laboratory evaluation of osa.pdf

8/14/2019 Laboratory evaluation of OSA.pdf

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Chapter 8

Laboratory evaluation

of OSAR.J. Soose, O. Yetkin and P.J. Strollo

Summary

Since its emergence, half a century ago, sleep laboratorymonitoring has revolutionised the evaluation and manage-

ment of patients with sleep disorders. Throughout this periodthe sleep laboratory has been used for countless clinical andresearch purposes, with its most widespread application lyingwithin the diagnosis and treatment of sleep-related breathingdisorders.

A comprehensive sleep history and physical examinationremains at the cornerstone of the initial evaluation for anypatient presenting with symptoms of sleep apnoea or anyother sleep-related complaints. Nevertheless, at the presenttime, clinical symptoms and risk factors alone are insufficientto accurately diagnose and assess the severity of a sleep-relatedbreathing disorder. Furthermore, despite the growingmomentum of unattended diagnostic and therapeutic strat-egies for evaluating and managing sleep aponea outside of thelaboratory, it is clear that attended polysomnography in thesleep laboratory will continue to play a prominent role inthe care administered to millions of patients with sleep apnoeaand other associated sleep disorders.

Keywords:Electroencephalography, multiple sleep latency test,obstructive sleep apnoea, polysomnography, sleep staging,spectral electroencephalography

UPMC Sleep Medicine Center,Division of Pulmonary, Allergy, andCritical Care Medicine, University ofPittsburgh School of Medicine,

Pittsburgh, PA, USA.

Correspondence: P.J. Strollo, UPMCSleep Medicine Center, Division ofPulmonary, Allergy, and Critical CareMedicine, University of PittsburghSchool of Medicine, Pittsburgh, PA15213, USA, [email protected]

Eur Respir Mon 2010. 50, 121135.Printed in UK all rights reserved.Copyright ERS 2010.European Respiratory Monograph;ISSN: 1025-448x.DOI: 10.1183/1025448x.00024409

Historical perspective

The birth and development of sleep disorder medicine is closely correlated with the innovation

and progress in the area of sleep laboratory diagnostic techniques. Until the 20th century theunderstanding of sleep and associated respiratory, cardiovascular and neuromuscular physiology,largely remained a mystery. Human electroencephalography (EEG) was first reported in 1929 byBERGERet al.[1] and set the foundation for the discovery of the classic EEG findings, which is nowreferred to as nonrapid eye movement (nonREM) sleep as stated by Alfred Loomis [2] in 1937.Over the next two decades, Nathaniel Kleitman in collaboration with Eugene Aserinsky [3] and

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William Dement [4] pioneered the characterisation of rapid eye movement (REM) sleep and thehuman sleep cycle. By 1953, continuous overnight recordings of sleep were made at the University ofChicago (Chicago, IL, USA) and the laboratory evaluation of sleep became an essential componentof this new and emerging field of sleep medicine. A major milestone in the laboratory evaluation ofsleep came with the publication in 1968 of standardised definitions and criteria for the scoring andstaging of sleep by RECHTSCHAFFENand KALES[5]; guidelines that stood firmly for almost 40 yrs.

Investigation into sleep-disordered breathing began in 1956 with the description of the

Pickwickian syndrome or obesity hypoventilation [6]. It was not until 10 yrs later, however,that the excessive daytime sleepiness of this syndrome was linked to upper airway obstructionrather than the previouslythought alveolar hypoventilation. Using EEG and respiratory data,Gastaut and colleagues documented repetitive disruption of airflow with subsequent arousals andsleep fragmentation [7]. The laboratory evaluation of sleep was officially termed polysomnography(PSG) by HOLLAND et al. [8] in 1974. Although PSG has been used for countless clinical andresearch purposes over the past four decades, its most widespread application today lies with thediagnosis and treatment of sleep-related breathing disorders.

Current indicationsAmerican Academy of Sleep Medicine practice parameters

The American Academy of Sleep Medicine (AASM) produced guidelines for the indications ofattended PSG in 2005 [9], these are shown in table 1.

