Sunil Roy T.N.,
MBBS, MD, DM*, Vikram Sankar, MBBS, MD*, Johnson Francis, MBBS,
MD, DM*, Hiroshi Tada, MD†
*Medical College Calicut, Kerala,
India; †Division of Cardiology, Gunma Prefectural
Cardiovascular Center, Maebashi, Gunma Japan.
Address for
correspondence: Dr. Sunil Roy T.N, Senior Lecturer in
Cardiology, Medical College Hospital, Calicut, Kerala, India. E mail:
sunilroytn@hotmail.com
Abstract
Several strategies for mapping ventricular outflow tract tachycardia
have been reported as useful indices for differentiating between those
originating from the right and the left side. Recently, tissue tracking
imaging (TTI) has been demonstrated as a novel non-invasive modality
for identifying the origin of outflow tract tachycardias. Tissue
tracking imaging is an ultrasonographic technique that measures the
myocardial motion amplitude towards the transducer in each region
during systole, identifying regional myocardial displacement on the
basis of myocardial velocities using color Doppler myocardial imaging
principles. In this technique, the origin of the arrhythmia could be
recognized as the site where the earliest color-coded signal (ECCS)
appeared on the myocardium at the onset of the systole. In preliminary
studies this modality was found to be useful in differentiating outflow
tract ventricular tachycardias. ECCS was always found below or at
the level of the pulmonary valve in all arrhythmias which could be
ablated from the right ventricular outflow tract, while in those where
the ECCS was above and close to the pulmonary valve it could be ablated
from the left
sinus of valsalva. These results indicate that TTI can provide detailed
and accurate information on the arrhythmia origin of outflow tract
tachycardia and may be
useful for differentiating between an outflow tract tachycardia
originating from the LV
epicardium remote from the LSV and that from the LSV. Newer advances in
echocardiographic technologies like high resolution, high frame rate
real time three dimensional echocardiography with speckle tracking may
further improve the precise localization of arrhythmias in the future.
Keywords: Tissue tracking imaging; Ventricular
tachycardia; Earliest color-coded signal; Myocardial motion amplitude;
Speckle tracking
Radiofrequency
(RF) catheter ablation has been established as an effective and
curative therapy for ventricular tachycardias (VTs) or symptomatic
premature ventricular contractions (PVCs) originating from the outflow
tract (OT-VT/PVCs) in structurally normal hearts.
1 Most of these arrhythmias have their
origin in the septal aspect of the right ventricular outflow tract
(RVOT), but some originate from the free wall of the RVOT, or from the
endocardium of the left ventricular outflow tract (LVOT).
2 Thus OT-VT/PVC can originate from
several
different portions of the outflow tract and RF ablation should be
attempted at the optimal site of the outflow tract to cure the
OT-VT/PVC.
Several non invasive modalities have been used to predict tachycardia
focus. However these methods have only modest diagnostic accuracy.
Recently, idiopathic OT-VT/PVC originating from the left ventricular
(LV)
epicardium (Epi-VT/PVCs) have been reported.
3
Correct identification of the latter type of OT-VT/PVC before the RF
energy applications is important in order to avoid futile RF
applications from the left sinus of Valsalva (LSV) and the ensuing
complications.
Several
strategies for mapping RVOT VT have been used including activation
mapping, pace mapping, use of basket catheter,
4
and non-contact mapping systems.
5
Precordial R wave transition, QRS morphology in lead I, R wave duration
and R/S amplitude of lead V1 or V2 have been reported as useful indices
for differentiating OT-VT/PVCs originating from the right side from
those
originating from the left side.
6
However some overlap for each index was noted between these two groups
especially in children.
7
Activation mapping can be used in sustained arrhythmias and pace
mapping in case where clinical arrhythmia is unstable. Pace mapping and
activation mapping are still the preferred methods to guide ablation in
majority of cases of RVOT-VT as their effectiveness have been well
proven. Non contact mapping provides a global endocardial activation
map to guide ablation.
8
However RVOT is a complex and narrow three dimensional structure. There
is safety concern of positioning the non contact mapping catheter in
such a narrow area.
9
Algorithm
based on surface ECG has overall sensitivity of 88 % and specificity of
95% for localization of OT-VT/PVCs.
6
Body surface mapping has diagnostic accuracy varying from 77% to 92%.
2 In a comparative study of non
contact mapping with conventional mapping by Ribbing M et al,
higher success rate was obtained by non contact mapping (100% versus
71.5 % ) against conventional mapping.
5
But non contact mapping was associated with higher cost, longer
procedure time and Fluoroscopy time. Preliminary studies by 3 D
mapping techniques have shown it to be as effective as other
conventional methods for localization of OT-VT/PVCs.
10
Recently,
tissue tracking imaging (TTI) has been demonstrated as a novel
non-invasive modality for identifying the origin of OT-VT/PVCs.
11 TTI is an ultrasonographic technique
that measures the myocardial motion amplitude toward the transducer in
each region during systole, identifying regional myocardial
displacement on the basis of myocardial velocities using color Doppler
myocardial imaging principles.
