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A microprocessor controlled timing device for cardiac electrical stimulation
J. ELECTROCARDIOLOGY 12 (1), 1979, 83-87
A Microprocessor Controlled Timing Device For Cardiac Electrical Stimulation BY DAVID M. MIRVIS, M.D. AND FRANCIS W. KELLER, B.S.E.E.
SUMMARY A microcomputer controlled device for delivering precisely timed stimuli to cardiac chambers is described. Stimulation patterns are determined by software, rather than hardware design, resulting in enhanced flexibility. Additionally, by providing timing inputs to conventional laboratory stimulators, this instrument can convert these limited use devices into ones capable of generating complex cardiac stimulation patterns at moderate cost. A n a l y s i s of b o d y s u r f a c e a n d i n t r a c a r d i a c electrical recordings during paced cardiac r h y t h m s h a s s i g n i f i c a n t l y aided t h e u n d e r s t a n d i n g of n o r m a l a n d a b n o r m a l c l i n i c a l electrophysiology. 1 Fixed-rate atrial pacing p e r m i t s e v a l u a t i o n of p r o p e r t i e s of the sinoa t r i a l 2 a n d a t r i o v e n t r i c u l a r nodes, a a n d of the s u b j u n c t i o n a l c a r d i a c conduction s y s t e m ? T h e a d d i t i o n of a p p r o p r i a t e l y t i m e d e x t r a s t i m u l i allows d e t e r m i n a t i o n of r e f r a c t o r y periods of cardiac conducting tissues 5 and measurement of s i n o a t r i a l c o n d u c t i o n t i m e s . ~ B o t h f o r m s of s t u d y h a v e led to t h e c u r r e n t concepts of t h e p a t h o g e n e s i s of t a c h y a r r h y t h m i a s 7's includi n g t h o s e due to v e n t r i c u l a r p r e e x c i t a t i o n ? Single c h a n n e l , fixed r a t e cardiac s t i m u l a tion devices a r e r e a d i l y a v a i l a b l e . G e n e r a t i n g p r e c i s e l y t i m e d i m p u l s e s o n o n e to t h r e e c h a n n e l s , however, r e q u i r e s t h e d e s i g n a n d purchase of complex devices costing t h o u s a n d s of d o l l a r s to r e p l a c e m o r e ubiquitous s i m p l e cardiac p a c e m a k e r s or l a b o r a t o r y s t i m u l a t o r s . One a l t e r n a t i v e to t h i s replacem e n t , as described h e r e , is t h e d e v e l o p m e n t of a microprocessor controlled timing device c a p a b l e of c o n v e r t i n g a s t a n d a r d l a b o r a t o r y single or double c h a n n e l s t i m u l a t o r into a n i n s t r u m e n t c a p a b l e of precise a n d c o m p l e x cardiac stimulation.
From the Section of Medical Physics, Division of Circulatory Diseases, Department of Medicine, University of Tennessee, Center for the Health Sciences, Memphis, Tenn. 38163 This research was supported by grants HL-01362, HL09495 and HL-20597 and National Research Service Award HL-05323 from the National Heart, Lung and Blood Institute, and by a grant from the Deborah Heart and Lung Institute. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked " a d v e r t i s e m e n t ~' in accordance with 18 U.S.C.,w 1734 solely to indicate this fact. Reprint requests to: David M. Mirvis, M.D., University of Tennessee, Center for the Health Sciences, 951 Court Ave., Room 339M, Memphis, Tennessee 38163
MATERIALS AND METHODS Hardware: A schematized, block diagram of the hardware controller is presented in Fig. 1. An appropriately amplified single channel electrocardiogram (ECG) signal is first delivered to a previously described h a r d w a r e QRS trigger. 