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Cardiac ablation is used extensively for treating cardiac arrhythmia, but has been hindered by the lack of an adequate feedback mechanism for determining tissue temperature during ablation. This article describes a system based on microwave sensing technology that allows both heating at RF or microwave frequencies and radiometric temperature sensing to be performed during the ablation process using a common catheter. The technique can reduce or eliminate the problem of inadequate feedback, which can potentially increase the precision with which the procedure can be performed.

The MMS radiometric sensing system in catheter
More than 1 million people have been diagnosed with cardiac arrhythmia, a statistic that no doubt excludes the thousands or millions more who have the condition but either do not know it or have not been diagnosed. The condition is characterized by the heart beating too fast, too slow, or irregularly, and covers a wide range of conditions from transient loss of consciousness and reflex anoxic seizures, to cardiomyopathy and Sudden Adult Death Syndrome (SADS). Depending on the type of arrhythmia, it can be treated either by the process of ablation (tissue destruction) or though drugs and implanted defibrillators. Cardiac ablation is carried out using one of several technologies, but each one suffers from a level of imprecision based on the inability to accurately measure the temperature of the targeted tissue. As a result, a less-than-optimum amount of tissue may be ablated to ensure patient safety. A system designed by Meridian Medical Systems of Woolwich, ME, has the potential to increase the precision of cardiac ablation by providing more accurate feedback during the procedure through the use of microwave technology.

Cardiac Arrythmia

Under normal conditions, tiny currents activate the top part of the heart (atrium) just above the bottom part of the heart (ventricles), which are the muscular chambers that pump blood around the body. An arrhythmia occurs when the normal electrical cycle of the heart is disturbed. When the heart beats too fast, the condition is called a tachyarrhythmia and when too slow a bradyarrhythmia.

Most arrhythmias that occur from the top of the heart are problematic but not life-threatening. However, when the condition arises from the bottom of the heart, the condition is more severe and can cause death. The catheter ablation technique is applied to atrial arrhythmias, and in many patients can provide a full restoration of normal heart function. Lower ventricular arrhythmias present a far greater challenge, and the treatment methodologies are either the aforementioned powerful drugs or implantable defibrillators.

The ablation technologies employed to treat arrhythmias at the top of the heart include electromagnetic (from RF through microwave frequencies), ultrasound, and cryoablation, all of which have demonstrated varying degrees of success. However, in each case optimum results are hindered by the inability to provide real-time feedback during the procedure concerning the correct amount of energy to deliver to the target tissue. Consequently, cardiac ablation systems generally apply energy at levels below optimum to avoid overheating.

Typical feedback schemes rely on the measurement of temperature as a feedback mechanism using conventional temperature sensors (thermocouples, thermistors, and fiber-optic thermosensors).These probes, which are embedded in the catheter, can measure only the temperature of the catheter tip that is in contact with the heated tissue rather than the target tissue itself. Surface cooling is often required, which impacts both the catheter tip and the adjacent tissue, rendering the conventional sensors ineffective in detecting the temperature of the target tissue. In addition, since the catheter is continuously being washed by the patient’s blood, temperature measurement is further degraded.

The radiometric sensing system created by Meridian Medical Systems (MMS) employs a catheter/antenna technique that allows both heating at RF or microwave frequencies and radiometric temperature sensing to be performed during the ablation process using a common catheter. Radiometric sensing provides accurate, non-invasive temperature measurement regardless of whether surface cooling is employed. The system is undergoing final development and has demonstrated the potential to significantly increase the efficacy of cardiac ablation by providing accurate feedback in real time so that more precise tissue ablation can be performed.

Years of Development

MMS was founded in 1985 by Dr. Kenneth Carr, a veteran of the microwave industry, a discipline more commonly known as the enabler of all types of wireless communications systems, radar, and military systems, than for its role in the medical equipment industry. However, the use of electromagnetic energy from lower frequencies through the microwave and millimeter-wave region has long been employed with varying levels of efficacy to treat tumors, as an alternative means of heating blood and other fluids, and other medical applications.

Dr. Carr is regarded as a pioneer in applying the inherent characteristics of EM energy to provide solutions for medical applications that have eluded more conventional approaches. He spent the majority of his career developing traditional microwave products, and founded a ferrite products manufacturer that was later acquired by M/A-COM. He retired from M/A-COM in 1990 as group vice president and technical director, and has since applied his talents exclusively to developing microwave medical technology at MMS. The result is as string of awards for innovation from government and industry and more than 40 patents related to the use of “microwaves in medicine”. The company’s products use RF and microwave energy to generate heat, measure emissions and motion, and monitor and maintain the temperature of blood and other fluids. Many have received FDA approval.

The Design

To solve the problem of inadequate feedback in cardiac ablation, MMS has developed a catheter that combines the ability to deliver microwave energy for tissue heating and a radiometer (essentially a remote sensing device) fabricated in part as a Monolithic Microwave Integrated Circuit (MMIC) to sense the temperature of the heart wall. The Dicke radiometer employed in the design obtains tissue temperature measurements noninvasively and operates by comparing an internal reference temperature with an actual radiometric measurement and using the difference to calculate body temperature. Results thus far show the techniques to be extremely accurate.

Although radiometers have been used for years in applications ranging from measuring atmospheric and terrestrial radiation from space to oceanographic remote-sensing, the radiometer designed by MMS incorporates several proprietary techniques that optimize its use for cardiac ablation. In particular, the use of a tiny MMIC in the catheter itself significantly reduces the problem of signal loss that occurs over the long run of coaxial cable between the external portion of the instrument and the catheter.

A microwave radiometer measures EM radiation, which the body emits as a function of temperature. Body tissue is also reasonably transparent at microwave frequencies, so microwave radiometry makes it possible to non-invasively monitor temperature beneath a surface and provide detection in the presence of cooling that is often applied to the catheter tip to prevent tissue overheating.

The Dicke radiometer employed by MMS has the ability to eliminate errors caused by RF amplifier gain and noise temperature drift as well as DC offset errors. Noise power at the antenna in the catheter is compared to a reference, and errors common to both the reference and antenna cancel at the output, providing a highly accurate result. The radiometer detects a temperature rise when the sensing antenna is in the vicinity of the target tissue. This increase is highly predictable and can be graphically presented to the physician on a convenient display. The radiometric sensing system combines both the RF electrode and microwave antenna within the catheter, and the two function simultaneously.

Development

MMS designed the MMIC, which is the foundation of the radiometric system, using Microwave Office high-frequency design software from AWR Corporation, along with a process design kit (PDK) developed jointly by AWR and TriQuint Semiconductor that accurately represents TriQuint’s foundry process. The company’s experience with various EDA tools made the choice of the Microwave Office software an easy one, since in addition to being extremely straightforward to use, it makes the process of transferring the circuit layout to TriQuint’s foundry painless. The circuit was designed within the Microwave Office software, features were selected from the PDK library, design rule checking was performed, and the results were transferred quickly to TriQuint. The fabricated MMIC demonstrates performance in close agreement with its Microwave Office simulation, allowing the MMS team to quickly begin final development of the complete system.

 

 

Bob Allison is vice president and engineering manager of Meridian Medical Systems. He can be reached at allison@mms-llc.com.

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