Although attended PSG evaluation is considered standard practice in the above situations, acomprehensive sleep history and physical examination remains the cornerstone of the initialevaluation of any patient with sleep-related complaints. Moreover, a number of diagnoses can be

Table 1. The American Academy of Sleep Medicine recommended practice parameters for the indication ofattended polysomnography (PSG)

Diagnosis of sleep-related breathing disorders

PAP titration in patients with sleep-related breathing disorders

Pre-operative evaluation prior to upper airway surgery for snoring or OSA

Assessment of treatment results after:

Oral appliance therapy with good clinical response in patients with moderate-to-severe OSA

Surgical therapy with good clinical response in patients with moderate-to-severe OSA

Recurrence of symptoms after initial good response to surgical or oral appliance therapy

Clinical follow up after:Incomplete response or symptom recurrence after initial good response to PAP therapy

Substantial weight loss in a patient with OSA to assess whether treatment is still necessary

Substantial weight gain in a patient with OSA who was previously controlled with treatment

and has symptom recurrence to assess whether further treatment is necessary

Evaluation of patients with related medical comorbid conditions and suspicion of sleep apnoea

Patients with systolic or diastolic heart failure and nocturnal symptoms suggestive of sleep apnoea

Congestive heart failure patients with persistent symptoms despite optimal medical

management

Neuromuscular disorders with sleep-related symptoms

Diagnostic evaluation of narcolepsy (overnight PSG preceding a multiple sleep latency test)

Diagnostic evaluation of periodic limb movement disorder in patients with complaints of

repetitive leg movements and associated symptoms of sleep disturbance

The standard practice designation of each indication listed above is the result of a consensus high degree of

clinical certainty (direct level I or overwhelming level II evidence) and is generally considered as accepted patientcare. PAP: positive airway pressure; OSA: obstructive sleep apnoea.

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made solely on the basis of a clinical evaluation. Examples of these clinical presentations that donot require PSG are parasomnias, nocturnal seizure disorders, restless legs syndrome, confusionalarousals, sleep bruxism and circadian rhythm disorders.

Laboratoryversushome monitoring

Type I attended full-montage PSG in the sleep laboratory continues to hold its position as themost comprehensive and reliable monitoring method for the diagnosis of sleep disorders. Portablemonitoring devices (types II, III, and IV), both in the laboratory and home settings, have beenused with increasing frequency. Table 2 outlines the parameters measured in each of the basiccategories of the sleep monitoring systems. Practice parameters and clinical guidelines for the useof portable monitors have been published [10, 11]. This will be discussed further by P ENZELet al.[12] in the chapter of this current monograph entitled Ambulatory diagnosis of obstructive sleepapnoea and new technologies.

Improved cost-effectiveness and availability have been cited as driving forces in the recent shifttowards a more ambulatory evaluation of sleep apnoea outside of the sleep laboratory. However,these benefits in cost and availability must be cautiously weighed against the stability and

redundancy of the sensors in an attended in-laboratory full montage PSG, with subsequentsuperior sensitivity, specificity and reliability. Furthermore, costs of portable monitoring may bemore than expected when taking into account loss or damage of equipment and the potential needfor repeat testing. In an era of rapid growth in the field of sleep medicine, and the concurrentcommitment to subspecialty board certification and the training of new sleep fellows, sleeplaboratory evaluation also provides the intangible benefits of continued education in thelaboratory evaluation of sleep pathophysiology.

One method of improving cost-effectiveness and patient access is the consideration of a split-nightstudy in certain clinical circumstances. A split-night study combines diagnostic PSG and a positiveairway pressure titration in one night, rather than the traditional process of two separate nights

[13, 14]. Proper clinical judgment in patient selection is essential, as inaccurate estimation ofobstructive sleep apnoea (OSA) severity and/or incomplete therapeutic titration may occur.Specific guidelines have been published by the AASM for the use of split-night studies [9].

