750
CIRCULATION
5. Crampton RS, Hunter -FP Jr: Low-energy ventricular defibrillation and
miniature defibrillators. JAMA 235: 2284, 1976
6. Schuder JC, Stoeckle H, Dolan AM: Transthoracic ventricular defibrillation with square-wave stimuli. Circ Res 15: 258, 1964
7. Schuder JC, Rahmoeller GA, Nellis SH, Stoeckle H, Mackenzie JW:
Transthoracic ventricular defibrillation with very high amplitude rectangular pulses. J Appl Physiol 22: 1110, 1967
8. Stoeckle H, Nellis SH, Schuder JC: Incidence of arrhythmias in the dog
following transthoracic ventricular defibrillation with unidirectional rectangular stimuli. Circ Res 23: 343, 1968
9. Schuder JC, Gold JH: Design of an ultrahigh-energy hydrogen
thyratron/SCR research defibrillator. Med Instrum 10: 146, 1976
10. Garner HE, Mather EC, Hoover TR, Brown RE, Halliwell WC:
Anesthesia of bulls undergoing surgical manipulation of the vas deferentia. Can J Comp Med 39: 250, 1975
11. Weingarten M, Lowe HJ: A new circuit injection technic for syringe-
12.
13.
14.
15.
16.
VOL 56, No 5, NOVEMBER 1977
measured administration of methoxyflurane: a new dimension in
anesthesia. Anesth Analg (Cleve) 52: 634, 1973
Schuder JC, Stoeckle H, Gold JH: Effectiveness of transthoracic ventricular defibrillation with square and trapezoidal waveforms. In Proceedings
of Cardiac Defibrillation Conference, Purdue University, West
Lafayette, Indiana, 1975, p 109
Peleska B: Cardiac arrhythmias following condenser discharges and their
dependence upon strength of current and phase of cardiac cycle. Circ Res
13: 21, 1963
Peleska B: Cardiac arrhythmias following condenser discharges led
through an inductance: Comparison with effects of pure condenser discharges. Circ Res 16: 11, 1965
Ten Eick RE, Wyte SR, Ross SM, Hoffman BF: Postcountershock
arrhythmias in untreated and digitalized dogs. Circ Res 21: 375, 1967
Pansegrau DG. Abboud FM: Hemodynamic effects of ventricular
defibrillation. J Clin Invest 49: 282, 1970
A Comparison of Unipolar and Bipolar Electrograms
for Cardiac Pacemaker Sensing
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VINCENT DECAPRIO, PH.D., PHILIP HURZELER, PH.D.,
AND
SEYMOUR FURMAN, M.D.
SUMMARY Simultaneous unipolar and bipolar electrograms were
recorded and compared from 49 pacemaker patients with bipolar
endocardial electrodes. Average bipolar depolarization signal voltage
equalled that of unipolar but showed greater variation. Bipolar and
unipolar slew rates were equal in both mean and variance. The proximal pole voltage had little effect on the bipolar result in 8% of the
cases, tended to cancel the tip voltage in 49% of the cases and augmented the tip voltage in 43% of the electrograms.
The average bipolar R wave duration was 28% less, the T wave
amplitude 34% less, and the ST-segment elevation 37% less than the
unipolar values.
By consistently attenuating the undesirable T waves and ST elevations, while leaving the depolarization signal unaffected, the bipolar
electrode offered the advantage of a superior signal-to-noise ratio for
sensing depolarization. In one case, however, the bipolar signal was
so small as to cause a clinical sensing failure.
FROM THE FIRST DAYS of cardiac pacing, two varieties of stimulating electrodes have been used: unipolar and
bipolar. The unipolar electrode has one pole (cathode or
negative stimulating pole) in contact with cardiac tissue, and
the other (anode or positive pole) outside of the heart, either
in subcutaneous tissue or on the surface of the body. The
bipolar electrode has both the cathode (sometimes called its
distal or tip pole) and the anode (proximal or ring pole) at
the cardiac tissue being stimulated.
