European Heart Journal – Cardiovascular Imaging (2014) 15, 32–40
doi:10.1093/ehjci/jet112
Two-dimensional echocardiographic features
of the inferior right atrial isthmus: the role
of vestibular thickness in catheter ablation of
atrial flutter
Pedro Marcos-Alberca 1*, Damián Sánchez-Quintana 2, José A. Cabrera3,
Jerónimo Farré4, José M. Rubio 4, Jose A. de Agustı́n1, Carlos Almerı́a1,
Leopoldo Pérez-Isla 1, and Carlos Macaya1
1
Cardiology Department, Instituto Cardiovascular, Hospital Clı́nico San Carlos, c/ Prof. Martı́n Lagos s/n. 28040 Madrid, Spain; 2Department of Human Anatomy, Universidad de
Extremadura, Badajoz, Spain; 3Arrhythmia Unit, Hospital Quirón, Madrid, Spain; and 4Cardiac Electrophysiology Unit, Fundación Jiménez Dı́az, Madrid, Spain
Received 6 March 2013; revised 12 May 2013; accepted after revision 17 May 2013; online publish-ahead-of-print 9 June 2013
Objectives
The aim of this study was to examine the feasibility of transthoracic two-dimensional (2D)-echocardiography in defining the
cavo-tricuspid isthmus (CTI) anatomy and its value concerning the ease of catheter ablation of isthmic atrial flutter (AF).
.....................................................................................................................................................................................
Methods
CTI analysis was accomplished in 39 cases: 16 necropsy specimens and 23 patients. Sixteen were patients with isthmusdependent AF and seven controls with other supraventricular re-entrant tachycardias. Two-dimensional transthoracic
echocardiography and a right atrium angiogram were performed before radiofrequency catheter ablation (RFCA).
.....................................................................................................................................................................................
Results
The measurements of the CTI with angiography were compared with those taken with echocardiography and correlation
was excellent (r ¼ 0.91; P , 0.0001). In normal patients, the dimension of the vestibular thickness was successfully
compared and validated with the histological examination of the necropsy specimens: histology median 6.8 mm, range
4.4 –10.5 vs. echo median 6.2 mm, range 5.4 –8.7; P: NS. Vestibular thickness was greater in complex than in simple
RFCA (13.6 + 1.9 mm vs. 10.0 + 2.3 mm; P ¼ 0.01). When vestibular thickness ≥11.5 mm, the ablation prone to be
complex (sensitivity 83.3%, specificity 80%, positive predictive value 71.4%, and negative predictive value 88.9%).
.....................................................................................................................................................................................
Conclusions
Two-dimensional transthoracic echocardiography clearly depicts the inferior isthmus and, displaying the thickness of the
tricuspid vestibule, it was related with complexity of the ablation procedure in isthmus-dependent AF.
----------------------------------------------------------------------------------------------------------------------------------------------------------Keywords
Echocardiography † Atrial flutter † Catheter ablation
Introduction
Isthmus-dependent common atrial flutter (AF) is the most frequent
type of macro-re-entrant atrial tachycardia and the treatment of
choice is radiofrequency catheter ablation (RFCA) with bidirectional
conduction block across the cavo-tricuspid isthmus (CTI).1 – 9 Nevertheless, ablation cannot be easily achieved in some patients, requiring
multiple energy applications and prolonged procedure times. Morphological and histological studies have shown the complex endocardial topography and the considerable variation in the content of
myocardium and fibro-fatty tissue of the CTI.10 – 12 These architectural factors may make it difficult to obtain complete and transmural
linear lesions across the isthmus, thus, resulting in conduction persistence or recovery at the ablation areas. The right atrial angiographic
dimensions and topographic configurations of the inferior isthmus
have been shown to be related with the complexity of the ablation
procedure, but the atrial wall of the isthmus cannot be visualized
with this technique.13 – 15 Intracardiac echocardiography may be
useful to characterize the isthmus anatomy and wall thickness.16,17
In contrast, there are a paucity of data about the role of transthoracic
echocardiography to well define the anatomy and the atrial wall thickness of the inferior right atrial isthmus.18,19
We examined the two-dimensional (2D)-transthoracic echocardiographic features of the isthmus region and their differences with
* Corresponding author. Tel: +34 913303290; fax: +34 913303290, Email: [email protected]
Published on behalf of the European Society of Cardiology. All rights reserved. & The Author 2013. For permissions please email: [email protected]
Two-dimensional echocardiographic features of the inferior right atrial isthmus
33
Figure 1 Echocardiographic and angiographic imaging of the inferior right atrial isthmus. (A): Transthoracic echocardiography: the CTI plane is
obtained applying to the transducer a clockwise rotation and simultaneous posterior tilting. This projection permits correctly visualize the area
between the opening of the inferior vena cava (IVC) into the RA and the tricuspid valve, at the same level of the opening of the coronary sinus
(OsCS); CTI area is clearly depicted. RV, right ventricle; RA, right atrium; LV, left ventricle. (B) Magnification (zoom) of the CTI area emphasized
in frame B (dot line). The elements of the CTI are easily disclosed. Asterisk denotes the thickness of the vestibule (endo-epicardial distance, perpendicular to the vestibule length). Double asterisks denote the depth of the recess, as the perpendicular distance from the recess length to the endocardial boundary. (C) Example of an angiography of the right atrium; processing; and measurement of the CTI and their elements, vestibule and recess.
