Temperature Measurement Improves Patient Care

Figure 1. Assorted Small Thermistors
Courtesy of Cornerstone Sensors

The term thermistor is a blending of thermal and resistor—a device that changes resistance as a function of temperature. Various metallic oxides are combined with binders and fired to form a thermistor. They can have either a positive temperature coefficient (PTC) of resistance or a negative one (NTC), negative being much more common.

Several thermistors are shown with and without leads in Figure 1. The smallest ones measure only a few thousandths of an inch in size, have very fast reaction times, and can be accurate to at least 0.1°C. According to Daniel McGillicuddy, vice president of sales and marketing at Cornerstone Sensors, “There is a new frontier in medicine where surgeons are using controlled localized heat to treat different maladies, including destruction of temperature-sensitive cancer cells. Some thermistor companies offer ­micro interchangeable NTC thermistors that can be incorporated into a catheter or needle. Such a small probe enhances the surgeon’s ability to control the amount of heat applied to reduce damage to surrounding healthy cells.”

Thermistors are nonlinear devices widely used in all kinds of temperature measurement applications because of their stability, accuracy, and low cost. Nevertheless, being nonlinear, the voltage across them or current through them must be linearized in some way to derive a corresponding temperature. The problem may be appreciated by considering the standard equation many manufacturers use to characterize their thermistors:

where: R_T = resistance at the temperature (T)
R_T0 = resistance at calibration temperature, usually 0°K or 273.15°C
? = the slope of the thermistor R-T characteristic⁵

This relationship is accurate at a single temperature but degrades as the temperature span is made larger. An equivalent expression for the R-T characteristic relates 1/T to the natural logarithm of R_T. To improve the accuracy of this approximation for larger temperature ranges, Steinhart and Hart, oceanographers at the Woods Hole Oceanographic Institute, proposed linearization via a third-order polynomial:

where: a, b, c, and d = constants measured for a particular thermistor

This equation can linearize temperature measurements based on a thermistor’s resistance to an accuracy between 0.005°C and 0.01°C over a 100°C-span between 0°C to 260°C. Removing the second-order term has very little effect, increasing the error limit to 0.01°C for the same span and range.⁵

When a thermistor is used to measure temperature, its power dissipation must be kept very low to avoid errors caused by self-heating. In one such application, a thermistor probe is included in a PAC to facilitate thermodilution studies. In this procedure, chilled saline or glucose solution is injected into the right atrium, upstream of the thermistor probe. The temperature difference measured is used to infer probable heart function impairments, a small ?T corresponding to a high rate of flow. In this application, absolute accuracy isn’t as important as high resolution and good repeatability.

Rather than inject cooled fluid, a more modern form of thermodilution applies heat pulses via a second catheter. The heat pulses are accurately and rapidly generated so their position in time is precisely known. In contrast, injecting a chilled solution takes several seconds, and the length of time will vary for each injection. Downstream, temperature is measured by the PAC thermistor and correlated with the heat pulse timing. The method is claimed to be sensitive to 0.02°C temperature differences.⁶

Several years ago, Yellow Springs Instruments introduced the 400 Series of interchangeable thermistor-based temperature probes, which has become an industry standard. These probes follow a B-series curve with a nominal resistance of 2,252 ? at 25°C. Although the R-T relationship is nonlinear, these probes exhibit a 100-? change in resistance per °C change around 25°C.

David Rowe, product manager for medical consumables and sensors at Philips Healthcare, added, “All Philips temperature probes are Series 400 and EN-12470 compliant. They are accurate to 0.1°C and compatible with patient monitors using Series 400 technology. We have many reusable probes that are autoclavable as well as a range of disposable probes.”

Thermistors also are used in the self-heating mode to indirectly measure airflow. The thermistor’s temperature rise above ambient balances the amount of heat generated by current flowing through the device and heat lost to its surroundings. The corresponding change in resistance is proportional to the cooling effect of the airflow.

This kind of flow sensor must be calibrated to account for different types of anesthesia gases, for example. The same principle is used to measure airflow in sleep apnea machines and patient breathing monitors.

