Anita Arya MBBS
Feifan Ouyang MD
Karl-Heinz Kuck MD FESC FHRS
Roland Tilz MD
Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia in the Western world, where it affects more than 2.3 million people in the USA and 4.5 million people in Europe. It is associated with significant mortality, stroke risk and reduced quality of life and is the most common cause of arrhythmia-related hospitalisations.(1–3) Treatment options for AF have conventionally included either a rhythm-control strategy with anti-arrhythmic drugs, or rate control of the rapid ventricular response. Over the last decade, however, the technique of catheter ablation for AF has provided an additional and efficacious treatment option for rhythm control. AF ablation has evolved from being an experimental procedure to the most common ablation performed in most electrophysiology laboratories throughout the world.
Previous theories for AF generation and sustainment were based on the ‘multiple-wavelet model’; a depolarising wavefront would encounter an area of non-uniform anisotropy or slowed conduction, become fractionated and divide about islets or strands of refractory tissue forming multiple daughter wavelets.(4) It has become apparent, however, that a proportion of patients might have a focal mechanism of AF that both triggers and maintains the arrhythmia.(5,6) Haissaguerre et al(5) observed that 94% of initiating ectopic foci originated from the pulmonary veins and that ablating these foci led to a short-term elimination of AF. Since then, variations on the pulmonary vein isolation (PVI) procedure and substrate modification have become a recognised and effective method of treating AF.(7,8)
The technique of AF ablation
The most commonly used ablation technique is circumferential PVI around the ipsilateral PV ostia using irrigated radiofrequency current. Following trans-septal catheterisation, selective angiography of each pulmonary vein is performed. An electroanatomical map of the left atrium is then created and the PV ostia identified during angiography are tagged using the mapping catheter to guide the subsequent ablation (Fig. 1).
PVI provides symptomatic benefit in patients with AF, improves quality of life and can lead to potential reductions in hospital admissions. In patients with paroxysmal AF, success rates of about 80% during five years follow-up have been reported(9,10); however, redo procedures are frequently required. Recovered PV conduction is the dominant finding upon restudy and is responsible for the majority of AF recurrences in patients with paroxysmal AF.(11)
A number of factors may be responsible for PV reconnection – from lesion quality to pulmonary vein regenerative properties – and contact force of the mapping catheter has been identified as a potential major determinant influencing lesion quality during radiofrequency ablation.(12,13) A recent study showed a significant variability in contact force applied between operators and upon different sites in the left atrium during pulmonary vein isolation, including sites with very low contact force.(14) Low contact force may predispose to incomplete lesion formation, thereby resulting in the recovery of pulmonary vein conduction after ablation. Establishing continuous and transmural radiofrequency lesions via good catheter stability and tissue contact is therefore crucial in preventing PV reconnection and securing a successful outcome to the ablation.
To date, the quality of catheter tip to tissue contact has been measured indirectly by measures such as impedance changes during ablation, unipolar or bipolar electrogram attenuation and catheter tip temperature. The accuracy of these surrogate measures have not, however, been validated extensively, and measures such as electrode temperature are particularly imprecise during irrigated tip ablation, where registered tip temperatures inadequately reflect actual tissue heating.
To circumvent these issues, new catheter systems to measure contact force are under development. SmartTouchTM catheter is one such technology(15); this catheter uses a 3.5mm irrigated tip electrode that is connected by a tiny spring to the shaft (ThermoCool® SmartTouchTM, Biosense Webster, Inc) (Fig. 2). Contact force and direction are measured every 50ms by the degree of spring bending via a transmitter at the tip and three location sensors force at the shaft (resolution <1g in bench tests). This allows real time measurement of contact force between the tip of the ablation catheter and the myocardial tissue while providing positional information through integration with the electroanatomical mapping system (CARTO® 3, Biosense Webster, Inc) (Fig. 3).
