Download Distribution of Heart Potentials on the Thoracic Surface of Normal

Distribution of Heart Potentials on the Thoracic
Surface of Normal Human Subjects
By Bruno Taccardi, M.D.
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• Multiple electrical recordings from the thoracic surface of the dog have recently shown2
that during some portions of the QRS interval
only one maximum and one minimum of potential are simultaneously present on the chest
wall, as one would expect from the theory of
the single equivalent dipole.3 During larger
portions of the ventricular depolarization
time, however, several simultaneous maxima
and minima can be observed. Such a complex
distribution of surface potentials cannot be
reconciled with the above-mentioned theory.
A multipolar equivalent generator should be
considered,4"7 in order to account for the experimental data. The complexity of the surface patterns should be referred to the existence, within the heart, of multiple, separate
groups of electrically active fibers,8'9 each
group exerting a preponderant influence on
certain regions of the chest surface (proximity potentials).
It follows from the foregoing observations
that, in dogs, some portions of the precordial
electrocardiogram should be interpreted according to the dipolar theory, and no information about "local" activities may be expected during these periods; other parts of
the same tracings, however, do contain proximity potentials and should be interpreted
accordingly.
The several points of transition from the
"dipolar" to the "multipolar" portions of a
given QRS complex are not detectable by inspecting or measuring the tracing and it is
From the Department of Electrophysiology, Istituto
cli Cardiologia Sperimentale Servizi Seientifici Simes,
Milano, Italy.
Supported by Professor G. E. Ghiradi, Founder of
the Istituto di Cardiologia Sperimentale.
A preliminary report was presented at the 13th
Congress of the Italian Physiological Society.1
Received for publication November 8, 1962.
Circulation Research, Volume XII, April 1963
therefore impossible to decide in this way
which parts of the complex contain local potentials. This discrimination becomes possible
by examining instantaneous isopotential maps
of the chest surface.
Similar difficulties are encountered when
interpreting the human precordial electrocardiogram. Basic work of Wilson and his associates,10' " Rijlant,12"14 Nelson8 and others
demonstrate that proximity potentials must
exist on the surface of the human thorax, but
their location and time course are still imperfectly known. In particular, it has not been
established whether in man proximity potentials are present during the whole depolarization time or during some parts of it; nor is
it certain whether they are exclusively located
in the regions explored by routine chest leads
or whether they are also present elsewhere.
On the other side, several authors3- 13 consider
that the precordial electrocardiogram does not
contain significant information about "local"
activities and assert that all the precordial
tracings can be predicted from distant leads.
In this study an attempt is made to determine the location and time course of proximity and dipolar components in the thoracic
potential distribution of normal human subjects.
Methods
Electrocardiographic exploration of the thoracic
surface was performed in 15 healthy, male human
subjects aged 15 to 43. In each individual 200 to
400 points, regularly distributed on the surface
of the trunk, were marked by means of a rubber
.stamp. The subjects were photographed before
and after the marking. The distance between
explored points was 1.5 cm in the precordial
region; 3 cm in the other regions of the anterior
and left thoracic wall; 6 cm on the posterior
chest wall (fig. 1).
The chest electrodes were small silver or steel
plates, 5 mm in diameter. No conducting jelly
was used, to avoid distortion of the electric field.
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FIGURE 1
Anterior and posterior thoracic surface of subject no. 1. The points to be explored
v.ere marked by means of a rubber stamp. Below, left: QRS complex in lead 2. The
vertical lines indicate 16 instants of time corresponding to the 16 isopotential maps
which were constructed for subject no. 1. BelotG, right: Q7iS complex in lead 2; the
vertical lines indicate 9 instants corresponding to the maps which were selected to be
reproduced in figures 3a to 5a.
As in these conditions the skin-electrode resistance
may become very high, a cathode follower was
interposed between the chest electrode and the
amplifying chain in order to prevent amplitude
loss and distortion of the tracings.
