Overview of Obstructive Sleep Apnea
Obstructive sleep apnea (OSA) is a common, widely underdiagnosed condition that is associated with significant morbidity and mortality. Due to intermittent anatomical blockage of the upper airway, reduction or cessation of airflow occurs during sleep, resulting in recurrent oxygen desaturation and sympathetic neural activation, with resultant nighttime hypertension and cortical arousal. This cycle results in sleep fragmentation and limits the amount of time spent in deeper sleep stages. Common symptoms include snoring, restless sleep, daytime fatigue, and morning headaches.
If not treated, OSA is associated with an increased risk of cardiac, respiratory, and metabolic conditions, including hypertension, stroke, congestive heart failure, and sudden death. [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13] The estimated prevalence of OSA with associated daytime sleepiness is 4% in adult men and 2% in adult women.
The prevalence of OSA as defined by an apnea-hypopnea index (AHI) of 5 or higher is considerably higher and may include up to 24% of males and 9% of females.  The vast majority of these patients snore. Approximately 80-90% of patients with OSA remain undiagnosed. [15, 16]
Go to Obstructive Sleep Apnea for more complete information on this topic.
Efficacy of Home Sleep Tests
In-facility polysomnography (PSG) is the historical standard for the diagnosis of OSA. For the appropriate patient, however, home sleep tests (HSTs) can be used to diagnose this condition. PSG and HSTs use the same respiratory equipment, pulse oximetry equipment, and movement and position sensors. [17, 16]
Data generated from each test is analyzed in the same manner. Multiple studies have demonstrated excellent correlation between the results of multichannel HSTs and PSGs. The typical design of these studies pairs an individual HST against concurrent and/or serial PSGs.
Machines that have been studied include Edentec, [18, 19, 20] PolyG,  AutoSet, [22, 23, 24, 25, 26, 26] Embletta,  Sibel Home,  Bedbugg,  NovaSom,  WatchPAT, [31, 32, 33, 34] SNAP,  ApneaLink, [36, 37] SOMNOcheck,  Stardust II,  Apnomonitor,  and Apnea Risk Evaluation System (ARES) Unicorder. 
If a patient is unable to comply with the instructions for home sleep testing due to age or cognitive impairment, an attended sleep study is required. An attended study can be performed either in-facility or at home.
Sensitivity and specificity
Reported sensitivities for the HSTs range from 86-100%, while specificities range from 64-100%. The reported coefficient of correlation for AHI between 2 tests ranged from 0.74-0.98. Although these studies only validated the individual machine that was tested, the aggregate results of these studies indicated that HSTs, in general, provide an accurate and reproducible method of identifying patients with OSA.
Of note, some studies found better agreement between PST and HST when the 2 tests were recorded simultaneously, as compared with separate recordings. Overall, patients have similar treatment outcomes whether OSA is diagnosed by PSG or HST.
Advantages of HSTs
HSTs offer the advantage of allowing testing in the patient's own home and with less instrumentation, thereby theoretically providing a more natural sleeping environment. Additionally, HSTs are substantially less expensive and more widely available than in-facility PSGs.  They can be used for the diagnosis of OSA and for home titration of positive airway pressure (PAP) treatment.
No known risks are associated with home sleep tests (HSTs).
Disadvantages of HSTs
Disadvantages of HSTs include the limited capability to immediately identify and resolve technical issues, the inability to diagnose other types of sleep disorders, and an increased role for the patient in terms of the application and use of the device, which may make some patients uncomfortable. In addition, the total sleep time cannot be calculated from an HST recording, since sleep and wake states are not directly assessed.
As with PSG, improperly preparing the patient for HST or misplacement of the equipment may result in inconclusive results or inaccurate readings; however, with an HST, no attendant is present to solve these issues as they arise, so the patient may have to repeat the study.
Indications for HSTs
In patients in whom OSA is suspected based on history and/or physical examination findings, overnight recordings using HST monitors can be used to confirm the diagnosis of OSA.
Although its conclusions are debatable, the American Academy of Sleep Medicine (AASM) recommends portable monitoring only for patients free of significant comorbid conditions and with a high pretest probability of moderate to severe OSA. 
