Ablation Technologies and Delivery Systems

Chapter 16 Ablation Technologies and Delivery Systems

Catheter ablation has been predominantly performed with a single electrode radiofrequency (RF) catheter–based system, with or without cooling; however, a number of new ablation technology and delivery systems show promise in revolutionizing the treatment of arrhythmias.

Comparison of Ablation Technologies

Each energy source used for catheter ablation has particular characteristics that determine how the energy interacts with myocardial tissue. As seen in Table 16-1, each technology is distinguished by the characteristics of the energy source. In RF energy, the 300- to 700-kHz energy source is produced by an RF generator that uses resistive heating. In contrast, the 915 to 2450 mHz microwave energy produced by a microwave generator is absorbed by substances on the basis of their di-electric properties, resulting in increased molecular motion that translates into tissue heating. Because of the differences in frequency, microwave energy uses an antenna operating at the delivered frequency, whereas RF energy may be delivered from an electrode. Cryothermia relies on the extraction of heat rather than its delivery and usually is the result of a pressure drop of a suitable gas such as nitrous oxide. Laser energy may be delivered at a range of frequencies, most commonly from 300 to 2000 nm. Each wavelength has specific absorption properties that will determine its effect on blood and body tissue. The wavelength of light is delivered via a fiberoptic, usually at a pinpoint. The fiber may be coupled to a diffusing element, which will activate to deliver the energy over a specific region, permitting the energy delivery to be adapted to the target. Ultrasound energy may be delivered using a piezo-electric crystal that converts electrical energy to mechanical energy.

The ability of each energy source to affect the properties of the ablation is illustrated in Table 16-2. Only cryothermal energy can reliably cause reversible effects for true mapping. By reducing tissue temperature to a level above freezing, it may be possible to have the bulk of the tissue be reversibly affected. Heating also has some reversible effects, but it is difficult to avoid irreversible cell death. RF energy and cryothermia are nearly completely dependent on tissue contact, whereas microwave energy, laser, and ultrasound may be delivered without tissue contact. Lesion size is considerably variable across energy sources, but in general, RF energy and cryothermia are believed to produce smaller lesions for a given maximal surface area of contact. Ultrasound and laser permit focusing of energy, which may permit controlled delivery to a specific location or even depth.

The biologic effects of the energy sources are described in Table 16-3. Heating occurs by different mechanisms, depending on the energy source. Microwave energy is absorbed by tissue, causing increased molecular motion, particularly of water molecules, which results in heating. Laser energy has several effects, including heating, vaporization, and coagulation. Ultrasound energy acts by sonication, boiling of water, and tissue heating. Cryothermia acts by its freezing the tissue, which produces ice crystals and a change in the osmotic properties of tissue. With thawing of the ice crystals, further damage and rupture of the cell may occur. Cryothermia also disrupts the vascular supply to the tissue. Overall, cryothermia is felt to result in the greatest preservation of the architecture of tissue. Cryothermia also causes the least destruction of the endothelial surface, which may help reduce the incidence of thrombus formation. Although the risk of esophageal damage is largely unknown, studies indicate that esophageal damage resulting in fistula formation may be less with cryothermia compared with other energy sources.

A summary of the advantages and disadvantages of each energy source is given in Table 16-4. RF energy has been extensively used clinically, and its relative safety has been demonstrated. The need for tissue contact and the ability of impedance rise to limit lesion formation has helped prevent complications and the destruction of collateral tissue. Microwave energy permits a variety of antenna configurations to be developed and does not require tissue contact, although the distance between the antenna and the tissue must be held relatively constant for constant energy delivery. Cryothermia’s greatest advantage is the minimal disruption of tissue architecture and the endothelium and its reversible effects. In addition, tissue adherence permits excellent contact. Laser energy is capable of giving energy rapidly to create large-sized lesions and can be controlled. Ultrasound energy is capable of creating larger lesions and may permit combined imaging.

Table 16-4 Advantages of Energy Sources

Radiofrequency Proven safety; limited ability to cause collateral damage Limited lesion size; requires contact; charring
Microwave Contact not required; antenna configuration may be varied for different applications Requires control of distance to keep energy delivery constant; antenna must be efficiently coupled to tissue; heating of cable
Cryothermia Reversible; improved safety; minimal endothelial disruption; adheres to tissue Slow energy delivery; reversibility at margins; inability to drag lesion
Laser Rapid delivery of high energy; large lesions; directional No self-limiting feedback for energy delivery; risk of excessive energy delivery
Ultrasound Large lesions; heats tissue more than blood; may permit imaging Difficulty controlling depth; dependent on tissue properties

Ablation Technologies

Cooled Radiofrequency Ablation Systems

Cooling of RF ablation systems has been a major technologic advance in catheter ablation systems. The basic principle of cooling the electrode is avoidance of excessive temperatures that cause degradation of proteins and other tissue components, which leads to rises in impedance, which, in turn, prevents adequate delivery of RF energy. In addition, the avoidance of excessive temperatures may decrease the risk of catheter ablation complications such as perforation and thrombus formation.

Cooled RF ablation systems may be divided largely into the two groups: (1) internal cooling systems and (2) external cooling systems. Internal cooling systems use saline, which circulates within the catheter to reduce electrode temperature. The dimensions of the internal cooling system limit the flow rate that may achieved during RF application. Because most catheters using current designs are limited to 7 to 8 Fr diameter size (2.33 to 2.67 mm), the flow rate is limited.

An example of the currently available internally cooled RF ablation system is the Boston Scientific (Natick, MA) Chilli Catheter (Figure 16-1). Clinical trials using this system have demonstrated that this catheter may be used effectively during the ablation of ventricular tachycardia (VT).

Externally cooled RF ablation systems use small holes in the ablation electrode to permit saline to escape the catheter and be introduced into the body. Because a return path is not required, current designs permit the forward flow to be greater compared with internally cooled RF ablation systems. In addition, it is possible that the saline that escapes into myocardial tissue directly cools the myocardium in addition to cooling the ablation electrode. Present designs incorporate several holes in the electrode that are able to cool the electrode efficiently. The Biosense Webster (Waterloo, Belgium) Thermocool (Figure 16-2) catheter and the St Jude Medical (St Paul, MN) catheter (Figure 16-3) have lateral holes at the catheter electrode. The Thermocool catheter has been approved for ablation of atrial fibrillation (AF) and VT.

Temperature sensing and power regulation during cooled ablation, whether using internally or externally cooled systems, present some particular challenges. It is generally believed that in noncooled RF ablation, the temperature measured by thermocouples or thermistors reflects the surface tissue temperature to an imperfect degree. Even in these cases, the true maximum tissue temperature is thought to be approximately 1 mm below the surface; therefore the temperature measured by the electrode tip sensors does not reflect the maximum tissue temperature. In cooled RF ablation systems, the irrigation of the electrode results in cooling at the site of temperature measurement. During irrigation, the myocardial surface is cooled, resulting in the highest temperature occurring deeper in the myocardium. Some rise in electrode temperature is observed, reflecting heat transfer from tissue to the electrode, but it is difficult to extrapolate directly from this temperature to the highest myocardial temperature.

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Aug 12, 2016 | Posted by in CARDIOLOGY | Comments Off on Ablation Technologies and Delivery Systems

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