The same electrodes are used to sense cardiac activity as
well as to stimulate the heart. Bipolar and unipolar electrodes are not equivalent in transmitting the cardiac electrogram to the pacemaker. Only the electrical events at the tip
pole describe the unipolar electrogram; the remote anode
contributes negligible voltage, since its location is extracardiac. ' 2 The bipolar electrode exhibits a large anodal voltage
(ring signal), similar in magnitude to the tip signal, but the
resulting electrogram is also dependent upon the orientation of the electrode within the heart. It has long been
demonstrated that unfavorable electrode orientation can
produce low voltage bipolar signals, even in the presence of
high voltage tip and ring signals.3
The unipolar system is not orientation sensitive, because
of its virtually indifferent anode, and has been considered to
yield larger and morphologically consistent electrograms.2
The preference for unipolar sensing is such that, in the event
of unsatisfactory bipolar sensing, conversion to unipolar is
frequently attempted.2 3
Several clinical instances in which bipolar sensing was
superior to unipolar, or in which conversion from bipolar to
unipolar produced no beneficial effect, caused a re-evaluation of the previously held beliefs. The difference between
the two electrograms was evaluated by measuring and
recording signals returning via the pacemaker electrode during implant (acute) and during pulse generator replacement
(chronic) on satisfactorily functioning bipolar electrodes.
The signals compared were, in each case, the unipolar signal from the tip pole, and the bipolar signal developed
between tip and ring poles.
From the Department of Surgery, Division of Cardiothoracic Surgery,
Montefiore Hospital and Medical Center, Bronx, and the Polytechnic Institute of New York, Brooklyn, New York.
Supported in part by USPHS Grant HL 04666-17.
Taken from a dissertation submitted to the Faculty of the Polytechnic
Institute of New York in partial fulfillment of the requirements for the Doctor of Philosophy degree (Bioengineering), 1977.
Address for reprints: Dr. Philip Hurzeler, Cardiac Pacemaker Center,
Montefiore Hospital and Medical Center, 111 East 210th Street, Bronx, New
York 10467.
Methods
Right ventricular, high fidelity (0.1Hz - 2kHz) endocardial electrograms from bipolar pacemaker electrodes were
measured in 49 patients. Twenty-one electrodes were acute
and 28 were chronic (in service 2-83 months). A three-channel lead selector and high impedance isolated preamplifier of
PACEMAKER SENSING/DeCaprio, Hurzeler, Furman
751
limb lead
I
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FIGURE 1. The recording system routes simultaneous unipolar and bipolar signals from the catheter electrode and a
standard limb lead to an isolated preamplifier and onto magnetic tape. The tape is later played into a photographic recorder to obtain a hard copy. The illustrated configuration records the following: a lead IH ECG on channel 1, a bipolar
electrogram on channel 2, a unipolar electrogram from the tip electrode on channel 3, and added voice comment on channel 4. To record a unipolar ring signal, the tip and ring connections to the lead selector are interchanged.
custom design* allowed simultaneous bipolar and unipolar
tracings to be recorded with a peripheral lead II ECG (fig.
1). A Hewlett-Packard Model 3960A instrumentation grade
tape recorder stored the data for later playback to Models
DR-8 and DR-12 Electronics for Medicine multichannel,
high speed photographic recorders. Readings from the tracings were then statistically analyzed by a General Electric
computer time sharing network.
Those parameters of the ventricular electrogram that
most affect pacer sensing have been described elsewhere5 and
in this study were similarly measured for both unipolar and
bipolar signals, viz., peak-to-peak voltage and maximum
slew rate (slope, or dv/dt) of the QRS complex, the ST-segment elevation and the peak-to-peak voltage and maximum
slew rate of the T wave. The duration of the intrinsic deflection was taken as the width of the intracardiac R wave measured between crossings of the isoelectric line; however, the
durations of some irregularly shaped curves could not be
determined without extrapolation (e.g. fig. 2, middle) and
are, therefore, estimates. Parameter values were averaged
over one or two respiratory cycles before tabulation.
The electrograms were recorded from electrodes with a
low, stable, clinically useful stimulation threshold and impedance, and satisfactory radiographic visualization. The
electrode surface areas and bipole separations are shown in
table 1. Only complexes from the most prominent focus,
either conducted or idioventricular, were analyzed. All
recordings were free from pacemaker stimuli and artifacts.