1~ The filtered, time derivative of the incoming signal is generated and is passed to a one-shot multivibrator which acts as the trigger detector. As depicted in Fig. 1, a polarity switch has replaced the originally described absolute value amplifier stage. The digital pluse generated when the multivibrator is tripped sets a flip-flop that is sensed by the microcomputer module. Additionally, the trigger signal may be viewed on a monitor oscilloscope (Fig. 2); the point on the QRS at which triggering occurs is indicated by the onset of a discontinuity in the ECG waveform. The m i c r o c o m p u t e r module selected was a KIM-1 microprocessor (MOS Technology, Norristown, PA). This sytem consists of an 8-bit microprocessor with 2048 bytes of read only memory (ROM), 1024 bytes of random access m e m o r y (RAM), a 1 MHz crystal timing oscillator and two programmable interval counters. Four types of input/output (I/O) operations are possible. First, a hexadecimal keypad and light emitting dipole (LED) display are used for operator interventions. Second, program input and/or data storage may utilize an audio cassette interface. Third, an asynchronous I/O interface is provided for connection to another computer terminal. Last, 15 I/O ports are available for user applications. Audio cassette connections, as well as interfaces to the QRS trigger and the stimulator itself, operate through these pins. A third element of the system, the Stimulator Control Interface, amplifies the output signals of the microprocessor to a level compatible with the stimulator being used. As configured here, voltages of greater than -+6 volts are needed to trigger a Grass S-88 (Grass Instruments, Quincy, Mass.) stimulator modified for clinical safety purposes only. Two individually controlled channels CS~ Synch" and "$2 Synch") are provided as external trigger or synchronizing inputs into one or two channels of any appropriate laboratory stimulator. Power to all units is provided by regulated supplies delivering 5.0 volts to the microprocessor, 12.0 volts to the audio cassette interface and 6.0 and 9.0 volts to the QRS trigger.
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Input EKG ~_ ~-
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Programming. Software development utilizes a specially written cross-assembler for the KIM-1, using a PDP-7 digital computer. This allows assembler language p r o g r a m m i n g of the microprocessor, eliminating the need for hexadecimal machine-language coding. The output of this Fortran program, i.e., a set of hexadecimal machine instructions, can be loaded directly into KIM-1 memory by connecting its asynchronous interface to a similar one on the laboratory computer.
To S2 SYNC To Sl SYNC
j
Fig. 1. Block diagram of the microprocessor controlled timing device, described in detail in the text.
RESULTS A p p l i c a t i o n . One clinical a p p l i c a t i o n will be described illustrating the utility and versatility of t h e t i m i n g controller. T h i s p r o g r a m , des i g n e d to d e l i v e r p r e c i s e l y t i m e d e x t r a s t i m u l i d u r i n g t h e c a r d i a c cycle, h a s b e e n u s e d in o v e r fifty clinical s i t u a t i o n s to d e t e r m i n e ref r a c t o r y periods of t h e e l e m e n t s of t h e cardiac c o n d u c t i o n s y s t e m . 3'5
9
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Fig. 2. An example of the input to (lead III) and output from (TS) the hardware QRS trigger. The arrow indicates the discontinuity in the QRS complex, introduced by the trigger to delineate the moment of triggering. The patient studied here was in atrial fibrillation, with a variable ventricular rate. Regardless of this R-R variation, the trigger was tripped by identical portions of each QRS. J. ELECTROCARDIOLOGY, VOL. 12, NO. 1, 1979
COMPUTERIZED STIMULATOR
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Hardware Configuration. An ECG input is provided from a single ECG lead recording a predominantly monophasic positive or negative deflection. Because only single cardiac chamber stimulation will be required, both S~ and $2 <
J. ELECTROCARDIOLOGY, VOL. 12, NO. 1, 1979
]
85
Fig. 3. Use of the programmable stimulator to evaluate sinoatrial node function. A: Normal sinus r h y t h m at a cycle l e n g t h of 822 msec. Surface leads II, I and V1 and an intracardiac lead (HBE) are recorded. A, H and V identify low atrial, His bundle and ventricular deflection, respectively. B: Atrial pacing during the initial portion of the trace (cycle length 400 msec) terminates after 120 seconds at the broken line. Spontaneous atrial depolarizations resume after a 1208 msec delay.