By means of two to five Tektronix 122 preamplifiers (modified time constant: three seconds)
connected to a multi -channel Rycom or Tektronix
cathode ray oscilloscope, two to five tracings were
simultaneously recorded (fig. 2a) : a reference
electrocardiogram (usually standard lead 1 or 2 ;
occasionally lead VL) and one to four thoracic
electrocardiograms, obtained by leading off from
a common reference point (usually Wilson's central terminal, made with 50,000 ohm resistors)
and one to four points on the thorax. The electrocardiograms were photographed from the screen
of the cathode ray tube.
The chest electrodes were subsequently moved
to new points on the thoracic wall and new
tracings recorded until all the marked points
had been explored. A square wave generator,
triggered by the heart beat, injected into the
lead circuit a calibration signal after each systole.
The electrocardiograms were always recorded after
the end of a normal expiration and before the
beginning of the following inspiration.
The tracings were optically magnified. The timebase speed, when measured on the enlarged tracings, was 1.000 to 1.250 mm/sec and the calibration
2 to 16 cm/mv. The width of the enlarged QRS
complexes was 8 to 10 em and the amplitude 4
to 8 cm. Ten to twenty instants of time were
chosen in the QRS complex of the reference
tracing and the amplitudes of the 200-400 thoracic
electrocardiograms were measured at the selected
instants; the measured values were converted
into millivolts and plotted on maps of the explored region, each map corresponding to one
instant.
Continuous lines were drawn on the maps by
joining points between which there was no potential difference at the instant considered (isopotential lines). Linear interpolation between experimental points was used when necessary.
In some of the earlier experiments the right
arm was taken as the reference point instead
of the central terminal. In these cases the maps
Circulation Research, Volume XII, April 19G3
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HEART POTENTIALS ON THORACIC SURFACE
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FIGURE 2
fa) A: Lead 2 of subject no. 1; this lead was recorded throughout the whole exploration
and served as a time-reference tracing. B: Chest lead recorded from the pre-slernal
region, midway beUveen the upper and lower end of the sternum. C: Chest lead recorded
from the upper end of the pre-sternal region. Time marking: 5 msec. Calibration:
350 microvolt. The tracing was obtained by photographing the screen of a Tektronix
cathode ray oscilloscope, (b) Standard and unipolar limb and chest leads of subject
no. 1, obtained by ineams of a direct-writing multi-channel electrocardiograph.
were subsequently transformed into those which
would have been obtained with the central terminal
by means of a simple calculation, as described
in a previous paper. 1 This transformation does
not alter the shape and distribution of isopotential
lines: in fact it merely consists of adding a
constant to all the potential values of a given
map. (See also Nelson,8 page 1022).
Immediately after the chest exploration, standard and unipolar limb and precordial leads were
recorded in all the subjects by means of a
multi-channel, direct writing electrocardiograph
(Hellige or Battaglia-Rangoni) (fig. 2b). The
exploration was generally completed in about
three hours.
In three cases the entire experiment was repeated after an interval of one month or more,
in order to ascertain the reprodueibility of
the results; there was a satisfactory agreement
between the results of the first and second
exploration.
Results
In the early stages of ventricular depolarization a precordial maximum and a dorsal
minimum* of potential have been observed in
Circulation Research, Volume XII, April 1963
all the subjects (fig. 3a). The maximum is
generally located near the mid-sternal or left
para-sternal line, at the level of the third to
fifth intercostal space. The dorsal minimum
is generally located near the posterior midline, at the same level; in rare eases, it can
be observed in the vicinity of the left posterior axillary line or even more anteriorly.
During the rise time of the R wave in lead 2
the following events generally occur: the precordial maximum moves slowly, pointing
caudally and toward the left mammiUary line;
the dorsal minimum migrates much more rapidly toward the right scapular and deltoid
region, and appears then anteriorly in the
*The expressions "potential maximum'' (and
"minimum") indicate the region of the map where
the potential is higher (or lower) than that existing
in all the neighboring regions. The term ' ' saddle''
indicates a region such that an observer, staying
in it, can see growing potentials before and behind
himself, decreasing potentials at his right and left.
