Discovering The Impact of Airflow Measurements in Sleep Testing

RIP SUM is a non-invasive respiratory monitoring technique used in devices like Wesper Lab to combine thoracic and abdominal signals for precise airflow and respiratory effort measurements. Validated by studies and included in AASM guidelines, it provides accurate, real-time data for diagnosing sleep apnea, offering a superior alternative to traditional methods.

Dr. Chelsie Rohrscheib, PhD 
Head Sleep Expert & Neuroscientist

The Wesper Lab Level-III home sleep apnea test (HSAT) is a wearable device designed to assist in the diagnosis of sleep apnea. It utilizes two wireless patches and a pulse oximeter that patients can easily apply at home. The patches record various parameters, including respiratory effort, airflow, pressure, and body position to create a comprehensive sleep study analyzed by sleep clinicians. This data utilizes the well-established respiratory dual-chamber SUM method, applied broadly in gold-standard polysomnography studies as the RIP (Respiratory Inductance Plethysmography) SUM signal, which measures airflow by tracking changes in thoracic and abdominal respiratory effort. These signals are critical for assessing breathing dynamics and identifying events like apneas and hypopneas. This article will discuss the principles of operations for detecting airflow and respiratory effort in a novel and patient engaging way.

RIP: Respiratory Inductive Plethysmography

In the context of sleep apnea and respiratory monitoring, RIP SUM is a non-invasive technique to measure respiratory effort and airflow referenced by Konno and Mead [1]. RIP uses inductive sensors (belts) placed around the chest and abdomen, to detect changes in thoracic and abdominal circumference during breathing. The RIP SUM signal is based on the principle that the breathing system is a closed system and changes in airflow and air pressure are caused by modulation of the respiratory effort from the chest and abdomen. Over the years the principle was further validated [2-4] and was included in the formal guidelines for scoring by the AASM [5].

RIP: Respiratory Inductive Plethysmography

• Measures chest and abdominal movements (expansion/contraction) during breathing.
• Provides direct information about respiratory effort during respiration.

SUM: Summing Signals from the Chest and Abdomen

The SUM represents the combined signal from the thoracic and abdominal sensors. This reflects the relative airflow during breathing which is crucial for detecting apneic events. The signal can also assess whether the breathing is synchronous (normal) or asynchronous (indicative of respiratory distress or obstruction).

This combined measurement can calculate the tidal volume (the amount of air moved in and out of the lungs) with an appropriate calibration.

RIP can derive airflow measurements by capturing the thoracoabdominal motion signals (RIP SUM) and processing them using algorithms to calculate airflow. This method is particularly effective because it does not require any facial sensors and the summed signals from the chest and abdomen can reliably reflect the patient’s airflow [6].

RIP SUM Accuracy

Studies have shown that RIP Sum measurements strongly correlate with direct airflow measurements obtained using devices such as pneumotachometers, achieving diagnostic sensitivity comparable to polysomnographic standards for assessing sleep apnea [7, 8]. These results highlight RIP’s reliability as a non-invasive alternative to traditional airflow measurement methods, making it particularly valuable in both clinical and home sleep testing settings. Consequently, RIP Sum has been incorporated into the AASM guidelines for scoring apneas and hypopneas in sleep studies [5].

How Wesper LAB Airflow Works:

Two biosensors are placed:

• On the right rib cage, measuring thoracic respiratory effort.
• Above the navel, measuring abdominal respiratory effort.

As the patient breathes, the sensors measure hyper-localized respiration-induced motion driven by expansion and contraction.

The direct physiological signals from both patches are processed and presented in the following manner:

• Respiratory effort: Indicates direct respiratory effort from each body location (chest and abdomen).
• Airflow: Represents the SUM of the respiratory effort signals by highlighting the difference between chest and abdominal efforts. This is done according to the RIP Sum principle.
• Pressure: Using Bernoulli principle, representing the changes in airflow during breathing to detect the relative air pressure of the individual [9].

Diagnosis of Obstructive Sleep Apnea (OSA):

• The Wesper Airflow signal can help to detect breathing patterns and reductions in airflow during apneas (full airflow blockages) or hypopneas (partial airflow blockages).
• During an obstructive Apnea event, the apnea appears as a lack of change in the airflow signal for a period, indicating no airflow despite respiratory effort.

Central Sleep Apnea (CSA):

During a central apnea event, there is no respiratory effort. This can be detected directly by the respiratory effort measurement and lack of airflow.

The FDA considers the Wesper Lab airflow signal equivalent to direct airflow measurement, traditionally conducted with a nasal cannula [10]. This determination comes from the 510(k) premarket review process, which evaluates new devices against existing, legally marketed ones for “substantial equivalence” (in this case, Embletta MPR PG K122516 was the predicate device). Devices that go through this process, such as Wesper Lab, must demonstrate equivalent clinical performance when compared to direct airflow measurement via reference standards like polysomnography (PSG) [11].

Benefits of Having Airflow Signals in Respiratory Monitoring:

• Non-invasive: Comfortable for patients.
• Real-time data: Provides continuous monitoring of respiratory effort and airflow.
• Detects abnormalities: Can help providers differentiate between central and obstructive sleep apnea by analyzing effort and airflow synchronization.

Wesper Lab vs. Other Type III HSATs

Being an Airflow-based system makes Wesper Lab more desirable than Peripheral Arterial Tonometry (PAT) and Photoplethysmography (PPG) because it directly measures respiratory effort by tracking thoraco-abdominal movements during breathing. This allows physicians to utilize Wesper Lab to detect apneas and hypopneas based on the actual mechanics of airflow, providing real-time data on respiratory dynamics.

