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Guiding Lesion Formation During Radiofrequency Catheter Ablation
Key Points
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Radiofrequency (RF) energy is the most commonly used energy source in cardiac catheter ablation procedures. The goal of RF power titration is to maximize the safety and efficacy of energy application.
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Stable catheter-tissue contact is inadequately assessed by fluoroscopy, tactile feedback, and electrogram characteristics. Contact force sensing catheters may improve the safety and efficacy of catheter ablation procedures.
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Careful titration of energy delivery can avoid local complications, including coagulum formation, steam pop, and cardiac perforation. Collateral damage to nearby structures, including the atrioventricular node, esophagus, and phrenic nerves, can also be prevented.
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Each method of guiding RF energy delivery has advantages and limitations. Common methods include monitoring ablation electrode temperature, changes in ablation circuit impedance, and electrogram amplitude reduction.
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The discrepancy between catheter-tip temperature and myocardial tissue temperature is greater for large-tip and irrigated-tip catheters. Special precautions should be taken to avoid excessive myocardial heating and collateral damage.
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RF ablation in nonendocardial anatomic sites, such as in the pericardial space or coronary sinus, requires modification of the general power titration approach.
General Principles of Power Titration
Catheter ablation is first-line treatment for many cardiac arrhythmias. The energy source used most often in these procedures is unipolar radiofrequency (RF) energy, usually at 300 to 1000 KHz, which can be modulated to allow precise destruction of targeted tissue. The goal is to successfully destroy tissue critical to the tachycardia circuit or focus while avoiding local complications and collateral damage to adjacent anatomic structures.
Various sources of information are available to guide the operator in producing adequate, but not excessive, tissue heating and lesion size. Systematic methods of RF power titration are discussed in detail. Alternative energy sources for ablation and the biophysics of RF lesion formation are reviewed in other chapters.
Assessment of Catheter-Tissue Contact
RF ablation is critically dependent on tissue contact. RF current is usually delivered in a unipolar mode from the ablation catheter tip electrode to a grounding patch (dispersive electrode) on the patient’s skin. Current density is high at the catheter tip because of its small surface area, resulting in resistive heating at the catheter tip-tissue interface. The zone of resistive heating extends only 1 to 2 mm from the catheter electrode tip, because energy delivery and direct heating are inversely proportional to the fourth power of distance from the catheter tip. Without good contact, only intracavitary blood is heated, without sufficient energy delivery to targeted myocardial tissue.
Before contact force (CF) sensing catheters were available, only surrogate measures of catheter-tissue contact could be used, including local electrogram quality and beat-to-beat variability, baseline ablation circuit impedance, changes in electrode temperature and impedance during ablation, catheter movement on fluoroscopy, visual assessment by intracardiac echocardiography, proximity to outer surface on electroanatomic mapping system, pacing capture threshold, and tactile feedback. Catheter ablation procedures can be performed without fluoroscopy, relying instead on intracardiac echocardiography and electroanatomic mapping. Yet, even using all this information, substantial differences between estimated and actual CF are common.
Contact Force Sensing
Catheters that measure and report real-time CF are now available. Using either a fiberoptic sensor (TactiCath, Abbott Medical, Abbott Park, IL) or a magnetic spring coil (SmartTouch, Biosense Webster, Diamond Bar, CA) that deform in response to force at the tip, these systems display both contact pressure in grams and the instantaneous force vector at the catheter tip ( Fig. 2.1 ). Similar efficacy has been shown for ablation of atrial fibrillation (AF) with use of CF sensing as compared with standard irrigated catheters in several studies.
The ideal method of incorporating CF data to guide lesion formation requires further investigation. In the TOCCASTAR (TactiCath Contact Force Ablation Catheter Study for Atrial Fibrillation) study, patients treated with CF greater than 10 g in 90% or more of lesions had higher success rates. Preclinical studies have shown that the force-time integral (FTI), defined as the total CF integrated over the time of RF delivery (the area under the CF vs. time curve), correlates with tissue temperature and lesion volume at a given power setting. One study reported that FTI during RF ablation can predict lesion transmurality, with the best cutoff FTI value of more than 392 gram‐seconds (gs). In another study, ablation with minimum FTI of less than 400 gs had a higher likelihood of isolation gaps and pulmonary vein (PV) reconnection, which is associated with higher recurrence rates after ablation for AF. Other algorithms, including lesion size index (incorporating FTI and power) and force-power-time-index (FPTI), have been devised to incorporate other variables, but further research is needed to determine the optimal parameters for predicting a durable lesion.
In theory, CF monitoring can help prevent cardiac perforation and other complications of ablation associated with excessive CF, and may also increase procedural efficacy. CF-guided ablation might also allow reduced RF and procedure time, as well as zero-fluoroscopy catheter ablation procedures.
Power Titration for Ablation Efficacy
The goal of catheter ablation is to cause irreversible damage to targeted tissue and permanent loss of conduction, which results from sustained tissue temperatures over 50°C. Lesion size is defined as the dimensions (width and depth) or volume of the lesion. The best predictor of lesion size is achieved tissue temperature, because the zone of necrosis corresponds to tissue heated to 50°C or higher. Key factors influencing the size of an RF ablation lesion include current density at the electrode–tissue interface (determined by delivered power and electrode surface area), duration of energy delivery, electrode-myocardium CF, orientation of catheter tip, achieved electrode tip temperature (for nonirrigated catheters), electrode size, heat dissipation from intracavitary blood flow or nearby cardiac vessels, dispersive (patch) electrode size, and polarity of RF system (unipolar vs. bipolar). Because some of these factors are unknown during ablation, power is often increased to reach a prespecified goal (e.g., 30–50 W for ablation of the right atrial isthmus) or to a desired effect (e.g., loss of preexcitation or silencing an ectopic focus). Power is also titrated by monitoring changes in electrode impedance and catheter-tip temperature.