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right subclavicular fossa (fig. 3b and c) ;
meanwhile another minimum, which will become, later on, the principal minimum, appears in the precordial region near the
mid-sternal line, at the level of the fourth
sterno-costal joint (figs. 3d and 4a). Thus,
during 3 to 10 milliseconds, two potential
minima, one maximum and one "saddle" are
simultaneously present on the chest wall of
normal human subjects, the "saddle" being
interposed between the two minima.
The time course of the above-described
events varies considerably from one subject
to another: in some cases the last described
situation is observed in coincidence with the
foot of R in lead 2; in other cases, it coincides
with the rise of the R wave, or even with the
peak of this wave (fig. 5b). In the subsequent
stages of ventricular activation the upper
minimum moves progressively from the right
sub-clavicular region toward the area occupied by the mid-sternal minimum, whose voltage reaches one millivolt or more beneath
the level of the central terminal (fig. 4a).
The two minima finally merge together and
the potential distribution becomes again
"dipolar" (fig. 4b) ; at this stage the minimum of potential is located near the midline, at the level of the fourth sterno-costal
joint and the maximum is located at the left
of the minimum and more caudally (fig. 4b).
This pattern is most frequently observed during the last part of the rise, the peak and the
first part of the fall of the R wave in lead 2.
In the subject whose potential distribution is
illustrated in figures 2b to 5a, this "dipolar"
stage had a duration of 15 msec (figs. 4c and
5b). In some cases, however, two simultaneous
minima of potential are still present after the
peak of the R wave in lead 2 (fig. 5b, case 3).
During the last stages of ventricular activation a series of new events occurs in the
majority of eases: the potential maximum
migrates progressively toward the left axillary and dorsal region, covering a distance of
15 to 30 cm, while the minimum maintains its
former position. Meanwhile, another maximum appears near the upper end of the sternum (figs. 4d and 5a). Thus, during this
period, two separate potential maxima and
one minimum are simultaneously present on
the surface of the trunk. The two maxima are
antipodally located with respect to the center
of the heart (fig. 5a). The anterior maximum
reaches a level of 0.15 to 0.40 mv above the
central terminal; similar values are attained
by the dorsal maximum. In many cases this
situation remains unaltered up to the end of
ventricular activation time (fig. 5b) ; in some
instances, during the latest stages of ventricular depolarization the dorsal maximum disappears while the anterior one is still present;
in other cases, the anterior minimum divides
into two separate minima during the last milliseconds of the QRS interval.
The above-described events have been observed in the great majority of the subjects
examined. Occasionally the potential patterns
were slightly different from those described
above. In one subject, however, (no. 2),
FIGURE 3
(a, b, c, d) Isopotential maps of the thoracic surface of subject no. 1 corresponding
to the instant of time indicated at the lower right of the figures, (a>) At the beginning
of ventricular activation the potential distribution is dipolar, with one precordial
maximum and one dorsal minimum, (b) The dorsal minimum migrates toward the
right shoulder, (c) The preeordial maximum migrates slowly towa/rd the left mamtnillary
line; the minimum appears anteriorly in the right pectoral and sub-clavicular region.
Isopotential lines 0.25 and 0.5 shoiv a double bending xvhen passing through the
para-sternal region, at the level of the nipples. In this region, a new minimum will
soon appear, (d) The new minimum has now appeared, in the pre-sternal region,
at the level of the 4th intercostal space. In this phase of ventricular depolarization,
two minima, one maximum and one saddle can be observed on the anterior thoracic
surface. The potential values are in millivolts.
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only one maximum and one minimum were
observed during the whole activation time,
though it may be possible that the lacking
details of the potential pattern would have
appeared, had the number of instants studied or the accuracy of the measurements been
increased. In rare cases on the contrary, additional maxima or minima have been observed.
In case 13, for instance, two precordial maxima were present during the fall of the E
wave in lead 2 (fig. 5c). A similar pattern
is often observed in dogs, at an earlier stage
of ventricular depolarization. In other cases,
an additional minimum of potential was observed in the left axillai^ region at the beginning of the QES interval.
