Simultaneous bipolar pacing and sensing during EPs

Simultaneous Bipolar Pacing and Sensing During EPs J. S n o e c k , M D , M . B e r k h o f , E n g , a n d C. Vfints, M D

Abstract: Bipolar pacing and sensing-recording on the same electrode is a

problem during pacemaker implantation as well as during electrophysiological studies. A method is described that makes this possible. By using bipolar simultaneous sensing-pacing on the same electrode, the system can record local repolarization after a depolarization controlled by stimulation. The bipolarpaced evoked potential is useful as an indicator to drive a bipolar QT rate responsive pacemaker. Even more, during EPS it can be used to detect capture during tachycardia stimulation and to measure the exact refractory period. Key words: paced evoked response, programmed stimulation, stim-T interval, impedance, bipolar QT-rate responsive pacemaker.

Optimal cathodal pacing requires a small distal electrode surface area, while optimal sensing requires the opposite. It is therefore obvious that sensing and pacing are conflicting functions w h e n they are to be performed simultaneously on a bipolar lead. 4 Therefore, we describe a m e t h o d that makes it possible to stimulate and to record on a bipolar lead during pacemaker implantation and electrophysiological testing. We analyzed the problems and developed microcomputer-controlled electronic circuits to minimize these problems.

Pacing and sensing-recording are two different functions that can easily be accomplished by a quadripolar electrode.l'2 During pacemaker and electrophysiological studies, commonly a temporary quadripolar electrode is used, with the two distal electrodes used for pacing and the two proximal electrodes for sensing and recording. This is for both the atrial and ventricular electrodes. The advantage is that pacing and sensing do not interfere with each other, due to the separation of their electrode contacts. An optimal quadripolar electrode should be one, with a small distal electrode area for pacing and great proximal electrode areas for sensing and recording. 3 A disadvantage is that the recording place differs from the stimulation place: the ECG waveforms sensed on the proximal leads, having the smallest amplitudes, are used for recording. The distal leads, having the highest amplitudes, are used for bipolar stimulation. Furthermore, there is a time difference between the pacing and sensing place in detecting intracardiac signals and therefore tachycardias as well.

Sensing and Pacing Contradictions The bipolar lead must conduct electrical pulses from the pacemaker or stimulator to the heart, and vice versa, and both functions must be performed by the same lead. This requires good sensing and efficient pacing possibilities. The energy losses that occur during both functions must be taken into consideration. 5 Figure 1 shows a closed electrical loop during pac-

From University Hospital Antwerp, Belgium.

Reprint requests: Dr. Joseph Snoeck, University Hospital Antwerp, Wilrijkstraat, 10, Edegem 2520, Belgium.

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ing. The available voltage across the myocardial tissue is smaller than the battery voltage of the pacemaker-stimulator due to several losses, such as the pacing polarization impedance, called Zpp; the lead resistance, called RL (= Rlprox + Rldist); and the output circuit of the stimulator, called Zout. Optimal pacing is performed if these losses are kept to a minimum. Optimal sensing is performed if the available voltage developed by the heart can reach the sensing amplifier of the stimulator (Fig. 2). These bipolar intracardiac ECG amplitudes are attenuated due to losses such as: the tissue resistance, called RT; the sensing polarization impedance, called Zps; the lead resistance, called RL (= Rlprox + Rldist); and the input impedance of the sensing unit, called Zin. Optimal sensing is performed when these losses are kept to minimum.

Polarization at the ElectrodeHeart Interface During cathodal stimulation (connecting the negative pole of the stimulator to the distal tip), corn-

monly a 5-V-0.75-msec impulse is delivered. After the onset of the pacemaker spike, positively charged ions accumulate at the electrode-heart interface. The positively charged ions, which are surrounded by water molecules, move to the negatively charged electrode, which are also surrounded by water molecules. The effect of this ion transport is that a double layer is formed, with charges separated by water molecules, and thus a Helmholtz capacitance is formed. A minimum of 50 ~F/cm2 can be produced. 6 During pacing at high current densities, oxydoreduction reactions create an ohmic pathway, called the Faraday resistance. The Helmholtz capacitor accumulates charge and voltage at the electrode-heart interface. This useless voltage, called the polarization voltage, is opposite to the polarity of the battery in the stimulator, resulting in a reduced current during stimulation. The greater the surface area of the electrode tip, the smaller the polarization voltage. 7 This polarization voltage blocks the intracardiac ECG amplifier for sensing and registration for approximately 100-200 msec. After this period, the pacing polarization voltage decay lowers the voltage generated by the patient's heart. Polarization is the greatest energy loss during pacing. Other losses, such as lead resistance (normally

