Abbreviations
AI – Ablation Index
AF – atrial fibrillation
CTI – cavotricuspid isthmus
ID – impedance drop
Ω - Ohms
PVI – pulmonary vein isolation
LVEF – left ventricular ejection fraction
Wgs – Watts/grams/second
Introduction
The advent of contact force-sensing catheters has delivered important
data on ablation lesion size, safety and efficacy, and their application
is well-described during pulmonary vein isolation (PVI) for atrial
fibrillation (AF) . Using a weighted formula, catheter contact force (g)
can be combined with the duration (s) and power (W) of a radiofrequency
application to calculate Ablation Index (AI). Expressed as a continuous
value in Wgs, AI has been shown to predict lesion diameter and depth
during AF ablation , and left atrial procedures guided by site-specific
AI targets have demonstrated more enduring PVI and a comparable safety
profile versus conventional ablation techniques . AI infers the energy
delivered by the ablation catheter; this differs from impedance drop
(ID) which describes local impedance changes at the blood-tissue
interface and infers tissue receipt of injury. Whilst AI is known to
correlate with ID, the strength of this relationship attenuates with
procedural variables such as catheter angle of incidence and irrigation
techniques; as such, there may be important limitations when relying on
AI alone to guide lesion delivery .
Radiofrequency ablation of cavo-tricuspid isthmus (CTI) dependent
(‘typical’) atrial flutter achieves acute success (i.e. bidirectional
block) in over 90% of cases, and carries a class IA recommendation as a
treatment strategy in the ESC’s 2019 Supraventricular Tachycardia (SVT)
guidelines . Despite this, a significant proportion of CTI ablations can
prove technically challenging, and hence novel predictors of acute and
long-term efficacy remain desirable. Autopsy studies have demonstrated
marked heterogeneity in CTI architecture; Klimek-Piotrowska et al.
(2016) dissected 140 human hearts and found that, when compared to the
anterior or posterior margins, the middle CTI frequently harbours
distinct morphological variations such as trabeculae (62.1%) or
recesses (25%) . Peri-ablation imaging data has also shown that
structural anomalies – such as the presence of pouches, angular crypts
or tricuspid regurgitation – result in prolonged procedure times and
poorer outcomes . Accordingly, data from contact-force sensing catheters
suggests that site-specific inconsistencies in isthmus tissue contact
may be responsible for procedural failure . The prevalence of complex
CTI anatomy has led some authors to suggest that pre-procedural imaging,
such as cardiac MRI or right atrial angiography, would promote more
patient-specific ablation strategies and hence improve outcomes .
Hypothesis
We hypothesised that AI may provide important insights into lesion
delivery across the CTI during ablation of typical atrial flutter. Using
established 3D electro-anatomical mapping systems and ablation
catheters, examining local variations in ablation lesion characteristics
may elucidate the mechanisms which impede enduring bidirectional block
or contribute to complications. An appreciation of these relationships
may promote a more prescriptive approach to CTI ablation without the use
of additional resources such as peri- or intraprocedural imaging
techniques.
Methods
Ethics
This project was registered with the local clinical effectiveness unit.
Consenting patients underwent procedures which were clinically
indicated, without randomisation or allocation, and which made use
well-established mapping and ablation techniques. As such, the work was
consistent with Clinical Service Development in line with the UK’s
Health Research Authority recommendations, and no specific additional
ethical approval was required.