The variables entered are: A. - - the rate of the basic driving stimulus, Sl, entered as the interval between impulses in msec. If 0 is entered, no d r i v i n g s t i m u l i are delivered, and spontaneously occurring QRS complexes are sensed; B. - - the number of cardiac cycles between introduced extrastimuli; C. - - the number of cycles to wait after an e x t r a s t i m u l u s before continuing $1 pacing or r e s t a r t i n g QRS counting. This delay permits observation of alt e r a t i o n s in s p o n t a n e o u s c a r d i a c r h y t h m induced by the p r e m a t u r e beat; D . - - t h e m a x i m u m coupling interval, in msec, between the basic $1 driving stimulus or the trigger point on the spontaneous QRS and the premature, or $2, pulse; E . - the m i n i m u m permissible coupling interval, in msec; F. - - the decrement, in msec, between successive $2 impulses. After variables are entered, they may be reviewed and altered if required. The program is then initiated upon command and proceeds without interruption or intervention to introduce e x t r a s t i m u l i beginning with the longest coupling i n t e r v a l and c o n t i n u i n g u n t i l t h e m i n i m u m c o u p l i n g i n t e r v a l is reached. Pacing is then automatically halted, with cessation of both $1 and $2 stimulus outputs. Parameters m a y then be modified and the program rerun. During execution, the program may be halted at any point by the operator.
86
MIRVIS AND KELLER
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B
...~_ ~.~305bpm
] Fig. 4. Use of the programmable stimulator to convert paroxysmal supraventricular tachycardia to sinus rhythm. A: The stimulator was programmed to deliver a single 10 second burst of pulses at a rate of 260 beats per minute (BPM) as leads I, II and V1 were monitored. After cessation of pacing, ectopic P-waves (p1) of the tachycardia continued. B: An increase of pacing rate to 305 beats per minute likewise failed to convert the tachycardia. C: Tachycardia was convereted to sinus rhythm with normal P-waves (P) after pacing at 385 beats per minute. Durations and rates of pacing were determined by software programming.
II V1
A l t h o u g h i n t e n d e d to p r o v i d e s i n g l e chamber stimulation for determination of refractory periods, appropriate adjustment of variables m a y permit other operations. For example, entry of a high value for variable B, a narrow range of permissible coupling intervals (variables D and E) t h a t approximate variable A, a low decrement value (variable F) and a zero for variable C, results in carefully defined but prolonged fixed rate cardiac stimulation, at a frequency defined by variable A, such as m a y be needed for sinus node t e s t i n g , 2 (Fig. 3) or c o n v e r s i o n of a t r i a l tachycardias (Fig. 4). A l t e r n a t i v e l y , the s t i m u l a t o r h a r d w a r e c o n f i g u r a t i o n m a y be a l t e r e d w i t h o u t software modification. If, for example, the $1 and $2 synch outputs from the microcomputer device are directed to two stimulator output channels and two intracardiac catheters, one chamber (e.g.,the right atrium) m a y be driven at a fixed rate ($1) while the premature impulses ($2) are used to stimulate a second chamber (e.g., the right ventricle).
DISCUSSION The described system provides a moderately priced instrument capable of providing
complex, precisely timed cardiac stimulation patterns under computer control. Components, i n c l u d i n g microprocessor, power supplies, audio tape deck and components of the QRS trigger are commercially available for under $500. Thus, one advantage of the system is its reasonable cost. Second, and more significant, is the degree of flexibility in stimulation sequences possible by s o f t w a r e u t i l i z a t i o n w i t h r e d u c e d hardware-related limitations. For example, a single channel stimulator may be used to del i v e r c o m p l e x t r a i n s of v a r i a b l y s p a c e d stimuli through a single intracardiac catheter. Conventionally, separate but interfaced stimulator channels are required to deliver driving (S1) pulses and each type of premature stimulus ($2, $3, etc.). Addition of other stimulator channels to this sytem is required only for multiple chamber stimulation, as for study of certain cardiac dysrhythmias? Development of software was simplified by the PDP-7 cross a s s e m b l e r , described above, which obviated the need for programming in hexadecimal machine code. This further enhances the flexibility of the total system by m a k i n g it relatively easy to rewrite and/or modify the control program. J. ELECTROCARDIOLOGY, VOL. 12, NO. 1, 1979
COMPUTERIZED STIMULATOR
E x p a n s i o n s and o t h e r applications of t h e described device are probable. Clearly, s t i m u l a t i o n p a t t e r n s o t h e r t h a n the one detailed are feasible, including complex dual c h a m b e r p r e m a t u r e s t i m u l a t i o n , s t i m u l a t i o n by t r a i n s of p r e m a t u r e beats, a n d d e f i b r i l l a t i o n synchronization. U n u s e d I/O ports m a y be used to control a n a l o g tape drives, or to provide t i m e a n d e v e n t code s i g n a l s f o r r e c o r d i n g o n m a g n e t i c a n a l o g tape or p a p e r records simult a n e o u s l y w i t h ECGs.