Multiple Sleep Latency Test

A frequent complaint among patients with OSA is excessive daytime sleepiness. As such, thelaboratory evaluation of sleep apnoea may also include an objective measure of the patientslikelihood of falling asleep. The most widely used sleep laboratory test of daytime sleepiness is the

Table 2. The parameters measured in each of the basic categories of the sleep monitoring systems

Type Monitoring system Parameters measured

1 Standard technician-attended

polysomnography in the laboratory

setting

Minimum of seven channels, including EEG,

EOG, chin EMG, ECG or heart rate, airflow,

respiratory effort, and oxygen saturation

2 Comprehensive portable Minimum of seven channels, including EEG,

EOG, chin EMG, ECG or heart rate, airflow,

respiratory effort, and oxygen saturation

3 Modified portable Minimum of four channels, including airflow,

respiratory effort, heart rate, and oxygensaturation

4 Continuous single or dual channel One or two channels, including oxygen

saturation and/or airflow

EEG: electroencephalography; EOG: electro-oculography; EMG: electromyography.

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Multiple Sleep Latency Test (MSLT), best known for its utility in diagnosing narcolepsy.Developed by CARSKADON and DEMENT [15] at Stanford in the 1970s, the MSLT traditionallyassesses the time to sleep onset, as well as the presence of REM episodes, in a series of five napsthroughout the day.

The test is conducted in a standardised laboratory environment, conducive to sleep and avoidingwake-promoting factors, such as caffeine, physical activity and bright light. Specific guidelines andpractice parameters have been published elsewhere to standardise procedures before and during

the test [16, 17]. For example, a 2-week sleep diary is recommended prior to the MSLT so theresults are interpreted in the context of the patients preceding quantity and quality of sleepcompared with the usual, as well as information on daytime naps, bed/wake times and caffeine/alcohol/drug use. At the time of testing, a urine screen to exclude the occult use of drugs (legal orillegal) that may influence alertness is advisable.

In OSA patients the MSLT is primarily used to evaluate symptoms of persistent or residualdaytime sleepiness, despite adequate treatment with positive pressure therapy. Since sleep-relatedbreathing disorders, in addition to other sleep disorders, can shorten the mean sleep latency, a full-montage attended PSG is recommended immediately prior to the MSLT. Findings of hypersomniaor pathological sleepiness in a patient with well-controlled OSA, without evidence of anothersleep-related aetiology, may prompt the consideration of stimulant therapy.

Recording and monitoring of physiological signals

The first widely accepted PSG scoring manual was published in 1968 by RECHTSCHAFFENand KALES[5]. Although this system contributed greatly to the field of sleep medicine for four decades, theneed for modifications gradually became apparent. In 2007, the AASM published a new manualthat included updated definitions, scoring criteria and technical specifications [18]. The use ofattended PSG in the evaluation and management of sleep-related breathing disorders generally

includes the following parameters: EEG, electro-oculogram (EOG), chin and leg electromyography(EMG), airflow, arterial oxygen saturation, respiratory effort, and ECG or heart rate.

Staging of sleep

Electroencephalography

EEG is the primary PSG tool used to differentiate sleep from wakefulness and to determine sleepstaging [18]. EEG is also essential for the scoring of arousals during sleep. EEG electrodes areplaced using the standard International 1020 System [19] with reference electrodes, M1 and M2,

placed on the bony surface of the mastoid bilaterally (previously labelled A1 and A2 in the oldscoring system). The three EEG derivations currently recommended by the AASM scoring manualare: 1) F4-M1, 2) C4-M1, and 3) O2-M1. The set-up of back-up electrodes on the contralateralside (F3-M2, C3-M2, and O1-M2) is recommended in the event of electrode malfunction duringthe study. In the evaluation of nocturnal seizures and other sleep-related neurological disorders,additional electrode derivations may be necessary.

Electro-oculography

EOG recordings are fundamental to the scoring of REM sleep, which in turn can be an important

component of the evaluation of sleep-related breathing disorders. The M1 and M2 mastoidelectrodes are also used as reference points in EOG. The EOG records conjugate eye movements inan out-of-phase fashion. The out-of-phase deflections are achieved by placing one electrodesuperiorly and the other inferiorly with respect to the outer canthus. AASM recommendedderivations are: 1) E1-M2, where E1 is placed 1 cm below the left outer canthus, and 2) E2-M2,where E2 is placed 1 cm above the right outer canthus.

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Chin electromyography

Chin EMG is another key component to sleep staging, particularly the scoring of REM sleep. TwoEMG electrodes are placed over the mental and submental muscles to form a bipolar recording,with a third electrode placed as back-up in the event of malfunction or artefact.

