For many cardiac arrhythmias, catheter cryoablation is now a safe and effective alternative to RF ablation. Freezor®, a 7F cryocatheter, has been in clinical use in Europe, Canada and the US for several years. In that time, it has established a record of safety and effectiveness in the treatment of arrhythmias, particularly in peri-nodal procedures. With its demonstrated safety profile – especially important when working around the atrioventricular (AV) node – Freezor® represents a new paradigm in treatment alternatives for electrophysiologists.
The Biology of Cryotherapy
Cryotherapy is the use of cooling and freezing in living tissue to affect cellular function and activity.To produce cryotherapeutic temperatures, heat energy is removed from the tissue, rather than ‘injecting’ cold energy into the tissue. Multiple factors influence the effectiveness of catheter-based cryotherapy. Chief among these are tip contact (to remove heat from the tissue, the catheter tip must be in contact with the tissue); tip temperature (the catheter tip temperature applied determines if the cells are subjected to hypothermia or killed); and freeze duration (longer freeze times will cause more tissue damage).
During cryoablation, the removal of heat from the tissue creates a temperature gradient that extends from the catheter tip to the edge of the ‘cryotherapy zone.’At the tip/tissue interface of the catheter, the cryotherapy temperature can be either -30°C and -75°C, depending on the type of intervention (cryomapping or cryoablation).The temperature in the cryotherapy zone ranges from the sub-zero cryogenic temperature at the tip-tissue interface to body temperature at sites distant from the catheter tip. This zone is ‘dynamic’ as it expands during freezing and shrinks upon re-warming. In living tissue, vascularity always provides a heat source and will, as a result, cause local variations in temperature (see Figure 1).
Cryomapping and Cryoablation Mechanisms
Catheter-based cryotherapy includes two complementary procedures – cryomapping, which allows for the confirmation of target sites, and cryoablation, which destroys the cells responsible for the arrhythmia. When cells are cooled below body temperature, a series of events occurs. Beginning at a tissue temperature around 32°C, the cell membranes lose transport capability, and ion pump activity decreases.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, along with a corresponding increase in refractory period.