5.
6.
7. REFERENCES 1. NARULA,O S: His Bundle Electrocardiography and Clinical Electrophysiology. F.A. Davis, Philadelphia, 1975 2. MANDEL, W, HYAKAWA, H, DANZIG, R and MARCUS, H S: Evaluation of sinoatrial node function in man by overdrive suppression. Circulation 44:59, 1971 3. WIT,A L, WEISS,M B, BERKOWITZ,W D, ROSEN, K M, STINER,C and DAMATO,A N: Patterns of atrioventricular conduction in the h u m a n heart. Circ Res 27:345, 1970 4. NARULA,O S, COHEN,L S, SAMET,P, LISTER,J W, SCHERLAG,B and HILDNER,F J: Localization
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of A-V conduction defects in man by recording of the His bundle electrogram. Am J Cardiol 25:228, 1970 DENES,P, Wu, D, DHINGRA,R, PIETRAS,R J and ROSEN, K M: The effects of cycle length upon refractory periods in man. Circulation 49:32, 1974 STRAUSS,H C, SAROFF,A L, BIGGER,J T and GIARDINA,E G V: Premature atrial stimulation as a key to the understanding of sinoatrial conduction in man. Circulation 47:86, 1973 DENES, P, Wu, D, DHINGRA,R C, CHUQUIMIA,R and ROSEN, K M: Demonstration of dual A-V nodal pathways in patients with paroxysmal s u p r a v e n t r i c u l a r tachycardia. Circulation 48:549, 1974 JOSEPHSON,M E and KASTOP~J A: Paroxysmal supraventricular tachycardia. Is the atrium a necessary link? Circulation 54:430, 1976 WELLENS,H J J: Electrical Stimulation of the H e a r t in t h e S t u d y and T r e a t m e n t of Tachycardias. University Park Press, Baltimore, 1971 BRANDON,C W and BRODY D A: A hardware trigger for temporal indexing of the electrocardiographic signal. Comp and Biomed Res 3:47, 1970
A Microprocessor Controlled Timing Device For Cardiac Electrical Stimulation BY DAVID M. MIRVIS, M.D. AND FRANCIS W. KELLER, B.S.E.E.
SUMMARY A microcomputer controlled device for delivering precisely timed stimuli to cardiac chambers is described. Stimulation patterns are determined by software, rather than hardware design, resulting in enhanced flexibility. Additionally, by providing timing inputs to conventional laboratory stimulators, this instrument can convert these limited use devices into ones capable of generating complex cardiac stimulation patterns at moderate cost. A n a l y s i s of b o d y s u r f a c e a n d i n t r a c a r d i a c electrical recordings during paced cardiac r h y t h m s h a s s i g n i f i c a n t l y aided t h e u n d e r s t a n d i n g of n o r m a l a n d a b n o r m a l c l i n i c a l electrophysiology. 1 Fixed-rate atrial pacing p e r m i t s e v a l u a t i o n of p r o p e r t i e s of the sinoa t r i a l 2 a n d a t r i o v e n t r i c u l a r nodes, a a n d of the s u b j u n c t i o n a l c a r d i a c conduction s y s t e m ? T h e a d d i t i o n of a p p r o p r i a t e l y t i m e d e x t r a s t i m u l i allows d e t e r m i n a t i o n of r e f r a c t o r y periods of cardiac conducting tissues 5 and measurement of s i n o a t r i a l c o n d u c t i o n t i m e s . ~ B o t h f o r m s of s t u d y h a v e led to t h e c u r r e n t concepts of t h e p a t h o g e n e s i s of t a c h y a r r h y t h m i a s 7's includi n g t h o s e due to v e n t r i c u l a r p r e e x c i t a t i o n ? Single c h a n n e l , fixed r a t e cardiac s t i m u l a tion devices a r e r e a d i l y a v a i l a b l e . G e n e r a t i n g p r e c i s e l y t i m e d i m p u l s e s o n o n e to t h r e e c h a n n e l s , however, r e q u i r e s t h e d e s i g n a n d purchase of complex devices costing t h o u s a n d s of d o l l a r s to r e p l a c e m o r e ubiquitous s i m p l e cardiac p a c e m a k e r s or l a b o r a t o r y s t i m u l a t o r s . One a l t e r n a t i v e to t h i s replacem e n t , as described h e r e , is t h e d e v e l o p m e n t of a microprocessor controlled timing device c a p a b l e of c o n v e r t i n g a s t a n d a r d l a b o r a t o r y single or double c h a n n e l s t i m u l a t o r into a n i n s t r u m e n t c a p a b l e of precise a n d c o m p l e x cardiac stimulation.