Calibration of the pixel length with the known dimension of the electrode of the HALOw catheter (RV catheter in controls) was undertaken (arrow
heads) (D) Zoom view over the right inferior isthmus showing their elements (vestibule and recess). Double arrows denotes the dimensions measured as representative of both. Asterisk denotes the measurement of the recess depth. Examples are representatives of all performed.
regard to the laboriousness of catheter ablation in patients with
isthmic AF. The results were compared with right atrial angiography
and human heart specimens.
Methods
In this observational study, a total of 32 consecutive patients were
evaluated for their participation. Sixteen consecutive patients with
isthmus-dependent AF and seven control patients (75% men; 68 + 10
years) referred to our institution for RFCA (57% men, 66 + 5 years)
who underwent slow pathway ablation for clinically documented AV
nodal re-entrant tachycardia. Requirements for inclusion as a control
were the absence of any history of AF and inability to induce this arrhythmia at electrophysiological study. Groups were age and sex matched. In all
the patients, we performed a complete transthoracic echocardiographic
examination before the ablation procedure and, at the time of the ablation, a right atrial angiography before starting to apply radiofrequency
pulses. There were nine exclusions: five in whom the echocardiogram
was not performed before the RFCA procedure, two patients who
declined RFCA for the index arrhythmia, one patient finally diagnosed
of non-re-entrant atrial tachycardia, and one patient due to poor acoustic
window. Detailed information was given to all the patients and written
consent approved by the local ethic committee was obtained. Echocardiographic and angiographic anatomical isthmic variables for the global
and the simple or complex procedures of isthmus ablation were
compared. Focused to validate the feasibility of 2D-transthoracic echocardiography to analyse the atrial wall thickness across the isthmus, we
have included the histological examination of the isthmus in 16 necropsy
specimens and compared with normal control patients.
Echocardiographic examination of the CTI
The 2D-transthoracic echocardiogram was performed using Phillips
Sonos 5500 and Sonos 7500 (Koninklijke, Philips-Electronics, The Netherlands) equipped with harmonic fusion imaging and a broad-band S3
transducer operating at a frequency of 1.8 MHz of emission and
3.6 MHz of receipt. The patient was placed over its left side to perform
a standard 2D, transthoracic, and Doppler (colour, continuous, and
pulsed wave) examination. To visualize the CTI, we have developed a
novel echocardiographic projection by means of the modification of
the apical four-chamber views (Figure 1A and B).20 This projection
34
permits the visualization of the inferior right atrium (RA), which is proved
trough the exhibit of the opening of the coronary sinus and the opening of
the inferior vena cava (IVC) into the atria. Thus, between the tricuspid
valve (TV) and the Eustachian ridge of the IVC, the inferior CTI and
their elements are clearly delineated: the vestibule, as the plateau immediately behind the anterior leaflet of the TV and the recess, as the pouch-like
subeustachian zone close to the vestibule. Adjustment of gain settings and
grey scale was optimized to allow a precise endocardial and epicardial
definition, avoiding blooming or drop-out echoes which could influence
an accurate quantitation. Echocardiographic images were digitally
acquired and stored on a magneto-optical disk for subsequent retrieval
and detailed analysis of the total dimension of the CTI and their elements.