Only tissue in direct contact with the electrode tip is significantly affected by resistive heating; most lesion volume results from conductive heating, which occurs more slowly. The process can be modeled as nearly instantaneous production of a heated capsule at the catheter tip followed by conductive heating of adjacent tissue until thermal equilibrium is reached. Increasing the power output increases lesion size by raising the temperature of the resistively heated rim, allowing a larger volume of tissue to reach the critical temperature (50°C) required for tissue necrosis during energy application. Due to conductive heating, ablation lesions continue to grow even after interruption of RF energy, a phenomenon called thermal lag or thermal latency .
Power Titration for Ablation Safety
Although efficacy is important, it is also critical to avoid complications of excessive energy delivery. Careful titration of RF power can minimize the probability of coagulum formation, steam pops, cardiac perforation, and collateral damage to intracardiac and extracardiac structures. Table 2.1 lists some warning signs of impending complications, which can be avoided by discontinuing RF application or reducing RF power.
TABLE 2.1
Warning Signs of Impending Complications With Conventional Radiofrequency Ablation Catheters
| Indicator | Cause | Notes |
|---|---|---|
| Excessive ablation catheter electrode temperature rise (over 65°C for 4-mm electrode, 55°C for 8-mm electrode, 45°C for irrigated electrode) | Excellent catheter contact with inadequate convective cooling | Risk of steam pop or coagulum, less likely in temperature-controlled ablation mode |
| Impedance drop over 10 Ω, especially if rapid | Excessive tissue heating | Increased risk of subsequent impedance rise and steam pop |
| Increase in ablation circuit impedance | Formation of coagulum on electrode tip, trapping elaborated gas and insulating the electrode | Formed by denatured blood proteins, not prevented by anticoagulation |
| Shower of microbubbles on intracardiac echocardiography | Boiling at electrode–tissue interface | Correlates with surface temperature, not tissue temperature |
| Audible “pop” or sudden change in electrode temperature or impedance | Boiling within myocardial tissue | Can result in myocardial tear, effusion, or tamponade, especially in thin-walled chambers |
| Esophageal temperature rise | Heating of esophagus during ablation of posterior left atrium | Risk of esophageal injury or atrio-esophageal fistula (usually fatal) |
| Loss of diaphragmatic capture with pacing from ablation distal electrode pair | Thermal injury to phrenic nerve | Seen especially with ablation at right-sided pulmonary veins and epicardial ablation |
Coagulum Formation
During RF catheter ablation procedures, excessive energy delivery can cause a sudden increase in impedance because of blood boiling at the electrode–tissue interface when temperature exceeds 100°C. This causes accumulation of steam along the electrode surface, which acts as an electrical insulator, leading to abrupt increase in impedance. Boiling at the electrode-tissue interface, called interfacial boiling , is necessary but not sufficient for this abrupt impedance rise. If gas is not trapped by intimate myocardial contact, but instead dissipated by brisk blood flow or open irrigation, overall ablation circuit impedance may not change despite interfacial boiling.
Coagulum is caused by excessive heating of blood near the electrode-endocardial interface, denaturating proteins in blood cells and serum. This results in “soft thrombus” or char that initially anneals to the endocardium at the electrode–tissue interface ( Fig. 2.2 ). Coagulum is not formed by activation of clotting factors like typical thrombus and is not prevented by heparin or other anticoagulants. During temperature-controlled RF delivery, the tip temperature necessary for interfacial boiling is usually not reached, and therefore the dramatic impedance rise from gas at the electrode is not seen. However, because proteins denature at temperatures well below boiling, around 60°C, coagulum can form even in the absence of impedance rise. Matsudaira and colleagues found that coagulum formed in heparinized blood even when electrode temperature was limited to 65°C with a 4-mm electrode and 55°C with an 8-mm electrode.
Coagulum annealing to tissue rather than the electrode tip may not affect electrode temperature or impedance yet could detach from tissue and embolize. Embolic complications have been reported even in patients undergoing relatively short ablation procedures when few lesions were created, and no abrupt increases in impedance were observed.
Myocardial Boiling (Steam Pop)
When tissue temperature exceeds 100°C, water in myocardial tissue can boil and cause a sudden buildup of steam, which can be heard or felt as a “steam pop” ( 
). This is often associated with a shower of microbubbles on intracardiac echocardiography, composed of steam ( ). The escaping gas can cause barotrauma with dissection along tissue planes. Damage ranging from superficial endocardial craters to full-thickness myocardial tears resulting in cardiac perforation and tamponade can occur ( Fig. 2.3 ). The consequences of a steam pop vary widely depending on location, myocardial thickness, and proximity to vulnerable structures such as the atrioventricular (AV) node.
Temperature-controlled ablation with a conventional 4-mm-tip catheter carries a low risk for steam pop because tissue and electrode temperature are similar, and temperature is limited to well below 100°C. However, this is not necessarily true in regions with brisk blood flow, in which convective cooling can cause significant discrepancy between tissue and electrode temperature. Steam pops are more likely with large-lesion technologies, such as large-electrode ablation catheters (8–12-mm tips) and cooled-tip ablation catheters. A common feature of these large-lesion catheters is that tissue temperature greatly exceeds electrode temperature, sometimes by as much as 40°C. Therefore steam pops can occur even when electrode temperature is limited to ostensibly safe levels ( Fig. 2.4 ).
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