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TECHNIQUES AND TECHNOLOGYEditor: Hugh Calkins, M.D.

Recording Techniques for Clinical ElectrophysiologyWILLIAM G. STEVENSON, M.D., and KYOKO SOEJIMA, M.D.∗

From the Brigham and Women’s Hospital Boston, Massachusetts, USA; and ∗Keio University, Tokyo, Japan

Clinical Electrophysiology. The precise techniques employed in the electrophysiology laboratoryinfluence the nature of the electrograms that are recorded during mapping procedures. Unipolar recordingsthat are minimally filtered can be useful for mapping focal arrhythmia sources, but have substantial far-field signal that can obscure low-amplitude signals of interest in abnormal regions. Bipolar recordings arestandard in most laboratories because rejection of far-field signal facilitates identification of local potentialsin abnormal areas, but the signal of interest can be beneath either recording electrode and far-field signalsdo occur. Simultaneously obtained unipolar recordings are a useful adjunct to bipolar recordings in somesituations. High pass filtering and digital sampling also influence electrogram characteristics. High passfiltering of unipolar recordings can be useful to reduce far-field components, but limits inferences fromelectrogram morphology. (J Cardiovasc Electrophysiol, Vol. 16, pp. 1017-1022, September 2005)

mapping, ablation, conduction block

Introduction

Interpretation of recorded electrograms is fundamental tothe clinical investigation of arrhythmias during electrophys-iologic studies. We learn to recognize and interpret complexpatterns in recordings from different cardiac locations duringdifferent rhythms to diagnose, locate, and ablate a variety ofarrhythmias. A basic knowledge of the fundamental aspectsof recording methods, and their strengths and limitations fa-cilitates electrogram interpretation and is still leading to newand useful methods, such as those recently described for de-tecting conduction block during ablation of atrial flutter, andfor recognizing far-field signals that complicate identifica-tion of arrhythmia sources. When ablation targets a discreteregion with relatively normal surrounding tissue, such as id-iopathic focal tachycardias and accessory pathways, we rou-tinely obtain bipolar, unipolar high pass filtered at 0.5 Hz,and unipolar high pass filtered at 30 Hz recordings from thedistal electrodes of the mapping catheter as well as bipolarrecordings from proximal and middle electrode pairs. Whenmapping arrhythmias that are associated with myocardialscar, such as macroreentrant atrial tachycardia after atrialsurgery, or postinfarction ventricular tachycardia, we omitthe minimally filtered (high pass at 0.5 Hz) unipolar sig-nals. Our rationale for these methods is the subject of thisreview.

J Cardiovasc Electrophysiol, Vol. 16. pp. 1017-1022, September 2005.

Address for correspondence: William G. Stevenson, M.D., Cardiovascu-lar Division, Brigham and Women’s Hospital, 75 Francis St, Boston,MA 02115. Fax: 617-277-4981; E-mail: wstevenson@partners.org

doi: 10.1111/j.1540-8167.2005.50155.x

Unipolar Recordings

Cardiac electrograms are generated by the potential (volt-age) differences recorded at two recording electrodes duringthe cardiac cycle. All clinical electrogram recordings are dif-ferential recordings from one source that is connected to theanodal (positive) input of the recording amplifier and a sec-ond source that is connected to the cathodal (negative) input.Unipolar recordings are obtained by positioning the explor-ing electrode in the heart and the second electrode (referredto as an indifferent electrode) distant (theoretically an infinitedistance) from the heart such that it has little or no cardiacsignal. The precordial ECG leads are unipolar recordings thatuse an indifferent electrode (Wilson central terminal), createdby connecting the arm and left leg electrodes through highimpedance resistors (Fig. 1, bottom). Most clinical record-ing systems have the option for selecting a unipolar record-ing mode that connects the negative input of the recordingamplifier to the Wilson central terminal. In some laborato-ries utilizing the Wilson central terminal introduces substan-tial electrical noise. We routinely use an alternative to theWilson central terminal to reduce electrical noise. The ampli-fier is left in the bipolar configuration, but the negative inputis connected to an intravascular electrode that is remote fromthe heart, in the inferior vena cava.1 This remote electrode,commonly referred to as an “indifferent” electrode, is on acustom hexapolar catheter that has the two most proximalelectrodes (electrodes 5 and 6) located 15–20 cm proximalto the distal tip electrode. With the distal electrodes of thiscatheter positioned at the His bundle position, electrodes 5and 6 are in the inferior vena cava; one serves as the indifferentelectrode for unipolar recording, the other as an indifferentelectrode for unipolar pacing when desired.