In vitro studies have shown a high correlation between contact forces measured at the catheter tip independent of catheter orientation and correlations between contact force and lesion quality.(12) Okamura et al(13) performed a blinded analysis of catheter contact, as visualised by intracardiac echo, and contact force as registered at the catheter tip. They found a significant correlation between contact force and with lesion transmurality. The most suitable contact force is unknown, but it appears that lesion size is optimised by the application of 10–20g. In addition, lesion size can be controlled by modifying power to compensate for suboptimal contact force. In an animal model, Nakagawa et al(14) showed by increasing power at times of low contact force and lowering power during high contact force, that an equivalent lesion size could be produced and that this was applicable to lesions made in either left or right ventricle.
The potential benefits of this catheter is that while previous markers of lesion transmurality have been indirect, the SmartTouchTM catheter now provides a direct means of assessing contact, thereby enabling the contact force to be quantified and applied consistently and, if necessary, power settings adjusted to optimise lesion size. This technology also has the potential to reduce adverse effects of ablation such as myocardial perforation, steam pops and thrombus formation.(15)
The catheter technology itself is 8 F with a 3.5mm tip. It has also been integrated completely into the CARTO® 3, which has a Smart-Touch™ module. Slight differences exist in terms of catheter set-up and calibration compared to conventional irrigated catheters: the current version of SmartTouchTM catheter requires 15 minutes to warm up following connection to the system and a further five minutes in a blood pool to achieve equilibrium. The catheter can be inserted through commonly used guide sheaths. An included insertion aid protects the springs at the tip of the catheter.
The CARTO® 3 system integrated SmartTouchTM module provides graphical and numerical displays of direction and contact force, and stores data regarding power and electrogram voltage at each point during mapping and ablation, thus making it easy to use.
By providing contact force information and potentially improved catheter tip contact during ablation, it is believed that the SmartTouchTM catheter will allow for a higher procedure efficacy and therefore better chronic success rates for AF patients.
Clinical data are emerging regarding the use of the SmartTouchTM catheter during radiofrequency ablation. Although retrospective studies show some variability of contact force measurements during radiofrequency ablation, there might be some relationship between left atrial position, for example, ridge, roof, posterior wall, and contact force. In patients with drug-refractory AF, studies are underway to evaluate the safety and efficacy of the SmartTouchTM catheter. The SMART-AF study began recruiting in July 2011 and should report preliminary findings later this year.
To date, we have performed catheter ablation using the SmartTouchTM catheter in 23 patients. In our experience we have found the technology to be very useful in avoiding excessive high contact force and very low contact force. In the short-term, we observed forces ranging from 0–100g. During mapping and ablation we aimed to keep the catheter tissue contact in a range between 10 and 40g.
The uptake of AF ablation to treat both paroxysmal and persistent AF is increasing worldwide. Catheter ablation is an effective therapy for treatment of paroxysmal AF. Obtaining durable lesion formation and long-term PVI is likely to be a critical factor influencing the success of AF ablation. New technologies, such as the SmartTouchTM catheter, enable the incorporation of real-time contact force measurement in an irrigated ablation catheter, thereby allowing the optimisation of radiofrequency power and lesion formation and potentially improving the overall success of the procedure.
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- Ouyang F et al. Recovered pulmonary vein conduction as a dominant factor for recurrent atrial tachyarrhythmias after complete circular isolation of the pulmonary veins: lessons from double Lasso technique. Circulation 2005;111(2):127–35.
- Yokoyama K et al. Novel contact force and sensor incorporated in irrigated radiofrequency ablation catheter predicts lesion size and incidence of steam pop and thrombus. Circ Arrhythmia EP 2008;1:354–62.
- Okamura Y et al. A systematic analysis of in vivo contact forces on virtual catheter tip/tissue surface contact during cardiac mapping and intervention. JCE 2008;19:632–40.
- Nakagawa H et al. Controlling lesion size and risk of steam pop by controlling contact force and radiofrequency power in canine beating heart. Circ 2010;122:A15777.
- Biosense Webster Inc. Summary of safety and effectiveness data (SSED), PMA: P030031/S011. February 06, 2009.