Viewing Flow Management as a Critical Subsystem of Devices

Originally Published MDDI May 2002

COVER STORY

From infusion pumps to valves and IV sets, flow-control systems are providing new capabilities in care while helping to ensure patient safety.

Gregg Nighswonger

Although pumps and other flow-control technologies have been keeping pace with the more specialized needs of the medical industry, they are now being challenged to meet increasing demands for reliability and longer life. Today's specifications also often call for smaller size and lighter weight, as well as new levels of functionality that can support efforts to improve patient safety. In short, emerging medical systems are requiring designers to embrace new perspectives on the mechanisms and components used to transport and control the flow of fluids and medical gases.

Fundamental characteristics and functionality of the broad range of flow-related components—including pumps, valves, connectors, and filters, among others—are also changing with the use of new materials and design innovations. A benefit to medical device manufacturers is the ability to more easily match product specifications with optimal flow components.

The current generations of infusion pumps, for example, are designed not just to transport volumes of fluid, but also to be tools that can simplify the work of clinicians and help prevent use errors. Rather than thinking of pump components as commodity items, designers should view them as critical subsystems of the finished device.

NEW CONSIDERATIONS FOR SAFETY

The National Academy of Sciences Institute of Medicine estimates that more than half of all medical errors occurring during the actual administration of care involve the use of intravenous (IV) infusion equipment. Medical device manufacturers are among those who are working on new systems and technologies that can help reduce the number of preventable adverse incidents involving medications—especially those involving IV delivery systems.

A prime example is the automated infusion system, which requires a precise and reliable delivery system—that is, an electronic pump capable of maintaining flow rates and pressure levels within strict specifications. The system's pump component can provide either an additional opportunity for error or an additional safety check. In a worst-case scenario, a pump failure can result in harm to the patient, such as the free flow of medication. However, today's advanced systems can incorporate features that protect against such potential outcomes by ensuring that component failures will not cause adverse events.

A greater effort is being made to raise the level of inherent safety in many products that incorporate flow-control systems—infusion pumps, implantable devices, pain management equipment, and IV sets are all the focus of efforts to incorporate greater levels of safety. Development of new products of these types now entails closer scrutiny of those components used to manage the flow of liquids, bodily fluids, nutrients, and medications. A manufacturer's failure to do so can result in product recalls and loss of goodwill and revenue.

FDA records document numerous instances in which infusion systems have been subject to recalls because of failures involving the operation of the pump component. And researchers at the Bath Institute of Medical Engineering (United Kingdom) have noted that "infusion pumps are involved in 100 to 200 adverse incidents each year, as reported to the Medical Devices Agency's own Adverse Incident Centre." They emphasize that a significant portion of such incidents are life-threatening, and that an estimated 40% are caused by pump failure.

ENSURING PROPER PUMP PERFORMANCE

Medical device dependability is derived to a great extent from the reliability of the equipment components. Using the example of the infusion pump, performance will depend principally on the quality of the pump unit. Pump performance can vary dramatically, depending on the equipment and the environment in which the system is being used.

According to some observers, pump failure is often not the fault of the pump itself, but results from a communication failure between the system's designer and the component supplier. The pump selection process, which can often take place well into product development, needs to include a careful consideration of all device requirements.

Determining which pump unit is the proper choice for a given application must take into account not only the product specifications, but also how the components might be affected by changes in the operating conditions. Will operation in environments with significantly higher ambient temperatures cause a pump malfunction? If such a malfunction does occur and the entire device subsequently fails, what safeguards must be incorporated to ensure the safety of the patient being treated with the device?

Among the factors that can influence the performance of an infusion pump are:

For example, Eric Pepe of KNF Neuberger Inc. (Trenton, NJ) explains that "It is critical that designers specify the pumping system's tolerance to various performance requirements, including electrical power, temperature, flow rate, vacuum, and pressure." He suggests that "it is vital to clarify early in the design process the system's performance requirements and to match them with those of the device's design."

Pepe gives the example of a pump that has the ability to create a vacuum greater than that required by the device. If the device contains soft tubing, the pump's excessive vacuum could cause the tubing to collapse, resulting in a system shutdown or equipment damage, according to Pepe. "Likewise, a pumping system that can create pressure greater than a medical instrument's tolerance can break connectors and other system parts when the pressure becomes excessive."

The use of an IV infusion pump to administer blood-based products to a patient underscores the importance of considering the material to be pumped when developing such a device. The practice of administering pump-assisted transfusions is becoming more prevalent—driven largely by the convenience and improvements in flow-rate and volume control that are made available by infusion pumps, compared with conventional gravity-based methods. When automated IV pumps are used to infuse blood products, however, there is concern that passing human red blood cells through the pump mechanism can result in significant levels of hemolysis.

Thus it is critical to consider specific device requirements as early as possible in the development process. Ideally, this would be as soon as the factors that will shape component decisions are determined. Delaying the selection of items such as pumps can often delay completion of the product development process while an appropriate component is found, or it can necessitate the use of custom components.

NEW MATERIALS AND SIMPLER DESIGNS

Although pump mechanisms are a key element in the transport of fluids in medical applications, valves, stopcocks, lines, and other components are also critical flow-control elements. New materials that offer resistance to various chemicals in the medical environment, control mechanisms that incorporate more human factors elements, and increased functionality are factors that are shaping other aspects of flow-control component design, and expanding the range of device applications.

Chemical Resistance. Materials used in flow-control mechanisms must be capable of withstanding exposure to a range of chemicals and body fluids that may affect the material characteristics. B. Braun Medical Inc. (Bethlehem, PA), for example, has begun converting production of all its Discofix standard-bore stopcocks to a lipid-resistant material. The conversion is intended to expand the range of anesthesia and critical-care applications for the products to include the administration of lipids.