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Rd ~-Rp

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lO-lO0 ~), stimulator pacing output impedance (normally negligible), and, during sensing, input impedance of the stimuIator (ideal infinite resistance), are definitely smaller but can influence all in their own way, the sensing, pacing, and the poststimulus potential.

Material and Methods Activated Carbon Electrode (ELA T84F) The ideal electrode for bipolar cardiac pacing requires low threshold values as well as the ability to sense small intracardiac signals. To provide these characteristics, the electrode-stimulator circuit must have good biocompatibility--the ability to reduce the thickness of fibrotic tissue and polarization losses to improve charge transfer at the electrode-heart interface. A bipolar carbon electrode, in combination with a low output impedance/high input impedance stimulator has optimal biocompatibility. Polarization losses are inversely related to surface a r e a : The activated carbon tip (ELA T84F) electrode, used in this

study, has an increased surface area due to a high microporosity as a result of a carefully adjusted oxidation process during manufacturing. This activation process reduces the transfer impedance between the electrode and the electrolyte. Polarization losses were measured in vitro by stimulating on a carbon tip electrode using a bath of physiological saline. Tissue resistance of the patient was stimulated by a 300 fl resistor in series. A 5-V pulse of 0.75 msec duration was applied on the electrode and the afterpotential voltage was measured. Ten measurements were performed, The m a x i m u m amplitude of the decaying voltage was 160 mV (p = 0.002). The final part of the waveform is a quasiexponential decay over a long period of time. This is part of the stimulus artifact, and in a paced patient the evoked response is superimposed upon this much larger voltage. The greater the impulse duration, the greater the polarization voltage. 6 Our first conclusion is that to ensure bipolar sensing and pacing during pacemaker implantation or electrophysiological testing, one should use carbon tip electrodes with a polarization voltage that is reduced due to their electrode material, geometry, macrolow density, and microhigh density surface area.

Simultaneous Bipolar Pacing and Sensing

Sense-Pace Electronic Circuit Adaptations

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The entire system was first tested with hardware electronics. Then, a microprocessor-based circuit was developed to perform on-line computerized measurements. In summary, a conventional stimulator requires the following:

In a conventional pacing device used during electrophysiological testing, the output capacitor is recharged by a very large (25 k~) resistor (which means very slow recharging). Using this type of stimulator, high polarization voltages are generated. 9 Therefore, the output stage of the programmable stimulator is changed into a fast reload circuit. A preprogrammed voltage with amplitude and decay curve approximately the same as that of the stimulation afterpotential, but opposite in polarity, is generated. Using this analog subtraction technique, the sensed voltage becomes within the range of the amplifier input. ~~ To shorten the amplifier saturation time, two zener diodes are used to clamping the input voltage. An adjusted bipolar T wave filter, with breakpoints of 0.1 H and 10 H, 11 and an amplifier were added to the system. Finally, the input amplifier is short-circuited during the delivery of a pacemaker impulse.

1. A fast reload circuit 2. An electronic analog subtraction unit 3. Clamping of the input range of the sensing amplifier 4. A bipolar T wave filter (0.1-9.9 Hz) and highgain amplifier 5. Short-circuiting of the input amplifier during the impulse duration 6. Or the use of a software controlled pacing system. A microcomputer-based software stimulator is designed to store pacemaker and electrophysiological stimulation protocols and to perform automatically the first five adaptations for optimal bipolar pacing and sensing. All of the programs run on a IBM-compatible computer with several input-output and communication boards. There are two in-

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tracardiac sensing interfaces for input and two isolated amplifiers for output and one analog-digital board for intracardiac ECG and impedance measurement. Further, a printer for protocol and a screen for information display during the investigation is used,

msec), then the S3 is not captured with an $2-$3 interval of 260 msec (Fig. 4). This intracardiac measurement is the exact measurement of the refractory period of $2, because the positions for pacing and sensing are the same. The principle of bipolar paced evoked response measurement is useful not only during electrophysiological testing but even more so with implantable antitachycardia pacemakers,