A sample patient intake form used at the Head and Neck Surgery Sleep Clinic at the University of California at San Diego is seen below.
Indications for PSG
Home sleep testing has no known contraindications. However, if the differential diagnosis for a given patient includes other types of sleep disorders, such as insomnia, narcolepsy, or parasomnias, then a polysomnogram is the preferred diagnostic test. 
The routine addition of electroencephalogram (EEG) and electrooculogram (EOG) monitors in PSG allows recording of sleep stages and cortical arousals, which are typically unnecessary in the diagnosis of OSA but are critical in the diagnosis of other sleep disorders.
Types of HST Monitors
Three categories of portable monitors (Type II, III, and IV) are used in the diagnosis of OSA in either an attended or unattended setting.
Note that type III and IV portable monitors do not detect sleep stages. Instead, they extrapolate the respiratory disturbance index (RDI) or AHI based on the time period over which the recorder was switched on.
Type II monitors
Type II monitors have a minimum of 7 channels (eg, EEG, EOG, electromyogram, heart rate, airflow, respiratory effort, oxygen saturation). This type of device monitors sleep staging in addition to allowing calculation of AHI.
Type III monitors
Type III monitors are limited channel devices (usually 4-7 channels). They have a minimum of 4 monitored channels, including ventilation or airflow (at least 2 channels of respiratory movement or respiratory movement and airflow), heart rate, and oxygen saturation.
Type IV monitors
Type IV devices historically measured only 1 or 2 parameters (eg, oxygen saturation or airflow); they typically included oximetry, but not oximetry alone. This changed, however, after the Centers for Medicare and Medicaid Services (CMS) decided to cover continuous positive airway pressure (CPAP) treatment for positive tests from Type IV devices with at least 3 channels. 
HSTs have a variable number of channels for monitoring respiratory metrics, oxygen saturation, heart rate, chest and abdominal movement, leg movement, and snoring.
Measurements of airflow typically use nasal prongs like those used to provide nasal oxygen. The sensor measures pressure changes.
Alternatively, or in addition to the nasal sensor, an oral thermistor can be used to measure mouth breathing. This is useful for patients with complete nasal obstruction or significant nighttime nasal obstruction.
Information from respiratory channels is crucial to the diagnosis of OSA. Ayappa et al demonstrated a close correlation between the respiratory disturbance index (RDI) calculated from nasal cannula analysis and that obtained during in-facility PSG. 
Oxygen saturation channels
For most HSTs, a pulse oximeter is attached to either the finger or earlobe for peripheral oxygen saturation monitoring. These sensors use the differential absorption of red and infrared light by oxygenated and deoxygenated hemoglobin to calculate the saturation of peripheral oxygen (SpO2).
Although not as sensitive as the pulse oximetry monitors used in the operating room or the intensive care unit, these sensors provide data on the number and degree of oxygen desaturations during the night. Often, this is reported as the number of oxygen desaturation events and the lowest oxygen saturation (LSAT).
Heart rate channels
Heart rate can be monitored via a single-lead ECG, using an electrode attached to the upper chest.
Chest and abdominal belts are used to measure respiratory effort and thereby differentiate between central and obstructive apneic events. Typically, bands with piezoelectric sensors or inductive plethysmography technology are used. A central apneic event is seen as the complete absence of movement of either belt. In contrast, an obstructive apneic event is measured as paradoxical movement during respiration, with abdominal expansion seen during inspiration without chest expansion.
Leg movements can be recorded using a small cuff or belt placed around each ankle. These are generally not included with HSTs.
Snoring can be measured by placing a microphone on the neck.
Alternatively, it can be calculated from nasal pressure, since higher flow rates and greater force on vibrating structures produce lower-frequency pressure changes.
Currently, neither of these methods has been validated as an objective means of measuring snoring.
Use of HST Monitors
Patients should be able to sleep as they normally do, even if that includes turning over and sleeping on their abdomen. The electrodes and sensors should be attached in a manner that prevents them from coming off.
