Abstract
Titrating radiofrequency (RF) energy in cardiac catheter ablation procedures is a key component of creating effective, durable lesions while minimizing the risk of procedural complications. Because RF catheter ablation requires stable contact between catheter tip and myocardial tissue, this must be assessed in real-time using fluoroscopy, tactile feedback, electrogram characteristics, or direct measurements of contact force available on some ablation catheters. Titrating energy delivery can eliminate conduction in targeted tissue while minimizing the risk of coagulum formation, steam pop, and cardiac perforation. Aside from delivering the appropriate power, collateral damage to nearby structures, such as the atrioventricular node, esophagus, and phrenic nerves, can be prevented by monitoring for unwanted heating. There are different methods of guiding RF energy delivery, such as monitoring ablation electrode temperature, changes in ablation circuit impedance, contact-force-derived parameters, and electrogram amplitude reduction, each with specific advantages and limitations. When using large-tip and irrigated-tip catheters, special precautions should be taken to avoid excessive myocardial heating and collateral damage, because there is a greater discrepancy between catheter-tip temperature and myocardial tissue temperature. When performing RF ablation in other sites besides endocardial surfaces of cardiac chambers, such as in the pericardial space or coronary sinus, changes to the general power titration approach are recommended.
Keywords
cardiac arrhythmias, catheter ablation/adverse effects, catheter ablation/methods, contact force sensing, power titration, radiofrequency energy
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.
| 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 ).
Cardiac Perforation
RF energy delivery can contribute to perforation even without a steam pop. This is more likely in a thin-walled chamber such as the left atrium, especially with high power and excessive CF. Long deflectable sheaths allow highly effective contact with myocardium. Unless caution is exercised (e.g., by limiting power), this may increase the chances of cardiac perforation during delivery of RF energy.
Left atrial ablation for AF is often performed with an irrigated catheter through a long sheath and carries a particularly high risk for cardiac perforation, effusion, and tamponade—over 1% in a worldwide survey. Limiting energy delivery (power or duration) to the minimum required to achieve the procedural end point reduces the risk for local complications, including coagulum, steam pops, and perforation.
Damage to Surrounding Structures
In addition to these local complications, collateral damage to structures outside the heart can result from excessive energy delivery. Depending on the arrhythmia location targeted, catheter ablation can result in damage to lung tissue, coronary arteries, phrenic nerves, aorta, or esophagus. Although many strategies have been developed to protect these structures during ablation, one of the simplest and most effective is to reduce power to the minimum necessary level and limit duration of RF energy application.
Monitoring for heating of extracardiac structures is important, especially with irrigated and large-tip catheters. During posterior left atrial ablation, temperature monitoring in the esophagus can detect potentially dangerous heating of esophageal tissue, allowing the operator to reduce power or reposition the catheter. During ablation at the right-sided pulmonary vein ostia or in the epicardial space, phrenic nerve injury can occur, with symptoms ranging from transient and mild to permanent and disabling dyspnea. To avoid this complication, many operators avoid RF application in sites with phrenic nerve capture on high-output pacing. Another option is to ablate at lower power during continuous pacing just above phrenic capture threshold, interrupting energy delivery if diaphragmatic stimulation is lost. Several techniques for phrenic nerve protection have been developed to allow safer ablation at a critical site in which phrenic nerve injury is otherwise likely.
Methods of Titrating Energy Delivery With Conventional Radiofrequency Ablation Catheters
Multiple methods of titrating power with conventional nonirrigated catheters have been used, alone and in combination, in response to real-time data. Commonly used parameters are electrode-tip temperature, ablation circuit impedance, local electrogram amplitude, and electrophysiologic end points.
Temperature-Titrated Energy Delivery
Power and duration of RF application do not accurately predict extent of tissue destruction because unmeasured variables such as catheter orientation, cavitary blood flow, and catheter CF significantly affect the volume of the resulting lesion. Ablation catheters contain a thermistor near the tip, and temperature of the electrode-tissue interface has been shown to be useful in predicting lesion volume. Current RF generators decrease power automatically when temperature exceeds a specified cutoff. Power, temperature, and impedance are continuously displayed to the operator as time plots during the energy application. In one large series, an 80% reduction in rate of coagulum formation and sudden impedance rise was seen with closed-loop temperature control.
Controlling catheter-tip temperature reduces, but does not eliminate, the risk for coagulum formation and steam pops, because coagulum can form at temperatures well below 100°C and tissue temperature is often higher than catheter-tip temperature. Besides power and electrode temperature, other important determinants of tissue temperature include catheter orientation, electrode size, catheter contact, and convective cooling. Not all of these can be controlled or monitored in a clinical ablation procedure.
True tissue temperature control, as opposed to electrode-tip temperature control, has been tested in vitro. RF energy delivery was titrated using a thermocouple needle extending 2 mm from the catheter tip into the myocardium. This achieved adequate lesions without excessive intramyocardial temperature rise and prevented steam pops. In theory, tissue temperature–guided power titration would result in more predictable lesion size, reducing variability because of differences in catheter contact and convective blood flow cooling. However, significant engineering obstacles must be overcome, such as demonstrating the safety of inserting a needle into the beating human heart and reliably measuring tissue temperature regardless of catheter orientation.
Impedance-Titrated Energy Delivery
Because neither applied power nor electrode-tip temperature adequately reveals tissue temperature, investigators have sought other surrogate measures of tissue heating. One such parameter is ablation circuit impedance, which reflects the resistance to current flow, from the tip of the ablation catheter to the skin grounding pad. At the high frequencies used for RF ablation, tissue impedance can be modeled as a simple resistor. As the tissue is heated, ions in the tissue become more mobile, resulting in a fall in local resistivity, measurable as a fall in ablation circuit impedance. Significant tissue heating is associated with a predictable fall in impedance, usually in the range of 5 to 10 Ω. Absence of impedance fall may reflect inadequate energy delivery to the tissue, poor catheter-tissue contact, or catheter instability.
Impedance titration has been used successfully to guide ablation procedures. In one protocol used for accessory pathway ablation, power was adjusted manually to achieve a fall in impedance of 5 to 10 Ω, to a maximal power of 50 W. A randomized comparison showed similar results for temperature and impedance power monitoring with 93% procedural success in each group, and no difference in the rate of coagulum formation. However, the same investigators found that impedance titration was not useful for AV nodal slow pathway modification, in which lower power and temperature are desirable to avoid AV block, with smaller resulting lesions. Successful slow pathway sites showed a lower mean electrode temperature (48.5°C) and no significant change in impedance. This suggests that impedance drops are smaller, and impedance monitoring less useful, for ablations in which smaller lesions are desired. Theoretically, a closed-loop system using impedance instead of electrode temperature to regulate power could be developed, but such systems are not commercially available.
Impedance monitoring can also be used to increase the safety of ablation procedures. Large drops in impedance, reflecting excessive tissue heating, predict subsequent impedance rises caused by interfacial boiling. In one study, RF applications caused coagulum only when impedance fell at least 12 Ω.
An important finding from early studies was that impedance and electrode-tip temperature do not always correlate. For example, Strickberger and colleagues found a statistically significant inverse association between impedance drop and electrode-tip temperature, with each ohm corresponding to 2.63°C on average ( Fig. 2.5 ). However, there was significant scatter between the two variables, with a correlation coefficient (R = 0.7, R 2 = 0.49), suggesting that only half the variability in impedance was associated with corresponding changes in electrode-tip temperature. Although impedance reflects tissue characteristics, impedance drop is an imperfect means of assessing the true outcome of interest, tissue heating.
