Med. & Biol. Eng. & Comput., 1978, 16, 243-249
New controller for in-series cardiac-assist devices P e t e r J. M a r t i n
and Dov Jaron
BiomedicalEngineeringProgramme,Departmentof ElectricalEngineering,Universityof RhodeIsland,Kingston,RhodeIsland02881, USA Abstract
A new control unit for in-series cardiac-assist devices is presented, This unit allows the control of the phase difference between the fundamental components of aortic pressure and flow using a single timing adjustment. To accomplish this the conventional "delay" from the e.c.g. R-wave to device activation was replaced by the time interval from the R-wave to midpump systole (TMPS). The second adjustment, the "duration" of pump systole, which in this control unit is symmetrical about the TMPS, does not affect the phase difference. A computer simulation of the control unit suggests that the phase difference is controlled by TMPS alone when the "duration" is maintained above a threshold level. The control unit is being tested using intraaortic balloon pumping in dogs. Preliminary results from these experiments have substantiated the simulation results.
K e y w o r d s - - C o n t r o l unit, Cardiac assistance, Diastolic augmentation, Counterpulsation, Intraaortic balloon pumping, In-series assistance
I Introduction AT PRESENT, methods of controlling in-series cardiac-assist devices are subjective in nature, and adjustments are performed manually by trained operators. Only recently have research efforts been directed towards finding objective criteria for controlling these devices (CLARK et aL, 1973; WILLIAMS et aL, 1975). One parameter which has been suggested as a possible objective control criterion is the phase difference between the fundamental components of aortic pressure and flow. This parameter will be referred to here as the afterload phase angle. Previous experimental observations have suggested that the benefits derived from in-series assistance are highly dependent upon the afterload phase angle (JARoN et aL, 1970; JARON, 1977). To perform a well controlled study of the relationship between the afterload phase angle and the haemodynamic effects of cardiac assistance, a control unit is needed which allows accurate, reproducible, and convenient control of the phase angle. Such control is not possible with conventional units. The objective of this work was to design and construct a device capable of controlling the afterload phase angle.
2 Design considerations The control unit generates a driving waveform which activates the assist device at the proper time during the cardiac cycle. This waveform is synchronised to the cardiac cycle using the R-wave of the electrocardiogram (e.c.g.) Received
Present commercial units employ two timing adjustments to control the assist device. The first is the time interval from the e.c.g. R-wave to pump activation (termed 'delay') and the second is the time interval from activation to deactivation of the device (termed 'duration' of pump systole). A change in either of these adjustments will alter the phase of the driving waveform, and hence tlhe afterload phase angle. In the new control unit the conventional 'delay' has been replaced with the time interval from the R-wave to midpump systoles (TMPS). The 'duration', which is designed to be symmetric about the TMPS, can now be varied without disturbing the phase of the driving waveform. The TMPS alone controls the phase of the driving waveform. A comparison of the new and conventional timing adjustments is shown in Fig. 1. The times of device activation (Tact) and deactivation (Tdeact) are found by subtracting and adding one half of 'duration' to the t.m.p.s., respectively. They are given by: Tact= TMPS - 1 ]2 duration (1) Td,act=TMPS + 1 ]2 duration . . . (2) The following criteria were included in the design of the unit: (a) The phase angle can be controlled to an accuracy of one degree. Timing adjustments accurate to 1 ms satisfy this criterion for heart rates up to 166 beats/min. (b) A safety feature to deal with premature ventricular contractions (p.v.c.s) was included. The control unit automatically deactivates the assist device if a p.v.c, occurs while the assist device is active.
0140-0118/78/0734-0243 $1 950/0
9 IFMBE: 1978 M e d i c a l & Biological Engineering & C o m p u t i n g
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(c) The unit was designed so that it could be operated either manually or by computer as part of an automatic control system. Either mode is selectable by the operator from a front-panel switch. In the manual mode, timing adjustments are made via a set of b.c.d, leverwheel switches on the front panel. When the unit is operated automatically, the adjustments are made directly by the computer. (d) The design provides a time interval after detection of the R-wave for computer processing of data before new adjustment for the current cardiac cycle is determined. This 'processing' period can be used to process all previous haemodynamic data including data obtained up to the most recently detected R-wave. These data then
high-speed counting technique and are completed in less than 1 ms. The control unit continues counting in milliseconds until the assist device is activated at Tac t and deactivated at Tde~t. Upon deactivation of the assist device the entire control unit is reset and waits for the next R-wave. If a second R-wave occurs before the control unit is reset (p.v.c.), the assist device is deactivated and the control unit reset. The control unit then resumes operating using the new R-wave as the reference.
