Accurate Detection Of Left Atrial Thrombus Prior To Atrial Fibrillation Ablation In Patients With Therapeutic Anticoagulation: Does Transesophageal Echocardiography Beat Conventional Wisdom?
Credits:Dhanunjaya Lakkireddy MD, FACC
Director – Center for Excellence in Atrial Fibrillation & EP Research, Bloch Heart Rhythm Center & Mid America Cardiology
Corresponding Author :Dhanunjaya Lakkireddy MD, FACC. Director
– Center for Excellence in Atrial Fibrillation & EP Research, Bloch Heart Rhythm Center & Mid America Cardiology, University
of Kansas Hospitals, Suite G600, 3901 Rainbow Blvd, Kansas City, KS 66160.
Atrial fibrillation (AF) significantly increases the risk of
left atrial (LA) thrombus and systemic thromboembolism [1-4]. Screening transesophageal echo (TEE) to rule out left atrial
thrombus has become standard of care over the years .
Conventional thinking of therapeutic anticoagulation for 4-6 weeks prior to
cardioversion may not reduce the risk of left atrial thrombus completely. Left
atrial thrombi can be seen on 2-9% of screening TEEs in AF patients with
various levels of anticoagulation . Radiofrequency
ablation of atria with pulmonary vein isolation (PVI) with or without various
additional ablative techniques has evolved into very important strategy in the
treatment of patients with AF [6-10]. Even
though the relative risk of systemic thromboembolism after non TEE guided
cardioversion after 3 weeks of anticoagulation remains lower (approximately
0.8%) despite 7% prevalence of LA thrombi, the same may not be applicable to
invasive treatment modalities like AF ablation [6, 11-13]. The presence of LA thrombi may
increase the risk of clot dislodgment and subsequent thromboembolism with
catheter manipulation during AF ablation and is considered to be an absolute
In this issue of JAFIB, de Bono et al try to assess the risk
of LA thrombi prior to AF ablation . It was a retrospective
study that looked at 244 preprocedural TEEs in 148 consecutive patients who
underwent AF ablation at their institution. After therapeutic INRs for 4weeks
and warfarin holding 3 days prior the ablation, 4 out of the 244 screening TEEs
showed LAA thrombus at 1.6%. Two out of the four patients met the criteria for
high thrombogenic risk with depressed LV function, persistent AF, severely
dilated LA, prior transient ischemic attacks. However, the other two had
paroxysmal AF with CHADS2 score less than or equal to 1 and a high AF burden.
Based on their low risk profile the latter two patients would not have been
bridged with low molecular weight heparin (LMWH) or heparinoids by conventional
wisdom. This is a very small study with intriguing findings. It raises an
important question if every patient should undergo preprocedural screening to
rule out left atrial thrombus.
There are three vulnerable time periods that increase the
risk of periprocedural thromoembolism during AF ablation. Preprocedurally,
persistence of LA thrombi despite conventional anticoagulation or formation of
a new thrombus during the warfarin free interval prior to the ablation, if
undetected can add to the risk of thromboembolism. Intraprocedurally, thrombus
formation on the transseptal sheaths and catheters and coagulum formation at
the ablation site due to protein denaturation can result in thromboembolism.
Post procedurally, endothelial denudation and extensive left atrial ablation
and LAA stunning from conversion to sinus rhythm can promote thrombus formation.
Even though the incidence of clots in patients without the
thrombogenic risk factors is low (1.1%) it is still not zero [14,
15]. It raises the possibility of clot dislodgement as the
cause of systemic thromboembolism in at least some of these patients
periprocedurally. In our ongoing efforts to make AF ablation safer, attention
should be paid to this period of warfarin free interval where there is small
but real risk of thrombus formation.
Pre ablation CT scans are usually obtained for assessing the
LA and pulmonary venous anatomy, integration with 3D electroanatomic mapping
and as a baseline for assessing the progression of PV stenosis. These CTs also
provide valuable information about the status of LA thrombus. From a registry
of 1221 patients from the Cleveland Clinic’s AF ablation program, 9 patients
had findings suggestive of LAA thrombus on preablation CT scan but only 3 of
them were confirmed by a TEE . All of the 3 patients had
predisposing factors that increased their risk of thrombus formation including
– chronic AF and low left ventricular ejection fraction. No thrombus was seen
in patients with paroxysmal AF and normal LVEF.
Recently, Patel et al have shown that LAA/ascending aorta
Hounsfield Unit (HU) cut off ≤ 0.75 on the preablation CTs correlate to
LAA thrombus and dense non clearing spontaneous echo contrast (SEC) with high
sensitivity (100%) and moderate specificity (72%) . CTs
also have a low positive predictive value (29%) with a very high negative
predictive value (100%). So in patients who undergo preablation CTs, a
LAA/Ascending aorta HU ratio > 0.75 may preclude the need for preprocedural
TEE suggestive of the absence of a LAA thrombus. Unless these CTs are done
within 24 hours of the ablation, the brief hiatus of no warfarin period before
ablation can still continue to be a challenge with propensity to clot formation.
This can be addressed by performing ablation on therapeutic INRs avoiding the
warfarin free period prior to the ablation.
Wazni et al reported the superiority of uninterrupted
anticoagulation with warfarin in decreasing the risk thromboembolism without
increasing the risk of bleeding compared to full dose and half dose
subcutaneous LMWH bridging after stopping warfarin prior to ablation . But one has to be cautious in interpreting these results
as this experience is limited to a very high volume tertiary care center with
operators on the higher end of the learning curve and those patients with
paroxysmal AF. Replication of similar results in other groups of patients in
low volume institutions with operators on lower half of the learning curve can answer
a lot of questions that surround the periprocedural anticoagulation.
On the other side of the coin, there are anecdotal reports
of very long and tortuous LAAs in which thrombus may not be detected on TEEs due
to portions of the LAA being out of plane. CT scans may perform better in
detecting thrombus in these oddly shaped LAAs . In this
issue of JAFIB, a featured review article by Mears et al highlights the current
state of anticoagulation and bleeding related issues in a very comprehensive
In the absence of significant risk factors it is still hard
to justify a preablation TEE in every patient from a cost effectiveness and
patient comfort stand point. The risk of atrial stunning and propensity to
thrombus formation in patients who have a very high daily burden of paroxysmal
AF may be similar to those with persistent AF. Anticoagulation strategy in this
subset of paroxysmal AF patients should be on the same lines as those of
persistent AF. Efforts should be made to utilize the preablation CT data to
effectively screen patients. To conclude this study does make a good point for
the possibility of LAA thrombus in unsuspecting cases of paroxysmal AF.
However, the overall incidence of thrombus is much lower than the prior
reported series, especially in those who are deemed to be low risk. The
presence of high AF burden should be considered as a risk factor for higher
thrombogenecity. The subset of patients with paroxysmal AF and well preserved
LVEF, low to moderate AF burden and non severely dilated LA need to be further
studied before any definitive recommendations can be made on the need for
routine preablation screening TEE.
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