The genesis of a unipolar recording is illustrated schemat-ically in Figure 1. By convention the exploring electrode thatis in contact with the myocardium is connected to the pos-itive input of the recording amplifier. In this configuration

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Figure 1. The basic generation of unipolar and bipolar recordings is shown. At the top of panels A and B are horizontal bars that represent a sheet ofmyocardium with depolarization propagating from left to right (arrow). The potential in front of the wavefront is positive and that behind the wavefront isnegative, generating a vector that points toward the positive potential. Electrodes resting on the tissue are shown as grey circles. Unipolar recording is shownin the top right-hand figure. Theoretical electrograms are shown in boxes. From the top schematic, as the wavefront propagates toward the electrode a positivedeflection, an R wave, is inscribed. As the wavefront propagates past the recording electrode, an S wave is inscribed and thus an RS complex occurs (middleschematic). Recording from the initial site of depolarization (origin at the left side of the tissue) produces a QS complex as the wavefront moves away fromthe recording electrode. Recording at the right side of the tissue (dead end) produces a monophasic R wave. Panel B shows bipolar recording. Electrode 1is connected to the positive input of the amplifier and electrode 2 is connected to the negative input. Electrograms created by a mathematical simulation areshown below the schematic. Compared to the signal from electrode 1 (Uni-1), the signal from electrode 2 (Uni-2) is slightly delayed (because the wavefrontreaches it later) and is inverted because it is attached to the negative input of the recording amplifier. Adding these two signals together generates the bipolarsignal (Bi 1–2) that removes much of the far-field signal. High pass filtering is simulated by numerically differentiating the signal with respect to time.Differentiating the Uni-1 signal decreases the far-field component and produces a signal quite similar to the bipolar signal but slightly shifted with respect totime. Differentiating the bipolar signal produces additional deflections and further complicates the signal. Indifferent electrode configurations for unipolarrecordings are shown in panel C.

a wavefront of depolarization that is propagating towardthe exploring electrode generates a positive deflection. Asthe wavefront reaches the electrode and propagates away, thedeflection sweeps steeply negative. Thus an R/S complex isgenerated. In a sheet of uniformly conducting tissue, the max-imum negative slope (−dV/dt) of the signal coincides withthe arrival of the depolarization wavefront directly beneaththe electrode.

Unipolar Recordings for Identifying the Site of EarliestActivation

The morphology of the unipolar recording indicates thedirection of wavefront propagation, provided that the cor-ner frequency for the high pass filter is kept low; we usuallyuse 0.5 Hz (see below). When the exploring electrode is lo-cated at the site of initial activation (e.g., the left-hand sideof the tissue in Fig. 1B), depolarization produces a wavefrontthat spreads away from the electrode generating a monopha-sic QS-complex (Figs. 1 and 2).1,2 During mapping of focal

arrhythmias, such as idiopathic RV outflow tract tachycar-dia and focal atrial tachycardia, a QS complex is typicallyrecorded at the successful ablation site. However, the sizeof the area with a QS complex can be larger than the fo-cus, exceeding a centimeter or more in diameter. Thus, aQS complex should not be the only mapping finding used toguide ablation. Successful ablation is unusual, however, atsites with an RS complex, as these are generally distant fromthe focus. It is also important to recognize that a QS com-plex may be recorded when the exploring electrode is not incontact with the myocardium, but is floating in the cavity.In that situation, the initial negative slope of the recording istypically slow, suggesting that the electrogram is a far-fieldsignal, generated by tissue some distance from the recordingelectrode.

