What’s Your HST Super Power?

Clinicians today have numerous options for prescribing home sleep apnea testing (HSAT). While sleep laboratory type 1 tests based on polysomnography (PSG) utilize an array of sensors, belts, technologies, and experts to monitor the patient, at-home tests generally gather less information in favor of convenience, comfort, and accessibility.

Two respiratory inductance plethysmography (RIP) belts measuring respiratory movement—positioned around the torso beneath the armpits and at the umbilicus—are the “gold standard” in the lab setting, according to Paul Raymond, MD.

However, two belts aren’t always used at home, notes Dr. Raymond, medical director of the Anchorage Sleep Center and chair of the American Academy of Sleep Medicine (AASM) Payer Policy Review Committee.

“If you’re doing home testing, some units have two belts, but most do not use them because it becomes cumbersome to the patient,” he explains. “You have to put it in context when you’re doing home testing to look at cost, patient comfort, ease of use, and the accuracy that you’re going to get out of [the system],” as well as the test’s objective.

Diagnostic OSA Tests

Type 1 in-lab sleep tests utilize the most comprehensive, precise equipment to monitor electroencephalography (EEG), electro-oculography (EOG), electromyography (EMG), heart rate or electrocardiography (ECG), airflow, breathing/respiratory effort, and arterial oxygen saturation via pulse oximetry (SpO₂).¹ In-lab tests using two RIP belts can identify an array of complex sleep disorders including obstructive sleep apnea (OSA). Type 2 at-home tests enable a full PSG covering a minimum of seven channels using RIP belt technology and an array of sensors that are applied by a technologist before sleep.

Type 3 testing, the most common HST, monitors four channels, including ventilation or airflow, with at least two channels of respiratory movement or respiratory movement and airflow.¹ These at-home tests utilize equipment applied by the patient with instruction that typically includes a nasal cannula, finger oximeter, often a body position sensor, and either one or two respiratory belts.

“Unlike in-lab polysomnography, home sleep testing equipment varies significantly. Some systems use two respiratory inductance plethysmography belts identical to in-lab setups, but others use patches to assess chest and/or abdominal wall excursion, and others do not use either,” explains Christopher Cielo, DO, attending pulmonologist in the Division of Pulmonary and Sleep Medicine at Children’s Hospital of Philadelphia (CHOP). At CHOP, they use in-lab PSG for pediatric clinical cases and, occasionally, home sleep testing in research protocols.

Finally, type 4 tests collect fewer parameters of airflow, blood oxygen level, and heart rate but do not meet all the criteria of a higher-category device.¹ These beltless technologies provide less information and often serve to screen patients for further diagnostic testing or measure treatment efficacy for those already under medical management, explains Helgi Helgason, MS, RPSGT, RST, senior product specialist at Nox Medical.

While EEGs have typically been utilized in lab settings for type 1 tests, Helgason says sleep experts began to “just look from the nose down to the waist,” for OSA, causing the recent explosion in sensor and belt technology that are smaller, more lightweight, and easier to tolerate for type 3 HSAT.

For patients suspected of having a high probability of moderate to severe OSA, HSAT is more for confirmation, says Dr. Raymond. He warns that without a measurement specific to respiratory-related arousals, clinicians must consider that home studies likely underestimate the degree of sleep apnea.

Direct measurements—including from sensors in RIP belts and EEGs—provide the clearest pictures of respiratory function and sleep architecture.

Belt Benefits and Design

Respiratory belt technology contains a wire-loop conductor that creates inductance to get a value that is modulated proportionally with respiratory movement.² While one belt measures respiratory effort, the pairing of two RIP belts helps clinicians distinguish between central and obstructive apneas.

“Without measuring directly the respiratory effort from both the chest and abdominal walls, you can’t tell what type of apnea it is,” Helgason says.

Two belts allow the clinician to assess for change in the phase of paradoxical breathing between the chest and abdomen, a “supportive measure for determining obstructive events,” adds Dr. Cielo.

Since the chest and abdomen are physiologically separate, they “go out of phase” with cessation in airflow, a hallmark of OSA, explains Helgason. “If you have a diagnostic device that only has one belt, the best you can get is diminution in the tidal volume or lung volume… but there will be inferring or guesswork to do,” he says. “It won’t give you a full picture of what the patient is going through.”