Our recording arrangement causes a positive voltage at
the tip electrode to register as an upward deflection. Unipolar signals were recorded with the negative input of a high
impedance bioelectric amplifier connected to a large surface
*Courtesy of Medtronic, Inc.
area metallic plate, temporarily inserted in the subcutaneous pulse generator site. Bipolar signals were measured with
the tip electrode connected to the positive input and the ring
electrode connected to the negative input.
Early results showed that the interpretation of some
,p - 1-Pk-to-PNsk Volta*
SIs. Rate
SRO". - 5i
D - Duration
S-S - S-S S555nt Diop1coen't
5t.
T
<
S
~~~~SRX
S
- S 55
-
olt
5.t.
S1*
lw
ISOLCDtRIC LIPC
FIGURE 2. Three diferent representative and diagrammatic acute
electrograms, both unipolar and bipolar. Intermediate configurations exist but the parameters can be similarly defined.
VOL 56, No 5, NOVEMBER 1977
CIRCULATION
752
(31%), equal in 22 cases (45%) and less in 12 cases (24%).
The unipolar tip signals always showed a larger slew rate
than the corresponding unipolar ring signal.
Statistical evaluation of the unipolar and bipolar electrograms as independent groups revealed that average unipolar and bipolar voltages and slew rates are equal. The
average bipolar R wave duration is 28% less, the T wave
voltage 34%, and the ST-segment elevation 37% less than the
average unipolar parameter (table 2). In the bar graphs of
the bipolar and unipolar voltage distributions (fig. 3) the
bipolar signals appear to show a larger variance, which is
not significant according to an F test with 90% confidence.
By comparing the amplitudes of the unipolar tip and ring
signals for each electrode, a ring-to-tip voltage ratio was
computed. If the ring electrogram is small in comparison to
the tip electrogram a large voltage difference exists and the
bipolar signal then approximates the unipolar tip signal; it is
a quasi-unipolar signal (fig. 4-1). Twenty-two cases with a
ratio greater than 0.5 (a ring signal amplitude at least half
the tip) were considered true bipolar signals, the result of tip
and ring contributions of similar magnitude. The true
bipolar signals exhibited a standard deviation of ± 6.8 mV
with a mean amplitude of 11.3 mV and a greater variance
than the unipolar group at the 90% level of the F test.
Data from each bipolar electrogram were then subtracted
from the values of its unipolar mate, and the individual
changes in voltage, slew rate, and duration were computed
for all 49 cases and averaged. As six of the 21 acute bipolar
electrograms had no discernible T wave, and the ST segment of chronic electrograms is isoelectric, in neither instance could a change be calculated. Therefore, for the T
wave voltage and slew rate, and ST displacement, the mean
values of the independent groups (table 2) were used to compute the percentage change (table 3). The probability (Pr)
that the unipolar value may be equal to the bipolar value,
based on Student's t-test,7 is included in the righthand
column. Any Pr value less than 0.05 indicates a significant
change (table 3).
Bipolar R wave amplitudes were an average 3.03% less
than unipolar ones. This difference, however, is not supported at any reasonable level of significance. The R wave
duration, T wave amplitude, T wave slew rate, and ST-segment elevation are all significantly reduced with bipolar
sensing (table 3).
Finally, the unipolar-bipolar pairs were further divided
into acute and chronic subgroupings. A comparison of the
unpaired (not from the same electrode) acute and chronic
values revealed similar changes for both unipolar and
bipolar signals; amplitude is maintained, though only with
marginal confidence; and slew rate is decreased by about
40% (table 4).5 Here, Pr is the probability that no acute-tochronic difference exists. Values less than .05 indicate
TABLE 1. Bipolar Electrode Specifications
Quantity Manufacturer, model
33
8
6
2
Medtronic 6901
Medtronic 5816
G. E. bipolar
Pacesetter systems
Cathodal
surface area
11 sq mm
87 sq mm
12 sq mm
12.2 sq mm
Bipole
separation
28 mm
15 mm
20 mm
29 mm
Downloaded from http://circ.ahajournals.org/ by guest on June 18, 2017
bipolar signals was facilitated by simultaneously observing
the unipolar ring signal. Therefore, in the last 30 instances
unipolar ring signals were also recorded by moving the positive input terminal from the tip to the ring electrode, and
continuing the simultaneous recording. At the conclusion of
the recording, simultaneous tip and bipolar electrograms,
and simultaneous proximal and bipolar electrograms were
available for analysis.
