Cardiac Mapping: Utility or Futility?
Gupta
Anoop Kumar, Alok Maheshwari, Ranjan Thakur, Yash Y. Lokhandwala.
.Thoracic and Cardiovascular Institute, Sparrow Health
System, Michigan State University, Lansing,
MI. USA
Address for correspondence: Anoop K. Gupta, MD, Michigan State
University, Thoracic and Cardiovascular Institute, 1200 E. Michigan
Avenue, Suite
525, Lansing, MI 48912
Email: anoopgupta@msn.com
Introduction:
Cardiac mapping is a broad term that covers several modes of mapping
such as body surface,1 endocardial,2 and epicardial3
mapping. The recording and analysis of extracellular electrograms,
reported as early as 1915, forms the basis for cardiac mapping.4 More commonly, cardiac mapping is performed with
catheters that are introduced percutaneously into the heart chambers
and sequentially record the endocardial electrograms with the purpose
of correlating local electrogram to cardiac anatomy. These
electrophysiological catheters are navigated and localized with the use
of fluoroscopy. Nevertheless, the use of fluoroscopy for these purposes
may be problematic for a number of reasons, including: 1) the inability
to accurately associate intracardiac electrograms with their precise
location
within the heart; 2) the endocardial surface is invisible using
fluoroscopy
and the target sites can only be approximated by their relationship
with
nearby structures such as ribs, blood vessels, and the position of
other
catheters; 3) due to the limitations of two-dimensional fluoroscopy,
navigation
is not exact, time consuming, and requires multiple views to estimate
the
three-dimensional location of the catheter; 4) inability to accurately
return
the catheter precisely to a previously mapped site; and 5) exposure of
the
patient and medical team to radiation.
Newer mapping systems have revolutionized the clinical
electrophysiology laboratory in recent years and have offered new
insights into arrhythmia mechanisms. They are aimed at improving the
resolution, three-dimensional spatial localization, and/or rapidity of
acquisition of cardiac activation maps. These systems use novel
approaches to accurately determine the three-dimensional location of
the mapping catheter and local electrograms are acquired using
conventional, well-established methods. Recorded data of the catheter
location and intracardiac electrogram at that location are used to
reconstruct in real-time a representation of the three-dimensional
geometry of the chamber, color-coded with relevant electrophysiological
information.
However, these mapping systems are very expensive and not required for
the
commoner clinical arrhythmias like atrioventricular nodal reentry
(AVNRT), accessory pathway mediated tachycardia (WPW syndrome and
concealed pathways) and typical atrial flutter. The purpose of this
article is to discuss the
possible contribution of newer cardiac mapping system to treat various
arrhythmias.
Non-contact Endocardial mapping
Endocardial Solutions (ESI) system:
Myocardial potentials are reconstructed from data acquired by recording
cavity potential from an array of electrodes sitting in the blood pool
within
the cardiac chamber and not in endocardial contact. Inverse- solution
methods
are applied to these non-contact signals in order to reconstruct
endocardial
potentials from the raw cavity potentials.
Endocardial
Solutions mapping showing the earliest activation (in red color) in a
case of focal atrial tachycardia. The upper right hand panel displays
AP projection on the left and LAO projection on the right side. The
lower half of the
picture shows intracardiac tracings. TV= Tricuspid annulus; IVC=
Inferior
vena cava; HIS= His bundle; APP= Anteroposterior projection.
The
Endocardial Solutions electrode array mounted on a 9F catheter.
Endocardial Solutions (ESI) system is used for clinical non-contact
intracardiac mapping. It can display high-resolution color maps of
endocardial activation in the intact beating heart (figure 1).
The electrode array is
mounted on a 9F catheter and consists of a 7.5 ml balloon around which
is woven a braid of 64 insulated 0.003 inch diameter wires, with a
single
break in its insulation, producing 64 non-contact unipolar electrodes
(figure
2). A multi-channel amplifier and computer workstation processes the
raw
far-field electrographic data. The system is applied in three steps:
1. Cardiac chamber geometry is established.
2. Site(s) critical for maintenance of reentry circuit(s) are
identified.