4 Circuit description The circuitry in the control unit is divided into five functional blocks. These are shown in Fig. 2.
[_ 1/2 DUR
1/2 DUR __1
DURATION OF PUMP SYSTOLE
time Fig. 1 Comparison of timing adjustments between n e w and conventional control units, The e,c.g. R- wave occurs at time t = 0
become available for generating new timing adjustments. These new adjustments are accepted by the control unit at the end of the 'processing' period. In our present design the 'processing' period was chosen to be 100 ms. Such a scheme is feasible because cardiac systole is longer than 100 ms and the assist device will never be active during this time. The control unit was designed and built in a modular fashion using digital circuitry. This allows alteration and expansion of logic functions and rapid trouble shooting and repair. The following sections describe the basic operation and circuit design of the control unit. A detailed description can be found elsewhere (MARTIN,1976).
3 Operation The control unit is driven by a 1 kHz clock signal. When the R-wave is detected (t=0), timing of the 100 ms 'processing' period is begun. At the end of this period the control unit computes the activation and deactivation times from the timing adjustments. Half the duration is subtracted from the TMPS to give T.a and half the duration is added to the TMPS to give Tae,a. The subtraction and addition are performed simultaneously using a 244
Fig. 2 Control unit showing basic functional blocks
Each block will be discussed and a diagram presented where appropriate. The computer-to-control-unit interface is used to convert serial data from the computer to parallel form for the control unit. The modular design of the control unit allows any type of interface to be used, depending upon the requirements of the computer.
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For this reason a detailed description of the interface will not be presented. The choice of either computer-generated or manually set timing adjustments is made by the operator. A switch on the unit controls the data selectors which can use either source as input. The control unit contains three data selectors, shown in Fig. 3. The first, DS1, selects the TMPS and the second, DS2, selects half the duration from either the computer or the front-panel switches. The CE tu
with the value of the TMPS. The W O R K counter is initialised with half the duration. The W O R K counter is then decremented at a 1 MHz clock rate. Simultaneously, the START counter is decremented and the STOP counter is incremented at the 1 MHz rate until the W O R K counter reaches zero. This essentially subtracts half the duration from the START counter and adds half the duration to the STOP counter so that they contain T~r and Td~,r respectively. The W O R K counter is now reset with
uO LU t.)
t'v" LIJ I.--
It3 ILl -r
DS3 (3) 7~157
OUTPL TO START AND STOP COUNTERS
SEL3FROM TIMING CONTROE
OUTPUT TO WORK COUNTER
Fig. 3 Data selectors
output of DSI is used to initialise the START and STOP counters in the timing unit. The third selector, DS3, selects either the TMPS from DS1 or half the duration from DS2 and uses these data to initialise and reset the W O R K counter in the timing unit. The timing unit (Fig. 4) is used to generate T, ct and Tdeact from the input parameters and to activate and deactivate the assist device at these times. This section contains three counters with their associated decoders. The START counter is used to control activation of the assist device, the STOP counter is used to control deactivation of the assist device, and a general purpose W O R K counter is used in generating Tact and 7~,ct from the TMPS and half the duration. When the 100 ms 'processing' period is complete both the START and STOP counters are initialised
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the TMPS. All three counters are then decremented using the 1 kHz clock rate. When the START counter reaches 100 a pulse is generated to activate the assist device (t=T.r When the W O R K counter reaches 100 a reference pulse is generated to indicate t = TMPS. Finally, a pulse is generated to deactivate the assist device when the STOP counter reaches 100. The three counters are decoded at 100 to compensate for the initial 100 ms 'processing' period. The purpose of the timing-control logic is to ensure proper operation of the timing unit and data selectors. Fig. 5, which shows the timing-control logic, is divided into three sections. The first section receives pulses indicating when significant events have occurred. The second section determines the input to data selector DS3 and the state at which the W O R K counter is decoded. This section also
generates the pulses used to initialise and reset the counters in the timing unit. Finally, the third section generates the appropriate clock signals for these counters.