Signals which vary with time and may be displayed as a
plot of amplitude as a function of time (e.g., electrograms)
are said to be in the "time domain" in the mathematic terminology of waveform analysis. Such signals, by means of
the "Fourier Transform" can be recast into the mathematically equivalent "frequency domain" and be visualized
as a spectrum or plot of amplitude versus sinusoidal frequency.6 Both representations are equally accurate and precise
although each offers specific conveniences. The frequency
domain form is especially useful to designers of electronic
circuitry, but as the time domain form preserves the
morphologic features of the waveform, it permits correlation of physiologic events with pacemaker sensing circuit
response. Since the frequency domain offers no further insights into the electrophysiologic process, it was not used in
this study.
Results
One-third (16 of 49) of the bipolar depolarization signals
resembled the bottom curve of figure 2 with a narrow triangular deflection crossing the isoelectric line more than
once. Of these, the peak was positive, indicating propagation from ring to tip in 11 cases, and negative, indicating
propagation from tip to ring, in five cases. The remaining 33
bipolar waveforms could not be classified into unique
morphological subgroups.
The bipolar depolarization signal voltage (measured
within 0.1 mV) was greater than its unipolar mate in 21
cases (43%), equal in four cases (8%) and less in 24 cases
(49%). In one of the latter 24 cases the bipolar voltage was
too small to be sensed by the pacemaker; the matching unipolar tip electrogram was larger and sensed. The maximum
slew rate always occurred during an intrinsic deflection,
which was usually a downward slope, virtually a straight line
segment. The maximum bipolar slew rates, measured within
0.2 V/sec, were greater than their unipolar mates in 15 cases
significance.
TABLE 2. Unipolar and Bipolar Parameters
Unipolar
Mean
Amplitude (mV)
Slew rate (V/sec)
Duration (msec)
T wave voltage (mV)
T wave slew rate (V/sec)
Acute ST displacement (mV)
12.2
2.8
88.3
2.4
.02
2.6
SD
5.2
1.7
30.3
2.2
.03
1.6
Range
4.0-29.2
0.6-7.0
25.0-150.0
0-8.4
0-.11
0-5.7
Mean
11.8
2.8
61.9
1.6
.02
1.6
Bipolar
SD
6.0
1.8
29.3
1.3
.02
1.5
Range
3.3-31.3
0.6-7.3
10.0-145.0
0-5.2
0-.08
0-4.0
PACEMAKER SENSING/DeCaprio, Hurzeler, Furman
PEAK-TO-PEAK
753
AMPLITUDE
VOLTAGE
lA Fvr
I h
2.0
4.0
3.0
8.0
*0.0
12.0
14.0
19.0
20.0
22.0
24.0
320.0
32.0
15
14
13
12
11
10
9
8
7
6
S
4
3
2
1
NUMBER OF CASES
m
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
NUMBER OF CASES
Downloaded from http://circ.ahajournals.org/ by guest on June 18, 2017
FIGURE 3. Comparison of the bargraphs of the 49 unipolar and bipolar electrograms suggest greater bipolar variance,
significant marginally only when those with a ring signal less than half the tip signal were excluded.
lOi,w
don
-B
J
0
,-
3
A
P---.
--\4
1, mnv
4
lm
B
c
FIGURE 4. Four sets of electrograms indicate several clinicalfindings. Curve A is a lead HI ECG, B is the bipolar electrogram, C is
the unipolar tip electrogram and D is the unipolar ring signal. Only
A, B and C are simultaneous recordings. Calibrations accompany
each intracardiac electrogram. Time lines are 200 msec apart. 1) A
quasi-unipolar signal. With a small ring signal, the bipolar signal,
though shorter in duration, resembles the unipolar tip signal. 2)
Bipolar signal enhancement. A difference in activation time at each
pole causes a bipolar signal larger than either tip or ring signal. 3)
Bipolar signal attenuation. Synchronous tip and ring depolarizations, though both large, produce a bipolar signal much smaller
than either. 4) The superior bipolar signal-to-noise ratio provides an
exceptionally clean signal of activation, with greatly reduced T
waves and ST-segment elevations.