Discussion
The above-described results show that in
man, as in dogs, the distribution of heart potentials on the body-surface is either dipolar
or multipolar, depending on the instant which
is considered. During the dipolar periods the
potential pattern is similar to that which
would be observed if a single equivalent dipole were substituted for the heart. The only
information obtainable from surface measurementduring these intervals concerns the magnitude and orientation of the dipolar moment,
and possibly the location of the equivalent
dipole. Consequently, nothing can be deduced
about the multiple depolarization waves traveling simultaneously through the heart. The
"intrinsicoid" deflections observed during
these periods are correlated to rotation or
shifting of the equivalent dipole and do not
necessarily indicate activation of the myoeardial regions facing the exploring electrode.
During the "multipolar" phases, however,
chest leads do yield information about local
electrical events. Thus, for instance, the positive wave appearing at the end of the QRS
interval in leads exploring the upper end of
the sterna] region (fig. 2a) is typically a local
phenomenon, being only present over an area
covering a few square cm. Unfortunately, all
the local details contained in the precordial
electrocardiograms are not as evident as the
above-mentioned wave and, generally speaking, it is impossible to detect the points of
traiisition from the dipolar to the local components of a given human electrocardiogram
by merely inspecting or measuring it. As
previously observed for the dog, this discrimination can be done by examining isopotential
maps.
In normal human subjects the thoracic potential distribution is dipolar during a large
part of the QRS interval; however, a multipolar distribution can be observed in most
cases during a total time attaining 20 to 35
msec (fig. 5b). In cardiac patients the timerelationships between bipolar and multipolar
phases, as well as the number, location and
time course of potential maxima and minima
are often quite different from those observed
in normal subjects. The results of thoracic
explorations performed in heart patients will
be described in a subsequent paper.
The data yielded by isopotential maps of
the chest surface do not by themselves permit
establishing a close relationship between surface and intracardiac electrical events. NeA'ertheless, a tentative explanation of the observed
phenomena might be suggested on the basis
of the most generally accepted descriptions of
the ventricular activation processes (Lepeseh-
FIGURE 4
(a, b, c, d) Isopotential maps of the thoracic surface of subject no. 1 corresponding
to the instant of time indicated at the lotuer right of the figures, (a) The upper
minimum moves toivard the region occupied by the new minimum, (b) The two minima
merge together. The shape of isopotential line —1, lioivever, reveals that the fusion
of the two minima is not yet complete, (c) The fusion of the two minima is now
complete and the potential distribution is once again of the dipolar kind, (d) The
precordial maximum moves to the left; meanwhile, another maximum appears in
the upper pre-sternal region. (The region surrounded by isopotential line —.1 is a
maximum, its potential being higher than that of the neighboring regions.)
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kin). The precordial maximum and the dorsal minimum appearing at the beginning of
QRS are probably due to septal activation
and to early depolarization of the right ventricular wall. In the following stages the main
wavefronts are oriented in a base-apex direction and this fact explains the migration of
the potential minimum toward the right deltoid and clavicular region and the shift of
the maximum toward the left mammillary
line. Thereafter, an activation wave reaches
the surface of the right ventricular wall; the
minimum appearing at this instant near the
sternal midline may possibly be related to
this event. It is particularly interesting to
remark that in subjects with right bundlebranch block the sternal minimum appears
very late, up to 80 msec after the beginning
of ventricular depolarization. In most normal
cases the sternal minimum excavates, so to
speak, a "niche" in the region previously
occupied by isopotential lines surrounding the
maximum (figs. 3d and 4a). This behavior
could be expected if a part of the electromotive forces directed toward the right precordium suddenly disappeared because of complete activation of a circumscribed zone of the
right ventricular wall, while similar forces
are still present all around. This may well
be the external manifestation of Sehaefer's
epicardial " Quellpunkt, " 1 7 the region of the
right ventricular wall where the earliest epicardial negativity can be observed. However,
this is only a tentative explanation of the
events observed ; an experimental confirmation
of this view is still lacking. The maximum of
potential observed in the left and inferior portion of the precordium in the phases of ventricular activation illustrated by figures 3d
and 4a may be ascribed to the main wavefronts traveling toward the apex while the
"saddle" is probably due to another wavefront traveling in the upper parts of the right
ventricular myocardium. After the fusion of
the two minima, a dipolar situation occurs,
during which no conclusion about local intracardiae events can be drawn. However, during this period, the methods suggested by
Gabor and Nelson18 or Bayley19 would be
helpful in determining the intra-thoracic location of the equivalent dipole.