Measurements were carried out using the GPL Java-based-software
ImageJ-v1.3, based on the software NIH-Image of National Institutes of
Health. Similar to our previously reported angiographic method13, we
have measured the distances between the anterior leaflet of the TV
and the Eustachian valve as representative of the total echocardiographic
dimensions of the CTI, the length of the vestibule and the depth and
length of the posterior recess. In addition, the harmonic
2D-echocardiogram allows to disclose the epicardial and endocardial
boundaries of the atrial wall at the vestibular area, being the quantification
of this distance representative of the wall thickness of the anterior
isthmus. The measurement of isthmus length and their elements were
performed using the image immediately before the opening of the TV
(end-diastolic), similar to those obtained with the right atrial angiograms.
The post-procedural echocardiogram and measurement of the vestibular
thickness was not performed taking into account the potential effect of
the oedema in the dimension obtained.
Angiography of the RA: technique, analysis,
and measurement of the right atrial isthmus
As previously described13, right atrial angiography was performed
through an 8F sheath, 40 cm long, inserted into the right internal
femoral vein. The inferior CTI was profiled as the area between the Eustachian valve and the plane of the TV annulus. We distinguish the two
isthmic components: the ‘pouch-like’ posterior recess and the anterior
vestibule of the TV. Angiographic studies were acquired as a serial of
frames in the 458 RAO view and digitally recorded on PC-workstation
and magneto-optical disk. The frame representative of the inferior
isthmus and their elements were selected by the agreement of two
experienced investigators.
The measure of the CTI and their elements was performed as follows
(Figure 1C and D). The digital image in format TIFF of the angiogram was
stored and transferred to a personal computer. Thereafter, a postprocessing of the image was carried out to correct visualize and quantify
the CTI, using the GPL Java-based-software Image J v1.3. Spatial calibration was accomplished previously to any measurement using as reference
the length in pixels of the electrode (7-French HALO catheter) closer to
the isthmic area. In controls, calibration was performed over the 5-French
bipolar endocavitary ventricular catheter, the nearest to the CTI. We
selected these references to minimize the effect of foreshortening in
fluoroscopic images. In the full image, the CTI were measured as the distance between the Eustachian valve and the tricuspid plane. Applying a
zoom view over the CTI, the vestibule was correctly identified and measured. The recess was measured as the distance between the end of the
vestibule at the hinge point of the TV and the opening of the inferior caval
vein (ICV). The depth of the recess was measured as the distance
between the imaginary line that defines the recess and the bottom of
this. Using the methodology reported elsewhere, 14,15 we labelled the
CTI as straight if the perpendicular distance from the line traced from
P. Marcos-Alberca et al.
the TV to the ICV to the deepest point of the recess was ,2 mm,
concave for distances from 2 to 5 mm and pouche-like if distance was
.5 mm.
Electrophysiological study and isthmus
ablation
The mapping, pacing, and ablation protocols in AF has been published
elsewhere.4 – 8,21 – 25 Briefly, if the patient was in AF, electrical recordings
of right atrial activity were obtained, ensuring a counter-clockwise activation. Thereafter, conventional entrainment was accomplished using the
ablation catheter. If re-entry was confirmed and excitable gap identified
over the CTI area, an ablation line was performed at the central/inferior
isthmus (at the 6 o’clock region in a fluoroscopic left anterior oblique
view). It is worth emphasizing that standard 8-mm tip and four electrodes
ablation catheter was used in all patients. A dragging technique starting at
the ventricular side of the CTI and pulling the catheter back to the ICV
after predetermined timed application was used. When sequential activation was interrupted, pacing from both sides of the ablation line (coronary
sinus and right atrial free wall) was performed, ensuring the total interruption of the electrical conduction into the circuit of the flutter in
basal conditions and with isoprenaline infusion. Data collected included
the number of radiofrequency applications, total duration of them, and
total duration of fluoroscopy, including positioning of the sheaths and
all the endocavitary catheters (skin-to-skin). All procedures were performed in an unselected basis by one of two senior cardiac arrhythmologist, blinded to the result of the echocardiogram.