Discussion
Ventricular electrograms may be resolved into four bioelectric events: the intrinsic deflection (the intracardiac R
wave), repolarization (the T wave), far-field phenomena (distant electrical activity) and the injury current (ST-segment
elevation).5 The most important event for pacer sensing is
the intrinsic deflection, the rapid, nearly straightline downward deflection of the unipolar electrogram, which occurs
when the muscle adjacent to the electrode becomes electronegative as the depolarization wave passes. With bipolar
electrodes, the wavefront appears first at one pole, then
travels to the other. An exception is the quasi-unipolar signal where the wavefront does not produce similar intrinsic
deflections at each pole. The bipolar voltage, i.e., the potential difference between the two poles at any time, is a function of three variables, tip voltage, ring voltage and travel
time between poles, against only one (tip voltage) for the
unipolar electrogram. Because of the increased number of
variables, the greater variance in bipolar amplitude is not
surprising. Further evidence for inconsistency in bipolar
amplitude is found in the large signal variation associated
with normal respiration.8
The interelectrode distance (bipole separation), the velocity of the spreading depolarization wave and especially
the direction of the depolarization pathway through the ventricle determine the timing of the tip and ring intrinsic
deflections. In an idealized model of myocardial tissue activation (fig. 5) identical signals appear on both poles of a
bipolar electrode. In the two extreme cases the bipole may
be oriented either normal (N) (at a 90° angle) or parallel (P)
to the path of the depolarization wave, here considered a
TABLE 3. Percent Change from Unipolar
Voltage
Slew rate
Duration
Mean T voltage
Mean T slew rate
Mean ST displacement
Mean
SD
-3.03
.32
-27.64
-33.60
-29.17
-37.40
30.30
26.54
30.13
Pr
.32
.98
<10-6
.007
.045
<10-6
VOL 56, No 5, NOVEMBER 1977
CIRCULATION
754
TABLE 4. Acute-to-Chronic R Wave Change
Acute
SD
Mean
Chronic
Mean
SD
12.4
3.5
5.2
1.7
12.1
2.3
a.3
Slew rate (V/sec)
1.6
.825
.012
Bipolar
Voltage (mV)
Slew rate (V/sec)
13.4
3.6
6.1
1.7
10.5
2.2
5.8
1.7
.094
Unipolar
Voltage (mV)
Pr
.008
Downloaded from http://circ.ahajournals.org/ by guest on June 18, 2017
sheet of charge extending the full width of a strip of cardiac
muscle. For the N orientation, the wavefront strikes both
poles simultaneously and no bipolar potential results
(fig. 4-3).
Alternatively, the P orientation causes a difference in arrival times at the two poles resulting in an additive bipolar
signal (fig. 4-2). The amplitude of the bipolar signal is also
influenced by the distances from each pole to the active
tissue and the width of the depolarization wavefront.9
A parallel (P) orientation, with a bipolar signal larger
than either ring or tip, occurred in 10 of the 30 cases in which
the ring signal was explicitly recorded. In two of the 10, the
bipolar was larger than the tip signal because of the ring contribution. In the other eight, the cause was a timing difference between the two smaller unipolar signals.
In the 21 cases in which the bipolar signal amplitude was
greater than the associated tip signal, the tip signal alone
was always large enough for clinically satisfactory sensing.
No bipolar signal of sufficient amplitude and slew rate to
trigger a pacer has been formed from two insufficient unipolar signals. The probability of such an occurrence is extremely small since the small, wide intrinsic deflections
N
p
-e .'-
N
DIPOLAR
BIPOLAR
T
p
associated with poor electrode position or an infarcted myocardium'0 arrive nearly simultaneously at both poles and
tend to be attenuated rather than enhanced with bipolar sensing.
(However, since only clinically satisfactory signals were
recorded, and a unipolar tip signal greater than 2 mV was a
criterion, such bipolar signals were not demonstrated in this
study.) Thus, the clinical practice of unipolarizing a bipolar
electrode with an N orientation to improve its sensing performance,* does not have a useful converse: bipolarizing a
pair of poorly performing unipolar electrodes will not help.