The disappearance of the saddle at this
stage (fig. 4b) does not necessarily imply that
the wavefront originating it has extinguished,
but only that it does not generate enough current to distort in a measurable way the dipolar equipotential lines.
It is generally admitted that during the last
FIGURE 5
(a) Isopotential maps of the thoracic surface of subject no. 1 corresponding to the
instant of time indicated at the lower right of the figure. One maximum of potential
is noiu located in the dorsal region; the sternal maximum has attained a level of
0.4 mv above the central terminal. Two maxima are antipodally located with respect
to the center of the heart, (b) The reference lead (QB.S complex of standard lead
2, when not otherwise indicated) of the 15 subjects examined. The hatched rectangles
indicate the phases of ventricular activation during which a multipolar potential distribution was observed on the thoracic surface. In each tracing the left rectangle
generally indicates the presence of two simultaneous minima; the right rectangle
indicates the presence of two maxima. In case 2 a dipolar pattern was observed
during the whole QRS interval, (c) Distribution of heart potentials on the chest ivall
of subject no. 13, during the fall of the R wave in lead 2. Tivo potential maxima,
on the right and left side of the thorax, are simultaneously present. A similar
distribution is sometimes observed on the dog's chest.1 (d) (From Hertault et al.,
Arch. Mai du Coeur, 55: 193, 1962) D 1 and .V 1: standard lead 1 and precordial
lead V 1. E (left): Unipolar electrocardiogram obtained from cavity of pulmonary
artery (to be compared with tracing C in figure 2a). E (right): Unipolar electrocardiogram with tip of recording catheter immediately below the pulmonary valve.
The positive wave indicates activation of the "crista supraventricularis."
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stages of ventricular depolarization the electrical activity is confined to the posterior wall
of the left ventricle, to the "crista supraventricularis" and possibly to the upper and posterior portion of the inter ventricular septum.
The dorsal maximum of potential observed at
this stage (fig. 5a) is probably related to the
left ventricular activation while the anterior
maximum might be correlated to the activity
of the "erista supraventricularis." Comparison of isopotential maps with a chest radiograph of subject no. 1, taken in the dorsal
recumbent position, showed that the maximum
of potential appearing anteriorly during the
last third of QRS (figs. 4d and 5a) is located
exactly before the main stem of the pulmonary
artery.
Electrocardiograms obtained by Kossmann
et al.20 and, more recently, by Hertault and
his associates21 from the cavity of the pulmonary artery and from the right ventricular
cavity, with the tip of the catheter touching
the "erista supraventricularis" show a positive wave occurring during the last third of
the QRS interval (fig. 5d) which resembles
those recorded by ourselves from the upper
end of the sternal region (fig. 2a).
These findings support our view ascribing
to activation of the "erista supraventricularis" the maximum of potential appearing
in the pre-sternal region during the last third
of the QRS interval (figs. 4d and 5a).
The descriptions of the instantaneous distribution of heart potentials which have been
published in the past 3 ' 4 are rather dissimilar
and lead to contradictory conclusions. It
should be pointed out that the results of an
exploration of the thoracic surface are dependent on the technique used. Whether one
does or does not find proximity potentials is
dependent on the spacing between electrodes,
the size of the electrodes, the amount of amplification used, the individual being tested,
and the instant of time during the cardiac
cycle.s More details will be found in the future when the technical conditions are improved. In the present state of the technique
the limiting factor is the necessity of measur-
TACCABDI
ing tracings obtained from different systoles.
In a given subject the potential distribution
corresponding to different systoles is not always quite identical, owing to respiratory
changes and to variations of ventricular volume following slight changes of the heart rate.
The choice of a correct base-line and the
finding of isochronous points on the tracings
also imply a certain amount of error. The influence of such errors can be minimized by
accurate experimentation, but several controls
are necessary to make sure of the exactness
of the results. The best way to verify the results of an exploration is, probably, the repetition of the exploration in the same subject.
