Going with the Flow of Gas-Sensing Technologies

Originally Published MPMN June 2009

PRODUCT UPDATE

Going with the Flow of Gas-Sensing Technologies

Used in a variety of healthcare applications, gas-sensing technologies are becoming more accurate, reliable, and compact

Bob Michaels

Gases are ubiquitous in medical device and other healthcare applications. Treating patients in hospitals would be unthinkable without the use of anesthesia gases to perform surgeries or breathing facilitators such as helium to treat respiratory diseases. And of course there is oxygen, the ultimate breathing enabler. But to monitor and regulate the flow of gases in medical applications, a multitude of sensors is necessary.

Because sensors for monitoring gases and gas concentrations can make the difference between life and death, manufacturers are devoting increasing time and resources to making them more accurate, more reliable, and more stable. At the same time, they are striving for products with faster response times to ensure that medical equipment can react quickly to patient needs.
The Air You Breathe
Oxygen is vital for life, and that’s why regulated oxygen delivery is vital in the medical arena. Responding to the need for proper oxygen delivery is Servomex (Jarvis Brook, Crowborough, UK; www.servomex.com), a provider of gas-measurement products. “As medical advances and treatments have grown, so have the applications of our sensor technology into the healthcare market,” comments Martin Cox, the company’s business unit manager for Hummingbird Sensing Technology transducers. “This [development] includes gas-delivery systems and patient monitoring in ICU, OR, and other life-critical applications.”
Among the company’s Hummingbird line of products is the Paracube series of transducers—paramagnetic-based technologies that are used in critical-care applications. “Paramagnetic technology takes advantage of the unusual magnetic properties of oxygen by measuring the displacement of two nitrogen-filled subminiature glass spheres suspended within a nonuniform magnetic field,” Cox explains. When oxygen is present within a sample gas, it is attracted into the magnetic field, causing the sphere assembly to rotate. This rotation is sensed by a closed-loop control system, the output of which provides an inherently linear and accurate measurement of oxygen.
The robust measurements achieved using paramagnetic technology have enabled the company to develop products ranging from high-accuracy to fast-response sensors. “The accuracy, linearity, repeatability, and low zero drift offered by paramagnetic technologies allows us to incorporate them into breath-by-breath patient monitoring, critical-care ventilator, and neonatal incubator applications,” Cox remarks.
In addition to paramagnetic-based sensors, Servomex offers zirconia sensors, which monitor oxygen by measuring the difference between the concentration of the gas entering the sample port and the concentration entering the reference port. Using partially stabilized zirconia for ruggedness and stability, this sensor is suitable for parts-per-million oxygen measurements, according to Cox. “Zirconia sensors,” he adds, “are ideally suited to respiratory measurements that require the fastest response times.”
Complementing the trend toward greater reliability and faster response times is the demand from OEMs for smaller, more-resilient sensors suitable for portable equipment. Although the functioning of portable devices is similar to that of their static counterparts, portable systems require more-compact sensors that can withstand shocks. “The challenge to us as manufacturers has been to respond to customer requirements as best we can within the limitations of the technologies and manufacturing techniques available,” Cox comments. “But we do that by continuing to push the envelope in terms of R&D.”
Concentrating on Oxygen

Accuracy, reliability, and a small footprint are the name of the game not only for sensors that monitor particular gases, but also for those that monitor gas concentrations and pressures. “Modern oxygen concentrators and ventilators have become smaller, more portable, and less costly than older technologies,” says Chris Dixon, director of sales and marketing at Kavlico Corp. (Moorpark, CA; www.kavlico.com). “To address those changes, pressure-sensing technologies need to keep pace.” While shrinking in size, sensors for monitoring oxygen pressure have also become more precise, incorporating innovations such as amplification and temperature compensation to minimize the need for final system adjustments.

To meet the needs of evolving medical devices, Kavlico provides a range of sensors for monitoring oxygen pressure. The P6050, for example, regulates the pressure of the pump that delivers oxygen to the patient. And in ventilator systems, which require mixtures of oxygen and nitrogen or helium, separate P6050 sensors measure the pressures of the gases to control the overall mixture.
Used for oxygen concentrators, respirators, and ventilators, the P6060 family of piezoresistive-type sensors offers built-in electronics that provide stable and repeatable outputs under varying pressure conditions. Low-profile products that can simplify integration into OEM devices, these sensors provide enhanced 1% accuracy to optimize control algorithms. They are also available with pressure ranges as low as 0.5 and as high as 75 psi, which covers most of the pressures that medical equipment are required to handle.
Fulfilling the requirements of accurate, low-pressure sensing, the P993 detects patient inhalation, allowing delivery of oxygen to patients only when they inhale. Based on a proprietary capacitive microelectromechanical systems (MEMS) technology, the sensor is used to determine when to turn on the flow of oxygen, enabling OEMs to reduce the size of oxygen-delivery equipment. “Inhalation and exhalation are quite difficult to monitor,” notes Dixon, “and inaccuracies related to the precision of this measurement can have a large effect on the efficiency of the entire system.”
Anesthesiology—It’s a Gas
A crucial but risky area of medical science, anesthesiology requires specialized sensors to monitor a range of gases. To meet the need for anesthesia gas monitors (AGMs), LumaSense Technologies (Santa Clara, CA; www.lumasenseinc.com) markets Andros-brand sensors to OEMs for patient monitors and anesthesia machines. Capable of analyzing the anesthesia gases inhaled and exhaled by patients during surgery, AGMs also measure metabolic gases such as oxygen and carbon dioxide.
LumaSense’s AGMs are based on either nondispersive infrared (NDIR) or dispersive infrared (DIR) technology. The NDIR-based 1026A gas monitor utilizes a broadband IR emitter, which covers all of the wavelengths of interest for a given set of gases to be measured. Optical band pass filters allow that portion of IR wavelengths at which a specific gas absorbs IR energy. The DIR-based 1026B, on the other hand, uses an optical spreader to disperse the spectrum of IR light. Employed in Philips patient monitors and in the Taema Felix and Dameca Siesta anesthesia workstations, this sensor can analyze any wavelength in the entire spectrum using Fourier transform algorithms.
Like current designs from other manufacturers, the 1026A requires a chopper wheel to control the IR wavelengths. Having eliminated this source of failures, the 1026B is more accurate and more reliable. However, its more-complicated optics—including optical grating—still incorporate some moving parts. The company’s ability to eliminate these moving parts in a new-generation NDIR sensor has prompted it to revert to the original technology platform.
“While both the NDIR and DIR techniques are reliable and accurate,” states Harry Hirschman, LumaSense’s senior director of worldwide gas products, “the trick is, how do you make them more reliable? Basically, by having fewer moving parts.” There will always be some moving parts in anesthesiology sensors because a pump must be used for pumping the gas from the breathing line to and from the patient, Hirschman adds. But the new NDIR-based model, the 9500, removes them entirely from the optical path.
“The instruments have a tough job to do,” emphasizes Hirschman. “Therefore, they must be sophisticated.” They must be able to measure five anesthetic agents—enflurane, desflurane, halothane, isoflurane, and sevoflurane—in addition to nitrous oxide, carbon dioxide, and oxygen. They must also be able to measure the concentrations of the various gases of interest at a known flow rate and report the values to the patient-monitoring system, which then reports them to the anesthesiologist. While the current-generation NDIR and DIR sensors can alternately identify any two of the five anesthetic agents simultaneously, the next-generation 9500 will be able to identify any three.
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