Visual scoring of sleep stages

EEG, EOG and chin EMG data are integrated in order to assign a stage to each 30-s epoch. Specificrules and definitions of sleep staging are thoroughly documented in the AASM scoring manual,table 3summarises the key features of each stage [18]. PSG representations, in 30-s epochs, of thephysiological signals for each of the sleep stages, are shown in figure 1. The top two channelsdepict the occipital and central EEG signals, the next two channels represent the left and right EOGsignals, and the bottom tracing on each figure is the chin EMG.

An arousal is characterised by an abrupt shift of EEG frequency lasting for at least 3 s with aminimum of 10 s preceding stable sleep. An arousal during REM sleep also requires a concurrent

increase in submental EMG activity for the duration of at least 1 s. Arousals are often associatedwith apnoeas, hypopnoeas and even increasing respiratory effort, and therefore have importantclinical implications in the fragmentation of sleep in patients with OSA. A PSG example of an EEGarousal is depicted in figure 2.

Assessing respiratory function

Several different modalities are currently available for the detection of respiratory events duringPSG. The criteria for scoring respiratory events were recently redefined by the AASM [18]. Anapnoea is defined by the cessation of airflow for a minimum of 10 s, with the oral thermistor

providing the most accurate detection of an apnoea (fig. 3). An apnoea is further classified asobstructive, central or mixed based on the assessment of respiratory effort during the event. Incontrast, hypopnoeas are generally classified only as obstructive events. Hypopnoeas can be scored

Table 3. Polysomnographic staging of sleep and the associated electrophysiological characteristics of eachstage

Stage Description Key features

W Wakefulness Alpha rhythm of 813 Hz on EEG

Eye blinks: conjugate, vertical eye movementsREM: conjugate, irregular, sharply peaked eye movements

N1 Non-REM 1 SEM: conjugate, regular, sinusoidal eye movements

Low-amplitude, mixed-frequency EEG activity

Vertex sharp waves

N2 Non-REM 2 K-complexes

Sleep spindles

Low-amplitude, mixed-frequency EEG activity

N3 Non-REM 3 High-amplitude .75 mV, low-frequency 0.52.0 Hz

EEG activity

R REM REM

Low chin EMG activitySawtooth waves, sharply contoured 26 Hz EEG activity

Transient muscle activity

EEG: electroencephalography; REM: rapid eye movement; Non-REM: non-REM; SEM: slow eye movement;

EMG: electromyography.

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using various definitions although most commonly they are characterised by at least a 30%reduction in airflow in association with a 3% or 4% oxygen desaturation (fig. 3).

Airflow, respiratory effort, oxygenation and ventilation comprise the primary approaches toassessing respiratory function during sleep. More than one modality is usually required

to accurately detect and distinguishvarious types of sleep-related breath-ing events. Within each modality,a variety of technologies exist tomake the appropriate measurements,each with its own advantages and

disadvantages.

Airflow

The various advantages and disad-vantages concerning the differentmechanisms involved with the useof airflow detection are outlined intable 4.

Thermal technology

Thermocouples and thermistorsdetect changes in temperature as asurrogate for semiquantitative air-flow measurement. The sensor is a

CHIN

ROC

LOC

C3A2

O1A2

Time s

Figure 2. An arousal (&%) is marked by an abrupt burst ofelectroencephalography activity lasting at least 3 s. Note the

normal N2 sleep before and after the arousal. O1A2 and C3A2refer to electroencephalography derivations. O: occipital;

C: central; and both A and M: mastoid electrode placement;

LOC and ROC refer to electro-oculography electrode placement:

LOC: left outer canthus electrode; ROC: right outer canthus elec-trode placement; CHIN: chin electromyography electrode placement.

d) e)

a) b) c)

Time s Time s

Time s

O1A2

C3A2

LOC

ROC

CHIN

O1A2

C3A2

LOC

ROC

CHIN

O2M1

C4M1

LOC

ROC

CHIN

O1A2

C3A2

LOC

ROC

CHIN

O1A2

C3A2

LOC

ROC

CHIN

F4M1

Figure 1. The physiological signals for each sleep stage. a) Stage W, the fundamental finding in wakefulness isalpha electroencephalography (EEG) activity in the occipital lead. b) Stage N1, mixed-frequency EEG activity with

slow rolling eye movements characterising sleep onset. c) Stage N2, sleep spindles (#) followed by a K-complex