3. Ablation catheter is navigated to critical site(s).
A catheter locator system is
central to steps 1 and 3 while the inverse solution for reconstructing
endocardial electrograms is central to step 2. A 'locator' signal
emitted from the
mapping/ablation catheter allows its position to be determined relative
to the electrode array. It can be used to construct the
three-dimensional
computer model of the endocardium (virtual endocardium) by moving the
conventional
catheter around the cardiac chamber, thus building a series of
co-ordinates
for the endocardium. Reconstructed electrograms are then superimposed
onto
the virtual endocardium to produce isopotential maps.
Electrogram reconstruction has been validated by comparing the
morphology and timing of reconstructed electrograms with contact
electrograms from the
same endocardial location (as indicated by the catheter location
system). The accuracy of electrogram reconstruction is good, but
decreases with increasing distance between the electrode array and the
endocardium and this becomes significant for distances >34 mm.
The
main advantage of this system is that it requires only one beat to
reconstruct a complete activation map, which potentially allows mapping
of hemodynamically unstable arrhythmias. However, the distance between
the endocardial wall and the balloon center and the spatial complexity
of the activation patterns influences the accuracy of the electrogram
reconstruction negatively. This will affect the reliability when
mapping for example, dilated LVs or complex reentrant circuits.5,6,7
Contact Mapping system:
1) Electroanatomical mapping (CARTO System):
The
technology is based on a catheter location system, which determines a
mapping catheter's position and attitude within an ultra low magnetic
field emitted from radiators positioned under the operating table.
System Components:
The
catheter resembles a standard 7- or 8- Fr deflectable catheter with a
4-mm tip and proximal 2-mm ring electrodes. The location sensors lie
just
proximal to the tip electrode, totally embedded within the catheter.
The
three location sensors are located orthogonally to each other. The
locator
pad is placed beneath the operating table and includes three coils that
generate ultralow magnetic fields, which decay as a function of the
distance
from their sources. When the catheter is moved within this magnetic
field,
the currents generated represent movement in three dimensions. The
resolution
of the location capabilities of the system has been previously been
shown
to be <1 mm in both in vitro and in vivo studies.8
This information enables tracking of the tip of the mapping catheter
within
the heart, enabling navigation of the catheter independent of
fluoroscopy.
Signals received within the sensor are transmitted along the catheter
shaft
to the main processing unit.
Mapping procedure:
The
reference catheter is introduced and placed inside the coronary sinus
or in the right ventricle. The mapping/ ablation catheter is introduced
and
placed in the chamber being mapping. The mapping system determines the
location
and orientation of both the mapping and reference catheters. The
location
of the mapping catheter is gated to a fiducial point in the cardiac
cycle
and recorded relative to the location of the fixed reference catheter
at
that time, thus compensated for both subject and cardiac motion. By
moving the catheter inside the heart, the system continuously analyzes
its location and presents it to the user, thus enabling navigation
without the use of fluoroscopy.
The
mapping catheter is dragged over the endocardium, sequentially
acquiring the location of its tip together with the local electrogram
when the catheter is in stable contact with the wall. By sampling the
location of the catheter together with the local electrogram from a
plurality of endocardial sites, the three-dimensional anatomy of the
chamber is reconstructed in real-time. The local activation time is
then color-coded and superimposed on the anatomical map with red
indicating early-activated sites, blue and purple late activated areas,
and yellow and green areas intermediate activation times (figure
3,4,5).
The
stability of the catheter and contact is evaluated at every site by
examining the following criteria: 1) local activation time stability,
which is defined as the difference in ms between the local activation
calculated from two consecutive beats; 2) location stability, defined
as the distance in mm between two consecutive gated locations; 3)
morphological superpositioning of the intracardaic electrogram recorded
on two consecutive beats; and 4) cycle length stability, defined as the
difference between the cycle length of the last beat and the median
cycle length during the procedure.
The
three-dimensional geometry of the chamber is generated using a modified
"star" reconstruction algorithm. The sets of points from endocardial
surface are used for geometrical reconstruction. It is recommended to
map each chamber in a different reconstruction bin ('chamber setup"),
when mapping more than one chamber, which can be simultaneously
displayed.
The
electroanatomical maps can be presented in either two or three
dimensions as activation, isochronal, propagation, or voltage maps. The
activation maps display the local activation time color-coded overlaid
on the reconstructed three-dimensional geometry (figures 3,4,5).