HSTs are easy to administer. During the patient's clinic visit, show the patient how to apply the various recording devices and how these sensors are connected to the sleep machine (as demonstrated in the images below).
Show the patient how the recording will be initiated when he or she goes to bed and how it will be stopped when the patient arises in the morning.
Instruct the patient to return the machine to the clinician's office following the HST. At that time, the data are downloaded into a computer and analyzed either manually or automatically.
Interpretation of Results
Many machines have an auto-score function, which is sufficient if the results match those anticipated based on the patient’s history and physical examination. If any discrepancy exists, however, manual scoring is recommended. 
Respiratory metrics used in assessing OSA
Hypopneas are episodes of abnormally shallow breathing as defined by a 50-75% decrease in airflow. Some definitions require an oxygen desaturation of 2-4% or EEG arousal due to partial airway closure. Airway obstruction should last a minimum of 10 seconds in adults and 8 seconds in children (≥4 y).
Apneas are defined as 90% or greater decrease in airflow for 10 seconds or longer. 
The apnea-hypopnea index (AHI) is the sum of the number of apneic and hypopneic episodes per hour of sleep.
The respiratory disturbance index (RDI) is equal to the average number of respiratory disturbances per hour. It is similar to the AHI; however, it also includes respiratory events that do not technically meet the definitions of apneas or hypopneas but do disrupt sleep (as measured by cortical arousal) .
Respiratory event–related arousals (RERAs) are events characterized by increasing respiratory effort for 10 seconds or longer, leading to an arousal from sleep; these events do not fulfill the criteria for a hypopnea or apnea. They can be measured with esophageal manometry.
Grades of OSA
No consensus currently exists regarding how the severity of OSA should be graded; however, the AASM advocates the following guidelines:
Mild: AHI of 5-15 events per hour
Moderate: AHI of 15-30 events per hour
Severe: AHI of greater than 30 events per hour
Thus, for adults, an AHI of 5 or more is typically considered abnormal. However, many insurance companies only cover treatment if the patient has an AHI of 15 or greater, or an AHI of 5-14 plus at least 2 OSA-related comorbidities.
Medicare guidelines  designate a positive sleep study as an AHI or RDI greater than or equal to 15 events per hour. Alternatively, Medicare defines it as an AHI or RDI greater than or equal to 5 and less than or equal to 14 events per hour, with documented symptoms of excessive daytime sleepiness, impaired cognition, mood disorders or insomnia, or documented hypertension, ischemic heart disease, or history of stroke.
No universally accepted criteria are available for oxygen desaturation or abnormal oxygen desaturation index in sleep-disordered breathing. In general, patients with multiple or more pronounced desaturations are viewed as having more severe OSA.
Note, however, that the LSAT may be falsely lowered because of technical errors, such as displacement of the monitor. Conversely, the lack of oxygen desaturations does not rule out OSA. Apneas and hypopneas may occur without desaturations because of resolution of the event before desaturation occurs or due to lack of oximetry sensitivity. This is particularly true in children.
Central versus obstructive apneas
True central apneas are seen primarily in patients with neurologic disease or injury or who have advanced heart failure. Occasionally, however, patients with sleep-disordered breathing demonstrate both central and obstructive apneas on their sleep tests. For some individuals, this may be an artifact; an obstructive event may be sensed by the patient’s brainstem, resulting in the cessation of respiratory effort and, therefore, the cessation of chest and abdominal movement as well. This would be measured as a central event, despite its origination as an obstructive event. Not surprisingly, these "central apneas" typically disappear once the patient is treated. 
HST assessment in children
Significant controversy surrounds the use of sleep tests in children. In the authors' experience, HSTs can be successfully used in children aged 4 years and older. However, certain modifications must be made to the application and interpretation of these tests when they are used in children.
The child should be habituated to the nasal cannula by wearing it to bed for an entire week prior to the actual sleep test. Additionally, child-friendly gloves may be necessary to keep the child from pulling at the leads. Because children have a faster respiratory rate than adults, the criteria for apnea and hypopnea events need to be adjusted appropriately when scoring the sleep data.
Sample Sleep Studies
Sample sleep study results for mild, moderate, and severe OSA are shown in the images below.