Infrared

When a temperature measurement must be made noninvasively, IR-based instruments may be suitable. Exergen has patented the arterial heat balance (AHB) technique and applied it to outer-ear as well as temporal-artery temperature measurement. By measuring the ambient air temperature and the temperature of the ear or temple area, an estimate of heat loss can be made, and from that, the arterial blood or inner ear temperature inferred.

The company has conducted extensive testing that confirms good agreement between this technique and accepted invasive procedures. In an article describing AHB applied to measure the ear (E) canal temperature (AHBE), the heat-balance proportionality k-factor is discussed: “Specific k-factors programmed into AHBE instrument models vary depending on the site chosen for reference protocols, and are based on extensive published and unpublished clinical data. The calibration is not user accessible, in order to prevent accidental or unauthorized change.”⁷

Bob Harris, industrial division sales manager at Exergen, explained, “The AHB technology also is used in related applications such as noncontact temperature measurement of fluid being warmed or cooled and transported through disposable tubing. In medical equipment using this technique, AHB provides monitoring or can be the input to a temperature controller.”

An alternative IR approach to patient temperature measurement relies on inserting a probe sufficiently far into the ear canal that at least a partial view of the tympanic membrane is achieved. An article in the Swiss Medical Weekly explored possible measurement impairments such as depth of insertion, obstruction by earwax, environmental acclimatization time, and left or right ear selection.

During tests on the 333 patients included in the study, a wide range of probe insertion depths was measured and various degrees of earwax obstruction encountered. In spite of these conditions, the effect on measured temperature was minimal in all cases.

According to the article, “Neither the mean temperature nor the range of measurements was influenced to a clinically meaningful extent by the depth of penetration of the probe in the external auditory canal. This observation negates our hypothesis that [inaccurate] IR temperature measurement may be associated with… shallow penetration of the probe, assuming a less good angulation of the sensor to the tympanic membrane as compared to deep penetration.”⁸

Covidien’s tyco Healthcare FirstTemp™ Genius™ instrument used in the study takes 32 measurements as it scans the inner ear during a 2- to 3-second period. At the end of the period, it selects the highest reading as the true value. Oral, rectal, and core temperature offsets can be included in the reported value if required.

During operation, the Exergen device also scans the probe’s field of view, achieved by manually moving the probe as it is held against the outer ear. The highest temperature is selected from 10 taken during a 1-second period. It is this temperature that the Exergen algorithms operate upon to deliver the IR arterial temperature value.

Optical Fiber

A few distinct technologies are lumped together under the term optical fiber temperature measurement. The Bragg grating approach uses a specially prepared fiber to measure temperature. Most other methods transmit light to and from a sensor located at the end of the fiber, and it’s the tiny sensor that does the actual measuring.

A Bragg grating is a periodic physical change in a fiber’s index of refraction that is created by a special manufacturing process. The light frequencies affected by the destructive/constructive interference within this structure change with temperature. By illuminating a Bragg grating and analyzing the reflected light, the temperature of the grating can be determined.

Figure 2. Operating Principle of Birefringence-Based
Optical Fiber Temperature Sensing
Courtesy of Opsens

Opsens uses two different technologies in its fiber-based medical temperature sensors. The company has patented a white-light polarization interferometry (WLPI) method that is used to measure the difference in path length caused by temperature-sensitive birefringence. In this technique, described in Figure 2, a small crystal of a selected material is sandwiched between a linear polarizer and a dielectric mirror to form a two-beam polarization interferometer.

The crystal is attached to the end of an optical fiber that transmits white light to it and couples the reflected light back to a WLPI signal-conditioning instrument. Changes in path length through the crystal primarily depend on the temperature-induced changes in its birefringence, and the signal-conditioning instrument recovers this information from the reflected light. This approach allows various types of crystals to be used depending on the accuracy and temperature span required.⁹

Opsens and several other companies separately have developed GaAs-based optical-fiber sensors that exploit temperature-induced band-gap changes. As with the WLPI sensor, the GaAs crystal is attached to the end of an optical fiber. Light frequencies corresponding to energies less than the band gap are absorbed, and those greater are transmitted. By analyzing the light reflected from such a sensor, its temperature can be determined.

