Key Points
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Irreversible damage to myocardial cells can occur at temperatures as low as −5 to −10°C.
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Lethal temperatures are reached within 60 seconds within 3-mm depth and may not require much longer applications to achieve transmural lesions in thin areas.
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Refreezing thick areas potentiates cell depth because of repeated freeze/thaw cycle and increases lesion depth by different mechanism.
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Horizontal catheter position increases lesion volume and depth.
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Low-flow areas result in larger lesions (unlike radiofrequency applications).
Freezing the cardiomyocyte to a certain temperature results in cell death. At the tip of the cryocatheter, temperature rapidly reaches less than −20°C, which results in intracellular ice formation and immediate cell death. At the periphery of the ice ball, much lower temperatures are reached and extracellular freezing occurs, resulting in incomplete cell death. Some delayed cell death occurs at the periphery of cryolesions as well, because of different mechanisms of cell death, including apoptosis.
The target temperature of −60°C was somewhat empirically introduced into cardiac cryosurgery based on the knowledge obtained from freezing cancer cells and other tissues, as well as availability of cryoprobes in the United States operating at −60°C.
Obviously, not only absolute temperature is important, but also freezing duration, rate of cooling, speed of rewarming, and other factors. For example, atrioventricular (AV) node can survive −80°C application of less than 10-second duration but may sustain irreversible effects with −5°C to −10°C applied for 2 minutes or with repeated cycles of cryoablation.
This chapter discusses factors that determine cryolesion formation and morphologic characteristics of cryolesions.
Freezing Duration and “Lethal” Temperatures
Cryoapplication time is an extremely important issue clinically. Two-minute applications are typically recommended for atrial tissue. These recommendations are based on the fact that lesion depth does not change substantially after 120-second application. This is reasonable for focal cryoablation; however, these recommendations are probably not practical for creation of long, linear lesions in the atrium.
Specifically, temperatures in the myocardium are reaching plateau by about 60 seconds of freezing ( Figure 3–1 ). If the thickness of the atrial tissue is 2 to 3 mm, probably a 90-second application is adequate, given that maximal negative temperature was already achieved and added 30 seconds of exposure at given temperature.
The issue is that there are no sufficient experimental data available in regard to what is the minimal necessary exposure for specific temperatures to have a lethal effect on the cardiomyocyte. Atienza et al. in an experimental setting, evaluated the effects of freezing duration on the AV node conduction and found that if application was 10 seconds or less, AV node conduction fully recovered (temperature [T] = −80°C).
Perhaps experiments like this could be conducted using −30°C cryoapplications as well and changing freezing duration. In any case, because commercially available cryoprobes have just one or two temperature settings, only limited data are available on this issue. Measuring temperatures at different depths in the myocardium in vivo is also challenging.
Some extrapolations can be made comparing temperatures in the cryolesion in vitro with lesion depth obtained in vivo. Wood et al performed a series of elegant experiments measuring tissues in vitro under different conditions. According to their data, with 4-mm ablation catheter at 5-mm depth, maximal temperature reached was only +20.5°C, and at 3-mm depth, temperatures reached −6.1°C (no-flow conditions). Slightly lower temperatures were observed with horizontal catheter position, reaching −6°C with no-flow conditions at 5-mm depth.
Cryolesion depth measured in experiment setting with epicardial applications would be similar to no-flow conditions in vitro. For 4-mm catheters, it was found to be 4.7 mm acutely and 3.3 mm chronic (permanent). Temperature at that depth reaches about −6.1°C and apparently is adequate to result in irreversible cell damage with 240-second exposure.
With 8-mm catheter at 5-mm depth, temperatures reach −11.3°C to −16.8°C depending on whether catheter is horizontal or vertical to myocardium (no flow). Morphologically, with 8-mm-catheter cryolesion depth in acute experiments was 5.0 mm, and in chronic experiment 4.1 mm, with 240-second epicardial applications.
Similarly, Khairy et al demonstrated cryolesion depth with transvenous 8-mm cryocatheter applications to be 4.9 mm in the atrium and 4.8 mm in the ventricle; therefore, based on temperature measurements, it appears that cardiomyocyte is quite sensitive and may not survive even −6°C, as far as exposure is prolonged.
Most of the investigators evaluated 240-second duration lesions and longer, targeting maximal possible depth. At 5-mm depth of 240 seconds or longer, exposure is critical because temperatures are borderline. Only extracellular ice formation is accomplished; therefore, prolonged time is critical for osmotic cell membrane damaging effects to take place. Much lower temperatures and potentially intracellular freezing are accomplished at 3-mm depth and may not require such a long application time for irreversible changes to occur at that depth.
