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What are the new AASM rules that everyone is talking about?
The American Academy of Sleep Medicine recently published new standardized scoring rules for sleep: "The AASM Manual for the Scoring of Sleep and Associated Events: Rules, Terminology, and Technical Specifications." The new manual is the result of years of work and debate about standards that began with a meeting of industry representatives in July 2004.
Issues addressed include, sleep stage terminology, technical specifications for recording and data presentation, and the standardized scoring of sleep. The new recommendations took effect on July 1, 2008 for labs that are accredited by the AASM.. Two of the recommendations related to diagnostic sensors that are the subject of this discussion are found in section VIII; which lays out the technical considerations for respiratory effort:
The sensor for detection of respiratory effort is either esophageal manometry, or
calibrated or uncalibrated inductance plethysmography.
The section also describes the recommendation for sensors to be used for detection of apneas and hypopneas as:
An oronasal thermal sensor be used to detect the absence of airflow for identification of apnea. 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.
As one of the worlds leading diagnostic sensor companies, Ambu Sleepmate, followed the recommendations and developed sensors to help labs meet the new standards.
How does a Piezo respiratory effort sensor work?
Ambu Sleepmate introduced the first piezo respiratory effort sensor the "Resp-Ez" in 1989. 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 if it broke.
A piezo crystal is essentially a ceramic material that is capable of emitting a weak electrical signal when it is flexed. 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. In 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 and readings can occur when the patient moves and tension is lost. Piezo belts also can produce a phenomenon known as false paradoxing, particularly when the tension on the belt is altered by patient movement
The piezo respiratory effort sensor, has proven its reliability and reasonable accuracy over millions of sleep studies, and will continue to be used in sleep studies around the world. AASM accredited labs will have to use RIP ( Respiratory Inductance Plethysmography) as it is now the AASM recommended standard.
How does Respiratory Inductance Plethysmography work?
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. As the band is stretched and relaxed by the patient’s breathing the cross-section enclosed by 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. 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. The bands need only be tight enough to stay in place, 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 at the umbilical 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 determine 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 and used to create what is called a "flow-volume loop". Calibrated RIP systems have not been as popular because of the time required to calibrate the system and the expense of the systems.
How do I adjust a RIP belt on a patient?
The belts should be adjusted with the patient in an upright position. We recommend that you adjust the sliders on the belt so that it is slightly smaller ( 6 to 8 inches) than the patients chest or abdomen size. The belt has enough stretch so that it will close comfortably without being too tight. It does not need to be and in fact should not be as tight as a typical Piezo belt. As long as the belt is tight enough to stay in place it will generate a good signal. The wider RIPmate belts stay in place without a lot of tension. Over tightening will interfere with a good signal.
Why does the RIP signal look different sometimes?
The RIP system is very sensitive and can provide more information about the patient’s respiration (e.g. flattening of the signal during an event).
Further, RIP by its very nature is akin to an antenna and is sometimes subject to interference. The RIP systems are designed with modules that operate at different frequencies to minimize any interference. In the event that you do get some static in the signal, rerouting the interface cables so they are not directly touching or tightening or loosening one of the bands slightly can help separate the frequencies further and eliminate any interference.
Why are there specific sensors for the chest and abdomen?
RIP operates based on changes (caused by the stretching and contracting of the band) of the frequency of a small current that is passed through the wire in the bands. The module for each band operates on a different frequency so that one will not interfere with the other. If you tried to use two chest or two abdomen modules on the same patient they would be on the same frequency and you would likely experience "static" or interference in the signal, making it hard to read the patient's respiratory effort signal.
Do all RIP sensors meet the AASM recommendations?
The AASM recommendation for RIP is Respiratory Inductance Plethysmography. As of this writing another existing technology known Respiratory Impedance Plethysmography is not what the AASM recommends. There may be some confusion as both go under the RIP moniker, but they are very different. The technologist looking to meet current AASM recommendations should be aware of the difference and insist on Inductance Plethysmography.
What are the AASM recommendations for pressure and thermal airflow?
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. Also, 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 AASM is recommending one sensor for apnea detection and another sensor for hypopneas. 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.
In the past, practitioners have generally used one or the other sensor, 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.
What is the difference between a thermistor and a thermocouple?
Thermal sensors are generally either a thermistor or a thermocouple. A thermistor 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. On the other hand, thermistors tend to react more quickly to a temperature change. Thermistors require a battery box to generate the current for the resistor. At the end of the day it is often a personal preference of the technologist that determines which sensor is used as they both provide adequate signals to detect breathing events.
How should I clean my sensors after use?
Ambu Sleepmate recommends using an alcohol wipe to clean off the sensor and allowing it to air dry. Do not wrap them up or store them before the sensors are completely dry. Care should be used to avoid soaking the sensor in any solution or in using any cleaner that is corrosive to plastic. Adhesive removers are not recommended and should be used sparingly if at all, and wiped completely off before allowing the sensor to air dry in the open.
The belts for both Piezo and RIP effort sensors can be hand washed in mild soap and water and hung to dry. Do not wring the RIPmate belts as there is a wire imbedded in the fabric.
If sterilization is required use a gas process, do not autoclave sleep sensors.
What is the difference between a snore microphone and a snore sensor?
A snore microphone is as the name implies, a small microphone that can record the sound of a patient's snoring. The sound waves are picked up and shown as signals on the recorder's screen. A snore sensor is a small Piezo crystal that is encapsulated in a plastic housing. The Piezo picks up the vibrations that are present during snoring. The sensor only picks up vibrations it will not pick up other sounds. Some technologists prefer the sensor because it is generally more durable than the microphone and is not affected by moisture.
How do I position a snore sensor?
The best way to place a sore sensor is to place two fingers on the patient's neck and ask them to simulate a snore. Place the sensor where you can feel the most vibration. The sensor should be placed so that it will stay in contact with the skin and taped into place. Additional tape can be used on the cord about six inches away for strain relief. Snore sensors and microphones should be comfortable for the patient and should not be placed directly on the patient's larynx.
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