The clinical application of catheter ablation for cardiac arrhythmia has been made possible by the use of radiofrequency (RF) energy, a form of energy that is useful and practical in many aspects.

Figure 1.

Ice ball formation on the tip of a quadripolar cryocatheter after cooling in a water bath. (Reproduced with permission)

First of all, energy delivery can be accomplished simply by using the existing hardware within a standard diagnostic catheter. Using, more or less, the same electrodes, delivery of high frequency electrical signal in the range of radio wave is transformed into heat through cardiac tissue resistive mechanism. This heating creates tissue desiccation that is contained within a few millimeters surrounding the tip of the electrode. Thus, ablation using RF energy was found to be safe and, for purposes of localized tissue destruction, effective. Many patients with disabling arrhythmias caused by a focal substrate, such as a supraventricular tachycardia utilizing dual atrioventricular-nodal pathways (AVNRT), accessory pathways (Wolff-Parkinson-White syndrome), or focal atrial or ventricular reentry [such as idiopathic ventricular tachycardia (VT) arising at the right ventricular outflow tract] could benefit from a curative procedure using simple application of RF ablation. Other, somewhat more complex arrhythmia, where a critical pathway can be easily identified such as the common atrial flutter, can also be cured using RF ablation with multiple applications to create a short line of block. However, attempts at using this simple radiofrequency energy technology for more complex ablation whereby long linear lesion or deep penetration is required has been less successful. Investigators were then interested in other forms of energy for tissue destruction.

       Alternative Energy Sources. Many forms of energy have been utilized in the field of medicine. Tissue destruction for the treatment of tumor and other tissue anomalies has been attempted by using various forms of physical energy.1–9 In the field of arrhythmia, cryoablation has been used for many years and had become the standard form of energy as it was found to be effective for ablation of any type of arrhythmia substrate, including accessory pathways as well as VT from in chronic ischemic heart disease. Similarly, surgical laser procedures had also been found to be quite effective for complex arrhythmia. Thus, it was clear that more “powerful” delivery of energy could achieve better success, and hence the quest for using these “alternative” sources of energy through a catheter. In this article, several modalities will be discussed. Certainly, with the proliferation of medical technology, this review is nowhere near comprehensive nor does it carry enough detail information. It is only meant to be a preview of recent and ongoing development and clinical usage of these alternative forms of ablation energy. The author is directly involved with ongoing research in microwave ablation.

       Cryoenergy Ablation. Although the exact mechanism whereby cell death is produced by hypothermia is still not clearly understood, cryoablation is known to cause freezing, thawing, hemorrhage, inflammation, and fibrosis.10 Hypothermia causes the formation of extra- and intracellular ice crystals.11 This initial compression effect is followed by tissue destruction that occurs during the ensuing thawing period that lasts several hours later.12,13 The safety and efficacy of cryoenergy ablation on myocardial tissue have been well recognized and the effect of cooling on the conduction system was reported in 1964, whereby hypothermia was achieved using carbon dioxide.14 Its unique, reversible effect was also recognized in that study whereby AV nodal conduction resumed after withdrawal of cooling at -45º C. Permanent block was noted at temperatures below -60º C applied for 90 to 120 seconds.15,16 It was also noted that the size of myocardial damage depended on the duration of application.17,18 More importantly, it was noted that the lesions were free from thrombus formation. Thus, cryoenergy appeared to be an ideal form of ablation whereby large lesions are needed and minimization of thrombus is critical.
       Transvenous cryoablation using cryo-catheters was first reported in 1991,19 whereby AV block was accomplished using an 11 French (Fr) cryocatheter cooled by pressurized nitrous oxide. The use of a more user-friendly cryocatheter was reported in 1998,20 using Freon in acute and chronic animal studies.
       Compared to the lesion created by standard RF energy, cryolesion is considered to be, in general, larger, more homogenous, and has clearer and smoother demarcation (Figure 2)

Figure 2.

