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In vitro comparison of platinum–iridium and gold tip electrodes: lesion depth in 4 mm, 8 mm, and irrigated-tip radiofrequency ablation catheters

Markus Linhart, Hanke Mollnau, Alexander Bitzen, Sabine Wurtz, Jan W. Schrickel, René Andrié, Florian Stöckigt, Christian Weiß, Georg Nickenig, Lars M. Lickfett, Thorsten Lewalter
DOI: http://dx.doi.org/10.1093/europace/eup040 565-570 First published online: 26 February 2009


Aims We compared a newly developed irrigated gold tip electrode ablation catheter and a gold tip 4 and 8 mm catheter with the corresponding platinum–iridium (Pt) tip catheters in an in vitro setting.

Methods and results In a flow chamber simulating physiological flow conditions, radiofrequency catheter ablation was performed on tissue samples of porcine endomyocardium and liver. Lesion depth, energy and temperature delivery, and popping frequency were determined. Two hundred and fifty-three ablations were conducted. Four and eight millimetre, gold tip electrode catheters produced significantly deeper lesions compared with the Pt tip electrode (liver 4 mm: 4.67 ± 1.7 vs. 2.9 ± 1.0 mm, P < 0.0001; endomyocardium 4 mm: 3.88 ± 1.1 vs. 2.81 ± 0.7 mm, P < 0.001; liver 8 mm: 3.98 ± 1.0 vs. 2.03 ± 1.1 mm, P < 0.001; endomyocardium 8 mm: 4.00 ± 0.9 vs. 3.39 ± 0.8 mm, P < 0.001) and correlated with the amount of energy delivery. Popping frequency was significantly higher in gold tip electrodes. In irrigated tip electrodes, there was no difference in the lesion depth comparing gold with Pt (liver: 5.18 ± 0.7 vs. 5.01 ± 0.7 mm, P = ns; endomyocardium: 4.89 ± 0.7 vs. 4.78 ± 0.8 mm, P = ns). There was a trend towards less popping in the gold tip electrode.

Conclusion Both 4 and 8 mm not-irrigated gold tip catheters produced deeper lesions than the corresponding Pt tip catheter. In irrigated tip catheters, gold and Pt tip material did not show differences in the lesion depth.

  • Radiofrequency ablation
  • Gold tip electrode
  • Platinum–iridium tip electrode
  • Irrigated tip catheter
  • Cooled tip catheter
  • Lesion depth


Radiofrequency (RF) catheter ablation (RFA) is the therapy of choice for a variety of symptomatic cardiac arrhythmias.1 For enduring success of ablation, it is crucial to produce cell death and persistent necrosis. In several clinical settings, e.g. ventricular tachycardia (VT) ablation of the left ventricle, it is important to generate lesions that penetrate deeply into the myocardial tissue.2 However, temperature excess at the tip of the ablation electrode frequently leads to coagulum formation on the ablation electrode and char or crater formation in the tissue, the latter presumably due to steam formation.3 During the ablation procedure, this can be noticed as an impedance rise and simultaneous audible popping.4 This phenomenon indicates excessive cell damage with macroscopic disruption of the myocardial structure and can lead to thrombus formation or perforation with potentially severe complications.

Many efforts have been undertaken to optimize power delivery into the myocardial tissue without exceeding electrode tip temperature. Notably, active cooling of the RFA electrode by irrigation of the catheter tip with infused saline has been developed and is now a well-established method to enhance the depth of tissue penetration of RF energy and thereby enlarge lesion size.2,5,6 An alternative approach is the development of electrodes with gold tips, as gold tip electrodes exhibit an almost four-fold thermal conductivity compared with conventional platinum–iridium (Pt) tip electrodes (3.17 vs. 0.716 W/cm K).7 We could previously show that 4 mm non-irrigated gold tip electrodes produce deeper lesions than 4 mm standard Pt electrodes.8 Furthermore, other studies demonstrated that large tip electrodes (8 mm electrode length) are able to produce larger lesions than standard 4 mm tip electrodes.9,10

In order to combine the advantages of irrigated tip technology and gold tip electrode material, an externally irrigated gold tip electrode RF ablation catheter has been developed recently. The aim of the present investigation was to evaluate the performance of this catheter compared with conventional Pt and gold tip electrode catheters of 4 and 8 mm tip length in an in vitro setting.