The propagation map shows a dynamic color display of the propagation of
the activation wavefront across the reconstructed chamber. The voltage
map displays the peak-to-peak amplitude of the electrogram sampled at
each site. This value is color-coded, with red and purple indicating
areas with the lowest and highest amplitude, respectively (figures
3,4,5). The abnormal low voltage usually represents scar tissue and
thus may help in understanding the mechanism underlying the arrhythmia.
Electroanatomic
mapping on CARTO system of focal right atrial tachycardia showing
earliest activation (red color) below crista terminalis. LAO 40o
projection ( Figure 3A) and RAO 300 projection (figure 3B).
Isochronal
mapping on CARTO system of right ventricle showing earliest activation
(red color) in the outflow tract region suggestive of right ventricular
outflow tract ventricular tachycardia (AP projection).
Isochronal
mapping on CARTO system of left ventricle showing earliest activation
(red color) in the inferobasal region suggestive of left ventricular
tachycardia (RAO projection).
Clinical applications:
The
capabilities of the CARTO system to associate relevant
electrophysiological information with the appropriate spatial location
in the heart and to accurately determine the three-dimensional location
and orientation of the ablation catheter may be of great value in
guiding ablation. The technology enables one to perform the mapping
procedure and to potentially define the mechanism underlying the
arrhythmia, to design an ablation strategy, and finally to return
accurately to the desired site for the delivery of RF energy.
The
CARTO nonfluoroscopic electroanatomic mapping system enhances our
ability to analyze and ablate the arrhythmia in following ways: 1) it
allows us to associate the intracardiac electrogram with the
reconstructed three-dimension anatomy (electroanatomic mapping), which
helps to determine the site of arrhythmia origin, its mechanism, and
the importance of particular cardiac structures in sustaining the
arrhythmia circuit. Knowledge of the arrhythmia mechanism and circuit
(e.g., macroreentrant circuit versus focal origin) is critical in
planning arrhythmia ablation. Thus, macroreentrant tachyarrhythmias,
such as incisional atrial tachycardia, are best mapped using the CARTO
system2,9) Catheter can be relocated accurately to the points of
interest.
This enables RF ablation to be delivered by an anatomic approach. This
method is particularly useful in ablation of hemodynamically unstable
arrhythmias
such as ventricular tachycardia, because information can be acquired
and
stored from multiple short runs of tachycardia and ablation then
performed
in sinus rhythm3,10,11) important cardiac structures such as the His
bundle, ostia of venous structures, valve annuli and scar tissue can be
tagged
properly, which are avoided during ablation. The ability to tag
previously
unsuccessful ablation sites also helps to identify the correct location
for ablation, which may be useful in difficult cases of arrhythmia such
as atrial tachycardia or VT of focal origin or of right free wall
accessory
pathway. 4) Electroanatomical mapping, although not essential for
ablating
typical isthmus dependent atrial Flutter, is quite useful in a
recurrent
atrial flutter in locating breakthrough points in the line of block
that
has been created earlier. The advantage is greater when dealing with
postoperative
incisional flutter circuits. It also considerably reduces fluoroscopic
exposure
to radiation for the operator.
Limitations:
The
system is limited to point-to-point mapping strategy. Thus, if the
arrhythmias are non-sustained or quickly change to a different
morphology or mechanism, e.g., atrial tachycardia degenerates to atrial
fibrillation, mapping of
such arrhythmias will be difficult to complete and the procedure is
likely
to take considerable time.
2) Basket Catheter mapping:
Simultaneous mapping of multiple points has been performed using 64
-pole endocardial basket catheter (Constellation, EP Technologies),
which can be
deployed percutaneously. Current designs of basket arrays consist of a
series of equally spaced electrode pairs mounted on eight flexible
splines (A-H), and each spline contains eight 1.5 mm electrodes with
3-mm spacing, which can be straightened and advance from a percutaneous
sheath into the cardiac chamber of interest so that the splines deploy
and are apposed against the endocardium (figure 6A &B).
Figure 6A: Basket mapping of normal sinus rhythm. The right
upper panel shows the potential map, the earliest activation (red
color) at
the site of sinus node. In addition, the splines of basket catheters
are
aligned with the endocardial wall. The right lower panel shows
isochronal
map of the sinus rhythm. The left panel shows the electrogram from the
different splines. SN= Sinus node; TV= tricuspid valve; AVN=
Atrioventricular
node.
