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Credits:Elad Anter, MD, Mathew D. Hutchinson, MD, David J. Callans, MD
Cardiovascular Division, Department of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania .
Corresponding Address : David J. Callans, MD, Hospital of the University of Pennsylvania, Cardiovascular Division, 9 Founders Pavilion, 3400 Spruce St, Philadelphia, PA 19104.
Radiofrequency
catheter ablation of pulmonary veins has emerged as an effective therapy for
patients with symptomatic atrial fibrillation. Advances in real-time
intracardiac echocardiography with 2D and Doppler color flow imaging have led
to its integration in atrial fibrillation ablation procedures. It allows
imaging of the left atrium and pulmonary veins, including identification of anatomic
variations. It has an important role in guiding transseptal catheterization, imaging
the pulmonary vein ostia, assisting in accurate placement of mapping and
ablation catheters, monitoring lesion morphology and flow changes in the
ablated pulmonary veins, hence allowing titration of energy delivery.
Importantly, it allows instant detection of procedural complications.
Atrial
fibrillation (AF) is the most common arrhythmia encountered in clinical
practice, accounting for approximately one third of hospitalizations for
cardiac rhythm disturbances [1]. AF is characterized by seemingly disorganized
atrial electrical activity without effective atrial contraction. It was once
thought that all AF was caused by a single mechanism of multiple wavelets
propagating in random fashion throughout the atria. According to the multiple-wavelet
hypothesis, fractionation of wavefronts propagating through the atria results
in self-perpetuating “daughter wavelets.” Simultaneous recordings from multiple
electrodes supported the multiple wavelet hypothesis in human subjects [2]. However, it has become apparent that there
are likely other mechanisms underlying AF. In many patients, AF is caused by a
focal discharge at rapid rates. A focal origin of AF was originally supported
by experimental models of aconitine and pacing-induced AF, in which the arrhythmia
persists only in isolated regions of atrial myocardium [3]. This theory received minimal attention until
the important observation that a focal source for AF could be identified in
humans and ablation of this source could extinguish AF [4]. The observation that AF could be initiated
by ectopic beats originating in the pulmonary veins (PVs) sparked new interest
in the focal catheter-based techniques to isolate the PVs from the surrounding
left atrium [4]. Initial attempts to identify and ablate the
PV foci directly were only moderately successful and were associated with
frequent recurrences of AF and a significant risk of PV stenosis. The efficacy
and safety have improved using PV electrical isolation coupled with the use of
three-dimensional electro-anatomic mapping systems, intracardiac
echocardiography (ICE) and special mapping catheters [5]. ICE imaging has become an important player in
AF ablation. It guides transseptal catheterization, confirms the accurate
placement of mapping and ablation catheters, images the PV ostia, and assists
in the early detection of procedural complications.
Over
the past 30 years, electrophysiological procedures have been performed almost
exclusively under fluoroscopic guidance. Although, two-dimensional
"cardiac silhouette" imaging correlates reasonably well with cardiac
anatomy, it requires substantial operator experience. Moreover, the increased
complexity of some ablative procedures requires more accurate imaging tools.
Although, transesophageal echocardiography has been used for such cases, it
carries major disadvantages, including prolonged placement requiring heavy
sedation, and the risk of vagal nerve stimulation.
ICE
allows visualization of the heart from within the cardiac chambers or from
within the great vessels. Catheter-based ICE has advanced from devices bearing
single-element transducers and M-mode transducers to current technology, which
allows for higher resolution two -dimensional imaging with wave Doppler and
color flow evaluation of blood vessels and intracardiac structures. This
technology was initially limited due to the large size of the lower frequency
ICE catheters. Over the past 20 years, technology has progressed with the
advent of low frequency (12.5–9 MHz, 9 Fr) transducers allowing enhanced tissue
penetration and higher resolution. More recently, a 5.5-10MHz, 9 Fr electronic phase-arrayed
ultrasound catheter with pulsed/continuous-wave Doppler and color flow imaging
has been developed. This ultrasound catheter has a flexible tip that provides
higher resolution and deeper penetration of the left side of the heart from the
right atrium.