In vitro experimental setting

A specially constructed hemispheric examination chamber with a volume of 100 mL that allowed imitation of physiological cardiac flow conditions was used, as described and depicted before.8 Tissue slices from porcine liver and porcine heart were trimmed and placed in the flow chamber. In myocardial tissue, care was taken to use a surface section that was as smooth as possible without many irregularities. A heating pump kept the temperature of the saline fluid constant at 37.5°C; 0.9% saline was added to the water until an impedance of 250 Ω was obtained. A pulsed volume flow of 3.5 L/min passing through the examination chamber and over the tissue slice was generated by a pump. The catheter tips were placed perpendicularly on the tissue surface, with a constant contact pressure of F = 0.04 N, so that the contact area of the tip electrode with the tissue was the same in all catheters.

Ablations were performed with commercially available 4 and 8 mm Pt tip electrodes (AlCath, Biotronik, Berlin, Germany) and 4 and 8 mm gold alloy (99.99% Au) tip electrodes (AlCath G, Biotronik). For cooled tip ablation, 5 mm irrigated Pt and gold tip electrode catheters with an open irrigation cooling system (AlCath Flux TC and AlCath Flux TC G, resp., Biotronik) were used and connected to an infusion pump that provided a saline (NaCl 0.9%) flow of 10 mL/min at room temperature. Radiofrequency energy was delivered using the ablation generator AbControl MS (Biotronik). All catheter tips had thermocouple incorporated in it.

Ablation parameter settings and ablation procedure

In preliminary tests, ablation settings were evaluated for each tissue and catheter type, with the criteria of production of a visible lesion on the tissue surface and <50% occurring of popping. As in human application, a temperature-controlled setting was implemented for non-irrigated tip electrodes, whereas an energy-controlled setting was used in irrigated tip electrodes.

The ablation settings established during the pre-tests are displayed in Table 1.

View this table:
Table 1

Generator settings

TissueTemperature (°C)Power (W)Ablation duration (s)Irrigation flow (mL/min)
4 mm tip electrode (temperature-controlled)Liver55(max 75)60n/a
Heart50(max 75)60n/a
8 mm tip electrode (temperature-controlled)Liver47(max 75)60n/a
Heart47(max 75)60n/a
Irrigated tip electrode (energy-controlled)Liver(max 75)136010
Heart(max 75)136010

The applications were delivered to the tissue samples, with one to four separate applications on one piece with ∼7 × 4 cm in size and a distance of ∼1.5 cm between the lesions. Ablation duration was 60 s. Temperature, power, and impedance were digitally sampled throughout the duration of RFA. If an audible popping was noticed, ablation was stopped.

Lesion analysis

Lesions were analysed immediately after ablation as described before.8 Slices were cut, magnified, and digitalized, using a microscope with incident light and a magnification of 1 × 0.7 (Olympus, Hamburg, Germany) and a digital camera (Kappa Opto-electronics, Gleichen, Germany), which was connected to a PC running a measuring software (ImageBase, Kappa Opto-electronics). The lesion depth was determined by measuring from the deepest point on the tissue surface to the deepest point of lesion formation, recognizable by a transition from the whitish colour of the scar to reddish tissue colour (Figure 1).

Figure 1

Radiofrequency catheter ablation lesion in liver tissue produced by a 4 mm tip catheter (deeper gold tip lesion on the right, less deep Pt tip lesion on the left).

Statistical analysis

Lesion depths and ablation parameters were expressed as mean ± SD. Values were tested for normal distribution with Kolmogorov–Smirnov test. If normal distribution was determined, two-sided Student's t-test was performed for statistical comparison of lesion depth, temperature, and power delivery. If there was no normal distribution, non-parametric Mann–Whitney U-test was applied for analysis. Frequency of popping was compared using Fisher's exact test. A P-value of <0.05 was considered statistically significant.


A total of 396 ablations were performed with 143 popping events and 253 successful ablation procedures over 60 s.

View this table:
Table 2

Number and percentage of popping events

4 mm8 mmIrrigated tip
Ablation attempts295622311223313334295739
  • *P < 0.01.

  • P < 0.0001.

  • §P = 0.0942

The mean lesion depth, power, and temperature of the RFA are compared among the six different catheters in Figure 2.

Figure 2

Comparison of results between Pt and gold tip (Au) in successful ablations. (A) 4 mm tip: lesion depth and mean power reached for each RF application are significantly higher in gold tip (*P < 0.001; P < 0.0001; liver Pt n = 26, liver Au n = 26, heart Pt n = 22, heart Au n = 21; for comparison of mean power, non-parametric Mann–Whitney U-test was applied). Since a temperature-controlled setting was applied, there was no difference in the tip temperature (liver Pt 54.7 ± 0.4°C; liver Au 54.7 ± 0.4°C; heart Pt 53.9 ± 1.1°C; heart Au 54.3 ± 1.5°C; P = ns). (B) 8 mm tip: lesion depth and mean power reached for each RF application are significantly higher in gold tip (P < 0.01; §P < 0.05; liver Pt n = 12, liver Au n = 11, heart Pt n = 21, heart Au n = 22). Since a temperature-controlled setting was applied, there was no difference in the tip temperature (liver Pt 46.8 ± 0.2°C, liver Au 46.9 ± 0.1°C, heart Pt 46.9 ± 0.9°C, heart Au 48.9 ± 9.8°C; P = ns). (C) Irrigated tip: lesion depth and tip temperature are not different in gold tip compared with platinum tip (liver Pt n = 21, liver Au n = 24, heart Pt n = 26, heart Au n = 21). Since a power-controlled setting was applied, there was no difference in the delivered power (liver Pt 12.6 ± 0.1°C, liver Au 12.7 ± 0.2°C, heart Pt 12.4 ± 0.6°C, heart Au 12.6 ± 0.4°C; P = ns).