to preset the counters in the timing unit. This pulse also sets flip-flop CK1, which allows the 1 M H z clock to reach outputs CLK1 and CLK3. At this time, CLK1 is used to increment the STOP
LOAD 2 LOAD 1 CLK 1
- CLK 2 CLK 3
I PROGRAMMABLE DECODER
Fig. 4 Timing unit After detection of an R-wave and before the 'processing' period is complete the input to DS3 is half the duration (line SEL3 high) and no clock signals are generated at outputs CLK1, CLK2 and CLK3. At the end of the 'processing' period a pulse is received on line MHz ST. Flip-flop L D ! then generates a pulse on lines LOAD1 and LOAD2 SECTION 1
1MHZCLOCKI"~ ~ b FROM "i J ' - L ~ RESET FROM MAINCONTROL
,n, I K LD1 ~ L
MHZ ST F']~mn~"~-~
counter and CLK3 is used to decrement the START and W O R K counters. Line SEL3, which is now low, changes the input of DS3 to the TMPS. It should be noted that the W O R K counter will be decoded at zero (line D E C O D E low). When the computation of Tac, and Taeact is complete (the W O R K counter reaches zero) a pulse
L--~IC I I1 L ~ K C CK' ~ U ]
I[ ~ol ~L227 UNIT I
AK L"201- I
Fig. 5 Timing controllogic 246
M e d i c a l & Biological Engineering & C o m p u t i n g
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? Medical & Biological Engineering & Computing
will be received on K H Z ST. Flip-flop LD2 then set to 350 ms and the 'duration' was set to 260 ms. generates a pulse on LOAD2 which resets the A computer simulation of the operation of the W O R K counter to the TMPS. Flip-flop CK2 is control unit has been performed (MARTIN et al., then set. This terminates the 1 MHz clock and 1977). The results suggested that the afterload places the 1 kHz clock on lines CLK2 and CLK3. phase angle remains nearly constant as the 'duration' CLK2 is used to decrement the STOP counter as is changed provided that the 'duration' is maintained CLK3 continues to decrement the START and above a threshold value. Results from recent W O R K counters. DECODE is also changed so canine experiments substantiate the simulation that the W O R K counter will now be decoded at results. 100. The main control logic (Fig. 6) provides the 2O0overall control functions for the control unit. These include: detection of the e.c.g. R-wave and synchronAORTIC PRESSURE isation of the control unit to the heart cycle; de(mmH9) 100-'-~ - ~ . / ~ ~ p ~ ~ ~ . tection and processing of p.v.c.s; timing of the O-lOOms 'processing' period; and assist device control. ! ! ~ , , The R-wave detector consists of a highpass filter, CONTROL variable-gain amplifier, and a Schmitt-trigger circuit. PULSE ,-~ ~ ~ ~ ~ :~. ,,--. The highpass filter is used to remove any d.c. component from the e.c.g, signal and to suppress baseline shifts. The variable-gain amplifier is used , ! , , to adjust the amplitude of the R-wave so that it ECG exceeds the threshold level of the Schmitt-trigger circuit. The amplifier is also used as an impedance buffer for the Schmitt-trigger input of the one shot. 1 SEC "~ T'IME When the R-wave is detected, flip-flop FF1 is turned on. This allows the free running 1MHz Fig. 7 Sample of results obtained with the new clock to drive the MAIN counter. This counter control unit using intra-aortic balloon pumping divides the 1 MHz clock down to a 1 kHz clock and in dogs times the 1OOms 'processing' period. (Note that the apparent delay in aortic pressure waveform is due to mechanical delays in The detection circuit for premature ventricular balloon inflation and deflation and delays inbeats is made up of flip-flop PVC connected to the herent in the fluid-filled pressure catheter) reset logic. The PVC is set by an R-wave only if FF1 has previously been set but not cleared by the normal deactivation of the assist device. This condition activates the reset logic which in turn de- 6 Comment activates the assist device and resets the entire Control units presently available rely on operator control unit. judgment for subjective and qualitative adjustment The assist-device control circuit consists of flipflop D U R and 3 N A N D gates. D U R is set when the of assist-device operation. The control unit desassist device is to be activated and reset when the cribed here allows for accurate and objective control assist device is to be deactivated. The N A N D gates of the phasing of in-series cardiac assist devices. ensure that only the proper signals affect the assist Further, the unit was designed so it can be computer controlled. Consequently, the assist device can be device. incorporated into an automatic control system. Such a system will be capable of quantitative control 5 Test results of cardiac assistance and should be useful in proFollowing bench tests the control unit was used viding accurate and reproducible experimental for intra-aortic balloon pumping in dogs. Fig. 7 observations. This will increase our understanding shows a sample of the results obtained. The bottom of the haemodynamic and physiologic processes trace shows the e.c.g, signal used to synchronise during cardiac assistance and lead to additional the control unit to the cardiac cycle. The centre quantitative criteria for controlling in-series assist trace shows the output of the control unit. A refer- devices. Eventually, such a system would enhance ence pulse is superimposed over the control pulse to the effectiveness of clinical applications of mechanindicate the TMPS. The leading edge of the reference ical cardiac support. pulse indicates the occurrence of the TMPS. The top trace shows the resulting aortic root pressure. Acknowledgment--This work was supported in part by In the example shown here the period of the heart NSF Grant EN6 74-21085 and a grant from the Stride cycle was approximately 490 ms, the TMPS was Memorial Foundation. 248
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References CLARK, J. W., KANE, G. R., and BOURLAND, H. M. (1973) On the feasibility of closed-loop control of intra-aortic balloon pumping. 1EEE Trans., B M E 20, 404-412. JARON, D., TOMECEK, J., FREED, P. S., WELKOWITZ, W., FICH, S. and KANTROWrrz, A. (1970) Measurement of ventricular load phase angle as an operating criterion for in-series assist devices. Trans. A S A I O , 16, 466-471. JARON, D. (1977) Left ventricular afterload and systemic hydraulic power during in-series cardiac assistance: Studies using intra-aortic balloon pumping in dogs. Ann. Biomed. Eng., 5, 95-110.