Anatomic dissection and microscopic
examination of the isthmus
We examined 16 formalin-fixed hearts from patients who died of noncardiac causes (69% male, mean age 56 + 14 years). The walls of the
RA were dissected to display the isthmic area between the TV anteriorly
and the Eustachian valve and ridge posteriorly (Figure 2). As previously
described, the endocardial surface of the isthmus showed a smooth anterior region that corresponds to the vestibule of the TV, which posteriorly continues with a subeustachian pouch with irregular muscular
trabeculations.10,12 Full thickness of the atrial wall at the isthmus area
were prepared for light microscopic studies. The sections were then
dehydrated and embedded in paraffin, cut into 10-mm-thick slides, and
stained with Masson or Jones trichrome for histological analysis. We measured the isthmus wall thickness from the endocardium to the epicardium
along the isthmus.
Statistical analysis
All continuous measurements were expressed as means + SD and categorical variables in absolute values and percentage, except for the comparison of echocardiographic and histological subanalysis of the
vestibular thickness, expressed as median and range. The measurements
obtained from the echocardiographic and the angiographic exams were
compared using the Wilcoxon test for paired-data, Pearson’s bivariate
correlation, and the bias with lineal regression and Bland – Altman
plots. With the non-parametric test Mann– Whitney U test and receiveroperator characteristic curve, we analyse the accuracy of the echocardiographic and angiographic parameters in the ablation procedure.
Categorical variables were analysed using the x2 test or Fisher’s exact
test, when necessary. Software used was SPSS (SPSS 15, Inc., Chicago,
IL, USA) and Medcalc (MedCalc Soft, Mariakerke, Belgium). Statistical
significance was established in a P-value ,0.05.
35
Two-dimensional echocardiographic features of the inferior right atrial isthmus
Table 1
Base-line characteristics of the study patients
Atrial flutter
Control
P-value
................................................................................
67.6 + 10.3
66.0 + 5.4
0.34
Sex, male (%)
LA diameter, echo (mm)
75
41 + 4a
57
32 + 6a
0.35
0.04
RA diameter, echo (mm)
55 + 7a
47 + 5a
0.04
LVEF (%)
Hypertension (%)
60 + 13
50
68 + 6
57
0.54
0.55
Age (years)
COPD (%)
31
0
—
Cardiomyopathy (%)
Ischaemic (%)
50
25
0
—
—
—
Hypertrophic (%)
6
—
—
6
56
—
0
—
—
Valvular (%)
Current AAD (%)
a
Figure 2 Top: heart specimen with the isthmus viewed in profile.
Bottom: histological section of the isthmus with Masson’s trichrome
stain (myocardium in red and fibrous tissue in green). The anterior
sector corresponds to the vestibule of the tricuspid valve (TV) and is
related to the right coronary artery. The posterior sector is closest
to the orifice of the inferior cava vein and contains the Eustachian
ridge or valve.
Absolute values and percentages, except age (means + SD). LA, left atrium,
antero-posterior axis; RA, right atrium, supero-inferior axis; LVEF, left ventricular
ejection fraction; COPD, chronic obstructive pulmonary disease; AAD,
anti-arrhythmic drugs.
Results
Patient characteristics and ablation
procedure
Patients relevant clinical characteristic of the study are given in
Table 1. Successful radiofrequency ablation was achieved in all the
patients without complications. In patients with isthmus-dependent
AF, bi-directional conduction block through the CTI was demonstrated at baseline and after infusion of isoprenaline. Procedural parameters for isthmus ablation were as follows: total procedural
duration 54.6 + 29.6 min; total fluoroscopic time 44.9 + 29.3 min;
the average number of RF applications/patient was 4.1 + 3. The cumulative duration of RF applications was 991 + 762 s, and those of
the successful application was 227 + 124 s. The cumulative duration
of RF applications, adjusted to the number of applications, showed
differences amid 1– 4 applications and 5 applications through the
highest value, which were 11 applications in our group. The mean
RF-time was 508.5 + 204 s in the former (n ¼ 10, 63%) and
1797.5 + 650 s in the later (n ¼ 6, 37%), P , 0.001. A cut value of
1200 s of RF established in this group a clear division between the
simple procedure and the complex procedure. (Figure 3). The ratio
of simple/complex procedures according to the operator leading
the ablation procedure was proportionate: operator A (n ¼ 10),
rate of complex ablation 0.33 and operator B (n ¼ 6), rate of
complex ablation 0.4 (P ¼ 0.79).