(arrow) on a background of low-amplitude, mixed-frequency EEG activity. d) Stage N3, high-amplitude, low-frequency EEG activity (slow-wave sleep) occupying at least 20% of the epoch. e) Stage R, low-amplitude,

mixed-frequency EEG activity returns in rapid eye movement (REM) sleep along with a significant reduction in

chin electromyography (EMG) activity and the appearance of REM. O1A2, C3A2, F4M1 and C3A2 refer to EEG

derivations with: F: frontal, O: occipital, C: central, and both A and M: mastoid electrode placement; LOC andROC refer to electro-oculography (EOG) electrode placement: LOC: left outer canthus; ROC: right outer canthus

electrode placement; CHIN: chin EMG electrode placement.

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temperature-sensitive resistor com-posed of semiconductor materials.Thermal sensors are small, comfor-table and reusable. The sensor isplaced close to the patients nostrilsand lips, where a change in tempera-ture related to airflow causes a change

in the sensors electrical properties.Thermal technology is preferred forthe identification of absence of air-flow and, therefore, is most com-monly used in the scoring of apnoeas.Oral thermistors, in the absence offlow on the nasal pressure transducer,may also serve as an adjunctivemethod to detect mouth breathingand associated nasal obstruction.

Thermal devices do not correlatewell with actual airflow or minuteventilation and are not accurate fordetecting hypopnoeas. Comparedwith other technologies they have apoor dynamic response, with rela-tively slow response times, and acurvilinear response to tempera-ture change that flattens as airflowincreases [20, 21].

Polyvinylidene fluoride film sensorsprovide a newer alternative in ther-mal technology. The signal is propor-tional to the temperature differencedetected on the two sides of the film.Although they have faster responsetimes and a more linear responsecompared with other thermal airflowsensors, polyvinylidene fluoride film

sensors are more expensive and as aresult are not commonly used [22].

Pneumotachograph

A pneumotachograph is an instrument that measures flow in terms of the proportionalpressure drop across a fixed resistance consisting of numerous capillary tubes in parallel. Theprecise and direct calculation of airflow makes the pneumotachograph the current goldstandard for airflow measurement [22]. Nevertheless, the cumbersome nature of theequipment and requirement of an oronasal seal precludes its routine clinical use in the sleep

laboratory.

Nasal pressure cannula

Nasal pressure transducers are comprised of a standard nasal cannula connected to a pressuretransducer. Compared with thermistors and thermocouples, they provide a faster response to

SNOR

90 90

a)

b)

92 92 9091 91 91 89 84 83 7990 9089

RC

ABDM

SUM

NPNT

TC

Sa,O2

7989

SNOR

92 91 92 95 9596 93 92 91 93 9594 94

RC

ABDM

SUM

NPNT

TC

Time s

Sa,O2 9591 90 91

Figure 3. a) Apnoea (&%); the respiratory channels on a 90-s vieware shown. Cessation of airflow for at least 10 s on the

thermocouple (TC) and nasal pressure transducer (NPNT) signal

with continued respiratory effort and subsequent oxygen desatur-

ation (p). Note the paradoxical movement on the ribcage (RC) and

abdominal (ABDM) effort channels (#). b) Hypopnoea (&%); a

reduction in airflow is seen in the NPNT signal, followed by oxygen

desaturation of at least 4% and a subsequent period of hyperpnoea.

SNOR: snore monitor; SaO2: arterial oxygen saturation channel,

shown as %; SUM: sum channel.

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airflow changes and can detect airflow limitation. Nasal pressure transducers are more sensitive foridentifying flow limitation associated with increased upper airway resistance. They are therecommended technology for the identification of hypopnoeas [23].