For mapping focal tachycardias, we seek an area that has aQS complex with good electrode contact, often indicated bythe ability to capture the site with unipolar pacing. We thenattempt to hone in more precisely on the arrhythmia originby seeking the point of earliest activation assessed from the

Stevenson and Soejima Techniques and Technology 1019

Figure 2. Recordings from a remote site and at the successful ablation site during a focal ventricular tachycardia are shown. From the top are surface ECGleads followed by recordings from the mapping catheter: bipolar recordings from electrodes 1–2 (Bi 1–2), unipolar recording from electrode 1 (Uni-1) andelectrode 2 (Uni-2) that are minimally filtered (high pass filtering ≤0.5 Hz), and high pass filtered (30 Hz) unipolar recordings from these electrodes, Uni-1HP and Uni-2 HP. At the successful site (left-hand panel), the Uni-1 signal has a QS complex and the initial peak of the bipolar signal 1–2 is coincident withthe beginning of the downstroke of the unipolar signal. The dashed vertical line immediately precedes the onset of the electrogram to assist in comparison. Atthe remote site (right-hand panel), the Uni-1 signal has an RS configuration and the initial peak of the bipolar 1–2 signal follows the onset of the Uni-1 signal.The high pass filtered unipolar signals have substantially less far-field signal (s wave) compared to the minimally filtered unipolar signals and resemble thebipolar signal. At the successful ablation site Uni-2 has a slower downstroke and little deflection is evident when it is high pass filtered indicating that it isprobably not in contact with the myocardium.

bipolar signal (see below) as well as the rapid negative de-flection of the unipolar signal. The timing reference (fiducialpoint) for these measurements is a clearly defined point nearthe onset of the QRS complex (for VT) or p wave (for atrialtachycardia) or a stable intracardiac electrogram.

We also look at the unipolar electrogram morphologywhen mapping accessory pathways. At the atrial insertion

Figure 3. Recordings from a reentrant ventricular tachycardia illustratinghow a low-amplitude isolated potential evident in bipolar recordings can belargely obscured by minimally filtered unipolar recordings. From the top ofthe tracing are surface ECG leads followed by intracardiac recordings fromthe mapping catheter labeled as in Figure 2. Note that the gain is the same forall intracardiac signals. An isolated potential (arrows) is present precedingthe QRS onset in the bipolar recording but is not as easily appreciated inthe unipolar recording. In the high pass filtered unipolar recordings, thepotential is evident at electrode 1, but not at electrode 2.

site of an accessory pathway, the atrial electrogram has a QScomplex during ventriculoatrial conduction. During antero-grade (A-V) conduction, the ventricular electrogram at theventricular insertion has a QS configuration.3

The major disadvantage of unipolar recordings is that theycontain substantial far-field signal generated by depolariza-tion of tissue remote from the recording electrode. In normaltissue the maximum negative slope is a good indication of lo-cal depolarization. In abnormal regions, such as infarct scars,the tissue beneath the recording electrode may be small rela-tive to the surrounding myocardium outside the scar. A largefar-field signal can obscure the small local potential (Fig. 3).Therefore, we do not usually evaluate unipolar recordingswhen mapping arrhythmias associated with regions of scar,unless they are filtered to remove far-field signal (see below).

Bipolar Recordings

Bipolar recordings are obtained by connecting two elec-trodes that are exploring the area of interest to the recordingamplifier (Fig. 1, right-hand panel).2 At each point in timethe potential generated is the sum of the potential from thepositive input and the potential at the negative input. The po-tential at the negative input is inverted, and thus subtractedfrom that at the positive input. Because the far-field signalis similar at each instant in time, it is largely subtracted out,leaving the local signal. In a homogeneous sheet of tissue, theinitial peak of the bipolar signal coincides with depolarizationbeneath the recording electrode.

