Respiratory inductance plethysmography, or RIP, has been a part of sleep diagnostics for decades. But the information a RIP system can provide depends on how the technology is designed, configured and calibrated.
That distinction is becoming increasingly important as sleep testing expands beyond the lab. In home sleep testing, respiratory signals need to do more than indicate that thoracic and abdominal movement is occurring. They can also contribute to understanding how breathing changes across the night, support respiratory event characterization, and provide useful information when other respiratory signals are limited or unavailable.
A recent Sleep Review article highlights the evolving role of RIP in sleep diagnostics and home sleep testing. It examines questions around one-belt versus two-belt configurations, alternative respiratory effort sensors, and the growing interest in extracting more information from respiratory patterns as sleep testing becomes increasingly decentralized.
RIP, long considered a workhorse signal in the lab, is being reimagined for a world where many sleep studies happen at home, and where the pressure to simplify has never been greater. The discussion raises an important technical point that deserves closer attention: RIP is not a single, uniform measurement technology.
Different systems may all be described as RIP, yet they can differ substantially in what their signals represent and what information can be derived from them. In short, not all RIP is the same. One of the most important distinctions is whether the system provides a relative measure of respiratory movement or a continuously calibrated, volume-related measurement of breathing. This is where volumetric RIP becomes important.
Changing expectations of RIP
It can be tempting to treat all respiratory effort belts as variations of the same technology. But the clinical value of a respiratory signal depends on what the sensor is actually measuring.
Volumetric RIP is a specific implementation in which changes in electrical inductance are determined by changes in thoracic and abdominal cross-sectional area during breathing. When the belt fully encircles the body, conforms to natural respiratory expansion, maintains stable electrical contact, and is appropriately calibrated, the thoracic and abdominal signals can be combined to estimate respiratory volume. Differentiating that volume signal can then provide an estimate of airflow.1,2
This matters because airflow is central to the definition and assessment of sleep apnea. Apneas and hypopneas are defined by reductions or cessations in airflow, with effort, oxygen desaturation, and arousal information helping determine event type and clinical impact.3
By contrast, non-volumetric belts and proxy sensors may detect breathing-related movement, stretch, vibration, vascular changes, or downstream consequences of respiratory disturbance. These signals can be useful in some contexts, but they do not necessarily have a defined relationship to respiratory volume. They may indicate that effort is present, but they cannot reliably quantify ventilation or derive airflow in the same way a properly designed and calibrated volumetric RIP system can.1,2
Why two belts preserve physiology
In the Sleep Review article, Nox’s CTO makes a key point: measuring both thoracic and abdominal movement gives a fuller picture of respiratory mechanics than measuring only one compartment. The clinical issue is not simply whether one belt is easier to use than two. It is whether the system preserves the physiology needed to understand breathing.
During obstructive events, the airway may collapse while inspiratory effort continues. This can produce paradoxical motion: the rib cage and abdomen move out of phase. A single belt may show movement, but it cannot fully characterize the relationship between the thoracic and abdominal compartments. Dual-belt measurement is important because the combined thoracic and abdominal signals reflect the net change in respiratory volume more completely than either compartment alone.1
This is why the distinction between “effort signal” and “ventilation signal” matters. Effort tells part of the story. Ventilation tells clinicians how breathing changes across the night.
Calibration turns measurement into clinically useful information
A volumetric belt design is necessary, but it is not sufficient on its own. RIP-derived ventilation depends on calibration: scaling and combining thoracic and abdominal signals so their weighted sum is proportional to tidal volume.
Traditional calibration approaches, if used at all, can be useful, but patients move during the night – their posture changes, the belt positions shift, which can degrade a one-time calibration. Recent work has shown that advanced, continuous calibration and correction methods can improve the agreement between RIP-derived ventilation and gold-standard pneumotachography, including during obstructive sleep apnea events.2
This is an important practical point. For home sleep testing, the patient is moving, changing position, and sleeping without a technologist monitoring the signal in real time to catch and correct problems as they happen. A clinically useful respiratory signal must remain reliable in that real-world environment, hence the need for continuous calibration.