From the Section of Medical Physics, Division of Circulatory Diseases, Department of Medicine, University of Tennessee, Center for the Health Sciences, Memphis, Tenn. 38163 This research was supported by grants HL-01362, HL09495 and HL-20597 and National Research Service Award HL-05323 from the National Heart, Lung and Blood Institute, and by a grant from the Deborah Heart and Lung Institute. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked " a d v e r t i s e m e n t ~' in accordance with 18 U.S.C.,w 1734 solely to indicate this fact. Reprint requests to: David M. Mirvis, M.D., University of Tennessee, Center for the Health Sciences, 951 Court Ave., Room 339M, Memphis, Tennessee 38163
MATERIALS AND METHODS Hardware: A schematized, block diagram of the hardware controller is presented in Fig. 1. An appropriately amplified single channel electrocardiogram (ECG) signal is first delivered to a previously described h a r d w a r e QRS trigger. 1~ The filtered, time derivative of the incoming signal is generated and is passed to a one-shot multivibrator which acts as the trigger detector. As depicted in Fig. 1, a polarity switch has replaced the originally described absolute value amplifier stage. The digital pluse generated when the multivibrator is tripped sets a flip-flop that is sensed by the microcomputer module. Additionally, the trigger signal may be viewed on a monitor oscilloscope (Fig. 2); the point on the QRS at which triggering occurs is indicated by the onset of a discontinuity in the ECG waveform. The m i c r o c o m p u t e r module selected was a KIM-1 microprocessor (MOS Technology, Norristown, PA). This sytem consists of an 8-bit microprocessor with 2048 bytes of read only memory (ROM), 1024 bytes of random access m e m o r y (RAM), a 1 MHz crystal timing oscillator and two programmable interval counters. Four types of input/output (I/O) operations are possible. First, a hexadecimal keypad and light emitting dipole (LED) display are used for operator interventions. Second, program input and/or data storage may utilize an audio cassette interface. Third, an asynchronous I/O interface is provided for connection to another computer terminal. Last, 15 I/O ports are available for user applications. Audio cassette connections, as well as interfaces to the QRS trigger and the stimulator itself, operate through these pins. A third element of the system, the Stimulator Control Interface, amplifies the output signals of the microprocessor to a level compatible with the stimulator being used. As configured here, voltages of greater than -+6 volts are needed to trigger a Grass S-88 (Grass Instruments, Quincy, Mass.) stimulator modified for clinical safety purposes only. Two individually controlled channels CS~ Synch" and "$2 Synch") are provided as external trigger or synchronizing inputs into one or two channels of any appropriate laboratory stimulator. Power to all units is provided by regulated supplies delivering 5.0 volts to the microprocessor, 12.0 volts to the audio cassette interface and 6.0 and 9.0 volts to the QRS trigger.
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Polordy J,qbqqer Pulse 1 Switch ~ F T ~ i
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STIMULATOR CONTROL INTERFACE
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Programming. Software development utilizes a specially written cross-assembler for the KIM-1, using a PDP-7 digital computer. This allows assembler language p r o g r a m m i n g of the microprocessor, eliminating the need for hexadecimal machine-language coding. The output of this Fortran program, i.e., a set of hexadecimal machine instructions, can be loaded directly into KIM-1 memory by connecting its asynchronous interface to a similar one on the laboratory computer.
To S2 SYNC To Sl SYNC
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Fig. 1. Block diagram of the microprocessor controlled timing device, described in detail in the text.
