Radiofrequency ablation versus high-frequency ablatio in vivo comparison

A. Rouane; P. Bru¹ ; P.A. Chapelon ; A. Hedjiedj ; M. Nadi
L.I.E.N. ; Université H. POINCARE, B.P. 239 Vandoeuvre-lès-nancy, France
¹Hôpital Saint Louis, Service de cardiologie, 17019 La Rochelle ; France

Abstract: The ability to obtain atrioventricular block (AVB) with relatively low powers High frequency (HF) intracardiac ablation was established earlier. In this study, we describe the realisation of such injuries. Thus, and we made a comparison between radiofrequency (RF) and high frequency (HF) energy techniques. in vivo ablations were performed using, the two techniques in the same animal with reference to NAKAGAWA protocol [1].

INTRODUCTION

For arrhythmia treatment or conductive lesions, catheter intracardiac ablation is necessary for the destruction of morbid cells before replacing the natural electric impulse by artificial stimulator impulse. Radiofrequency ablation is now currently used in the treatment of different arrhythmias [2]. This technique is indicated for supraventricular tachycardia however, it is limited by the increase of impedance and coagulum apparition. This is some reasons explaining why swell injuries are obtained and ventricular ablation is then conter-indicated. The need for more important ventricular injuries leads us to define a new technique using high frequency ablation [3]. In vivo experimental series with sheeps demonstrated the ability of the HF ablation to obtain AVB with relatively low power.

MATERIAL

In order to avoid impedance mismatching, we have developed an automated impedance matching system. This device is placed at the amplifier output in order to optimise energy transfer between generator and load. The whole system is described on figure 1. We choose a p LC system with two air variable capacitors, and a fixed coil.

Figure 1. High frequency ablation system

The p LC system is driven in real time according to the Standing Wave Ration (SWR) measured values.drived. The two air capacitors, were piloted using step-motors. For in vivo case, the electric equivalent circuit of the loaded probe is represented by a parallel resistance with capacity. Thus the matching system adapt in real time the load impedance to the generator output (50 Ohms by maintaining a constant value (seen by the genrator) during the treatment. A perfect matching is obtained when no reflected energy is measured between the end of the catheter and the matcher.

METHOD

For this experiment, we used a only one pig (38 kg). The protocol used conforms to French and institutional regulatory requirements for invasive studies performed on animals . An intravenous injection of sodium pentobarbital (10 to 30 mg/kg of the animal’s weight) was given prior to intubation. General anesthesia was maintained by means of 1.0 and up to 1.5% of halothane, ketamine chlorhydrate under assisted ventilation. Blood was drawn on the same day both before and at the end of the procedure in order to measure creatine kinase (CK-MB), lactic dehydrogenase, blood count, and to determine standard biochemical blood levels. A standard ECG was displayed during the entire duration of the study.

A 15 cm long incision is then realised in the thigh muscle. We conserved the thinest fascia. Incision margins are elevated and we fill them with heated heparin at 36-37 °C, regenerated by a 20 cc by minute flow rate. Injuries are realised by a type 7F probe or by bipolar 27.12 MHz [3] probe ablation. Electrode is perpendicular (see figure 2) to the muscle axis and catheter pression is maintained constant. First incision must be closed before begining the controlateral thigh. Used energy varies between 5 and 50 watts for 5 to 30 seconds duration.

Figure 2. Heparined blood and position of bipolar electrod

After each application, electrode extremity was controled to verify the presence or not of clot. Two hours after the end of the intervention, triphenyl tetrazolium chlorure 2% is administrated (1.5 cc/kg) to separate necrosed tissu from normal tissu. The animal was then sacrified, thigh muscle excised end fixed in formol 10%. Injuries were numerated to allow measurement of maximal deeps, maximal diameter and superficial diameter injury.

RESULTS

The table 1 shows that injuries obtained by HF method present greater dimensions than RF method.

Table I :
Dimensions of lesions obtained by HF and RF methods

The photography (see figure 3) confirms that HF quality injuries is are of primary interest in clinical setting as regard to their large and depth dimensions.

Figure 3. Image of injuries examples obtained in the same condition
a)      left injury by RF (30 w, 9 seconds)
b)    right injuries by HF (30 w, 10 seconds)

DISCUSSION

When ablation is realised by using alternative current, and the probe is in contact with myocardium the electromagnetic energy is directly transferred inside the tissue. Classical phenomenons inducing tissues heating are conductive (ohmic) current and induced current. In HF method these two  contributions are involved leading to better penetration due to the induced current (capacitive heating).

HF method present the advantage of more uniform heating when associated to impedance matching in real time. Between two shoots, the fact that impedance is matched to the burned tissue optimise the energy transfert with better penetration inside the tissue. We oriented this study to obtain linear lesions during a block conduction. Unfortunately, this leaded to bad results with RF energy because points of ablation were separated. Even in bipolar mode, RF technique had never realised sufficient lesions. A contrario,HF method realises easily this kind of lesion, with no constraint due to the probe position, and seems thus an interesting alternative for indications were lesions must have large and deep dimensions. We are now trying to establish a correlation between optimal control of the lesion and the tissue temperature level.

CONCLUSION

We presented, as an alterantive for specific clinical indications, a HF technique able to realise linear lesions. In vivo preliminary results presented here must be reinforced by temperature control during ablation. 

REFERENCES

[1] H. Nakagawa, W. S. Yamanashi, Jan V. Pitha, M. Arruda, X. Wang, K. Ohtomo, K. J. Beckman, J. H. McClelland, R. Lazzara, W. M. Jackman ; Comparison of in vivo tissus temperature profile and lesion geometry for radiofrequency ablation with a saline-irrigated electrode versus temperature contrôle in a canine thigh muscle preparation; Circulation, vol. 91, N° 8, April 15, 1995.

[2] W.M. Jackman, K.J. Beckman, J.H. McClelland, Treatment of supraventricular tachycardia due to atrioventricular nodal re-entry by radio frequency catheter ablation of slow-pathway conduction. N. Engl. J. Med; 327 (1992), pp 313-318.

[3] A.Rouane,   M. Nadi.,   P.Bru,   A. Staiquly,   D. Kourtiche,  A. Hedjiedj,   G. Prieur,   Intracardiac HF catheter ablatherapy : Technical aspects. Med. Eng. Phys., 17 (1995), pp 36-41.