Needle-Free Systems. Federal authorities have mandated that healthcare providers adopt products and measures that reduce the risk of inadvertent needlesticks. Manufacturers have responded with new syringe concepts and with IV components that avoid the use of needles.

With an emphasis on both patient and user safety, B. Braun has developed the Ultrasite needle-free capless valve mechanism for IV administration systems. A redesigned housing used for the device is intended to simplify access with a syringe or IV tubing—but does not allow needle access. The one-piece device also uses full threads to provide a secure connection, and a stop ring to prevent overtightening of the component. The Ultrasite valve, based on a luer connection, is a passive device that meets government requirements. As an additional safeguard for patients, the device incorporates a positive-pressure design to help prevent catheter occlusions.

Temperature Control. In some medical applications, fluids need to be heated to maintain a constant temperature. In response, Watlow (St. Louis) has developed miniature polymer-tubing assemblies that incorporate a heating element capable of maintaining fluid temperatures during transfer from reservoirs to the point of use. The heating element is wound directly around the tubing, and a jacket over the assembly allows the tubing to be flexed or coiled on moving equipment or to be routed around other components.

A New Concept in Coupling. Colder Products Co. (St. Paul, MN) has developed an alternative to conventional luer connections for high-volume, disposable medical applications. The firm's SMF1 couplings can be used in such applications as blood pressure cuffs, surgical equipment, blood analyzers, and tube sets. Hose barbs on the devices provide leak-free connection with typical tubing sizes. The couplers are constructed of ABS and meet gamma sterilizability requirements. The SMF1 design also provides free coupling rotation with mating inserts. This allows the connected tubing to swivel, which can help prevent kinked tubing and unintentional disconnects.

INNOVATIVE TREATMENT OPTIONS

The use of some medical devices is moving progressively closer to the patients as systems become more mobile. Diagnostic equipment is now used at the bedside or in the home, and physicians and surgeons are exploring the use of new devices that offer less-traumatic, yet more-effective, treatment. Flow-control mechanisms are being adapted to these changes with the use of smaller, lighter parts for greater portability. At the same time, innovative thinking in the way that fluids and gases can be managed is offering new treatment options.

As a case in point, Medtronic Neurosurgery (Goleta, CA), a subsidiary of Medtronic Inc. (Minneapolis), recently received FDA clearance to market its Strata valve for use in treating hydrocephalus. This condition, which is generally marked by an abnormal accumulation of cerebrospinal fluid in the ventricles of the brain, affects an estimated 15 in every 10,000 newborns and one in every 10,000 adults, and can result in nervous system damage or death.

The Strata valve was developed to provide a greater degree of control in the use of hydrocephalic shunting, a technique for draining the excess fluid from the brain. A hydrocephalic shunt is fully implantable under the patient's skin and consists of a proximal catheter that is placed in the affected brain cavity, a pressure valve to regulate the rate at which the cerebrospinal fluid drains, and a distal catheter that diverts the excess fluid to the peritoneal cavity to be reabsorbed by the body. The Strata valve connects the two catheters.

The valve has five pressure adjustments that can be made noninvasively by clinicians and without x-ray verification as the patient's condition changes. A slim profile makes the device comfortable and discrete for patients. Other features include a siphon control mechanism to guard against excess drainage of cerebrospinal fluid, a complication that can compromise the fluid dynamics of the central nervous system and result in tissue tears or bleeding in the brain.

According to the company, use of the device can "potentially reduce the frequency of valve replacements, which require additional surgery, and limit exposure to radiation, which in large doses raises health concerns among both patients and clinicians."

CREATING THE WAVE OF THE FUTURE

Research is ongoing into advanced techniques for improving the control of fluids and gases in medical applications. These efforts require the collective expertise of researchers from various disciplines— computer modeling and computational flow dynamics, and materials science, among others.

One location where such cooperative research is taking place is Australia's Commonwealth Scientific and Industrial Research Organisation (CSIRO) BioMedical Devices. The group's focus is on the development of new components and materials for the creation of electronic and microelectronic devices. R&D is conducted via a cohesive step-by-step approach that involves devising analytic equations, computer models and physical models, all of which describe the movement and flow of particles, drops, bubbles, and fluids. The resulting data are incorporated into prototypes, then, after testing, into finished products.

Each phase of development involves the use of state-of-the-art testing equipment, including magnetic-resonance imaging (MRI). "By using MRI and high magnetic fields, we can excite material to produce radio waves, and from these we can measure the flow and movement of fluids by visual as well as other methods," says Kurt Liffman, PhD, of the CSIRO thermal and fluids engineering lab. "In fact, we use transparent fluids such as simulated blood to provide us with a visual advantage."

The lab's MRI system is the only one in Australia dedicated to the measurement of fluid and granular flows, according to Liffman. "Our equipment enables us to obtain accurate measurements of fluid flow using lasers, magnetic fields, and various photographic methods. To understand the phenomena that we are observing, we are assisted by a multidisciplinary team made up of physicists, mathematicians, and engineers."

The CSIRO unit is currently working on a device that monitors the flow of fluids used by certain sections of the medical community. The monitoring system sounds an alarm if fluid flow becomes too fast, too slow, or stops.

CONCLUSION

Whether it is a novel valve design for an IV set, a pump to precisely measure analytes, or a connector system that prevents accidental needlesticks, today's flow-control systems offer designers a range of choices. The key is to carefully assess potential components with regard to intended uses early in development. It is also important to consider the potentially critical nature of these subsystems.

Gregg Nighswonger is senior editor of Medical Device & Diagnostic Industry.

Photo by Roni Ramos

Copyright ©2002 Medical Device & Diagnostic Industry