12
It allows rapid semiquantitative visual assessment of the systolic
distance of the tissue motion along the Doppler axis using a graded
color display. In this technique, the origin of the OT-VT/PVC could be
recognized as the site where the earliest color-coded signal (ECCS)
appeared on the myocardium at the onset of the OT-VT/PVC. ECCS was
always found below or at the level of the pulmonary valve, in all
arrhythmias which could be ablated from the RVOT whereas the
OT-VT/PVCs in which the earliest ventricular activation was recorded
from the LSV had the ECCS in the myocardium above the pulmonary valve.
Most of the OT-VT/PVCs in which the ECCS appeared close to the
pulmonary
valve could be ablated from the LSV. These results
indicate that TTI can provide detailed and accurate information on the
arrhythmia origin of OT-VT/PVCs and may be useful for differentiating
between an OT-VT/PVC originating from the LV epicardium remote from the
LSV and that from the LSV.
13
Hence TTI may be useful for identifying tachycardia origin
noninvasively before the ablation procedure.
Tissue
tracking imaging visualizes the velocity-time
integrals (VTI) of different regions in a color, 2D mode online,
together with the systolic wall motion. Tissue tracking displays the
integral of the tissue velocity during systole, which equals the
distance motion along the Doppler axis during systole. There are seven
colour bands that indicate different distances of motion with a
stepwise increase in the distance of the motion
14.
The site of the
arrhythmia origin was defined as the site at which the earliest colour
coded signal (ECCS) appeared in the myocardium during the first beat of
the ventricular tachycardia from all of the obtained images.
In the article
by Tada H et al
11, TTI was
performed in 33 patients with idiopathic
ventricular arrhythmias before radiofrequency catheter
ablation. The ECCS during the arrhythmia was easily identified from the
echocardiography by tissue tracking in all patients. The site of the
arrhythmia origin, defined as the site where the ECCS appeared on the
myocardium at the onset of the arrhythmia,
corresponded to the site of origin as determined on fluoroscopy during
activation mapping in all patients. TTI provided detailed and accurate
information on the arrhythmia origin, especially in the outflow tract
and facilitates catheter ablation of idiopathic ventricular
arrhythmias. Radiofrequency energy delivery at the site where the
earliest ventricular activation was recorded abolished 29 arrhythmias.
Among the 7 arrhythmias in which ECCS were found in the myocardium
above the septum, 5 arrhythmias were successfully ablated from the left
sinus of Valsalva. The distance between the attachment of the pulmonary
valve to the septum and the center of the ECCS (8±4 mm) in these
OT-VT/PVCs with a successful ablation from the LSV was significantly
shorter than that in those with a failed ablation (18±6 mm,
p<0.05).
13 These results
indicate that TTI can provide detailed and accurate information on the
arrhythmia origin of OT-VT/PVCs and may be useful for differentiating
between an OT-VT/PVC originating from the LV epicardium remote from the
LSV and that from the LSV. However, total number of patients studied
were small and they did not compare TTI with other diagnostic methods.
Hence no clear conclusion could be drawn about the superiority of TTI
method against other non invasive methods.
However some
questions remains to be answered before it can be considered as the
modality of choice. Tissue tracking imaging has limitations inherent to
tissue Doppler imaging technique.
15
In the cardiac motion there are translational, rotational and
deformational movements. Besides, many tissues near the heart move due
to transmitted cardiac motion, vessel pulsation and respiratory motion.
In addition if the initial activation occurs in a plane perpendicular
to interrogating plane, tissue tracking may show tissue
displacement starting from adjacent areas due to tethering
movements. This issue is further
complicated in patients with regional wall motion abnormalities
16. We
need to further perform the procedure during the ablation procedure to
see whether the ablation site correspond exactly to the site at
which ECCS were recorded. It is also unclear how successful the
ablation would be in the absence of other invasive mapping techniques.
Myocardial
deformation can be assessed from the B mode images by automated point
wise tracking of speckle patterns. It has the advantage of angle
independence and may be superior to tissue Doppler based techniques for
strain estimation.
17
Whether speckle tracking imaging will be useful for identifying
breakthrough points of excitation in different arrhythmias and
accessory pathway localization needs to be evaluated. High resolution,
high frame rate real time three dimensional echocardiography with
speckle tracking imaging may be the modality for precise localization
of arrhythmias in the future.
Electrical
phenomena and its surrogate, the propagation of myocardial contraction,
are three-dimensional events with variable sites of origin and
distribution patterns. Next generation maps must be able to map the
full thickness of the atrial and ventricular wall. One major challenge
in clinical electrophysiology is to precisely visualize target cardiac
structures and simultaneously depict electrical events. Using
intracardiac ultrasound with high TDI frame rates, TDI is suitable for
displaying more rapid changes in tissue motion related to the onset and
propagation of electrical excitation in the myocardium at a very early
stage with high temporal resolution. This ultrasound imaging technique
should have an important impact on the diagnosis and management of
cardiac arrhythmias. Such technology will foster simultaneous single
modality visualization of anatomy, function and electrical events for
the purpose of refining the proper intervention.
Acknowledgement
We wish to
acknowledge the kind help of Prof. Mark E. Josephson in improving this
manuscript.
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