Mapping
and ablation in the left atrium are performed through a transseptal approach. Intracardiac
echocardiography provides the best available imaging tool for guiding
transseptal catheterization. Patients undergoing AF ablation typically need
dual transseptal catheterization (8Fr sheaths). Knowledge of the septal anatomy
and its relationship to adjacent structures is essential for safe and effective
access to the left atrium. The true interatrial septum is limited to the floor
of the fossa ovalis, flap valve, and anteroinferior rim of the fossa [6]. Many apparent septal structures are not truly
septal, and inadvertent puncture of some septal structures can lead to
perforation of the lateral wall of the left atrium or aortic root. These
potentially lethal complications can occur even with the most experienced operator.
The challenge for a successful atrial septal puncture is positioning the Brockenbough
needle at the thinnest aspect of the atrial septum. ICE provides excellent
views of the fossa ovalis and of the transseptal apparatus. Utilization of ICE
in conjunction with fluoroscopy allows the electrophysiologist to clearly identify
the interatrial septum and adjacent structures. When advanced in the right
atrium, the catheter provides a cross-sectional view of the fossa ovalis. It
allows checking the position of the Brockenbough needle and the Mullins sheath
in the middle of the fossa ovalis and tenting of the membranous septum at the
time of fossa ovalis puncture [figure 1] [7, 8].
Figure 1A: ICE-guided transseptal puncture
These ICE images, with the transducer placed in the right atrium (RA), show (a) a transseptal needle (arrow) tenting the interatrial septum at the fossa ovalis; (b) Advancement of the transseptal needle tip to the left atrium (LA) was then performed. After confirmation of optimal position in the LA, a sheath (arrow) was advanced over a wire to the LA.
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Ablation
of AF requires accurate anatomical information of the PVs. The PVs can have
variable anatomy, with most heart examined found to have four PVs with discrete
ostia, however the remainder (approximately 25%) having a common ostium, either
on the left or on the right. The right PV generally has upper, middle, and
lower pulmonary veins, with the right upper and middle PVs sharing a common
ostium separated from the lower right PV by a carina. The PV ostia in patients
with AF tend to be ellipsoid with longer superoinferior dimension, and
funnel-shaped ostia [9]. ICE provides detailed imaging of the
pulmonary veins including ostial diameter, which can assist in selection an
appropriately-sized circular multipolar mapping catheter [figure
2].
Figure 2A : ICE images of the pulmonary veins.
ICE images with the transducer placed in the right atrium, showing: (a) right upper (RUPV), middle (RMPV), and lower (RLPV) pulmonary veins; (b) separate left upper (LUPV) and lower pulmonary vein (LLPV) ostia; (c) color flow directed to the left atrium from the separate left pulmonary vein ostia; (d) Pulsed Doppler spectrum recording of the left upper pulmonary vein ostium with systolic and diastolic components.
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After
successful transseptal catheterization, the circular multipolar mapping
catheter and the mapping/ablation catheter are advanced through the sheaths
into the different PV ostia. ICE helps to optimally position the catheters in
place. Based on the size measured with ICE Doppler color flow imaging, an
appropriate circular multipolar catheter is placed on either an individual
ostium or a common one. When the common ostial diameter is larger than that of
the circular mapping catheter used, the latter can be positioned at the upper
portion of the ostium and then moved to the lower portion under ICE imaging
guidance. ICE imaging also confirms that delivery of radiofrequency energy via
the ablation catheter occurs proximal to the multipolar mapping catheter and
not inside the PV.
Doppler
color flow imaging has been effectively used for monitoring pulmonary vein
ostial narrowing during AF ablation [10]. Peak flow velocity at the pulmonary vein
ostium is measured at systole and diastole before and after ablation [figure 2D]. The ultrasound beam should be within 1 cm of the
PV ostium, and the pulsed Doppler sampling gate should be parallel to the PV
ostium. The peak pressure gradient can be estimated using the simplified Bernoulli
equation (∆P = 4V2) [11]. An increase in flow velocity greater than 100
cm/sec warrants redirection of the ablation lesions to a more a proximal zone.
