New AASM Recommendations for Sensors: A Simple Guide for the Sleep Technologist

The implementation of new rules for scoring and summarizing sleep recordings has finally arrived and marks one of the most important milestones in the history of sleep medicine. The American Academy of Sleep Medicine published the first comprehensive and standardized scoring rules for sleep recordings in 2007. The AASM Manual for the Scoring of Sleep and Associated Events: Rules, Terminology, and Technical Specifications is the result of years of deliberations and research among colleagues and industry concerning standardization of practices and technology in the field of sleep medicine. The final development of the new scoring manual includes changes in sleep stage terminology, technical specifications for recording and data management, and the standardized scoring of sleep. Since the publication of the AASM scoring manual, sleep disorders centers, physicians and sleep technologists have been preparing to comply with the new rules that became effective for all accredited sleep disorders centers on July 1, 2008. Understanding the changes and the use of new technology for sleep recordings presents a challenge for many technologists as well as sleep physicians.

The new scoring manual includes both recommendations and alternatives for parameters used in polysomnographic recordings. This article addressees two of the recommendations related to technical considerations and sensors used for recording of respiratory effort noted in section VIII of the scoring manual.

The AASM Scoring manual recommends that sensors used for detection of respiratory effort are “either esophageal manometry, or calibrated or uncalibrated inductance plethysmography.” Recommendations are also included in the new scoring manual for detection of apnea and hypopnea. An oronasal thermal sensor is recommended for detecting absence of airflow to identify apnea, and a nasal pressure transducer (with or without square root transformation of the signal) is recommended for identification of hypopnea.

This article addresses the recommendations related to the use of Respiratory Inductance Plethysmography (RIP) to measure respiratory effort and the recommendation that nasal pressure be used in conjunction with an oronasal thermal sensor. The following discussion is an effort to bring some clarity for the sleep technologist concerning RIP technology. It is important for sleep technologists to understand the difference in RIP technology and piezo technology which has previously been used to record respiratory effort for the past few years. We will also address the rationale for the AASM recommendations for the use of both thermal and pressure sensors during PSG recordings.

Respiratory Effort Sensors

Sleepmate Technologies introduced the first piezo respiratory effort sensor the “Resp-Ez” in 1989.2 It replaced the mercury strain gauge that had been the standard for many years with a small, convenient package that did not run the risk of mercury contamination.

A piezo crystal is essentially a ceramic material that is capable of emitting a weak electrical signal when it is flexed.³ The little LED lights on children’s shoes are often powered by a small piezo crystal in the sole of the shoe that is stressed with each footstep. When the stress is removed the electrical output stops. The repeated stress and release transmitted by the rise and fall of a patient’s chest through an elastic band creates the familiar sine wave pattern on the recorder. The waveform is only an approximation of the movement of the chest and abdomen. In particular the output of the piezo is not linear.4In other words the output created by a 1 inch change in chest or abdomen circumference is not twice the output from a 1/2 inch change. This lack of a linear response makes it more difficult to assess hypopneas.

Piezo-based effort belts also measure the tension where the crystal is located, a single point, where the band pulls during breathing. Problems with accuracy of the signal can occur when the patient moves and tension is lost. Piezo belts also can produce a phenomenon known as false paradoxing,³ particularly when the tension on the belt is altered by patient movement.

The AASM likely believed that the above mentioned problems associated with piezo technology were significant enough to warrant finding a more reliable way to measure respiratory effort. As a result of the new AASM recommendations the piezo respiratory effort sensor, though it has proven its reasonable accuracy over millions of sleep studies, is now replaced in AASM accredited sleep centers with RIP technology (the recommended AASM standard).

It should be noted that the AASM recommendation for RIP is Respiratory Inductance Plethysmography. There is currently another existing technology known as Respiratory Impedance Plethysmography which is not what the AASM recommends. There may be some confusion as both go under the RIPmoniker, but they work very differently. Accredited sleep centers that must use the new AASM scoring should be aware of the difference and insist on Inductance Plethysmography.