4 mm electrode tip

Lesion depth was significantly deeper using gold tip compared with Pt tip electrode. This correlated well with the higher energy delivery into the tissue. Maximum delivered power was 25.5 W. These effects were significant both in liver and in endomyocardial tissue. Since a temperature-controlled setting was applied, there were no differences in measured tip temperature. Ablation had to be stopped significantly more frequently due to popping in gold than in Pt electrodes (Table 2).

8 mm electrode tip

As in 4-mm, lesion depth was significantly deeper and energy delivery was significantly higher using gold tip compared with the Pt tip electrode, both in liver and in endomyocardial tissue. Maximum delivered power was 20.6 W. Again, as a temperature-controlled setting was applied, there were no differences in the tip temperature. In liver tissue, popping occurred significantly more frequently in gold than in Pt electrodes, whereas in endomyocardial tissue, there was no difference.

Irrigated electrode tip

There was no difference in the lesion depth comparing irrigated gold tip with irrigated Pt tip electrode, neither in liver nor in endomyocardium. Also, the tip temperature did not differ significantly, although the energy-controlled setting would have allowed variation in the tip temperature. There was a non-significant trend towards more popping events in Pt tip electrodes compared with gold tip electrodes.


Effects of electrode type on lesion depth and energy delivery

In non-irrigated catheters, the RF lesions were significantly deeper, and delivered energy was higher using 4 and 8 mm gold tip electrodes compared with the corresponding Pt electrodes. These effects could be observed both in liver and in endomyocardial tissue. The results translate the theoretical rationale about heat and electrical conductivity as physical properties into visible differences in ablation performance. We could confirm the findings of our previous study comparing 4 mm Pt and gold tip electrodes and now show the superiority of the gold tip in vitro also for the 8 mm non-irrigated electrode.8 The gold tip allows more efficient heat transmission both to the electrode–tissue interface and to the electrode–saline interface. This results in greater convective cooling by the saline flow and allows greater power to be delivered as had been suggested previously.6,10 Langberg et al.9 and Otomo et al.10 could demonstrate in animal models that large electrodes (8 mm) produce deeper lesions than standard 4 mm electrodes. In our experiments, we had to use different ablation settings with lower maximal temperature for 8 mm electrodes than for 4 mm electrodes since in the pre-tests there was a very narrow range between non-visible lesions at a lower energy output and very high popping frequencies at a higher output. Therefore, we were not able to directly compare 4 and 8 mm lesion depth.

Interestingly, the superiority of gold tip catheter regarding lesion depth was levelled out using the irrigated electrode tip catheter. Here, no significant differences could be demonstrated any more. Obviously, the active cooling of the electrode tip neutralizes the impact of the differences in the tip material, although a saline flow of 10 mL/min was applied as opposed to up to 20 mL/min in human application. Although the energy-controlled ablation setting would have allowed divergent catheter tip temperatures, there was no significant difference. However, Bruce et al.11 could show that during RFA with closed-loop (i.e. internally) irrigated catheters, tip temperature was markedly different from tissue temperature. Thus, catheter tip temperature might not be a useful parameter in the characterization of irrigated tip catheters anyway and in human application might even give a false sense of assurance that tissue overheating is not occurring. In experimental settings, this problem can be overcome by the insertion of temperature probes into the tissue.3

Overall, irrigated tip catheters produced the deepest lesions, although the effects are not directly comparable due to the different ablation setups (temperature-controlled vs. energy-controlled). Supposedly, using a higher flow rate, the advantage compared with not-irrigated catheter tips would have been greater. Nakagawa et al.12 showed that in irrigated catheters, a smaller tip electrode (2 mm) produced larger lesions than a 5 mm electrode. They suggest that in large tip electrodes, there is more current shunting to the blood, leading to a ‘voltage loss’ with less voltage available for tissue heating.