Although the relationship between nasal pressure and airflow is not linear, the signal can be madelinear through mathematical conversion [24]. The converted linear signal has been shown toclosely correlate with the flow signal from a pneumotachograph [25]. The signal amplitudedepends on the location of the sensor and the pattern of breathing. Because these two parameterschange constantly throughout the night, the airflow signal during a respiratory event can becompared only with the signal during the normal cycle just before the event. It should be notedthat nasal pressure cannulas do not measure airflow during mouth breathing.

Respiratory effort

An overview of the advantages and disadvantages regarding the methods of respiratory effortassessment are shown in table 5.

Inductance plethysmography

Respiratory inductance plethysmography (RIP) uses bands with inductive coils placed around the

chest and abdomen. RIP detects changes in the electromagnetic properties of the coils andprovides a semiquantitative measure of thoracic and abdominal pressure changes [26]. The sum ofthe thoracic and abdominal signals (sum channel) can be important in identifying paradoxingeffort in obstructive sleep apnoea. Nevertheless, the sum channel alone is a semiquantitativemeasure and cannot precisely derive tidal volume.

When properly calibrated, viaa spirobag, the inductance plethysmography signal can provide anestimate of tidal volume [27, 28]. Calibration is uncommon in routine clinical practice due todifficulty maintaining the calibrated signal during changes in body position. Change in body positioncan also result in slipping of the sensors and subsequent inaccuracy of the signal. Nevertheless, RIP isthe most widely used method of assessing respiratory effort in the sleep laboratory.

Piezoelectric sensors

Piezoelectric based systems work in a similar fashion compared with RIP. However, unlikeinductance plethysmography, the piezoelectric sensors measure tension only in a single location.In this location, a sensor crystal is pulled by the band as a result of changes in the thoracic and

Table 4. Summary of the advantages and disadvantages of the methods of airflow assessment

Method Advantages Disadvantages

Thermal technology Small and comfortable Slow, curvilinear response to

temperature change

Inexpensive and reusable Poor correlation with minute ventilation

Accurate detection of apnoeas Not accurate for detection of

hypopnoeasPneumotachograph Gold standard Cumbersome

Precise calculation of airflow and

volume

Requires oronasal seal

Nasal pressure

cannula

Simple, inexpensive, comfortable Nonlinear relationship between nasal

pressure and airflow

Better response time than thermal

technology

Signal affected by location of sensor

and pattern of breathing

Accurate detection of hypopnoeas Cannot measure airflow during mouth

breathing

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abdominal compartments. The technology has the advantages of being inexpensive andconvenient for the patient. It is generally not a preferable method for several reasons: 1)inaccuracy may occur with change in body position or with loss of tension in the belt over time; 2)unlike RIP, the sensors can not be calibrated and the signals can not be summed to estimate tidalvolume; and 3) overstretching of a belt can reverse polarity and result in a false-positiveappearance of paradoxical breathing.

Oesophageal pressure measurement

Oesophageal pressure monitoring or manometry, is considered the gold standard in the assessmentof respiratory effort and it can be measured using a variety of different methods. Oesophagealballoons, Millar catheters and fluid-filled catheters are examples of devices used to reflect changes inpleural pressure and subsequent changes in respiratory effort. Monitoring of oesophageal pressureduring PSG has been shown to have negligible effects on sleep architecture [29, 30].

High compliance oesophageal balloons have been widely used, primarily in the research setting,for many years. Millar catheters can be used to measure supraglottic pressure in addition tooesophageal pressure, which then can be used to calculate transpulmonary pressure. The small-bore fluid-filled catheters are generally preferred in the clinical setting as a result of improvedpatient comfort, lower failure rate and the ability to measure pressure changes at multiple sites.

In addition to the usefulness in distinguishing central and obstructive events, oesophageal manometryprovides the most accurate assessment for detecting respiratory effort related arousals (RERAs), inwhich increasing negative intrathoracic pressures precede an arousal before returning to baseline.