Figure 3 Bars chart showing the mean cumulative RF time
grouped by number of applications. A time value of 1200 s
(20 min) was chosen as the red line dividing simple from complex
procedures. Cut value is in consonance with others published elsewhere.
Imaging dimensions of the inferior RA:
angiographic and echocardiographic
comparisons
The overall angiographic and echocardiographic features of CTI were
similar, with a posterior pouch-like recess and the anterior linear
( plateau) vestibule of the TV. The correlation and the lineal regression of all echocardiographic and angiographic measures obtained
(Figure 4) disclosed an excellent result. The Bland– Altman plot was
in consonance with the global correlation. Angiographic and 2D
echocardiographic dimensions of the isthmus and their elements
are showed in Table 2. Difference between 2D echocardiographic
and angiographic measurement was statistically significant for the
depth of the posterior recess. As expected, angiographic and 2D
echocardiographic dimensions of the CTI were large in AF patients
36
P. Marcos-Alberca et al.
total time of RF during ablation trended to be longer whether the
CTI exceeds 35 mm (1207 + 968 s vs. 776 + 449 s; P ¼ 0.09). CTI
length did not differ between complex (.20 min) and simple
(,20 min of RF) subgroups in patients with AF: (37 + 8 vs. 37 +
5 mm; P ¼ NS). CTI was concave in 25% of the angiographies and
pouch-like in the remaining 75%. There were no patients with straight
morphologies or differences when complexity analysis was performed considering CTI qualitative morphology, concave or pouchlike (P ¼ 0.56, Fisher’s exact test).
The exclusive echocardiographic feature, the thickness of the vestibule, disclosed a greater value in the complex group, 13.6 + 1.9 vs.
10.0 + 2.3 mm; P ¼ 0.01 (Table 3 and Figure 5, top). The Receiveroperator characteristic curve pointed out an absolute value of vestibular thickness ≥11.5 mm as related with a complex procedure
with a sensitivity of 83.3% and a specificity of 80%, a positive predictive value of 71.4% and a negative predictive value 88.9% (Figure 5,
bottom).
Intra-observer and inter-observer
variability
Figure 4 Top: bivariate correlation of 2D-echocardiographic
measures and angiographic measures, fitted to a lineal regression
equation. Dot line represents the identity line. All measures in millimetres. Bottom: Bland – Altman plot of the 2D-echocardiographic
measures vs. angiographic dimensions. Continuous line points out
the bias and dot line + 2SD. All measures in millimetres.
opposite to the controls: 34 + 6 vs. 28 + 5 mm, P ¼ 0.05 for angiography and 37 + 8 vs. 30 + 5 mm, P ¼ 0.04 for 2D echocardiography.
Echocardiographic and histological wall
thickness of the inferior isthmus
12
As we previously described in the histological examination, the
smooth vestibular area showed the thickest overall atrial wall and
muscular content. We included as noteworthy, in echocardiography,
the vestibular thickness from the endocardium to the epicardium.
In control patients, and compared with structurally normal heart
specimens, no significant differences were found in the wall
thickness of the vestibule of TV between echocardiographic and
histological dimensions: median 6.2 mm (range: 5.4 – 8.7) for echocardiography vs. median 6.8 mm (range 4.4 – 0.5) for histological
dimension; P ¼ NS. Echocardiography revealed that patients with
AF disclosed a thicker vestibule of the TV than controls (11.4 +
2.8 vs. 6.6 + 1.2; P , 0.0001).
