The use of radio-frequency (RF) ablation is an important and growing technique at the heart of many new medical treatments. This equipment uses RF energy to ablate or destroy unwanted tissues. When delivered via a catheter, it offers a minimally invasive treatment for a wide variety of conditions, including atrial fibrillation. These systems typically include a peristaltic pump to provide cooling or temperature control. This pump must be able to produce and control the high pressures (up to 130 psi) required in this application.
It is critical to integrate pumps that are designed to handle these high pressures. Some new peristaltic pumps have been designed specifically for integration into ablation systems.
These pumps meet the demanding requirements of these applications.
Medical Application of RF Ablation
Medical devices use the RF ablation technique to treat many types of medical issues, including heart issues, tumors, and back pain (see Figure 1). For example, this technique can be used in treatment systems for atrial fibrillation (AFib), a condition that affects 2.7 million people in the United States. The condition is caused by an interruption of the normal, steady heartbeat. The heartbeat is controlled by electrical signals that travel through the tissues of the heart. But, when the heart is in AFib, these signals are disrupted, causing it to beat irregularly and often too fast. When the heart does not beat in rhythm, it cannot pump blood properly to deliver the energy and oxygen the body needs. This lack of oxygen- rich blood in the body and brain are what can cause physical and mental fatigue, and other symptoms of AFib. The ablation treatment is effective when the condition is caused by tissues of the heart wall that no longer conduct these electrical signals properly. The ablation treatment destroys the faulty tissues and provides patients with a permanent cure.
Surgical RF Ablation
A typical ablation system is made up of an RF generator, control and monitoring equipment, an antenna, and a cooling pump. The radio waves are used to create an electrical current that delivers heat to the targeted tissues, which creates a controlled lesion, or burn. The burned tissue is replaced with healthy tissue through the natural healing process.
The business-end of the device is the antenna, which applies the energy to the tissue being ablated. The antenna resides at the tip of a catheter or cannula, along with electrical and temperature sensors that are connected to the system controller. The RF generator delivers the energy through the antenna at the specific frequencies and amplitudes, as prescribed by the treatment. The pump delivers cooling fluid to maintain the correct temperature at the treatment site and helps to protect the surrounding healthy tissues. Catheters typically use open-loop cooling that delivers saline directly to the treatment area via small holes. Cannulas typically use a closed-loop system in which saline is recirculated through a double needle.
The pump must provide the prescribed flow rates to deliver the correct amount of cooling. To do so, it must overcome the back pressure created by the small inner diameter of the catheter or cannula tubing. These pressures can exceed 100 psi for closed-loop applications. Recently, the market trend among medical device OEMs is to develop smaller diameter catheters for greater precision and reach. These micro catheters require pumps that can overcome the resulting higher back pressures caused by internal diameters as small as 0.014 in. (0.36 mm).
The pump most often chosen for the job is a peristaltic pump, a type of positive displacement pump. With a peristaltic pump, the fluid is contained within a flexible tube fitted inside a semicircular pump casing known as the track. The tube is typically built into a disposable tube set that can be discarded after each procedure, which minimizes cleaning validation. The tube is pressed against the track by a rotor with two or more rollers attached to its external circumference. As the rotor turns, the part of the tube being compressed is occluded (pinched closed), forcing the fluid being pumped to move through the tube. As the tube reopens to its natural shape after the passing of the roller, a vacuum is created, which draws fluid into the pump. The advancing roller then pushes the fluid toward the pump outlet.
A peristaltic pump is designed to give consistent occlusion of the tubing; too much or too little can cause performance issues. Too little occlusion hurts flow and pressure capability and can cause leaking through the pump. Too much causes the tubing to rapidly wear out, hurting flow performance and causing excessive spallation.
There are two basic types of designs used in peristaltic pumps: fixed gap and spring-loaded. The fixed gap refers to the gap between the rollers and the track where the tube is occluded. This is a simple, yet effective design. However, it is very much dependent on the tube wall thickness to maintain consistent occlusion. Tubing supplied by the pump manufacturer is designed with the correct tolerances to work effectively with a fixed-gap pump. In many medical applications, tubing is not supplied by the pump manufacturer, and it is a nonstandard size. For a fixed-gap design, a custom roller must be specified to adapt to the dimensions and tolerances of the selected tubing.
A spring-loaded peristaltic pump has a spring mounted either on the track or on the rollers. The spring provides a positive force pressing on the tube that will compensate for the tube’s tolerances. The spring is selected to overcome not only the tubing’s compression strength, but also to achieve the desired pressure performance of the pumped fluid.
New Technology Used in Next- Generation Ablation Pumps
Recently, Watson-Marlow Fluid Technologies Group updated its existing surgical ablation pump, after extensive research and development in consultation with market-leading medical companies. The new pump was built on the technology of its predecessor pumps in RF ablation applications.
Some of the requirements that came out of that voice-of-customer exercise conducted during the R&D phase are a desire for high pressure, a predictable flow-pressure profile, and the need to minimize human errors from loading tubes. Users requested that the pump give positive feedback that the tube is loaded properly and ready to go. The loading mechanism should give operators confidence that the tube is properly loaded, and that it will work every time. Figure 2 shows the resulting technology, the 400RXMD surgical ablation pump.
Where the RXMD differs from other peristaltic pumps is the ability to produce high pressures — far higher than the 30 psi maximum of typical peristaltic pumps. Standard RXMD models are rated up to 90 psi, and custom configurations can achieve up to 130 psi. At high pressures, predictability and control is essential, and the RXMD can be tuned to meet a specific flow profile. For example, in RF ablation cooling, the pump must produce consistent flow at a high target pressure, but the flow must drop away once that target pressure is met. Figure 3 shows this performance with pumps set for three different target pressures. The figure shows the predictable “knee” shape of the graphs, which describes the flow profile.
What allows this extreme pressure performance and control is a robust track mechanism supported by two heavy duty coil springs. In addition, a new factory adjustment mechanism enables the pressure and flow profile to be tuned.
Other enhancements include a larger opening for tube loading and a new tube holder that ensures rapid, trouble-free tube loading and minimizes the risk of human error. The sprung dual holder design grabs the tube firmly and effectively without creating flow restrictions. The tube holder’s 90 angle centers the tube in the holder for optimal performance every time the safety cover is closed. The tube holder’s coil springs compensate for variations in tube dimensions and shore hardness within allowable tolerances.
Support beyond the Product
A purpose-designed off-the-shelf pump is a good start, but it is not sufficient to provide an effective solution. A company designing a new medical device must work with a supplier experienced with OEM medical device applications — one that will give access to their engineering staff and take the time to understand the unique requirements of the customer’s application. The customer/supplier team will work on designing custom configurations, testing them, and agreeing to a written specification. The end result of this process is an optimal pump solution that meets the customer’s precise flow and pressure requirements.
It is equally important that the supplier has strong quality systems in place, giving product traceability and change control. The product must meet the requirements — exactly — with the first order, and with orders placed years into the future.
RF ablation is an important technique in medical procedures that helps improve the lives of patients. For systems that require cooling or temperature control, the proven and enhanced RXMD peristaltic pump is ideally suited to these applications.
This article was written by Rodd Turnquist, Sales Manager OEM Division, and Christian Skantze, Product Manager, at Watson-Marlow Fluid Technologies Group, Wilmington, MA. For more information, click here .