Bipolar recordings are used to locate focal arrhythmias byidentifying the point of earliest activation relative to a sta-ble reference. Reducing the amplitude of the far-field signalfacilitates identification of local depolarization in abnormal

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Figure 4. Entrainment during scar-related VT demonstrating far-field po-tentials is shown. From the top are surface ECG leads and bipolar recordingsfrom the ablation catheter electrodes 1–2, 2–3, and 3–4. The last three stim-uli at a cycle length of 520 msec (unipolar pacing train from electrode 1 ofthe mapping catheter) is shown. All electrograms and QRS complexes areaccelerated to the pacing rate and VT resumes at a cycle length of 540 msecafter pacing. During pacing the recording at electrodes 1–2 is obscured bythe stimulus artifact. At least two potentials are seen in the 2–3 and 3–4recordings that are not generated by tissue directly captured by the pacingstimulus (brackets) and are therefore far-field potentials.5 Following pacing,a lower amplitude signal (indicated by the star) that was obscured duringpacing is evident on 2–3 and on 1–2 after the signal emerges at the rightside of the tracing. This likely represents local activation and the postpacinginterval (arrow 540 msec) is measured to that signal. The schematic at thebottom illustrates a possible relation of the mapping catheter to the reentrycircuit. All times are in msec.

areas of infarction or scar in bipolar recordings (Fig. 3). Bipo-lar recordings are preferred for catheter mapping in humansand particularly for scar-related arrhythmias.

The precision of locating the source of a particular electri-cal signal depends on the distance between the recording elec-trodes. For ablation procedures recordings between adjacentelectrode pairs are commonly employed (e.g., between elec-trodes 1–2, 2–3, and 3–4), with 1–5 mm between electrodes(the interelectrode spacing). In some studies wider bipolarrecordings (e.g., between electrodes 1–3 and 2–4) have beenused to provide an overlapping “field of view.” With bipolarrecordings the signal of interest may be beneath either thedistal or proximal electrode (or both) of the recording pair(Figs. 3 and 4). This is a relevant consideration for catheterablation that is performed by applying RF energy only at thedistal electrode, because the tissue generating the signal atthe proximal electrode is less likely to be damaged. With anarrow interelectrode distance this issue is less of a concern.We record bipolar signals from the distal (electrodes 1–2),mid (electrodes 2–3), and proximal (electrodes 3–4) pairsduring almost all mapping procedures (Fig. 4). Comparisonof the activation times among these three pairs provides anindication of the wavefront direction. If we are seeking anarea of early activation, and activation is earlier at the prox-imal electrodes than at the distal electrodes, we move thecatheter such that the distal electrodes approach the locationof the proximal electrodes until the activation is earliest atthe distal electrodes, from which ablation is performed.

In contrast to unipolar recordings, the amplitude of a bipo-lar electrogram is influenced by the direction of wavefrontpropagation.2 A wavefront that is propagating in the direc-tion exactly perpendicular to the axis of the recording dipoleproduces no difference in potential between the electrodes,and hence no signal. Theoretically, it would seem that thiscould have a major impact on “voltage maps” that displayareas of infarction and scar as low-amplitude regions. Withpresent methods, recording from ablation catheters with rel-atively narrow interelectrode spacing, the direction of wave-front propagation has a limited effect on electrogram ampli-tude, at least in abnormal ventricles. The impact of recordingbipole orientation on electrogram amplitude may be reducedby nonuniform, anisotropic conduction, particularly in low-voltage areas. Whether the different size and configurationof mapping catheter electrodes (large tip and smaller prox-imal ring electrodes) also tend to exaggerate the potentialdifference between the two recording sites is not clear.

In contrast to unipolar signals, the direction of wavefrontpropagation cannot be reliably inferred from the morphologyof the bipolar signal. A change in morphology, can however,be a useful finding. For example, when recording from the lat-eral aspect of the Eustachian isthmus during pacing from thecoronary sinus, the direction of activation changes when ab-lation produces conduction block in the isthmus.4 The bipo-lar signals typically invert, when the sequence of activationreverses in this situation (Fig. 5). Similarly, if bipolar record-ings are obtained with the same catheter orientation parallelto the AV annulus during retrograde accessory pathway con-duction, an RS configuration electrogram will be present onone side of the accessory pathway (where the wave front ispropagating from the distal electrode toward the proximalelectrode and a QR morphology electrogram on the otherside, where the wavefront is propagating from the proximalelectrode toward the distal electrode.

