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Key words: specific absorption rate, ablation, implantable cardioverter defibrillators, ICD, RF, tech support
The use of high-current, intracardiac electrostimulation devices is growing rapidly. Two devices included in this field are radiofrequency (RF) cardiac ablation systems and implantable cardioverter defibrillators (ICDs). Recently, the FDA-approved indications for the prescription of ICDs were greatly expanded to include many patients with serious cardiac electrical conduction diseases. Clinical use of RF cardiac ablation continues to grow rapidly since it offers a permanent cure for many arrhythmic conditions. While both devices utilize high levels of electrical energy delivered by intracardiac electrodes, their goals are very different. Defibrillation seeks to reset the electrical activity of the heart without damaging any myocardium, while ablation seeks to destroy selected arrhythmia-producing areas of the myocardium.
OST scientists are attempting to determine if ICD and ablation devices meet their respective goals. Specific projects involved measuring the spatial distribution and magnitude of energy transferred from the electrodes to the myocardium. As new ICD leads come on the market, the possible combination of electrical generators and intracardiac leads that can be used together expands exponentially. Ablation catheter design is still evolving with manufacturers pursuing many unique approaches. The rapidly changing device situation requires constant laboratory evaluation to keep up with the medical device industry's pace of development.
OST scientists developed a computerized system to evaluate the rate of energy deposited throughout simulated cardiac tissues by the electrodes of various electrical stimulation medical devices. The magnitude and spatial distribution of the specific absorption rate (SAR), which is the rate of energy deposition, were measured. SAR is measured in units of Watts per kilogram and is proportional to the square of the electric (E) field magnitude. SAR was measured around ICD and ablation electrodes while they were immersed in tissue-simulating liquid, and the SAR was measured throughout the volume of the liquid. The data from these measurements have also been used to validate computer models that predict SAR for situations and geometries that cannot be measured in the laboratory. For ablation catheters, SAR is determined at each point in a scan by applying a pulse of radiofrequency current to the catheter and measuring the rate of change in temperature due to the shock. A precisely calibrated glass-encapsulated microthermistor, embedded in the tip of a thermal probe, is used to measure temperature. Temperature versus time is measured at each particular point in the tissue simulating liquid. Measurements are made continuously before, during, and after the RF pulse is applied. Thermal gradients are allowed to decay, and then the thermal probe is moved automatically to a new point by the systems three-dimensional positioning system.
Figure 4 - Linearity Test of Ablation Electrode
Figure 4 shows results of linearity testing done at two positions with respect to an ablation electrode. At the position nearer the electrode (marked with the symbol +), thermodynamic factors introduce nonlinearities due to very high local SARs occurring above 3 Watts of applied RF.
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Figure 5 - SAR Pattern Around a 4mm Ablation Electrode
Figure 5 is the SAR pattern around a 4mm RF ablation catheter measured at 0.25 mm resolution.
The results of this work were used by OST scientists when they consulted on premarket reviews of ICDs and ablation devices. In addition, the information obtained in these studies will be applied to CDRH reviewers guidance documents for these devices.