Complexity of isthmus ablation procedure:
echocardiographic predictors
The echocardiographic CTI was large in patients with AF (37 +
8 mm vs. controls 30 + 5 mm.; P ¼ 0.04) and the duration of the
In our current practice, the best adherence to accepted standards for
the measurements of two-dimensional echo images is encouraged. It
was expected an intra-observer and inter-observer variability equal
to overall measurements accomplished in 2D echocardiographic
examinations using harmonic imaging. To better approach, the
obtained result to the real-anatomic dimension, vestibular wall thickness, and distances between endocardial boundaries (i.e. the CTI)
were measured from leading to leading echoes, so trying to
exclude any fat of the AV groove. Mean bias, intraclass correlation,
and 95% confidence intervals of the variability analysis are displayed
in Table 4. A good correlation was found; the confidence intervals,
similar for intra-observer and inter-observer analysis, were high
due to limitations inherent to sample size. Quantification of the
angiographic images is not subject to the limitations of those of the
transthoracic echocardiography, mainly due to its fixed reference
points for the acquisition.13 – 15
Discussion
This study evaluates the feasibility of transthoracic 2D echocardiography to characterize the complex anatomical structures of the CTI.
Our finding showed that (i) transthoracic echocardiography was able
to easily visualize the endocardial topography of the isthmus and their
elements as accurately as right atrial angiography; (ii) an unique
echocardiographic feature, the wall thickness of the vestibule, was
an anatomic marker in our series, related with a difficult isthmus
ablation procedure. Our study also produced excellent correlation
between thickness measurements of the isthmus made during life
and at post-mortem examination, thus establishing the validity of
the echocardiographic technique.
Anatomy of the CTI: role of the imaging
techniques
The RA angiography was the first imaging technique used to determine the gross morphology and dimensions of the CTI both in
normal and patients with common AF.13 Angiography permits the
37
Two-dimensional echocardiographic features of the inferior right atrial isthmus
Table 2
Angiographic and 2D echocardiographic results
Angiography
2D echocardiography
P-value*
Correlation
P-value**
...............................................................................................................................................................................
No patients
23
23
Right atrium, supero-inferior axis
Cavo-tricuspid isthmus
53.0 + 9.5
32.0 + 6.0
50.9 + 6.7
34.8 + 7.6
NS
NS
0.64
0.73
,0.01
,0.001
Vestibule
13.0 + 2.8
14.4 + 4.1
NS
0.62
,0.001
Recess
Recess, depth
19.2 + 3.9
7.3 + 2.2
22.0 + 7.5
9.9 + 3.4
NS
,0.01
0.54
0.50
0.01
0.02
Measures are length in millimetres and expressed as means + SD.
*P-value for comparison of means, ANOVA.
**P-value for correlation of paired-data. Pearson’s bivariate correlation.
Table 3
Anatomical variables and RF time
RF time
..............................
<1200 s
P-value
≥1200 s
................................................................................
Two-dimensional echocardiography
CTI
Vestibule
37.3 + 8.4
14.4 + 2.8
37.3 + 5.8
15.5 + 5.6
0.87
1.0
Recess, length
24.0 + 9.8
22.3 + 6.4
0.95
Recess, depth
10.1 + 4.2
13.1 + 3.7
0.15
Vestibule, thickness
RA, length
10.0 + 2.3
52.1 + 6.5
13.6 + 1.9
56.6 + 7.2
0.01
0.38
33.7 + 6.0
13.5 + 2.5
33.3 + 5.3
14.4 + 1.4
1.0
0.71
Recess, length
19.7 + 4.6
18.1 + 3.4
1.0
Recess, depth
RA, length
8.4 + 2.4
51.6 + 9.7
7.1 + 1.4
58.9 + 2.3
0.5
0.14
Angiography
CTI
Vestibule
RF, radiofrequency. Data are means + SD, in millimetres.
visualization of the structures such as the Eustachian valve, caval veins,
and TV with an excellent correlation with post-mortem cardiac specimens.13 Two subsequent studies have confirmed the utility of the
angiography to disclose the particular characteristics of the inferior
RA. It could facilitate ablation in difficult cases due to the highly variable anatomy of the CTI. For instances, these studies point out an
enlarged inferior isthmus, prominent eustachian valve, and a welldeveloped pouch-like recess as predictors of difficult isthmus ablation procedures.14,15 In our analysis, most patients had an isthmus
concave and especially pouch-like configuration. However, the
morphology of the CTI was not related with the complexity of the
procedure, but probably is unpowered for this categorical variable.