Figure
6B: Shows different splines of basket catheter in the right atrium.
The isochronal and potential maps are suggestive of ectopic atrial beat.
The basket catheter was connected via the amplifier to the
three-dimensional mapping system, which provides three-dimensional
color construction of
electrical activity. The signals are filtered from 30 to 400 Hz.
Bipolar
mapping is performed using signals from the electrodes on all eight
splines.
Resolution is limited to the proportion of electrodes that are in
contact with the endocardium and by unequal deployment and spacing of
the splines, especially if the geometry is distorted, for example by
left ventricular aneurysm. Nevertheless, successful ablation of
ventricular tachycardia in humans guided by basket mapping has been
described.12 Basket catheters have also
been deployed to assess right atrial activation patterns around the
crista terminalis before and after ablation, although the baskets were
withdrawn prior to the ablation procedures.13,14
A basket catheter has also been used to characterize atrial
fibrillation and demonstrate organization of atrial electrograms in
basket electrodes prior to atrial fibrillation termination.15 Although the spatial resolution requires
interpolation
of data, current design of basket mapping systems are able to display
animated
activation maps and also have catheter location systems, which allow
the
operator to guide an ablation catheter to a particular electrode on the
basket.
Advantages of Basket Catheter:
1) It provides simultaneous, multiple, stable recordings for most
of the endocardial surface. Depending on the location of the
arrhythmogenic substrate a complete coverage of the reentry circuit is
possible. 2) In addition to multiple simultaneous recordings, the
basket catheter provides stable, multiple pacing sites distributed
throughout the chamber where it is deployed. Thus, the entrainment of
arrhythmias could be remarkably facilitated. 3) The color-coded
animation images simplify the analysis of multielectrode recordings and
help in establishing the relationship between activation patterns and
anatomic structures. 4) In patients with atrial flutter, mapping with a
basket catheter provides the unique ability of recording from both the
sides of the Tricuspid valve and Inferior vena cava (TV-IVC) isthmus
simultaneously. 5) It allows high density mapping from infrequent
ectopic beats. 6)By mapping the precise location, the number of RF
ablations may be reduced substantially.
Limitations:
Basket catheters do not at all points orient themselves towards the
endocardium in the shape expected of them and this may vitiate our
judgment. Basket catheters have a relatively large area near the shaft
without electrodes, therefore during mapping some part of endocardium
is not in contact with the basket. The quality of recordings is
critically dependent on proper
selection of basket size. There is a potential risk of thrombo-embolism
with
left sided mapping.
3) Real-time positional management (Cardiac Pathways) EP system:
Reference and ablation catheters:
Two
reference catheters and one mapping/ablation catheter are introduced
percutaneously using subclavian and /or femoral approach. One reference
catheter is positioned in the coronary sinus, and the other in the
right ventricular (RV) apex. For ablation purpose, a 7F, 4-mm tip
bidirectional steerable cooled ablation catheter (Cardiac Pathways,
Sunnyvale, CA, USA) is used (figure 7). The cooled-tip ablation
catheter can be used in cooled or standard RF non-cooled mode. The
reference catheters (Cardiac Pathways) have a 6-F fixed curve distal
shaft. The shaft of the coronary sinus reference catheter contains nine
1-mm ring electrodes and one 2-mm tip electrode (interelectrode
distance 1 mm), whereas the RV reference and
the ablation catheters contain three 1-mm ring electrodes and one 4-mm
tip
electrodes (interelectrode distance 1 mm). The reference catheters are
equipped with four-ultrasound transducers and the ablation catheter
contains
three ultrasound transducers. The ultrasound transmitter device sends a
continuous cycle of ultrasound pulses (558.5 kHz) to the transducers of
the reference and ablation catheters. By measuring the time delay from
the departure of a transmitted ultrasound pulse and the reception of
this pulse at the other transducers, assuming a speed of sound in blood
of 1,550 m/sec, the distance between the individual transducers can be
calculated. These data are used to define the location of the
catheter(s) within the reference frame. Once the 3D-reference frame is
established, triangulation can be used to track the position of
additional transducers.
