Sleep apnea is a common disorder where breathing is repeatedly interrupted during sleep, leading to fragmented rest and reduced oxygen levels in the blood. These interruptions, called apneas or hypopneas, can occur dozens of times each hour, preventing deep, restorative sleep.
The condition is categorized into three main types: Obstructive Sleep Apnea (OSA), Central Sleep Apnea (CSA), and Mixed/Complex Sleep Apnea. OSA, the most common form, happens when soft tissues in the throat collapse and physically block the airway, while CSA occurs when the brain fails to send the necessary signals to the breathing muscles.
Untreated sleep apnea can have serious health implications, including increased risk for high blood pressure, heart disease, stroke, and excessive daytime fatigue. Accurate diagnosis and classification of the type and severity of these events are necessary for effective treatment.
Polysomnography: The Gold Standard
The most comprehensive method for diagnosing sleep-disordered breathing is Polysomnography (PSG), which is considered the clinical gold standard. This attended test takes place overnight in a specialized sleep laboratory, where a technician continuously monitors the patient. PSG is classified as a Type 1 sleep study because of the extensive array of physiological data it simultaneously records.
The test utilizes several sensor systems to capture a complete picture of sleep architecture and respiratory function. Electroencephalography (EEG) records brain wave activity, allowing specialists to determine the exact sleep stage (NREM and REM) during which breathing events occur. Electrooculography (EOG) monitors eye movements, and Electromyography (EMG) tracks muscle activity in the chin and legs, both necessary for accurate sleep staging.
Respiratory function is monitored through multiple channels, including sensors for airflow at the nose and mouth. Respiratory inductive plethysmography (RIP) belts are placed around the chest and abdomen to measure the effort of breathing. These effort measurements are necessary to distinguish between an obstructive event (effort present) and a central event (no effort present).
A pulse oximeter on the finger continuously records oxygen saturation and heart rate, identifying drops in blood oxygen that follow apneas and hypopneas. This comprehensive data allows for the precise calculation of the Apnea-Hypopnea Index (AHI), which is the number of events per hour of total sleep time. PSG is also required for diagnosing complex cases, such as Central Sleep Apnea, or when other sleep disorders like narcolepsy or periodic limb movement disorder are suspected.
Home Sleep Testing: Primary Diagnostic Alternatives
The high cost and logistical demands of in-lab PSG led to the development of Home Sleep Testing (HST). HST devices are unattended, simplified monitoring systems that patients use in their own homes for one or more nights. These devices, typically classified as Type 3 or Type 4 monitors, are generally sufficient for patients with a high probability of moderate to severe Obstructive Sleep Apnea (OSA).
Type 3 HST devices must record a minimum of four channels to qualify for clinical use. These channels include two measures of respiratory movement or airflow, a heart rate or electrocardiogram (ECG) channel, and oxygen saturation (oximetry). Airflow is typically measured using a nasal pressure transducer, while respiratory effort is tracked by two effort belts placed across the chest and abdomen.
This simplified sensor set gathers the necessary data to calculate a respiratory event index, often referred to as the Apnea-Hypopnea Index (AHI) or Respiratory Disturbance Index (RDI). The ability to record respiratory effort is crucial for distinguishing between obstructive and central events. However, the lack of EEG prevents determination of actual sleep time, so the event index is usually calculated based on the total recording time. HST is less appropriate for patients with significant co-existing medical conditions or when a diagnosis of Central or Mixed Sleep Apnea is suspected.
Accelerometers and Wearable Technology in Screening
Accelerometers are sensors that measure movement and are a component of many consumer-grade wearable devices, such as smartwatches and rings. In the context of sleep, this technology, known as actigraphy, is used to track body motion to estimate sleep duration and patterns. Actigraphy can be helpful for assessing insomnia, monitoring treatment effectiveness, or diagnosing certain movement-related sleep disorders, like restless legs syndrome.
Movement data alone cannot directly measure the physiological events that define sleep apnea. Accelerometers cannot detect the cessation of airflow, the respiratory effort of the chest and abdomen, or the corresponding drop in blood oxygen saturation. These physiological parameters are necessary for calculating the clinically diagnostic Apnea-Hypopnea Index (AHI).
Some advanced wearable devices incorporate accelerometers alongside other sensors, such as a built-in pulse oximeter. In these combined systems, accelerometer data might be used to track body position or to help algorithms filter out movement artifacts from the oximetry signal. While this combination can provide a suggestive screening tool by identifying oxygen desaturations, accelerometers alone are not sufficient to provide a definitive clinical diagnosis of sleep apnea. Devices approved for diagnosis, such as HST monitors, must include specific physiological sensors for airflow and respiratory effort.