Simultaneous Unipolar and Bipolar Recordings

We exploit the differences in unipolar and bipolar record-ings to assist in mapping by simultaneously recording bipolarand unipolar signals from the mapping catheter (Fig. 2).1 Be-cause the unipolar signal contains substantial far-field signal,sites that are remote from earliest activation are easily recog-nized as having an initial R wave in the unipolar recording.Furthermore, the onset of the unipolar R wave precedes thefirst peak of the bipolar electrogram (Fig. 2, right-hand panel).In contrast, at the earliest site of activation, the unipolar sig-nal has a QS configuration with the rapid downslope of theS wave coinciding with the initial peak of the bipolar signal(Fig. 2, left-hand panel).

High Pass Filtering

Placing a metal platinum electrode into an electrolyte so-lution (the blood stream) generates an electrical potential.This potential is obscured by automatically centering therecording on the screen and filtering. However, if intracar-diac recordings were not filtered the signal would wanderup and down as this potential fluctuated with respiration,catheter movement and variable catheter contact. These low-frequency oscillations are removed by high pass filtering thatattenuates the frequencies that are slower than the specifiedcut-off (corner frequency) of the filter. Filtering can change

Stevenson and Soejima Techniques and Technology 1021

Figure 5. Demonstraction of conduction block in the Eustachian isthmus during RF ablation. From the top are surface ECG leads followed by bipolarintracardiac recordings from the ablation catheter in the Eustachian isthmus (Abl 1-2 and Abl 3-4) and selected recordings from a 20-pole catheter positionedwith electrodes 19–20 at the mid-lateral right atrium, with preceding electrode pairs progressively 1 cm closer to the coronary sinus such that electrodes13–14 are at the rightward aspect of the Eustachian isthmus adjacent to the ablation line. Pacing is performed from the coronary sinus. The first paced beatshown propagates through the Eustachian isthmus and up the lateral wall. The second paced beat blocks in the Eustachian isthmus and conducts down thelateral wall. Note that the bipolar electrogram morphology adjacent to the ablation line, lateral to the block inverts as is more easily appreciated in theenlarged insets of recordings from RA 15–16 and 13–14 preblock and with block.

the electrogram amplitude, timing, and morphology. Unipo-lar signals are commonly filtered at 0.05–0.5 Hz to removebaseline drift. Filtering at higher corner frequencies (e.g.,30 Hz) alters the morphology of the signal, such that themorphology of the unipolar signal is no longer an indicationof the direction of wavefront propagation and the presence orabsence of a QS complex cannot be used to infer proximityto the site of earliest activation.

High pass filters with corner frequencies anywhere be-tween 10 Hz and 50 Hz are common depending on the labo-ratory preference. There are many different types of filters. Ingeneral, high pass filtering can be viewed as differentiatingthe signal, such that the height of the signal is proportionalto the rate of change of the signal rather than only the am-plitude. The effect of differentiating signals is shown in theright-hand panel of Figure 1. The bipolar signal becomesmore complex and additional peaks are introduced. Althoughfiltering distorts the signal and reduces its apparent ampli-tude, we have learned how to interpret these signals, as thishas been the standard practice in many laboratories.

High Pass Filtered Unipolar Recordings

High pass filtering a unipolar signal changes the electro-gram morphology such it is no longer a reliable indication ofthe direction of wavefront propagation and an RS complexno longer indicates that a site is remote from an arrhythmiafocus. We routinely record high pass filtered unipolar signalsfrom the distal two electrodes of the mapping catheter whenmapping scar-related arrhythmias (Figs. 2 and 3). Much ofthe far-field signal in a unipolar recording is comprised oflower frequencies than the signal generated by local depo-

larization because the high-frequency content of a signal di-minishes more rapidly with distance from the source thanthe low-frequency content. Therefore, high pass filtering aunipolar signal (at 30 or 100 Hz) reduces the far-field signaland can improve detection of lower amplitude local signalsfrom abnormal regions (Figs. 2 and 3).