Cryolesion in a canine ventricular tissue produced by -550º C cooling shown in this photomicrograph exhibiting sharp and smooth border. (Reproduced with permission)

; therefore may be less arrhythmogenic. The other potential significant benefit with cryoablation is the absence of thrombus formation,21,22 which is a major concern with numerous RF applications in left-sided ablation cases. The utility of cryoablation for left atrial ablation was recently reported, indicating its efficacy, safety, and ease of use. Cryoablation has indeed evolved as a potentially useful tool in complex ablation, whereby large or deep lesion is necessary. Its reversible effect may also prove to be useful for ablation of sites near the AV node. There are now several manufacturers producing cryocatheters and clinical investigation of these prototypes are ongoing in several countries.

       Laser Photoablation. Laser ablation was also found to be effective in surgical management of complex arrhythmia such as VT in the ischemic heart disease.23–27 Laser heating has distinct advantages over RF. Laser thermal heating, particularly using the continuous wave lasers at 790–1,064 nm wavelengths, is generated directly by photon absorption deep in the tissue. For surgical application, low power density non-contact irradiation was generally used, performed to both endocardial and epicardial surfaces. For catheter application, contact irradiation is necessary to avoid blood scattering and absorption.28 Contrary to general assumption, high power density (at 12–16 kw/cm2) contact irradiation using Nd:YAG laser did not produce uncontrolled vaporization of the myocardium. Instead, it created well-controlled lesions at 1–3 cm deep with 40–60 seconds application. Such transmural lesion is ideal for the treatment of VT in chronic ischemic myocardium (Figure 3)

Figure 3.

A transmural lesion from laser photocoagulation after 40-second application. The endocardial surface was relatively spared due to cooling from the circulating blood. (Reproduced with permission)

.29 Unlike radiofrequency, laser ablation is not impeded by fibrotic myocardium, and, in fact, studies have shown that scarred myocardium have more favorable light distribution than normal myocardium.
       Clinical application of transvenous laser ablation would require catheter manipulation. Early prototypes include insertion of the laser sheath into a pre-shaped or steerable guiding catheters with overall outer diameter of 10–11.5 Fr of the system. Newer prototypes are smaller in size, in the range of 9–9.5 French. With the advent of more flexible catheters and better knowledge in the method of delivery, effective and safe ablation of deeply seated myocardial VT may be able to be performed. Whether or not laser will be useful in other applications depends on the method of delivery. For example, an epicardial application to the left atrium may be an option for the treatment of atrial fibrillation. Another practical consideration in using laser for ablation is the cost. Nd:YAG lasers were typically costly apparatus; however, recent progress has enabled smaller Nd:YAG laser designs. Furthermore, one of the latest advances, the diode laser technology, which is similar to RF generator in cost, appears to be as effective as the Nd:YAG laser.29

       Ultrasonic Ablation. Ultrasonic ablation has been performed on cardiac tissue and was30–32 found to be effective in creating deep and uniform lesion33 and therefore potentially a suitable alternative to conventional radiofrequency energy. However, there has been little reported on the experimental or clinical use of ultrasonic energy for catheter ablation of cardiac arrhythmia. More recently, ultrasonic energy was reported to be useful in the delivery of balloon heating ablation for the ostium of the pulmonary vein.29 Through-the-balloon circumferential ablation (Figure 4)

Figure 4.

The balloon with the ultrasound transducer in the center is shown here. The balloon is fitted into the pulmonary vein ostium for circumferential ablation (Atrionix, Inc. Palo Alto, California). (Reproduced with permission)

is applied to the pulmonary vein ostium in attempt at isolating the pulmonary veins and prevent focal firing of atrial tachycardias to reach the left atrium.
       This application has been successful in terms of energy delivery. Some of the failure has been attributed to the mismatch between the balloon size and pulmonary venous ostial anatomy, such that complete occlusion of the pulmonary vein may not always be accomplished. Recently, other investigators have used similar circumferential balloon ablation using RF energy. Thus, ultrasonic energy may not be unique in this clinical application. There is limited data on the application of ultrasonic energy for ablation of deep substrate such as VT or for linear ablation for atrial flutter or MAZE procedure.