The purpose of a pacemaker's sensing amplifier is to respond selectively to cardiac electrical activity. An adequate
signal, returning via the electrode, triggers the sensing circuit into modifying the pacemaker's output. Adequacy
depends on the signal's amplitude and frequency content,
which can be characterized by its voltage, rate of change
(slew rate),2' and duration (the time between isoelectric
crossings of the depolarization signal). Signals of very short
duration tend to be rejected by the sensing circuit as spurious
high frequency noise, and will trigger the circuit only if
amplitude is high. The precise effect of signal duration on
sensing is determined by the circuit's high frequency filter
characteristics, and varies among designs. Pacemakers are
tested with a haversine test signal to simulate an intracardiac R wave.t A typical sensing amplifier is most sensitive to
a 25 msec duration haversine pulse and requires only a 2 mV
amplitude to trigger at that duration. As the duration is
decreased to 6 msec, a 4 mV haversine is required (personal
communication, J. Hartlaub, Medtronic, Inc.).
Because tip and ring voltage is nearly equal throughout
most of the R wave, a substantial voltage difference exists
only when the depolarization wavefront is between poles.
Usually, a bipolar signal of shorter duration and dissimilar
morphology from either tip or ring is produced (fig. 4-4).
Even if the amplitude of the ring signal is significantly less
than the tip, a bipolar signal morphologically similar to the
tip is generated, but again, with a shorter duration (fig. 4-1).
Forty-four of the 49 cases had bipolar durations shorter than
unipolar tip (mean decrease 28%), but as signal durations
never fell below 10 msec (mean 75 msec) it is unlikely that a
signal from
a
bipolar electrode would be rejected
as
noise.
The remaining three contributors to the electrogram (T
ST elevation and far-field activity) are physiologic
noise to a sensing system and should be rejected. The myocardial repolarization wavefront is wide" and appears nearly
simultaneously on both poles of the bipolar system. It is,
therefore, significantly attenuated. The bipolar T waves of
this study were reduced an average of 34% compared to
those of the simultaneous bipolar (figs. 4-3 and 4-4). The
acute ST-segment elevation is believed due to a local current of injury caused by pressure of the electrode tip against
wave,
FIGURE 5. An idealized model depicting the activation of a thee
of cardiac muscle adjacent to two bipolar electrodes represei Pts
uniform depolarization process as a line of dipoles spreading over a
group of very long muscle fibers (rectangle) (after Wilson e,,t al.7).
The tip (T) and ring (R) electrodes of bipolar catheters posi tioned
normal (N) and parallel (P) to thefibers are projected onto the'plane
of depolarization (dashed line). As the wave passes the poles of the
electrode, the ring, tip and bipolar curves are generated. With a normal orientation, the coincident arrival of equal potentials on tihe two
poles results in complete subtraction. The parallel orien'tation
delays the signal at one of the poles to produce a bipolar signal
which may be larger than either unipolar signal.
the
endocardium.
distant
from
the
The
electrogram
injury,
also
from
displays
the
an
ring,
ST
which
is
elevation
*In this instance good signals on tip and ring cancel each other to form an
attenuated bipolar signal. Elimination of one leaves a good signal fully available.
tA haversine pulse is one half cycle of a sine wave, squared. It may be electronically generated and its amplitude and duration independently varied to
test pacemaker sensitivity. See Pacemaker Standard (proposed), August,
1975, Para. 4.1.4.1., Association for the Advancement of Medical Instrumentation, 1901 Fort Meyers Drive, Arlington, Va. 22209.
PACEMAKER SENSING/DeCaprio, Hurzeler, Furman
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(though smaller than the tip electrogram) and the bipolar
signals therefore display lower ST elevations than the unipolar signals. Far-field potentials from distant electrical activity, (e.g., activation of the opposite ventricle, skeletal
muscle potentials, atrial activity, nonphysiologic signals,
etc.), if large enough, can falsely trigger the pacemaker.