In previous studies, we have demonstrated by histological examination the non-uniform wall thickness of the CTI area.10,12 Thus,
according to these anatomic observations, interest merges focused
in the visualization of the isthmus wall thickness during the ablation
Figure 5 Top: Boxplot (median, maximum, and minimum) of the
dimension of the thickness of the vestibule in simple vs. complex
procedures. Difference was statistically significant. Bottom: the
ROC curve and analysis showed the best result to predict the complexity of the procedure for a cut value of 11.5 mm of vestibular
thickness: sensitivity 83.3 (95% CI: 36.1 – 97.2); specificity 80.0
(95% CI: 44.4 – 96.9).
38
P. Marcos-Alberca et al.
Table 4
Variability analysis
Intraclass Mean Dispersion 95% CI (%)
(%)
correlation bias
(mm)
................................................................................
Intra-observer
Wall
0.87
thickness
Endocardial 0.82
boundaries
Inter-observer
0.3
1.0
231.1 to +33.1
1.7
3.8
219.5 to +27.1
0.70
20.1
21.8
235.6 to 32.0
Endocardial 0.88
boundaries
20.8
21.8
222.5 to 19.0
Wall
thickness
procedure in patients with AF. However, current imaging techniques
used in AF ablation are limited in this context. Fluoroscopic angiography only delineates the endocardial boundary, not allowing visualization of the wall thickness. Intracardiac echocardiography allows
the observation and measurement of the isthmus thickness and
found that the CTI was significantly thicker adjacent to the tricuspid
annulus (vestibular area).16,17 Nonetheless, and opposite to transthoracic echocardiography, its use not precludes invasive catheterization, is expensive and not widely accessible.
Transthoracic echocardiography in atrial
flutter and implications for catheter
ablation
The feasibility of transthoracic echocardiograpy to provide anatomic
information of the isthmus architecture remains to be systematically
analysed. The length of the CTI quantified by means of echocardiography influenced several parameters of the ablation procedure, such
as the duration and number of applications of RF.18 However, the
components and the wall thickness of the CTI were not analysed in
this study. A plane very similar to the right ventricular inflow view,
but clearly different to our named CTI-plane, has observed an association between the extension of the Eustachian ridge and difficult ablation, but measurements were not compared with RA angiography
and the wall thickness was not evaluated.19 Novel echocardiographic
techniques using real-time three-dimensional echocardiography
remark this association of Eustachian valve with more laborious
RFCA. However, it implies transoesophageal intubation, it was only
used in patients under general anaesthesia, quantitative correlations
with RA angiography are lacking and the low spatial resolution of 3D
imaging not allows wall thickness analysis.26 Other non-invasive
imaging techniques, such as cardiac MRI or cardiac computed tomography, could better depict the CTI, their elements and the vestibular
thickness, but the experience is either very limited 27,28 or implies exposure to ionizing radiation and iodine contrast.29
Our work characterizes and facilitates the echocardiographic
technique to study the anatomy of the RA through the use of the
tissue harmonic technology and improves the visualization of the inferior RA with the modified apical view, so-called by us CTI plane.20
Harmonic imaging displays a higher signal-to-noise ratio and an increase of the spatial resolution of the structures more lateral and
distant of the transducer, such as the RA and the opening of the
IVC. In addition, it reduces side-lobe artefacts, resulting in a darker
cavity and brighter walls, thereby improving image contrast and the
visualization of the endocardial and the epicardial wall boundaries.30
The advantages of tissue harmonic imaging is clearly remarked by the
fact that in our analysis, in spite of a 31% of patients with a past history
of chronic obstructive pulmonary diseases, only one patient was
excluded due to poor acoustic window, highlighting the feasibility
of harmonic transthoracic 2D echocardiography for our purposes.
The CTI and their principal elements were clearly delineated and
the measurements obtained correlated significantly with those
obtained by angiographic studies. Differences in the ortogonality of
the cross-sectional echocardiographic plane (apical and posterior)
opposite to the fluoroscopic plane (right anterior oblique at 458)
account for the overestimation with the 2D echocardiography.
This different cut-angle in relation to the CTI is evident in the estimate
of recess depth, as indicated by a correlation index of only 0.54.
Vestibular thickness and isthmus ablation
complexity
Noteworthy, our transthoracic echocardiographic approach
allowed us to visualize the epicardial and endocardial edges of the
vestibule and, therefore, the thickness of this isthmus component.