RESULTS A p p l i c a t i o n . One clinical a p p l i c a t i o n will be described illustrating the utility and versatility of t h e t i m i n g controller. T h i s p r o g r a m , des i g n e d to d e l i v e r p r e c i s e l y t i m e d e x t r a s t i m u l i d u r i n g t h e c a r d i a c cycle, h a s b e e n u s e d in o v e r fifty clinical s i t u a t i o n s to d e t e r m i n e ref r a c t o r y periods of t h e e l e m e n t s of t h e cardiac c o n d u c t i o n s y s t e m . 3'5
9
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Fig. 2. An example of the input to (lead III) and output from (TS) the hardware QRS trigger. The arrow indicates the discontinuity in the QRS complex, introduced by the trigger to delineate the moment of triggering. The patient studied here was in atrial fibrillation, with a variable ventricular rate. Regardless of this R-R variation, the trigger was tripped by identical portions of each QRS. J. ELECTROCARDIOLOGY, VOL. 12, NO. 1, 1979
COMPUTERIZED STIMULATOR
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Hardware Configuration. An ECG input is provided from a single ECG lead recording a predominantly monophasic positive or negative deflection. Because only single cardiac chamber stimulation will be required, both S~ and $2 <
J. ELECTROCARDIOLOGY, VOL. 12, NO. 1, 1979
]
85
Fig. 3. Use of the programmable stimulator to evaluate sinoatrial node function. A: Normal sinus r h y t h m at a cycle l e n g t h of 822 msec. Surface leads II, I and V1 and an intracardiac lead (HBE) are recorded. A, H and V identify low atrial, His bundle and ventricular deflection, respectively. B: Atrial pacing during the initial portion of the trace (cycle length 400 msec) terminates after 120 seconds at the broken line. Spontaneous atrial depolarizations resume after a 1208 msec delay.
The variables entered are: A. - - the rate of the basic driving stimulus, Sl, entered as the interval between impulses in msec. If 0 is entered, no d r i v i n g s t i m u l i are delivered, and spontaneously occurring QRS complexes are sensed; B. - - the number of cardiac cycles between introduced extrastimuli; C. - - the number of cycles to wait after an e x t r a s t i m u l u s before continuing $1 pacing or r e s t a r t i n g QRS counting. This delay permits observation of alt e r a t i o n s in s p o n t a n e o u s c a r d i a c r h y t h m induced by the p r e m a t u r e beat; D . - - t h e m a x i m u m coupling interval, in msec, between the basic $1 driving stimulus or the trigger point on the spontaneous QRS and the premature, or $2, pulse; E . - the m i n i m u m permissible coupling interval, in msec; F. - - the decrement, in msec, between successive $2 impulses. After variables are entered, they may be reviewed and altered if required. The program is then initiated upon command and proceeds without interruption or intervention to introduce e x t r a s t i m u l i beginning with the longest coupling i n t e r v a l and c o n t i n u i n g u n t i l t h e m i n i m u m c o u p l i n g i n t e r v a l is reached. Pacing is then automatically halted, with cessation of both $1 and $2 stimulus outputs. Parameters m a y then be modified and the program rerun. During execution, the program may be halted at any point by the operator.
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B
...~_ ~.~305bpm
] Fig. 4. Use of the programmable stimulator to convert paroxysmal supraventricular tachycardia to sinus rhythm. A: The stimulator was programmed to deliver a single 10 second burst of pulses at a rate of 260 beats per minute (BPM) as leads I, II and V1 were monitored. After cessation of pacing, ectopic P-waves (p1) of the tachycardia continued. B: An increase of pacing rate to 305 beats per minute likewise failed to convert the tachycardia. C: Tachycardia was convereted to sinus rhythm with normal P-waves (P) after pacing at 385 beats per minute. Durations and rates of pacing were determined by software programming.
II V1
A l t h o u g h i n t e n d e d to p r o v i d e s i n g l e chamber stimulation for determination of refractory periods, appropriate adjustment of variables m a y permit other operations. For example, entry of a high value for variable B, a narrow range of permissible coupling intervals (variables D and E) t h a t approximate variable A, a low decrement value (variable F) and a zero for variable C, results in carefully defined but prolonged fixed rate cardiac stimulation, at a frequency defined by variable A, such as m a y be needed for sinus node t e s t i n g , 2 (Fig. 3) or c o n v e r s i o n of a t r i a l tachycardias (Fig. 4). A l t e r n a t i v e l y , the s t i m u l a t o r h a r d w a r e c o n f i g u r a t i o n m a y be a l t e r e d w i t h o u t software modification. If, for example, the $1 and $2 synch outputs from the microcomputer device are directed to two stimulator output channels and two intracardiac catheters, one chamber (e.g.,the right atrium) m a y be driven at a fixed rate ($1) while the premature impulses ($2) are used to stimulate a second chamber (e.g., the right ventricle).