Low Pass Filtering and Band Pass Filtering

Low pass filters attenuate frequencies that are slower thanthe specified corner frequency (commonly 250–500 Hz). Thisis useful for reducing high-frequency noise and at these fre-quencies, does not substantially affect electrograms recordedwith clinical systems. Defining a band of frequencies torecord, such as setting the high pass filter to 30 Hz and thelow pass filter to 250 Hz defines a band of frequencies from30 to 250 Hz that are not attenuated. This is referred to asband pass filtering. A notch filter is a special case of bandpass filtering with specific attenuation of frequencies at 50 or60 Hz to reduce electrical noise introduced by the frequencyof common AC current.

Analog Versus Digital Recordings

Analog recording systems directly amplify the potentialfrom the recording electrodes, plot the potential on a displayoscilloscope and write it to recording paper, and/or store iton magnetic tape. Analog systems have largely been replacedby digital recording systems that utilize an analog to digital(A/D) converter that converts the amplitude of the potentialrecorded at each point in time to a number that is stored. The

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quality of digital data is influenced by the sampling frequencyand precision of the amplitude measurement.

The most common digital recording systems samplethe signal approximately every 1 msec (e.g., a samplingfrequency of 1,024 times per second (Hz)). With digital sam-pling, high-frequency signals that are faster than half of thesampling frequency are not detected and will simply be ab-sent or contribute to what appears to be noise. Hence, at asampling frequency of 1,000 Hz, a signal faster than 500Hz will not be detected. This sampling frequency also ren-ders low pass filtering at 500 Hz of little impact. For mostclinical purposes, a sampling frequency of 1,000 Hz is suf-ficient. However, high frequency, rapid potentials that canoriginate from the Purkinje system or areas of infarction,might be difficult to detect and will lack fine detail comparedto analog recordings or digital recordings at faster samplingfrequencies. Some recording systems offer faster sampling.The faster sampling places greater demands on the computerprocessor and increases the size of the stored data files.

Conclusion

Bipolar recordings provide sufficient information for mostmapping purposes in clinical EP laboratories. Unipolarrecordings and manipulation of filtering can be helpful insome situations and is easily implemented with most record-ing systems. Specific recording techniques differ between

laboratories and can importantly effect the nature of the elec-trograms obtained. These considerations may become moreimportant for interpretation of voltage maps used to locateregions of scar and infarction, and in studies assessing fre-quency content and characteristics of conduction during fib-rillation in the search of the arrhythmogenic substrate forarrhythmias.

References

1. Delacretaz E, Soejima K, Gottipaty VK, Brunckhorst CB, FriedmanPL, Stevenson WG: Single catheter determination of local electrogramprematurity using simultaneous unipolar and bipolar recordings to re-place the surface ECG as a timing reference. Pacing Clin Electrophysiol2001;24:441-449.

2. de Bakker JMT, Hauer RNW, Simmers TA: Activation mapping: Unipo-lar versus bipolar recording. In: Zipes DP, Jalife J, eds: Cardiac Elec-trophysiology. From Cell to Bedside. Second Edition. Philadelphia, PA:W.B. Saunders, 1995, p. 1068.

3. Haissaguerre M, Dartigues JF, Warin JF, Le Metayer P, Montserrat P,Salamon R: Electrogram patterns predictive of successful catheter abla-tion of accessory pathways. Value of unipolar recording mode. Circula-tion 1991;84:188-202.

4. Tada H, Oral H, Sticherling C, Chough SP, Baker RL, Wasmer K, KimMH, Pelosi F Jr, Michaud GF, Knight BP, Strickberger SA, Morady F:Electrogram polarity and cavotricuspid isthmus block during ablation oftypical atrial flutter. J Cardiovasc Electrophysiol 2001;12:393-399.

5. Tung S, Soejima K, Maisel WH, Suzuki M, Epstein L, Stevenson WG:Recognition of far-field electrograms during entrainment mapping ofventricular tachycardia. J Am Coll Cardiol 2003;42:110-115.