Procedure
Patients with no previous CTI ablation underwent radiofrequency ablation
of CTI-dependent atrial flutter at two tertiary Cardiothoracic hospitals
in the UK from 2019-2020. Procedures were performed on uninterrupted
oral anticoagulation, either under general anaesthetic or with conscious
intravenous sedation. Ultrasound-guided femoral venepuncture was
performed following local anaesthetic administration, and a quadripolar
or decapolar catheter was positioned in the coronary sinus. A
contact-force sensing ablation catheter (Thermocool
SmartTouchTM, Biosense Webster, Diamond Bar,
California) was passed to the right atrium. Electro-anatomical mapping
was performed using CARTO software (v3, Biosense Webster). For patients
presenting in atrial flutter, entrainment discerned CTI dependence,
whereas patients in sinus rhythm underwent empirical CTI ablation during
pacing of the proximal coronary sinus at 600ms. Point-by-point ablation
was delivered from the ventricular margin of the CTI progressing towards
the inferior vena cava, maintaining a 6 o’clock alignment in the left
anterior oblique projection as per standard clinical protocol. The use
of additional sheaths to guide ablation was at the operators’
discretion. Ablation VisiTagTM settings were
pre-specified to accept 5mm of catheter drift, and force-over-time (FOT)
constraints of 5s, 25% in conjunction with 3g minimum force. Operators
were advised to deliver ablation lesions with a peak AI of 600Wgs at
45-50W. These recommendations were based on a retrospective analysis
performed at our institution which demonstrated safe and effective CTI
ablation with clusters of lesions in this range of AI (18). In the event
of visual macro-displacement, locations were discarded. Saline
irrigation flow rate was 2ml/min during mapping and 17ml/min during
ablation. Impedance was measured between the catheter tip and a ground
patch on the patient’s right thigh. If bidirectional CTI block was
achieved, this was reassessed after 15 minutes’ observation and
consolidative ablation lesions delivered as required. In the absence of
complications, patients were discharged the same day if the procedure
was performed under sedation, or the following day if performed under
general anaesthetic.
Data extraction
Post hoc, VisiTags were anatomically trisected according to their
position on the CTI: IVC end (IVC), middle CTI (Mid), or ventricular end
(V). Lesion characteristics were subsequently extracted and aligned with
time stamps to allow assessment of temporal changes, including contact
force (every 50ms), impedance, impedance drop and ablation index (every
10-20ms), catheter angle in the axial and lateral planes (every
10-20ms), and power (every 100ms). It has been demonstrated that
precipitous rises in impedance are associated with thrombus and steam
formation, and a variety of methods for assessing change in impedance
during catheter ablation have been published previously, including total
impedance drop, overall median impedance drop, or median impedance drop
after 10 seconds of ablation (2) (9) (19). During our data cleaning,
very rare transient spikes and troughs (lasting <50ms) in
impedance – and consequently the running calculations of impedance drop
– were noted. The examination of other contemporaneous lesion
characteristics (e.g. contact force) suggested that these data points
were real, and as their ramifications on ablation safety were
potentially significant, they were included in further analyses.
However, these outlying values rendered the measurement of total
impedance drop less reliable, and so a mean ID was instead calculated
from all the values recorded throughout the duration of each
radiofrequency application. The mean ID was assigned as a surrogate
marker of ablation efficacy. Total energy delivery was assessed in terms
of peak ablation index; this was defined as the maximum recorded AI
(Wgs) measured during each lesion; for all VisiTags, this corresponded
to the final recorded value.
Follow-up
Patients underwent clinical review with ECG analysis at 3 months post
procedure, with a further review at 12 months or sooner if necessitated
by symptoms. Anticoagulation was continued according to
CHA2DS2-VASc score, and anti-arrhythmic
drugs were adjusted according to patient preference and physicians’
discretion.
Statistical analysis
Data were analysed in R (x64, v. 3.5.2.) and XLSTAT (Addinsoft, v.
2020.1). Categorical group parameters were compared using Chi square
tests. The Shapiro-Wilk test identified whether or not data were
normally distributed. Subsequently, continuous data were compared with
two-tailed T tests or analysis of variance (ANOVA) with post-hoc Tukey
HSD testing for normally distributed data, or with the Mann-Whitney U or
Kruskal-Wallis test for non-normally distributed data. Correlation was
assessed using Pearson’s coefficient for normally distributed data or
the Spearman rank correlation if non-normally distributed. Data are
presented as mean ± SD or median (interquartile range). The level of
significance was set at p<0.05.
Results
38 individuals were included in this study; clinical characteristics and
baseline ablation data are shown in table 1. 14 cases made use of
additional long sheaths (11 Swartz SR0, Abbott/St Jude Medical; two
AgilisTM, Abbott/St Jude Medical, one
VizigoTM, Biosense Webster) to assist ablation. Acute
success (bidirectional block) was achieved in all 38 cases (100%).
There were no complications, and no steam pops were recorded. After a
mean follow-up of 6.6 ± 3.3 months, 97.4% (n=37) of patients were in
sinus rhythm.
Table 1: clinical characteristics and baseline ablation data