Paced Evoked Response (PER) Recordings During P a c e m a k e r Implantation and EPs

Clinical Use of the Bipolar Paced Evoked Response Recordings

Recordings of the bipolar paced evoked response were made during sequential programmed stimulation with a constant S1-$2 interval of 500 msec and a decreasing S2-$3 interval (Fig. 3). During this sequence the S2-$3 is 330 msec and the PER of S3 is recorded (capture). If sequential programmed stimuiation with a decreasing $2-$3 interval is continued (decrement, 10

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Unipolar evoked T wave detection has been proven useful by implantable unipolar QT driven r a t e - r e s p o n s i v e p a c e m a k e r s . H o w e v e r , bipolar evoked T waves permit the development of a bipolar QT pacemaker. The bipolar PER signal can also be used for detection of capture during stimulation with extra stimuli 12,~s; detection of stimulation threshold,

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Fig, 4, Same recording as in Figure 3, There is a further decrease of the $2-$3 interval of 10 msec until the refractory period of $3 is reached at an $2-$3 interval of 260 msec.

Simultaneous Bipolar Pacing and Sensing resulting in a longer battery lifespan of implanted devices; measurement of the refractory period12"13; automatic discrimination between the QT interval at high rates during exercise and the QT inte~a~-morphology at the same rates during pathological tachycardia.Z4

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Conclusions Bipolar evoked potentials can be reliably measured if one uses a fast reload circuit, an activated carbon tip electrode, a microcomputer-stimulator with low output, and high-input impedance and electronically corrected amplifiers for optimal recording of the PER signal. It is almost impossible to record the bipolar PER if any of these is lacking. These bipolar paced evoked potentials will certainly play an important role in the development of future algorithms for antitachycardia pacing. The bipolar PER allows accurate measurement of the ventricular refractory period, which in turn allows safer and more efficient tachycardia termination sequences.

References 1. Fontaine G, Frank R, Aldakar M: The electrode-biointerface: pacing. In Barold S, Mugica J (eds): New perspectives in cardiac pacing. Futura, Mt. Kisco, NY, 1988 2. Adams T: The electrode-biointerface: sensing. In Bar-

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old S, Mugica J (eds): New perspectives in cardiac pacing. Futura, Mt. Kisco, NY, 1988 Ripart A, Mugica J: Electroce-heart interface: deftnition of the ideal electrode. PACE 6:410, 1983 Ohm OJ: The interdependence between ECG, total electrode impedance and pacemaker input impedance. PACE 2:465, 1979 Imich W: Intracardiac ECG's and sensing test signals. PACE 8:870, 1985 Walton C, Gergely S, Economides A: Platinium pacemaker electrodes: origins and effect of the electrodetissue interface impedance. PACE 10:87, 1987 Pioger G, Ripart A: Clinical results of low energy unipolar or bipolar activated carbon tip leads. PACE 9:1243, 1986 Thull R, SchaldachM: Electrochemistry of after pacing potentials on electrodes. PACE 2:1191, 1986 Edhac O, Lagergren H, Thoren A, Wahlberg I: Influence of output capacitor, electrode and pulse width on power consumption in cardiac pacing. PACE 1:16, 1978 Ohm OJ: Inhibition/filter characteristics and input impedance of QRS inhibited demand pacemakers. PACE 3:318, 1980 Kleinert M, Elmqvist H, Strandberg H: Spectral properties of atrial and ventricular endocardial signals. PACE 2:11, 1980 Edvardson N, Hirsch I, Olssen B: Right ventricular monophasic action potential in healthy young man. PACE 7:813, 1984 Boute W, Cals G, den Heijer P, Wittkampf FHM: Morphology of endocardial T-waves of fusion beats. PACE 11:1693, 1988 Begeman M, Boute W, Wittkampf FHM: Evoked endocardial potentials in tachycardia management. PACE 10:608, 1987 Donaldson R, Rickards AF: The ventricular endocardial paced evoked response. PACE 6:253, 1983