Most of the errors described are randomly
distributed and the probability of repeating
identical errors in different experiments is
small. The accuracy of the measurements,
however, could be greatly increased if it were
possible to record simultaneous electrocardiograms from all the explored points.
Conclusions
The problem of establishing how many electrocardiograph ic leads are necessary to collect
the entirety of the information obtainable
from body-surface measurements has not yet
been solved satisfactorily. It has been considered for years that the three standard leads
contain most of the essential data on the electrical activity of the heart; it appeared later
that these leads explore mainly the frontal
component of the cardiac electromotive forces,
and new leads were introduced with the purpose either to explore the dipolar components
of the cardiac electromotive forces lying outside the frontal plane, or to obtain "local"
information or both. The double role of chest
leads, that of exploring the non-frontal components of the resultant cardiac electromotive
force, and that of exploring "local" or "proximity" potentials makes their interpretation
rather difficult in some cases. The results of
experiments reported in this and previous
papers show that both interpretations of the
precordial leads are justified, provided they
are applied during the proper time-interval.
However, the onty way presently available to
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determine the limits of the dipolar and multipolar intervals, as well as the location of
proximity potentials, is the construction of
isopotential maps, a method which is unfortunately not applicable to current clinical
practice, at least in the present state of the
technique. The experimental use of this
method, however, yields useful data to physiologists and clinicians for the following reasons :
1. Isopotential maps indicate the number,
location and time course of potential maxima
and minima during various phases of the cardiac cycle; they also yield some information
about the probable intracardiac location of
the main groups of active fibers.
2. They show that significant information
on the electrical activity of the heart is to be
found on the thoracic wall outside the regions
explored by routine chest leads.
3. They indicate during which portions of
the QRS interval the thoracic potential distribution is "dipolar" and during which other
parts it is "multipolar." This is particularly
important when one applies methods for determining the intra-thoracic location of the
equivalent dipole,18-19 because these methods
are only applicable during the ''dipolar"
phases.
4. Isopotential maps give a representation
of the electrical activity of the heart which is
not dependent on the particular reference
point used for the measurements.
5. They make it possible to correlate measurements taken from one point of the thorax
with those taken from all the other points of
the body surface at the same instant. The
correlation of multiple simultaneous values is
particularly difficult to perform when examining ordinary peripheral and chest leads.
6. All the electrocardiograms which would
be obtained in a given subject by leading off
from any possible couple of points arbitrarily
chosen on the surface of the trunk, including
most "loaded" leads, can be easily predicted
from isopotential maps, by means of a simple
calculation.
Circulation Research, Volume XII, April 196S
Summary
The distribution of isopotential lines on the
thoracic surface of 15 normal subjects aged
15 to 43 was determined at 10 to 20 instants
of time during ventricular activation. At the
beginning of the QRS interval a precordial
maximum and a dorsal minimum of potential
can be observed. Thereafter, the minimum
migrates toward the right shoulder and appears then anteriorly in the right subclavicular fossa, while another minimum appears
near the mid-sternal line. The two minima
finally merge together, and the potential distribution becomes dipolar once again. During
the last third of the QRS interval the maximum of potential moves toward the left axillary and dorsal region, while a new maximum
appears near the upper end of the sternum.
A tentative correlation between surface and
intracardiac electrical events can be established on the basis of the most generally accepted descriptions of ventricular activation
processes. During some phases of ventricular
activation, the potential distribution on the
surface of the body is of the dipolar type,
and there are no measurable local components;
during other phases of ventricular depolarization, however, proximity potentials do appear
on the thoracic surface, yielding information
about local activation waves. When the ventricular activation is altered, the location and
time course of local and dipolar potentials
show important modifications.
Acknowledgment
The author wishes to express his gratitude to Professor G. E. Ghiiiirdi for the support and encouragement lie provided.
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•eh. Volume
XII,
April.1BOS
Distribution of Heart Potentials on the Thoracic Surface of Normal Human Subjects
BRUNO TACCARDI
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Circ Res. 1963;12:341-352
doi: 10.1161/01.RES.12.4.341
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