7. &r~a cardjilm is prernounted in a standard computer punch card. It is devrloped and delivered The cl~ctrocardiographic and vectorcardiographic data can then be read by using a standard viewer. be madr on a standard colnmercial printer. The card may be punched xvith pertinent data ‘1s an aid to The card Inay Ix handkd. fication, coding of diacrnosrs and storaEe of other oertinrnt information. trievc-rl by utilizing autornatrd methodology. I,

c/

phorescent paint. Ultraviolet lighting in the camera housing produces a uniform illumination for photography. The graticule and rhythm strip are photographed simultaneously. A push-button-operated, seven-digit numerical display is located on the console and also is photographed with the electrocardiogram for identification. The weep frequency of the oscilloscope is derived from a 64 cycle tuning-fork oscillator that is divided in binary fashion to 1 c.p.s. (Hz) and then used as a trigger. The fork maintains a frequency accuracy of better than 0.04 per cent, which ensures a precise horizontal trace frequency. If its length is maintained. trace velocity is assured. Two-axis intensity modulation relative to the trace velocity has been provided to improve visibility in the high velocity areas of the QRS complex. Half-scale amplitude can be selected for any channel, and a sweep speed of 50 mm.;/sec. is available for the rhythm trace. Later, planned additions to the system will provide (1) automatic punching of card with numerical patient identification directly from the console and (2) a vector control board for the simultaneous viewing and microfilm photography of X, Y and Z axes, plus presentation of these leads in scalar form. The vector board will have special electronic markers for control of the camera shutter in order to photograph the P, QRS. or T portion of the signals. RECORDING

CAMERA

camera records the electrocardiographic and vectorcardiographic traces on microfilm from the The

VOLUME

19,

JUNE

1967

823

in 45 seconds. or a copy can p,ltirnt idcnti~torrcl ,~rld re-

‘l‘he camera used is an slave oscilloscope (Fig. 4). electronically modified version of a commercially available camera (2000 Processor camera. Minnesota Mining and Manufacturing Co.. St. Paul. Minn.). The optical system consists of a lens with a stop value of f5.6 and a focal length of 65 mm. TI‘hr shutter is a leaf type and is activated by an electric colcnoid to open and close on command. The camera operates on a 100 to 1.30 v. singlephase alternating current and requires only a supply of camera cards (standard punch cards). one bottle of developer. fix and distilled-bvater \