Key Points
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Cryomapping may be performed during −30°C applications maintained for less than 60 seconds (classic cryomapping) or at the onset of −80°C applications (dynamic cryomapping).
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During −80°C cryoapplications, the cryolesion expands until the end of the third minute, mandating vigilance even in the presence of a cryomap indicating safety at −30°C.
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The typical successful site for atrioventricular nodal reentrant tachycardia (AVNRT) cryoablation is more superior and atrial than radiofrequency (RF) ablation.
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During ice-ball formation, a high-frequency signal artifact is noted as the cryocatheter adheres to the endocardial surface.
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Unlike RF ablation for AVNRT, an accelerated junctional rhythm is not characteristic of cryoablation.
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Six-millimeter electrode-tip cryocatheters provide superior results to 4-mm electrode tip cryocatheters for AVNRT, with a comparable safety record.
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Cryoablation of the AV node may be performed but is seldom indicated.
Catheter cryoablation offers several major advantages compared with radiofrequency (RF) ablation including catheter adhesion to endocardium that provides stability, limits collateral damage, and permits programmed stimulation during an application; absence of thrombus formation; and lack of pain ( Table 10–1 ). When targeting perinodal substrates, one of the most important characteristics of cryoablation is the ability to produce reversible effects, that is, cryomapping, to reduce the risk for inadvertent atrioventricular (AV) block. This chapter describes the concept of cryomapping, including its cellular basis and clinical utility. It then summarizes the techniques and literature associated with cryoablation of atrioventricular nodal reentrant tachycardia (AVNRT) and of the AV node.
| CRYOAPPLICATION CHARACTERISTICS |
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Reversible Effects of Cryoablation
Cellular Basis
When cells are cooled below body temperature, a series of events occurs. These effects begin when the tissue temperature declines to less than 32°C. Cell membranes lose transport capability, and ion pump activity decreases. The metabolic functions of cells are strained and biochemical reactions become uncoupled. These changes result in a decrease in the rate of spontaneous depolarization, loss of resting membrane potential, a decrease in action potential amplitude, and an increase in action potential duration, together with a corresponding increase in refractory period. Conduction velocity slows and local conduction block may occur in the region being cooled. These effects may be entirely transient, depending on the interplay between temperature and duration. The briefer the exposure to a hypothermic insult and/or the warmer the temperature, the more rapidly cells recover. In the electrophysiology laboratory, these temporary changes in excitability and conduction can be readily detected as local increases in the refractory period or transient local conduction block, or both.
Practical Cryomapping
When targeting perinodal substrates, this reversibility phenomenon is a major advantage of cryoablation compared with RF energy. Complete recovery of tissue conduction is the rule when a cryoapplication is promptly interrupted in the event of an untoward effect, such as AV block. From a practical perspective, cryomapping may be achieved by two approaches: “classic cryomapping” and “dynamic cryomapping.” With 4- or 6-mm electrode-tip cryocatheters, classic cryomapping is achievable when a temperature of −30°C is maintained for no more than 60 seconds (cryomapping mode). If the site proves successful (e.g., no adverse effect with or without abolishing the targeted arrhythmia), the operator may transition to “cryoablation mode” to create an irreversible lesion at −80°C. The second approach, dynamic cryomapping, is the one routinely used for AVNRT. Dynamic cryomapping inherently occurs at the onset of a −80°C application, as the temperature gradient spreads centrifugally from the catheter tip-to-tissue contact. Cooling of cells (e.g., to a temperature of −30°C) with reversible electrophysiologic effects necessarily precedes irreversible tissue destruction (e.g., at temperatures of <−50°C to −60°C). It has recently been demonstrated in animal models that reversibility is the rule for −80°C applications that are interrupted without delay after the onset of AV block, with a safety window of at least 10 seconds.
It is important to understand that, even with a −30°C cryomap that suggests the site is “safe,” an adverse effect may occur if the temperature is decreased to −80°C. At −30°C, local conduction block occurs in a very discrete area. As the temperature is decreased to −80°C, the frozen area of tissue will progressively grow. The lesion increases in size during the first 2 to 3 minutes and reaches a plateau thereafter. Thus, vigilance is required from the onset of a −80°C cryoapplication until the end of the third minute, as the temperature gradient spreads, despite an initially reassuring “cryomap.” It is typical, therefore, that at the beginning of the learning curve, operators gain experience with “classic cryomapping” at −30°C. Experienced operators often forego “classic cryomapping” and rely on the concept of “dynamic cryomapping.”
Atrioventricular Nodal Reentrant Tachycardia Cryoablation
Practical Approach
Transcatheter cryoablation for AVNRT shares many features with RF ablation for AVNRT but also involves unique aspects. First, an electrophysiologic study is performed to confirm the tachycardia mechanism and to identify a procedural end point that may be reproducibly verified. In our center, this typically involves positioning three or four electrode-tip catheters via femoral venous access under fluoroscopic guidance: high right atrium, His bundle, right ventricular apex, and coronary sinus. Incremental atrial pacing and extrastimulus testing is performed. Dual AV nodal physiology is identified by a divergence in A1-A2 versus A2-H2 curves, and is typically marked by a sudden A2-H2 jump of at least 50 milliseconds in adults (or 40 milliseconds in children), in response to a 10-millisecond decrement in the A1-A2 interval ( Figures 10–1 and 10–2 ). It is particularly important to demonstrate discontinuity in the AV nodal conduction curve and to define anterograde refractory periods of fast and slow pathways. If sustained tachycardia cannot be induced, isoproterenol is generally infused to increase the baseline heart rate by 30% to 50%. Programmed stimulation is then repeated.
Once the diagnosis is confirmed, the cryocatheter is positioned fluoroscopically across the tricuspid annulus ( Figure 10–3 ). Several systems for catheter cryoablation are in commercial use. Herein, we describe the cryoablation system manufactured by Medtronic CryoCath LP, Montreal, Canada. Commonly used quadripolar steerable catheters come in 7 French 4- and 6-mm electrode-tip and 9 French 8-mm electrode-tip sizes. In our center, barring research protocols, the 6-mm electrode-tip catheter is usually the standard catheter of choice in all but the smallest children. These catheters are equipped with a thermocouple at the distal electrode, where cooling occurs and temperature is recorded.
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