In our early experience, radiofrequency energy was deployed at a total of 219
PV ostia and changes in PV ostial peak flow velocities and pressure gradients
were measured [10]. The peak velocity of PV ostial flow measured 56
± 12cm/sec (range 21-98) before and 101 ± 22cm/sec (range 47-211) after ablation
(p<0.001). Turbulent flow features with spectral broadening of Doppler
signal recorded at the ablated PV has been observed when the peak velocity was
greater than 130 cm/sec [figure 3]. Patients with an acute
rise in PV flow velocities following ablation were followed for a period of six
to eighteen months. Periodic clinical evaluations for symptoms of PV stenosis
(dyspnea, exercise intolerance) were corroborated with magnetic resonance
imaging (MRI) or contrast-enhanced computed tomography. The study showed that
an acute increase in the PV ostial peak flow velocity of up to 158 cm/sec (estimated
pressure gradient ≥10 mmHg) appears to be well tolerated [10]. It is our practice to conduct ablation
lesions when the peak PV flow velocity change is less than 100 cm/sec. However,
a flow velocity change of more than 100cm/sec will warrant a more proximal
approach to lesion deployment. Interestingly, in the majority of patients, the
acute rise in PV ostial velocities probably reflects tissue edema, as we noted
almost complete reversibility in PV velocities in the subset of who returned
for a second ablation procedure [10].
Figure 2D: ICE images of the pulmonary veins.
ICE images with the transducer placed in the right atrium, showing: (a) right upper (RUPV), middle (RMPV), and lower (RLPV) pulmonary veins; (b) separate left upper (LUPV) and lower pulmonary vein (LLPV) ostia; (c) color flow directed to the left atrium from the separate left pulmonary vein ostia; (d) Pulsed Doppler spectrum recording of the left upper pulmonary vein ostium with systolic and diastolic components.
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Monitoring
ostial flow velocity during repeated ablation at previously ablated pulmonary
veins is also critical. We have previously reported the outcome of our first 13
patients undergoing repeat AF ablation procedure [10]. There were two patients with PV flow
velocities >100cm/sec before subsequent ablation. Following
subsequent ablation, in three PV, velocities greater then 158 cm/sec were
recorded. In one patient, the left upper PV flow velocity increased from 116 to
194 cm/sec. In another patient, the left upper PV flow velocity increased from
118 to 172 cm/sec and the left lower PV from 83 to 176 cm/sec. In these two
patients, MRI was performed at 2 and 4 months after ablation, demonstrating
mild to moderate PV stenosis (50–60%). The patient with two PV velocities
greater than 158 cm/sec developed exertional dyspnea at 4 months. The second
patient had no symptoms or progression of PV stenosis with late MRI imaging. No
patient with PV flow velocity < 158 cm/sec has been found to develop
symptoms consistent with PV stenosis after a repeat ablation procedure [10].
The
typical Doppler color flow imaging in PV stenosis is characterized by increased
ostial PV peak flow followed by a blunted systolic velocity and prolonged and
elevated diastolic velocity, resulting in a fused systolic and diastolic
components and long pressure half-time [10-13].
Isoproterenol
infusion is one of the most useful provocative maneuvers for potentiating
firing of both PV and non-PV triggers of AF. The effect of isoproterenol on PV
flow before and after AF ablation has been studied using ICE with Doppler color
flow imaging[14]. This study showed that isoproterenol increases
ostial peak flow velocity of both pre-ablated and ablated PVs. Moreover, this
effect of isoproterenol appears to be independent of the heart rate effect since
atrial pacing at similar rates had no effect on PV flow velocities. However,
although isoproterenol leads to higher peak velocity, the pulsed Doppler
imaging shows separate systolic and diastolic velocity components with normal
pressure half-time. These isoproterenol effects are important to recognize,
especially when the peak velocity of PV flow is used as an index of ostial PV
stenosis. The clinical implication is that an "isoproterenol effect"
on PV ostial flow could potentially be misinterpreted as clinically significant
PV stenosis [14, 15].
ICE provides
tissue imaging of morphologic changes induced by radiofrequency energy. These
changes include tissue swelling, dimpling, crater formation, accelerated bubbles
before popping-crater like lesion development, and increased echogenicity
during or immediately after lesion deployment. The left atrial wall thickness
can also be assessed with 2D or M-mode imaging. Based on real-time ICE
monitoring of lesion development, titration of energy power and/or duration can
control lesion formation and prevent tissue overheating or structural perforation.
The ligament of Marshall is occasionally an important trigger for AF, and may
therefore be a target for ablation. The thickness of the ligament of Marshall is usually greater than the surrounding tissue, and therefore has greater
echogenicity, enhancing its identification with ICE.
ICE
imaging is a valuable tool for early detection of complication during AF
ablation procedures and consequently allows earlier intervention. Moreover, the
recognition of certain complications has paved the way to changes in
anticoagulation and power titration protocols. Potential complications include
those occurring during transseptal catheterization and left heart mapping and ablation.