Direct Measurement of Respiratory Effort:

Wesper Respiratory Signals:

• Directly measures chest and abdominal movement, reflecting real-time respiratory effort.
• Can identify whether airflow obstruction is accompanied by physical effort (e.g., obstructive sleep apnea) or if there is a lack of effort (e.g., central sleep apnea).

PAT/PPG:

• Measure peripheral physiological responses, such as blood volume changes or arterial tone, which are indirect indicators of breathing and are influenced by other factors like autonomic nervous system activity.

In contrast, PAT and PPG do not measure respiratory signals directly. Instead, PAT relies on the body’s vascular response, detecting changes in arterial tone as a result of apneas or hypopneas, while PPG measures fluctuations in peripheral blood oxygen levels, which are delayed responses to respiratory events. Because PAT and PPG monitor the body’s secondary responses to breathing irregularities, they can be less sensitive and accurate in diagnosing sleep apnea, particularly in cases where respiratory effort and airflow are abnormal but oxygen levels may not yet reflect the event [12-14].

Better Differentiation Between Apnea Types:

Wesper Respiratory Signals:

• Allows to specifically distinguish between obstructive and central sleep apnea by showing the presence or absence of respiratory effort, and detecting Cheyne-Stokes breathing, respiratory asynchrony, hypoventilation patters, and periodic breathing.

PAT/PPG:

• Indirectly infer apneas by detecting changes in vascular tone or oxygen saturation but is not as accurate at differentiating apnea types [14].

Wesper is Not Affected by Peripheral Factors

One of the significant advantages of Wesper over alternatives like PAT or PPG is its ability to directly monitor respiration without being affected by peripheral factors. Wesper sensors track chest and abdominal movement, which is independent of conditions such as poor circulation, cold extremities, or movement artifacts. These factors often interfere with the quality of data from PAT or PPG devices, which rely on peripheral signals like skin temperature or blood volume changes.

For example, in cold conditions or with poor circulation, these devices may show inaccurate or noisy data because blood flow or skin tone can fluctuate, disrupting the signal integrity. Movement artifacts, which are particularly common in home settings, can also confuse readings from PAT or PPG, leading to false positives or missed apnea events [15-17].

Wesper is Not Affected by Heart Arrhythmias

PAT and PPG technologies can be less accurate in patients with atrial fibrillation (AFib). AFib disrupts the regularity of the heart’s rhythm, causing inconsistencies in pulse wave amplitude and vascular tone, which these devices rely on for detecting apneas. Furthermore, the disorganized heart rhythm disrupts the fundamental assumptions underlying these devices’ algorithms, potentially compromising the accuracy and reliability of their outputs. This can lead to false or missed readings, as the irregular pulse signal interferes with the measurement of respiratory events [10-12].

In contrast, Wesper Lab is unaffected by arrhythmias like AFib. The Wesper patches focus on monitoring chest and abdominal movements rather than pulse wave signals, making it more reliable in patients with irregular heart rhythms. Additionally, up to 50% of sleep apnea patients have AF, highlighting the importance of accurate diagnosis for effective treatment. Sleep apnea treatment can reduce AF episodes, emphasizing the need for technologies like Wesper that provide consistent measurements regardless of heart rhythm abnormalities [18, 19].

In conclusion, the Wesper Lab type III HSAT represents a significant advancement in sleep apnea diagnosis, offering a reliable and non-invasive alternative to traditional PSG. By utilizing direct respiratory sensing, the device provides real-time, continuous monitoring of respiratory effort and airflow through precise measurement of chest and abdominal movements. This direct measurement offers superior accuracy compared to methods like PAT and PPG, which rely on inferred signals such as heart rate or blood flow. Moreover, the Wesper Lab is not affected by factors like poor circulation, movement artifacts, and heart arrhythmias, making it ideal for home use. With FDA approval recognizing Wesper’s Airflow measurement as substantially equivalent to nasal cannula, Wesper Lab sets a new standard for effective and accessible sleep apnea testing, providing comprehensive and accurate data for both obstructive and central sleep apnea detection.

1. What is RIP SUM in respiratory monitoring?
RIP SUM (Respiratory Inductive Plethysmography SUM) is a technique that measures respiratory effort and airflow by combining signals from thoracic and abdominal sensors. It provides a reliable, non-invasive way to monitor breathing dynamics during sleep.

2. How does RIP SUM help diagnose sleep apnea?
RIP SUM detects airflow reductions and respiratory effort changes, identifying patterns like obstructive apneas (effort with no airflow) and central apneas (no effort or airflow). This detailed data helps differentiate apnea types and supports accurate diagnoses.

3. What makes RIP SUM better than PAT or PPG technology?
Unlike PAT and PPG, which infer respiratory events indirectly through vascular responses or oxygen saturation, RIP SUM directly measures thoracic and abdominal movements. This results in greater accuracy and reliability, especially in complex cases or when peripheral factors like poor circulation might interfere with other methods.

4. How does Wesper LAB utilize RIP SUM?
Wesper LAB uses RIP SUM principles by employing biosensor patches to monitor respiratory effort from the chest and abdomen. This technology provides real-time, precise airflow data, ensuring superior accuracy in detecting apneas and hypopneas in both clinical and home settings.

5. Why is RIP SUM ideal for home sleep apnea testing (HSAT)?
RIP SUM’s non-invasive design and ability to provide direct respiratory data make it highly suited for home use. Devices like Wesper LAB eliminate the need for cumbersome facial sensors and are unaffected by peripheral factors, offering reliable results in a comfortable, user-friendly manner.

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