Clinical Applications of NTC Thermistors

Thermistors are certainly the most used transducers for medical temperature measurements. Their properties facilitate clinically difficult measurements that include: hypothermia, transcutaneous measurements during cardiopulmonary bypass, testicular temperature measurements in reproductive medicine, continuous monitoring of preterm infants and personal heat strain monitoring in occupational medicine.

Thermistors are also extensively used in a variety of clinical and research applications to measure flow, thermal conductivity and diffusivity of biomaterials and to detect the presence of liquids.

We discuss some of areas in medicine where thermistor based technology is applied which include but not limited to:

• Cardiovascular monitoring
• Respiratory measurements
• Geriatrics
• Speech

Cardiovascular Monitoring

The cardiac output, the volume of blood ejected by the heart each minute is an important parameter in a cardiovascular medicine, which is used to obtain diagnostic information about the heart and for continuous monitoring of heart function in critically ill patients.

The thermodilution employs thermistor type catheters to estimate this parameter. The tip of a Swan-Ganz catheter is inserted into a large vein typically one in the right side of the neck and advances through the heart into the pulmonary artery. A cold saline or dextrose solution, whose volume and temperature are known, is injected into the blood stream through one of the catheter lumens. The solution mixes with the blood in the right atrium and is diluted as it is carried downstream to a thermistor located at the surface of another catheter lumen. At the thermistor location, the temperature of the blood-injected mixture is measured over a period of time, and then the cardiac output (efficiency) is computed from this temperature-time response data.

At the level of the capillary network, the tissue blood flow (perfusion) is a primary factor in the local transport of heat, drugs, oxygen, nutrients and waste products. This fundamental parameter holds the key to the diagnosis and subsequent treatment of numerous medical problems. The high sensitivity to small changes in temperature exhibited by a thermistor can be used to sense the small amounts of heat involved in the thermal techniques developed for the measurement of tissue blood flow.

Related: Cardiac Monitor

Minimally invasive probes constructed around two thermistors are usually applied in these techniques. In these probes, one thermistor is used in the self-heat mode, operating as both a heat source and a temperature sensor. Changes in tissue blood flow (perfusion) cause changes in its temperature, which is used as an indirect index of blood flow. As the baseline body temperature fluctuates a second thermistor with the same size and electrical characteristics is used to measure and compensate for the changes in reference temperature.

A measurement system like this may help clinicians in a number of application areas, providing an early warning for ischemic events, targeting therapy rapidly and accurately, monitoring of patients in organ transplantations and evaluating tumor and cerebral blood flow.

Respiratory Measurements

Obstructive and restrictive respiratory diseases are characterized by air flow limitation that results from modifications in lung parenchyma and airways. Thermal convention flowmeters measures the local speed of a fluid by measuring the heat loss from a heated element in the flow path. These instruments are used in respiratory medicine for the flow analysis and the regular thermistors are employed as the sensing elements. They are also known as thermistor pneumotachometers. In these instruments, a small thermistor is placed in the flowing fluid. It operates in the self-heat mode in order to maintain an average temperature above that of the surrounding fluid. This is accomplished using a feedback circuit below:

A change in temperature tends to cause a variation in the thermistor resistance which in turn affects the amplifier output voltage and the current through the sensor. Considering amplifier as presenting infinite constant gain and bandwidth, the bridge is always statically maintained balanced, and hence the thermistor resistance and temperature are constant. The sensor resistance is maintained constant equal to R₁ under any operating condition. This circuit operates adequately if the ambient temperature is constant. If changes in ambient temperature are anticipated, a second unheated thermistor can be included in the circuit to compensate it (R3).

One of the key advantages of this configuration is that high negative gain feedback divides, the sensor time constant by a factor equal to the loop gain, improving the frequency response. Thermistors lose heat at a rate dependent on the local mass flow, temperature, specific heat, kinematic viscosity and thermal conductivity of the fluid. Thus, when a patient exhales though breathing device in which a thermistor is mounted, it is cooled and the circuit needs to provide more electrical power to keep the thermistor temperature constant. The power is proportional to the airflow when gas flow properties are sufficiently constant, the output voltage of these circuits is a non-linear function of mass-flow rate only. Linear mass-flow relationships may be obtained using a linearization stage based on analog or digital implementation of piecewise linear or polynomial approximations.

Given that the thermistor is cooled equally for both directions of velocity, the system with a single sensor provides an output of the same polarity, independent of the flow direction. This feature limits the use of these sensors to unidirectional flow, which can be adequate in some applications e.g. in forced expiratory tests. Directional sensitivity can be provided by putting multiple sensors at separate points along the flow.

Related: Key Considerations in Respiratory Measurements & Instrumentation Involved

Geriatrics

The high sensibility and the small size of the thermistor contribute to its application in the management of urinary incontinence which is prominent in elderly. These monitors are based on the changes in temperature produced by the urinary incontinence event.

Speech Therapy

Patients suffering from cleft palate or similar defects exhibit a type of speech that is characterized by excessive nasal escape of air and by an abnormal resonance compared with the normal speakers. This may be treated by speech therapy, plastic surgery, palatial prostheses, or even a combination of these treatments.

In order to allow a qualitative and quantitative analysis of nasal air escape, anemometer based on thermistors have be proposed. The thermistor is positioned in the longitudinal path of the nasal airflow. The system uses a Wheatstone bridge, in one arm of the bridge a thermistor is connected while in the opposite arm a second thermistor is used, as temperature-compensation element.  

This system can provide a numerical figure of merit to indicate the extent of the defect and the effectiveness of a treatment e.g. palatial training.