The vestibular thickness in normal patients was closely related with
those found in post-mortem specimens, contributing to validate it.
In addition to the variable angiographic morphologies and length of
the CTI,14,15 the echocardiographic quantification of the vestibular
thickness merges as an unique non-invasive anatomic landmark
related to the complexity of the ablation procedure. Recently,
cardiac computed tomography has demonstrated the close relation
between isthmus wall thickness and complex ablation supporting our
results.31
The present study demonstrated that a vestibular thickness
≥11.5 mm was related with a superior complexity of the isthmus ablation procedure. Previous studies have shown the presence of ‘gaps’
or remnant tissue from isthmus ablated areas, commonly occurring in
the vestibule of TV.32 It has been suggested that transmural ablation of
the atrial wall could be required to achieve success.33 Our past anatomical study showed a thicker musculature and criss-crossing muscular fibres at the vestibular isthmus area that may require deeper
lesion.12 In addition blood flow of the right coronary artery near
the radiofrequency ablation area in the vestibule of the TV may
reduce lesion size by convective cooling. In the same way, an intramyocardial ‘small coronary vein’ extending through the vestibule
may produce similar cooling preventing transmural lesion formation,
and even preserving myocardial tissue in the isthmic scar.12,34
Practical implication of anatomic differences of the CTI for RFCA
of AF also has been investigated to enhance the efficacy of the procedure. Hence, when angiography revealed a straight isthmus morphology, ablation using a 8-mm tip catheter (the standard technique)
was superior to externally cooled-tip catheter while the opposite
tends to take place if the isthmus was concave, although the latter
not achieve a significant difference.35 A strategy of RFCA based on
targeting high-voltages electrograms obviating a complete ablation
39
Two-dimensional echocardiographic features of the inferior right atrial isthmus
line appears very attractive.36 – 38 It establishes a plausible link
between difficult RFCA and more or less prominent muscle
bundles and with our observed influence of the vestibular thickness,
supported with the close relation of echocardiography and necropsy.
A full contact of ablation catheter with the isthmus when the vestibule
is thicker could be anticipated with recent technologies.39
Limitations of the study
Although the patients were consecutive, the feature of structural differences between simple and complex procedures at the level of the
vestibular thickness is the result of a post hoc hypothesis based on the
differences observed in radiofrequency times, the feasibility of CTI
echo visualization and our previous experience with necropsy specimens10,12 or hystopathological examinations of RFCA lesions.40
Since, our hypothesis needs to be specifically tested. The exposition
to fluoroscopy seems to be high, but it is necessary to consider that
the time reflects the complete skin-to-skin procedure. When only
ablation-related fluoroscopic time is analysed, that was similar to previously reported (13.3 + 6.6 min).14,35
Our protocol of RFCA of common atrial flutter usually not
includes the use of electroanatomic mapping, which has also been
described to have a good correlation with RA angiography.41 Nonetheless, electroanatomic mapping not allows to depict how gross is
the vestibule, opposite to echocardiography.
We are aware of the general reservation in the medical community
related to the accuracy of transthoracic echocardiography in the clinical setting due to its strong dependence of personal skills and optimal
acoustic window, as such as possible interanatomic differences if other
populations are studied. As a feasibility study, further prospective
studies are mandatory to widely accept any novel diagnostic or prognostic utility using transthoracic echocardiography, ideally evaluating
intercentre reproducibility. Intrinsic difficulties to design a multicentre
study could limit to test any innovative echocardiographic plane.
Conclusion
Two-dimensional echocardiography implemented with tissue harmonic imaging is a useful and valid cardiac imaging technique to
clearly disclose and measure the CTI and their elements. It
emerges as a non-invasive approach to study the morphology of
the inferior RA, without the risks inherent to the current invasive procedure, the RA angiography using contrast. In addition, it permits the
visualization of a novel structural feature, the thickness of the vestibule, which measurements were closely related with the complexity
of the ablation procedure.
Funding
Funding received from Grant SAF2004-06864 (J.A.C. and D.S.-Q.) from
Ministerio de Educaión y Ciencia, Spain, Fondo de Investigaciones de la
Seguridad Social and Redes Temáticas de Cooperación; Red Cardiovascular C01/03 (J.A.C and J.F.)
Conflict of interest: none declared.
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