       Microwave Ablation. As in the case of laser ablation, microwave ablation also uses direct radiative energy to heat tissue. Microwave heating has been applied extensively in the field of medicine, primarily for tumor and other forms of tissue hyperplasia reduction. Electromagnetic field radiated by the microwave antenna raises the energy of the dielectric molecules and generates oscillatory movement of water molecules and frictional heat. Thus, unlike RF ablation, microwave heating, in theory, is not limited by changes in the tissue surrounding the electrode. Several in vitro34,35 and in vivo36,37 studies showed that indeed microwave ablation creates lesions that are larger than those produced by radiofrequency ablation. In addition to its potential in deep heating, microwave energy delivery is performed through a microwave antenna, which can be designed at various lengths, suitable for performing linear ablation. Such feature is potentially useful for ablation of atrial flutter38 and, possibly, atrial fibrillation.
       The application of microwave energy for catheter ablation has been slow to develop due to, largely, the difficulty in designing flexible catheters. Similar to laser delivery, effective microwave energy delivery requires conduits to prevent loss of or reflected energy. Surgical tools can be readily designed (Figure 5)

Figure 5.

The intraoperative microwave catheter used for MAZE procedure (AFx, Inc., Fremont, California). (Reproduced with permission)

and indeed its application has been shown to be quite successful for the treatment of atrial fibrillation.39,40
       Recent advancement in microwave technology has enabled the design of suitable (9 French) deflectable catheter with sufficient insulation for efficient forward energy delivery (Figure 6)

Figure 6.

Fluoroscopic view of the microwave catheter with a 4 cm antenna (Medwaves, Inc., San Diego, California) positioned at the roof of the left atrium for the creation of catheter-based MAZE procedure.

. In vivo studies are in progress to assess the efficacy of such system for the creation of linear lesion. Preliminary studies showed its efficacy and safety for atrial flutter. Its efficacy and safety for catheter-based MAZE is in progress and showing some promise (Figure 7)

Figure 7.

Linear lesions along the anterior borders of the left and right pulmonary venous ostia are shown here. The lesions are typically devoid of thrombus. A subtle lesion is noted along the roof, anterior to the superior pulmonary venous ostia.

.
       Another preliminary findings that may be beneficial for future ablation technology is the absence of thrombus with microwave ablation, even in cases where multiple ablations are applied. This finding is consistent with an efficient microwave energy delivery; i.e., radiative heating without creating overheating of its electrode component that would otherwise cause a local resistive heating effect similar to RF ablation.

       Advancement in Radiofrequency Ablation. Ablation using RF energy remains the most useful method in clinical practice today. Many types of arrhythmia can be cured using localized ablation. Cooled RF ablation has been shown to be effective in the production of larger lesion and useful in the management of patients with recurrent VT. Even when a “linear” ablation is required, radiofrequency energy can still be used in its standard form of delivery. Multiple-point ablation by continuous or intermittent dragging is the standard method for atrial flutter ablation. More extensive linear ablation can also be performed using multipolar RF catheter and has been shown to be effective. It is beyond the scope of this review to discuss the various methods of radiofrequency ablation modification.29

       Summary. The success in curing some arrhythmia using RF ablation has been, in itself, quite a milestone in the field of arrhythmia management. However, some common arrhythmia remains difficult to treat with simple ablation and thus the quest for other forms of ablation. Other forms of energy do provide some promise in terms of larger, deeper, and complex lesions but their application is still relatively in their early process. Nevertheless, preliminary data seem to support those promises and current progress remains strong. Many pre-clinical and clinical studies are now in progress using these forms of energy as well as modified radiofrequency ablation methods. Soon the clinicians and patients will benefit from a safe and effective form of ablation for many other types of arrhythmias that are currently remain elusive.