In clinical scenarios where cryoablation is applied in immediate proximity to pulmonary veins (PVs), tissues that are usually less than 3 mm thick, 90- or 120-second applications are likely to be transmural, although there are insufficient experimental data to directly support this. Lethal temperatures are achieved in first 60 seconds at 3-mm depth. However, the length of time in excess of 60 seconds required for freezing to achieve irreversible damage at 3-mm depth is not known. Freezing for 2 minutes at every point is not practical, however, and likely unnecessary. Based on EGM reduction (to <0.1 to 0.2 mV), an approach can be used for creation of linear lesions or application of other classic electrophysiologic criteria, like PV entry/exit block for PV isolation, just like using radiofrequency (RF).
Rate of Freezing
Rapid cooling is more lethal, contributing to fast death of myocardial cells within 2 to 3 mm of the probe. At the borderzone, both maximal negative temperatures achieved and rate of cooling are very slow, contributing to reversible cardiomyocyte damage. Because in currently available probes the rate of cooling is not controlled, this is not a practical consideration.
Double Freezing
Double freezing results in more extensive tissue damage and deeper, larger lesions because of repeated freeze/thaw effects on cell membranes. This was demonstrated both in experimental and in clinical settings on a variety of tissues, including myocardium. In our investigations, we have also demonstrated that repeated epicardial freezing results in deeper lesions.
Effects of Contact Pressure
Increased pressure results in faster freezing, because of compression of the tissue and decreased warming effects of intramyocardial flow. This was well demonstrated in experimental settings ( Figure 3–2 ).
Effects of Catheter Size on Cryolesion Formation
Larger catheter size results in larger lesions by several mechanisms: First, catheter tip in contact with myocardium is “protecting” that area from direct blood flow washout effects; second, larger catheter size can incorporate larger diameter channels, supplying more refrigerant and, therefore, increasing power of freezing. This is well demonstrated in animal models ( Figure 3–3 ).
Effects of Blood Flow on Cryolesion Size
Low-flow areas result in larger and deeper cryolesion formation. Therefore, cryoablation has an advantage in trabeculated areas, also ablating inside of low-flow “pouch” during typical isthmus ablation. Azegami et al, using 7 French, 4-mm cryocatheters, demonstrated cryolesion depths in the apex (low-flow area), 7.5 ± 1.8 mm, compared with the outflow tract (high flow), 5.7 ± 1.7 mm.
Effects of Probe Temperature on Cryolesion Size
Lower probe temperatures result in larger lesions. Only nitric oxide (−80°C)–based cryocatheters are available for electrophysiologists today. Other coolants available in surgical probes, however, have much lower boiling temperatures and are more effective.
The most widely used coolant in surgical settings is liquid nitrogen (LN2), which boils at −196°C; argon gas (−160°C)−based cryosurgical devices are also available. Such delivery systems are more complex, however, and it is more difficult or impossible to deliver these agents via very small diameter probes, suitable for transvenous applications. Argon gas–based probes have been used for ablation of prostate cancer and other tissues, and more recently introduced for epicardial ablation of atrial fibrillation.
Critical nitrogen (CN2) technology allows efficient delivery of coolant (CN2) and delivers at least −180°C to −190°C to the tip of the probe. Because gas is pressurized below the boiling pressure, it does not cause a vapor lock and could be efficiently delivered through smaller diameter transvenous catheters (see Chapter 9 for more details).
CN2 probes are not commercially available; however, in experimental settings, epicardial freezing with CN2 consistently produces transmural atrial lesions. Myocardial lesions produced with CN2 (T = −180°C) are deeper, compared with lesions produced with argon gas–based cyoprobes.
Effects of Blood Flow on Cryolesion Size
Convecting warming of the cryoprobe has tremendous effects on cryolesion formation and size. In contrast with RF applications, increased blood flow around the catheter significantly decreases cryolesion size and vice versa; cryoablation is more powerful in the low-flow areas, and lesions are much larger under those conditions.
Lesion Characteristics
Macroscopically, a cryolesion has smooth and sharply demarcated borders from normal myocardium ( Figure 3–4 ). Cryoablation results in tissue necrosis, followed by infiltration of the lesion with neutrophils and macrophages ( Figure 3–5 ). Dead cardiomyocytes are gradually removed and collagen deposition starts; gradually, inflammatory cells are replaced by more and more dense collagen deposition ( Figures 3–6 and 3–7 ). Some neovascularization also occurs in the cryolesion. Elastic fibers and the collagen network remain intact, somewhat preserving tissue architecture ( Figure 3–8 ), although no systematic studies are looking into the tissue architecture in a quantitative manner.