The major potential complications detected by ICE during left heart ablation
include:
Damage
to cardiac structures:
Inadvertent
manipulation of the catheter during transseptal catheterization or mapping/ablation
may cause damage to adjacent non-targeted structures, such as aorta, left
atrial appendage, mitral valve, and left atrial wall. As mentioned above, ablation
radiofrequency energy may cause intramural superheating and a “crater” lesion
during ablation.
Pericardial
effusion and tamponade:
Pericardial
effusion is one of the most serious complications associated with catheter
ablation for AF. It may occur immediately after transseptal catheterization,
during catheter manipulation and ablation and after withdrawal of a coronary
sinus catheter. ICE allows early detection of pericardial effusion [figure 4]. This is usually detected along the inferior
border of the RV and posterior LA. Early detection allows early
intervention with pericardiocentesis and continuous monitoring of
re-accumulation during the drainage process.
Figure 4A: Pericardial effusion.
These ICE images with the transducer placed near the tricuspid valvular orifice in the right ventricle , showing the interventricular septum (IVS), left ventricular (LV) wall and pericardium (arrow); (b) ICE image of a moderate pericardial effusion (arrow, echo-free space) surrounding the LV free wall
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Left
atrial thrombus formation:
Atrial
thrombus formation has been recognized as one of the major complications during
atrial ablation procedures [16]. These thrombi are usually single, linear, and
mobile, andare typically attached to the transseptal
sheath, and less commonly to the circular mapping or ablation catheters [figure 5]. The incidence of left atrial thrombus formation
during left atrial ablation has been reported as high as 10.3% when
anticoagulation is maintained at a target activated clotting time of 250–300
sec [16]. In 90% of patients with ICE detected left
atrial thrombus, successful withdrawal of the thrombus-attached catheter/sheath
from the left atrium into the right atrium has been reported to prevent serious
systemic embolic consequences. Increased anticoagulation with an activated
clotting time ≥350 sec reduces the risk of left atrial thrombus formation
during ablation procedures for AF.
Figure 5A: Visualizing Thrombi
ICE images of the left atrium, showing: (a) a thrombus (arrow) formed at the superior aspect of the right upper pulmonary vein following an ablation lesion; (b) a large thrombus is visualized at the left atrium, extending from the pulmonary valve to the septal leaflet of the tricuspid valve.
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Pulmonary
vein stenosis:
One
of the most serious complications of AF ablation is the development of PV stenosis.
ICE Doppler and color flow imaging are used to accurately evaluate and monitor
the flow velocities and pressure gradients before and after ablation lesions [figure 3]. As mentioned above, significant ostial PV stenosis
is morphologically characterized by swelling and enhanced echogenicity. Color
Doppler may demonstrate turbulence flow, and spectral Doppler shows increased
ostial PV peak flow followed by a blunted systolic velocity and prolonged and
elevated diastolic velocity, resulting in a fused systolic and diastolic
components and long pressure half-time [10-13]
Figure 3: Pulsed Doppler recorded peak flow velocity of the left superior pulmonary vein (LSPV) ostium shows the maximal velocity measured 50 cm/s with two systolic componenets (S) as well as early diastolic (D) before ablation (a). Following ablation, the ostial peak flow velocity increased to 134 cm/s (b).
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Esophageal
injury:
The
esophagus is contiguous with the thin posterior wall of the left atrium. With
the advance in catheter design and higher energy delivery, esophageal injury,
with or without left atrio-esophageal fistula, has been described and associated
with high mortality rate [17-19]. Radiofrequency lesions in the
posterior and lateral aspect of the right lower PV or posterior and medial
aspect of the left PV are within immediate proximity to the esophagus. ICE
real-time imaging monitoring of the posterior atrium and esophagus during
radiofrequency energy delivery may reduce the risk of injury.
ICE has
emerged as an extremely useful tool during electrophysiology procedures. In
particular, ICE plays a valuable role in left heart mapping and ablation
procedures, and has become standard in AF ablation procedures. It provides real-time
imaging of the complex anatomy of the left atrium and PVs, guides transseptal
catheterization, assists in accurate placement of mapping and ablation
catheters, and monitors lesion morphology and flow changes in the ablated PV. ICE
allows early detection of procedural complications, facilitating timely and
effective therapy.
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