Both the WLPI and GaAs sensors are ideal for very long-term monitoring because they don’t drift. The parameter being used is a characteristic of the material and inherently stable. Medical applications have time scales from hours to months.

For the medical clinician, optical-fiber-based temperature measurement provides a solution to the safety risks associated with electrosurgery. An ESU cuts by vaporizing the tissue in the immediate vicinity of a probe. RF at frequencies from 300 kHz to 3 MHz and amplitudes from 200 V to 10 kV is generated to accomplish this, and the potential for unintentional patient damage exists any time an ESU is used.¹⁰ Fiber-optic temperature sensing is nonconductive and immune to EMI so it can be attached to the patient as required and relied upon during ESU procedures.

Further Considerations

Measurement accuracy always is important when a parameter’s value must lie within prescribed limits. Is there cause for concern if the measured temperature is 0.1°C below the upper limit? How about 1°C above it? If the thermometer itself has a 1°C error, how can you decide which values are critical?

Figure 3. Range of Typical Measurements at Various
Sites Based on 37.4°C Arterial Temperature
Courtesy of Exergen

The picture is further complicated by considering that although Wunderlich’s 1861 report established 37°C as the mean body temperature, it also demonstrated a range from 36.25°C to 37.5°C as being normal.¹¹ In addition, the temperature offsets inherent among sublingual, rectal, and PAC measurements can account for 0.4°C to 0.5°C inconsistencies.¹² And, diurnal variation contributes as much as 0.5°C further uncertainty.¹³

The relationships among the various site-specific values and the typical upper and lower trigger points leading to further investigation are shown in Figure 3. Obviously, a patient’s temperature should be represented as a range rather than a single number. However, this implies an even greater need to minimize measurement error.

Conclusion

Fortunately, modern thermistor manufacturing techniques allow ±0.1°C device matching at low cost, making practical thermometers that are accurate and disposable. Most clinical temperature measurements are made with thermometers based on thermistor technology.

However, both IR and fiber-optic technologies also are important, offering advantages under certain circumstances. IR is noninvasive and can be very accurate. Exergen’s IR thermometers claim to have the added benefit of eliminating the careful positioning associated with other IR products that measure the inner-ear temperature. And, fiber-optic thermometry is immune to EMI, a big plus in electrosurgery.

Regardless of the measurement technology used, it’s clear from the large variation shown in Figure 3 that a patient’s temperature is just one input to a meaningful diagnosis, although an important one. Many other contributing factors also must be taken into account, which underscores the value of thorough clinician training and extensive experience. Nevertheless, an accurate temperature measurement provides reliable guidance for further treatment decisions.

References

1. Pompei, M., “Temperature Assessment Via the Temporal Artery: Validation of a New Method,” Exergen Clinical Study, 1999, p. 36.
2. Pompei, p. 32.
3. McCauley, G., “Understanding Electrosurgery,” Bovie Medical, 2010, p. 4.
4. “DS-X Lessons Learned,” MedSun, U.S. Department of Health and Human Services, FDA, Newsletter #4, May 2006, http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/medsun/news/newsletter.cfm?news=4
5. “NTC Thermistors,” http://www.thermometrics.com/assets/images/ntcnotes.pdf
6. “Continuous Cardiac Output,” ­Slideshow, Edwards Lifesciences, slide 39, 2003, http://ht.edwards.com/presentationvideos/powerpointslides/cco/cco_speaker_notes.pdf
7. Pompei, p. 13.
8. Twerenbold, R. et al, “Limitations of Infrared Ear Temperature Measurement in Clinical Practice,” Swiss Medical Weekly, 140:w13131, 2010, p. E4.
9. “White-Light Polarization Interferometry Technology,” Opsens, http://www.opsens.com/pdf/WLPIREV2.3.pdf
10. Hausner, K., “All Electrosurgical Units Are Not Created Equal,” http://www.elmed.com/Electrosurgery/Info/equal.htm
11. Kelly, G., “Body Temperature Variability : A Review of the History of Body Temperature and Its Variability Due to Site Selection, Biological Rhythms, Fitness, and Aging,” Alternative Medicine Review, December 2006
12. Pompei, p. 13.
13. Twerenbold, p. E5.