MARTIN, P. J. (1976) A digital afterload phase angle controller for in-series cardiac assist devices. M.S. Thesis, University of Rhode Island, Kingston, RI, USA. MARTIN, P. J., OHLEY, W. J., KARLSON, K. E., and JARON, D. (1977) Simulation analysis of a new control unit for in-series cardiac assist devices. Proceedings of the 5th New England Bioengineering Conference, 205-209. WILLIAMS, M. J., BEASLEY, J. 1., STUDWELL, M. L. and RuBI~, J. W. (1975), Control criteria for intra-aortic balloon pumping. Proceedings of the 28th ACEMB, 349
Un nouvel appareil de c o r n m a n d e pour les dispositifs d'assistance cardiaque en s6rie Sommaire--I1 pr6sente un nouvel appareil de commande pour les dispositifs d'assistance cardiaque en s6rie. Cet appareil permet le contr61e de la diff6rence de phase entre les composants fondamentaux de la pression aortique et le d6bit, en n'exigeant qu'un seul r6glage. Pour ce faire, le 'retard' traditionnel de l'onde-R d'61ectrocardiogramme ~. l'activation du dispositif est remplac6 par l'intervalle de temps de l'onde-R ~t la systole par mi-pompe (TMPS). Le deuxi6me r6glage, la 'Dur6e' de la systole par pompe, qui est sym6trique au voisinage du TMPS dans cet appareil de commande, n'influent pas sur la difference de phase. U n ordinateur, simulant l'appareil de commande, sugg6re que la diff6rence de phase est contr616e uniquement par le TMPS, lorsque la 'Dur6e' est maintenue audessus du seuil. Cet appareil de commande est mis h ]'essai chez les chiens par la m6thode de pompage de ballon intra-aortique. Les r6sultats pr61iminaires de ces exp6riences ont prouv6 les r6sultats de simulation.
Ein neues Steuerger~it f/Jr H i n t e r e i n a n d e r g e s c h a l t e t e H erzu nterst/Jtzu n g sg er~ite Zusammenfassung--Es wird eine neue Steuereinheit fiJr hintereinandergeschaltete Herzunterst~tzungsger~tte vorgestellt. Mit dieser Einheit kann die Phasendifferenz zwischen den fundamentalen Bestandteilen des aortischen Drucks und Flusses durch eine einfache Zeitjustierung gesteuert werden. U m dieses zu erreichen, wurde die herk6mmliche 'Verz6gerung' von der e.c.g. R-Welle bis zur Bet/itigung des Ger~its durch einen Zeitinterval v o n d e r R-Welle zur Mittelpumpen-Systole (TMPS) ersetzt. Die zweite Justierung, die 'Umlaufzeit' der Pumpensystole, die bei dieser Steuerung um die TMPS symmetrisch ist, beeinfluBt die Phasendifferenz nicht. Aus einer Computersimulation der Steuerung l~il3tsich schlie Ben, da 13diePhasendifferenz nur dann durch die TMPS gesteuert wird, wenn die 'Umlaufzeit' Ober einem Schwellenniveau gehalten wird. Die Steuerung wird zur Zeit mit intraaortischen Ballonpumpen an Hunden getestet. Erste Resultatsberichte fiber diese Versuche haben die Resultate der Simulation unterstiJtzt.
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