“Having a two-belt system is far superior because you cannot only see changes in amplitude… but you can also see the physiologic response to issues [when experiencing] occlusions,” says Helgason, noting Nox Medical’s approach of getting as many direct physiologic measures as possible to minimize interference.

When two belts measure both chest and abdominal expansion, Dr. Raymond reports correlation within 10% accuracy in terms of tidal volume compared to pulmonary function tests.

A two-belt system can also provide a reliable back-up signal for airflow when the nasal cannula gets dislodged in the middle of the night.

“Derived signals such as the RIP sum or RIP flow can be used as secondary airflow signals to score respiratory events when the thermistor or nasal pressure signals are not available,” Dr. Cielo says.

“A two-RIP-belt system can be an alternative sensor to measure flow by the frequency of the breathing and the volume of the breathing,” adds Dr. Raymond, enabling fewer invalid tests.

“The last thing the patient wants to do is retake the test, and the last thing the clinician wants is to see downloaded data that is inconclusive,” says Helgason.

Certain types of sensors and belt designs can also cause failed signals and invalid results. According to Helgason, piezo-electric crystal technology, which generally works as a strain gauge using a sensor covering a portion of a length-adjustable, reusable band, does not provide a true, clear signal of respiratory effect. The sensors in the band, he adds, fail to measure around the entire circumference of the torso in two locations as RIP belts do.

Dr. Cielo notes that since RIP flow is derived from chest and abdominal belts—not measured directly—the accuracy of RIP flow depends on the types of belts used.

Dr. Raymond references a 2021 study in which researchers retrospectively compared cannula flow to disposable cut-to-fit, semi-disposable folding, and disposable RIP belts in 767 HSAT studies.³ The disposable RIP belts that had integrated contacts and did not fold on top of themselves performed the best.³ The cut-to-fit RIP belts were most likely to be unreliable, and the semi-disposable folding belts produced the poorest RIP flow signals compared to the cannula flow signal, Dr. Raymond explains, noting a potentially compromised electromagnetic field with folding.

For clinicians seeking to diagnose patients with suspected OSA, it is vital to assess the comprehensiveness of the diagnostic testing equipment available in tandem with the patient’s preferences and needs. Intrusiveness, convenience, and expense may also play a part in selecting the optimal sleep study setting as well as the equipment used to acquire measurements.

“Clinicians should have a good understanding of the equipment they are using, including their limitations, in the population they are studying, so they can choose the most appropriate test for their patients,” Dr. Cielo concludes.

Kerri Reeves is a freelance journalist and regular Sleep World contributor based in Ambler, Pennsylvania.

References

1. US Centers for Medicare & Medicare Services. Decision memo: Sleep testing for obstructive sleep apnea (OSA). Medicare Coverage Database. Accessed April 23, 2024. https://www.cms.gov/medicare-coverage-database/view/ncacal-decision-memo.aspx?proposed=N&NCAId=227.
2. Nox Medical. Nox RIP Belts – Info Sheet. Accessed April 23, 2024. https://noxmedical.com/about/news-press/article/nox-rip-belts-info-sheet/.
3. Montazeri K, Jonsson SA, Agustsson JS, Serwatko M, Gislason T, Arnardottir ES. The design of RIP belts impacts the reliability and quality of the measured respiratory signals. Sleep Breath. 2021 Sep;25(3):1535-41.
4. Kapur VK, Auckley DH, Chowdhuri S, et al. Clinical practice guideline for diagnostic testing for adult obstructive sleep apnea: An American Academy of Sleep Medicine clinical practice guideline. J Clin Sleep Med.2017 Mar 15;13(3):479-504.
5. Troester MM, Quan SF, Berry RB, et al; for the American Academy of Sleep Medicine. The AASM Manual for the Scoring of Sleep and Associated Events: Rules, Terminology and Technical Specifications. Version 3. Darien, IL: American Academy of Sleep Medicine; 2023.
6. American Academy of Sleep Medicine. Blue Shield of California Payer Policy Modification letter. Accessed April 23, 2024. https://aasm.org/wp-content/uploads/2023/05/BCBS-CA-diagnostic-sleep-testing.pdf.