Isochronal mapping on Real Position Management
System, showing the reference catheter position in coronary sinus and
right atrium; and, ablation / mapping catheter in left atrium (AP
projection). CS= Coronary Sinus; RV= Right ventricle; ABL= ablation
catheter; RSPV= Right superior pulmonary vein; LSPV= left superior
pulmonary vein; LIPV= left inferior
pulmonary vein.
Because dimensional and structural characteristics of the catheter are
known,
it is possible to construct a real-time 3D graphic representation of
the
catheters, including the position of the electrodes and the transducer.
As one of the transducers is positioned distal to the deflection point
of
the shaft of the catheter, it is possible to display the curvature of
the
catheter as well. Furthermore, the real-time position management system
graphically
displays the beat-to-beat movement of the tip of the catheters.
Real-time position management (RPM) mapping system:
The
3D real-time position management and mapping system (Cardiac Pathways)
uses ultrasound-ranging techniques to determine the position of a
mapping/ ablation catheter relative to two reference catheters. The
mapping system consists of an acquisition module and an ultrasound
transmitter and receiver unit, both connected to a SPARC 20 computer
(Sun Microsystems, Palo Alto, CA, USA). The system is capable of
simultaneously processing (1) seven position
management catheters, (2) 24 bipolar/48 unipolar electrogram signals,
(3)
12- lead ECG and (4) two pressure signals. Signals are sampled at 3kHz
per
channel with a resolution of 14 bits. The high-pass filters are set at
30
Hz and the low-pass filters at 500 Hz. Electrograms and catheter
positions are stored on optical disk. The original position of the
reference catheters can be displayed on the real- time window, thereby
allowing repositioning of catheters after displacement (figure 8).
Isochronal
map of right atrium on Cardiac Pathways System showing the earliest
activation site (red color) at the junction of right atrium and
superior vena cava (AP Projection). The right panel of the picture
shows earliest activation signal in the ablation catheter (56 ms early
from the reference catheter). CS= Coronary Sinus; RV= Right ventricle;
ABL= ablation catheter.
The
real-time display of the catheter tip and the possibility of on-line
retrieving previous positions and curves of a catheter facilitate
repositioning of a catheter at previously marked sites. Use of this
system may result in
reduced fluoroscopy time. Routine application of the real-time position
management system requires only the use of special catheters; no
additional catheters or skin electrodes are needed. As any type of
catheter containing ultrasound transducers can be "tracked" within the
reference frame and used to locate positions, it is possible to
position, for example, two catheters simultaneously in the RA and the
LA to create lesions using this guidance system. This
may be useful in case of the creation of linear lesions in atrial
fibrillation patients.16 The ability to
create lesions within a defined area will allow systematic ablation of
endocardial zones of slow conduction critical for the perpetuation of
reentrant VT.17,18
The
limitation of this system is occasional failure of ultrasound
transducers, requiring replacement of a catheter. Secondly, a "Voltage
map" cannot be obtained unlike the other systems described.
Conclusion:
RF
ablation has become first-line therapy for many tachyarrhythmias. It
has a high short-term success rate, low complication rate, and results
in
improved quality of life compared with other treatments. One concern is
the potential effect of radiation exposure to both patient and medical
personnel
especially during long procedures. The newer mapping system may be
helpful
in this regard. Mapping systems does reduce fluoroscopic time for
ablation
without compromising efficacy or safety. The procedure time however may
be even longer with the newer mapping systems.
These newer mapping systems are useful tool for guiding catheter
ablation of unstable arrhythmias (e.g., ventricular tachycardia) and
complex atrial arrhythmias. The newer mapping systems at present are
not cost-effective for developing countries. Since the special
catheters used for nonfluoroscopic mapping are relatively expensive
than the conventional catheters, they
should not be used with due consideration, especially for arrhythmias
that
are relatively simple and easy to ablate, such as AVNRT, preexcitation
syndrome, or AV nodal junctional ablation.
Furthermore, these systems are continually being refined and the maps
obtained from these systems are not ideal yet. The learning curve with
the operators with this new and rapidly advancing technology is also an
issue. Currently, these systems remain an adjunct to the conventional
methods
of mapping to guide catheter ablation. More research and study is
required to define the primary indications for their usage.
Acknowledgements:
We
acknowledge the assistance of Dr. Ajay Naik in providing the CARTO
pictures, Joe Schuld for Endocardial Solutions Inc. mapping pictures,
Laura Sanchez for Basket catheter mapping pictures.
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