Unipolar signal amplitude is inversely proportional to the
distance between the electrode and the signal source. Bipolar
signal amplitude is inversely proportional to the square of
the distance from a point midway between the two poles and
the signal source.*12 The smaller lead-field of the bipolar
electrode renders it superior in rejecting electromagnetic interference and distant physiologic activity.18
By the attenuation or enhancement of select events the
bipolar electrode can provide a signal-to-noise ratio superior
to that of a unipolar electrode. It attenuates not only electrical noise"3 but also the physiologic signals which should be
treated as noise. T waves, ST-segment elevations and potentials from areas of the heart far from the electrode are
reduced before they enter the pacer's sensing amplifier
(where they may be further attenuated by filtering). At
times, however, the price of low noise can be R wave
attenuation unable to trigger a pacemaker in 2% of our
cases. Often a small but artifact-free signal may be an advantage over one larger but noisier (better signal-to-noise
ratio).'4 If the T waves are considered noise, the signal (R
wave)-to-noise ratio was greater for bipolar electrodes in 33
of our 49 cases (67%).
Conclusions
Bipolar and tip unipolar signals, simultaneously derived
from the same catheter electrode, are usually unequal in
amplitude, yet the averages of the two groups are equal. The
probability of obtaining a signal much larger or much
smaller than average is greater with bipolar sensing. The
orientation of a bipolar electrode within the heart deter-
*A unipolar electrode is equivalent to a monopole and the bipolar electrode to a dipole in electric field theory.
755
mines the amplitude and slew rate of the electrogram. The
orientation can cause either a subtraction or an addition of
the two unipolar amplitudes at each pole. The addition of
two smaller unipolar signals, and not a large ring signal, accounted for the majority of the cases having a bipolar signal
larger than the unipolar tip signal.
The depolarization signal of bipolar electrograms is
shorter in duration than unipolar, yet not so short as to be
mistaken for electrical noise by the sensing amplifier.
Bipolar sensing reduces both the physiologic (T waves, ST
elevations and far-field) and nonphysiologic (electromagnetic interference) noise in sensing systems providing a signal-to-noise ratio superior to unipolar.
References
1. Lewis T: The Mechanism and Graphic Registration of the Heart Beat.
London, Shaw and Sons, Ltd, 1925
2. Barold SS, Keller JE: Sensing problems with demand pacemakers. In
Cardiac Pacing, edited by Samet P. New York, Grune and Stratton, 1973
3. Barold SS, Gaidula JJ: Failure of demand pacemakers from low-voltage
bipolar ventricular electrograms. JAMA 215: 923, 1971
4. Greatbatch W: In Technical and clinical aspects of pacing electrodes.
Ann NY Acad Sci 167: 865, 1969
5. Hurzeler P, DeCaprio V, Furman S: Endocardial electrograms and pacemaker sensing. Med Instrum 10: 178, 1976
6. Geddes LA, Baker LE: Principles of Applied Biomedical Instrumentation, chap 14. New York, Wiley, 1968, pp 446A467
7. Crow EL, Davis FA, Maxfield MW: Statistics Manual. New York,
Dover Publications, 1960
8. DeCaprio V, Gaspar H, Escher DJW, Furman S: Respiratory cycle
variations in pacer sensing signals. Med Instru 10: 55, 1976
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action and injury displayed by heart muscle and other excitable tissue. In
Selected Papers of Dr. Frank N. Wilson, edited by Johnston FD,
Lepeschkin E. Ann Arbor, JW Edwards, 1954, pp 307
10. Chatterjee K, Davies G, Harris A: Fall of endocardial potentials after
acute myocardial infarction. Lancet 1: 1308, 1970
11. MacLeod AG: The electrogram of cardiac muscle: An analysis which explains the regression or T deflection. Am Heart J 15: 165, 1938
12. Brody DA, Bradshaw JC, Evans JW: A theoretical basis for determining
heart-lead relationships of the equivalent cardiac multipole. IEEE Trans
Biomed Eng BME8: 139, 1961
13. Kahn AR, Schlentz RJ: Design and construction methods for protecting
implanted cardiac pacemakers from electromagnetic interference. In
Cardiac Pacing, edited by Thalen H. Netherlands, Van Gorcum and Co,
1973
14. Chatterjee K, Swan HJC, Ganz W, Gray R, Loebel H, Forrester J,
Chonette D: Use of a balloon-tipped flotation electrode catheter for cardiac monitoring. Am J Cardiol 36: 55, 1975
A comparison of unipolar and bipolar electrograms for cardiac pacemaker sensing.
V DeCaprio, P Hurzeler and S Furman
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Circulation. 1977;56:750-755
doi: 10.1161/01.CIR.56.5.750
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