Respiratory Inductance Plethysmography

Inductance Plethysmography employs sensors that are able to measure changes in a cross-sectional area of the patient, specifically the thorax and abdomen during a respiratory cycle. The RIP sensor consists of a belt with a wire woven or sewn in a sine wave or zig-zag pattern along its length, and a driver module with a circuit board, oscillator and battery that passes a weak current through the wire in the band creating a small magnetic field.⁴ As the band is stretched and relaxed by the patient’s breathing the cross-sectional area within the band changes slightly. This change in cross-section produces a slight change in the magnetic field that results in a change in the frequency of the current. This change can be measured and converted to a voltage output that creates the waveform on the PSG recorder. The science behind this phenomenon has to do with currents induced by changing magnetic fields and certain laws attributed to physicists, none of which really matters for the purposes of our discussion here. The key concept is that the stretching and relaxing of the band can be measured accurately and depicted as a waveform.

The circuitry in the processor module detects the change in the frequency and produces a signal waveform that is represented on the PSG recorder. An important quality of Inductance Plethysmography is that the signal depicted is linear, that is to say it changes in proportion both when the band is stretched and when it is relaxed. Thus, if a 1 inch stretch of the band creates a 1 volt output then a 2 inch stretch would create a 2 volt output and the difference would be clearly seen in the size of the waveform on the PSG recorder.

It should be noted that with RIP there is no electrical current passing through the patient, and only a weak magnetic field is created. The signal does not require a specific tension in the band, making the fitting of the band less critical than with a piezo belt. The bands need only be tight enough to stay in place and in fact a band that is too tight can loose signal quality. The belts should be placed in the standard locations at mid chest and just below the umbilicus to assure maximum expansion during the respiratory cycle.

By placing the bands over the abdomen as well as the thorax, the sensor can measure the phase relationship between the two bands and can help distinguish central apnea from obstructive apnea during sleep studies. Some RIP systems also include a sum channel that is useful in detecting paradoxical breathing or slight phase shifts. When the thorax and abdomen signals are completely out of phase in theory they will cancel each other out and the sum channel will be flat. In practice this is highly unlikely given the signal processing requirements, but it is useful for the technologist to be able to note a decrease in the sum channel output during these out-of-phase or paradoxical breathing episodes.

RIP can also be calibrated to measure the actual volume of airflow to create a “flow-volume loop”. Calibrated RIP systems have not been as popular because of the time required to calibrate and the expense of these more sophisticated systems.

Pressure and Thermal Airflow

In addition to piezo effort belts being replaced by Inductance Plethysmography, the AASM has recommended that an oronasal thermal sensor be used to detect the absence of airflow for identification of apnea. As an alternative, the recommendations state that when the thermal signal is unreliable, technicians may use a nasal air pressure transducer. The AASM is recommending one sensor for apnea detection and another sensor for hypopneas. The recommended sensor for detection of airflow for identification of hypopnea is a nasal air pressure transducer, with or without square root transformation of the signal. The reasoning behind the recommendation for using two different types of airflow sensors is that nasal pressure transducers are more sensitive to slight changes in airflow (hypopneas), but may result in overestimating apneas, while thermal sensors are less sensitive to minor breathing changes, but are more reliable for identifying apnea.

Thermal sensors are generally either a thermistor or a thermocouple. Athermistor is a variable resistor that responds to temperature changes, while a thermocouple is made of dissimilar metals that generate a variable voltage in response to temperature fluctuations. Thermocouples are generally thought to produce a more stable signal and to react better to small changes in airflow. Pressure transducers are even more sensitive than thermocouples, but require a cannula to be purchased with each study, and are less comfortable for the patient. In addition, mouth breathing is difficult to detect with a cannula and can cause a loss of the signal.

In the past, practitioners have either used an oronasal sensor or nasal pressure for recording airflow, often based on personal preference. Thermocouples are the choice of many sleep labs as they represent a fair compromise between a pressure transducer and a thermistor. Thermocouples are small, generally fit well on the patient, and pick up both nasal and oral flows. It is not practical to detect oral airflow with a pressure transducer.

Although the new recording recommendations present a challenge because of the requirement of using both a thermal sensor and a pressure transducer, the technology is not new. Sleep technologists will eventually become accustomed to using both sensors and will devise innovative techniques for obtaining good quality sleep studies. Manufacturers will ultimately develop oronasal sensors that can easily be used in conjunction with nasal pressure. As the field of sleep medicine continues to grow, the technology also continues to improve. The new scoring and recording rules represent a step in standardizing techniques, terminology and rules in sleep medicine—a step that is long overdue. Sleep medicine technology is on the path to continued growth in the next decade.

J. Scott Cardozo
President, Sleepmate Technologies
Midlothian, VA