Influence of catheter type on popping phenomenon

In accordance with the higher energy delivery of gold catheters, ablation procedure had to be interrupted more frequently using 4 and 8 mm gold catheters because of impedance popping. In the irrigated tip catheter, however, there was a non-significant trend towards fewer popping events in the gold catheter in both tissue types. This might reflect an advantage of using gold as the tip material for irrigated catheters, although there was no direct superiority in terms of lesion depth. Popping is thought to be the result of sudden heat-induced expansion of intramyocardial gas and leads to tissue rupture with potential complications such as thrombus formation or perforation. Fortunately, it is an infrequent phenomenon in human RFA.13 In our experiments, the popping rate was much higher than it is known from clinical application, despite the use of relatively low power during RFA. This might be explained by the better maintenance of electrode stability throughout the ablation attempts in our experimental setting when compared with the clinical environment, as suggested previously.5

Comparison with clinical data and clinical implications

Our data implicate that in not-irrigated catheters, gold tip material produces deeper lesions than corresponding platinum tip electrode, independent of tip electrode length. In irrigated catheters, the use of gold as catheter tip material does not seem to enhance lesion depth. Clinical data are more controversial. In RFA of typical atrial flutter, superiority of 8 mm large tip platinum electrodes over 4 mm platinum electrodes has been shown clearly.14 Jais et al.15 could demonstrate the superiority of the irrigated Pt tip catheter over standard 4 mm Pt tip catheter, and Scavee et al.16 showed the advantage of externally irrigated over internally irrigated and 8 mm Pt tip electrodes. However, in the very recent first in vivo comparison of 8 mm gold tip electrode with 8 mm Pt and 4 mm externally irrigated Pt tip electrodes in RFA of atrial flutter, Sacher et al.13 failed to demonstrate differences in catheter tip type regarding clinical endpoints. They suggest that gold cannot play off its advantage of higher thermal conductivity over Pt in areas of low blood flow, such as in trabeculae, where there is only minimal convective cooling at the electrode tip.7 In a very interesting recent study, Da Costa et al.17 demonstrated that the anatomy of the cavotricuspid isthmus plays a more important role in RFA of atrial flutter than the type of the electrode used. They showed that 8 mm Pt tip catheters were more effective than irrigated tip catheters in straight isthmus morphology, whereas irrigated tip catheters tended to be more effective in the case of concave isthmus morphology.

In a retrospective study of VT ablation, Soejima et al.18 showed a greater efficacy of the irrigated tip catheter for terminating VT compared with the standard 4 mm tip electrodes.

Obviously, ablation site and anatomy are major determinants of applicability and efficacy of the various RF catheters. Experimental data provide the basis for the characterization of catheter types. Lesion depth can only be determined in non-human experiments since tissue preparation is necessary. However, experimental results need to be confirmed for clinically relevant endpoints in human application, perhaps independent of each anatomical region and characteristics (i.e. cavotricuspid isthmus, left ventricle, and epicardium).

Endomyocardial vs. liver tissue

As in our previous ex vivo study, we used both porcine liver and heart tissue for testing. This was based on the observation that unlike the trabeculized porcine endomyocardium, liver tissue samples have a smooth surface and a homogeneous structure, so slight differences in ablation effects could be detected more precisely. However, lesion depth was distinct also in endomyocardial tissue, and popping rate was comparable in both tissue types. In future experimental studies, one might rely on myocardial tissue only as the target organ for RFA of cardiac arrhythmia.

Study limitations

To allow stable conditions and to control electrode orientation and electrode–tissue contact pressure, the experiments were performed in ex vivo porcine liver and endomyocardial tissue, instead of the endocardium of a beating heart. Therefore, the contact pressure of the tip electrode could be controlled. The flat surface of the tissue also allowed the accurate determination of the lesion depth. However, the tissue had no intramural blood flow, so there, presumably, was less convective cooling inside the tissue. This may account, at least in part, for the employed ablation settings, which are by far lower both in temperature and in energy than those in human application and still produced a considerable popping rate. Internal cooling by the intramural blood flow might have altered our observations. It is suggested that resistive tissue heating at a distance from the site of current delivery plays an important role in RFA.5

In our model, tip electrode and neutral electrode were in close proximity, whereas in human application, the neutral electrode is placed at the patient's back. This might lead to different current streamlines with different ablation depths. For imitation of cardiac blood flow, we used saline instead of heparinized blood, which might have resulted in different degrees of convective cooling due to different heat conduction properties.9

Conflict of interest: S.W. is an employee of Biotronik GmbH & Co. KG, Berlin.


We thank Mr. Blum from Biotronik GmbH, Berlin, Germany, for the construction of the examination chamber and Mr. Reinhardt from VascoMed, Binzen, Germany, for technical support.


  • These authors contributed equally to this work.


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