What volumetric RIP enables
When RIP is dual-belt, volumetric, mechanically stable, and appropriately calibrated, it can support more than basic effort detection. It can help characterize breathing physiology across the full night.
Event classification. Obstructive, central, and mixed events are differentiated by the relationship between airflow and effort. A respiratory system that can estimate both ventilation and thoracoabdominal effort can support more physiologically informed interpretation.
Hypopnea assessment. Many clinically important events are partial reductions in breathing rather than complete cessations. These events can be difficult to characterize with sensors that are limited to detecting presence or absence of breathing.
Ventilatory burden. AHI counts events, but it does not capture how deep or long those events are, or how much ventilation is lost across the night. Quantitative ventilation signals make it possible to move toward richer measures of disease burden.1,2,4
Sleep and arousal insight. Breathing changes with wake, NREM sleep, REM sleep, and arousals. Emerging research shows that RIP signals can support estimation of sleep stages, arousals, and total sleep time, which is especially relevant for home sleep apnea testing where EEG is often not available.1,5
Precision sleep medicine. Endotyping seeks to characterize the physiological traits that contribute to an individual patient’s sleep apnea, including upper-airway collapsibility, loop gain, arousal threshold, and muscle responsiveness. These traits can be estimated by analyzing patterns of ventilation and ventilatory drive during sleep, providing additional physiological context beyond conventional event-counting metrics such as the AHI.1,6
The question clinicians should ask
As home sleep testing evolves, clinicians and health systems are seeing a wider range of approaches to respiratory monitoring: dual belts, single belts, strain sensors, accelerometers, PPG-based signals, PAT-based systems, and other proxies.
The important question is not simply whether a device captures a respiratory signal. The better question is: what does that signal represent?
For RIP-based systems, clinicians should ask:
- Is the system volumetric?
- Does the belt design preserve the relationship between inductance and thoracoabdominal volume change?
- Are both thoracic and abdominal compartments measured?
- Is the signal calibrated in a way that remains valid across the night?
- Can the system support quantifying airflow, effort, event depth, and ventilatory burden, not just event counting?
These questions matter because sleep apnea is not only a count of events. It is a disorder of breathing physiology. The more accurately we measure that physiology, the better positioned clinicians are to understand disease severity, identify event mechanisms, and support more personalized care.
A more complete view of breathing
The renewed conversation around RIP is welcome. Simpler home testing can improve access, reduce burden on patients, and help close diagnostic gaps. But simplification should not come at the expense of the physiological information clinicians need.
Volumetric RIP offers a way to preserve that information. When implemented with dual belts, stable hardware, and advanced calibration, RIP can move beyond effort detection toward a more complete view of ventilation during sleep.
LBL-0714
References
1 Bjarkason S, Finnsson E, et al. Toward physiology-based sleep assessment: the emerging clinical role of advanced RIP technology. Curr Pulmonol Rep. 2026;15:10.
2 Finnsson E, Arnardottir E, et al. Respiratory inductance plethysmography to quantify changes in ventilation in obstructive sleep apnea. IEEE Trans Biomed Eng. 2025. PMID: 41056175.
3 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.
4 Labarca G, Vena D, Hu WH, et al. “Sleep Apnea Physiological Burdens and Cardiovascular Morbidity and Mortality.” American Journal of Respiratory and Critical Care Medicine. 2023;208:802–813. doi:10.1164/rccm.202209-1808OC.
5 Finnsson E, Erlingsson E, Hlynsson HD, et al. “Detecting Arousals and Sleep from Respiratory Inductance Plethysmography.” Sleep and Breathing. 2025;29:155. doi:10.1007/s11325-025-03325-z
6 Sands SA, Edwards BA, Terrill PI, et al. “Phenotyping Pharyngeal Pathophysiology Using Polysomnography in Patients with Obstructive Sleep Apnea.” American Journal of Respiratory and Critical Care Medicine. 2018;197(9):1187–1197. doi:10.1164/rccm.201707-1435OC.
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