DISCUSSION The described system provides a moderately priced instrument capable of providing
complex, precisely timed cardiac stimulation patterns under computer control. Components, i n c l u d i n g microprocessor, power supplies, audio tape deck and components of the QRS trigger are commercially available for under $500. Thus, one advantage of the system is its reasonable cost. Second, and more significant, is the degree of flexibility in stimulation sequences possible by s o f t w a r e u t i l i z a t i o n w i t h r e d u c e d hardware-related limitations. For example, a single channel stimulator may be used to del i v e r c o m p l e x t r a i n s of v a r i a b l y s p a c e d stimuli through a single intracardiac catheter. Conventionally, separate but interfaced stimulator channels are required to deliver driving (S1) pulses and each type of premature stimulus ($2, $3, etc.). Addition of other stimulator channels to this sytem is required only for multiple chamber stimulation, as for study of certain cardiac dysrhythmias? Development of software was simplified by the PDP-7 cross a s s e m b l e r , described above, which obviated the need for programming in hexadecimal machine code. This further enhances the flexibility of the total system by m a k i n g it relatively easy to rewrite and/or modify the control program. J. ELECTROCARDIOLOGY, VOL. 12, NO. 1, 1979
COMPUTERIZED STIMULATOR
E x p a n s i o n s and o t h e r applications of t h e described device are probable. Clearly, s t i m u l a t i o n p a t t e r n s o t h e r t h a n the one detailed are feasible, including complex dual c h a m b e r p r e m a t u r e s t i m u l a t i o n , s t i m u l a t i o n by t r a i n s of p r e m a t u r e beats, a n d d e f i b r i l l a t i o n synchronization. U n u s e d I/O ports m a y be used to control a n a l o g tape drives, or to provide t i m e a n d e v e n t code s i g n a l s f o r r e c o r d i n g o n m a g n e t i c a n a l o g tape or p a p e r records simult a n e o u s l y w i t h ECGs.
5.
6.
7. REFERENCES 1. NARULA,O S: His Bundle Electrocardiography and Clinical Electrophysiology. F.A. Davis, Philadelphia, 1975 2. MANDEL, W, HYAKAWA, H, DANZIG, R and MARCUS, H S: Evaluation of sinoatrial node function in man by overdrive suppression. Circulation 44:59, 1971 3. WIT,A L, WEISS,M B, BERKOWITZ,W D, ROSEN, K M, STINER,C and DAMATO,A N: Patterns of atrioventricular conduction in the h u m a n heart. Circ Res 27:345, 1970 4. NARULA,O S, COHEN,L S, SAMET,P, LISTER,J W, SCHERLAG,B and HILDNER,F J: Localization
J. ELECTROCARDIOLOGY, VOL. 12, NO. 1, 1979
8. 9.
10.
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of A-V conduction defects in man by recording of the His bundle electrogram. Am J Cardiol 25:228, 1970 DENES,P, Wu, D, DHINGRA,R, PIETRAS,R J and ROSEN, K M: The effects of cycle length upon refractory periods in man. Circulation 49:32, 1974 STRAUSS,H C, SAROFF,A L, BIGGER,J T and GIARDINA,E G V: Premature atrial stimulation as a key to the understanding of sinoatrial conduction in man. Circulation 47:86, 1973 DENES, P, Wu, D, DHINGRA,R C, CHUQUIMIA,R and ROSEN, K M: Demonstration of dual A-V nodal pathways in patients with paroxysmal s u p r a v e n t r i c u l a r tachycardia. Circulation 48:549, 1974 JOSEPHSON,M E and KASTOP~J A: Paroxysmal supraventricular tachycardia. Is the atrium a necessary link? Circulation 54:430, 1976 WELLENS,H J J: Electrical Stimulation of the H e a r t in t h e S t u d y and T r e a t m e n t of Tachycardias. University Park Press, Baltimore, 1971 BRANDON,C W and BRODY D A: A hardware trigger for temporal indexing of the electrocardiographic signal. Comp and Biomed Res 3:47, 1970