Table 5. Summary of the advantages and disadvantages of the methods of respiratory effort assessment

Method Advantages Disadvantages

Inductance

plethysmography

Noninvasive, convenient More expensive than piezoelectric

belts

Can be calibrated to measure

tidal volume

Only semi-quantitative measurement

of tidal volume

Can detect paradoxicalrespiration

Inaccuracy can occur with bodyposition changes

Thoracic and abdominal channels

can be combined into sum

channel

Piezoelectric belts Noninvasive, convenient Cannot be calibrated

Inexpensive Inaccuracy can occur with body

position changes

Cannot be combined into sum

channel

Overstretch may result in false-

positive paradoxical pattern

Diaphragmatic EMG Noninvasive, convenient Cannot be calibrated

Inexpensive Less accuracy in obese patients

Signal interference from intercostal

muscle activity

Oesophageal balloon Gold standard Invasive, inconvenient

Most accurate estimation of

respiratory effort

Potential patient discomfort

May be displaced during study

Pulse transit time Noninvasive Limited usefulness in patients with

cardiac arrhythmias or peripheral

vascular disease

Inexpensive

EMG: electromyocardiography.

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Diaphragmatic EMG

Contraction of the diaphragm, as well as other respiratory muscles, occurs during inspiration, witha subsequent increase in the volume of the thoracic cavity. As such, surface EMG electrodes placedproperly on the chest wall can provide an indirect assessment of respiratory effort. This techniqueis noninvasive and inexpensive; however, reliable recordings are difficult to achieve in profoundlyobese patients and are negatively impacted by interference from ECG and intercostal muscle

artefact. Data on accuracy, reliability and validity is also lacking and so it is not a commonly usedmethod in the clinical setting.

Pulse transit time

Pulse transit time (PTT) is a newer method of measuring changes in respiratory effort. It isnoninvasive and utilises standard ECG leads and a pulse oximeter. PTT is defined as the time ittakes for the pulse wave to travel from the aortic valve (detected by the R wave on the ECG) to theperipheral arteries (detected by the pulse oximeter).

Transit times fall in a narrow normal range during regular respiration. During an obstructive

respiratory event increased blood pressure and arterial wall stiffness, from either an increasein sympathetic activity or changes in intrathoracic pressure, increases the amplitude of the PTTsignal [31].

PTT is useful in differentiating obstructive from central respiratory events and can also be used todetect microarousals. It has limited usefulness in patients with cardiac arrhythmias or peripheralvascular disease. This technology is still relatively new and has not gained widespread use inclinical practice.

Oxygenation

Oxygen saturation measurement is a critical component of respiratory monitoring during PSG.Decreases in haemoglobin oxygen saturation of at least 3 or 4% are utilised in the current criteriafor scoring hypopnoeas. In the absence of airflow limitation, nocturnal oxygen desaturation maysuggest other sleep-related breathing disorders, such as obesity hypoventilation or chronicobstructive pulmonary disease (COPD).

Pulse oximetry

Pulse oximetry, viafinger probe, provides a practical, inexpensive and noninvasive method ofmeasuring oxygen saturation and is, therefore, the most commonly used technique. Mostoximeters also provide a continuous estimate of heart rate. Two wavelengths of light, in the redand infrared regions, are measured through a translucent part of the skin, usually the fingertip.A photodetector on the opposite side measures the amount of transmitted light. The techniqueis based on the principle that oxygenated and deoxygenated haemoglobin absorb lightdifferently.

The accuracy of pulse oximetry in detecting respiratory events can vary depending on the type andaveraging time of the oximeter [32]. An averaging time of 3 s or less is preferred [33]. Pulseoximetry is also vulnerable to patient movement, anaemia, poor perfusion (e.g. vasoconstriction,hypovolaemia), elevated carboxyhaemoglobin or methaemoglobin levels, and nail polish.

Transcutaneous oximetry

This method measures arterial oxygen partial pressure is similar to the transcutaneous carbondioxide measurement described below. Like pulse oximetry, it is noninvasive and easy to use;however, problems with calibration and possible thermal injury have limited its use.

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Arterial blood gas

Arterial blood gas (ABG) is the most accurate method and the gold standard for measuring bloodoxygenation. Nevertheless, it is invasive, expensive and impractical for providing continuousmonitoring. The associated patient discomfort may adversely affect arousal from sleep. As such,ABG has not gained widespread use in the sleep laboratory setting.

Ventilation

The arterial carbon dioxide partial pressure (Pa,CO2) is considered a reliable marker of alveolarventilation. Assessing Pa,CO2 is an important component of respiratory monitoring during sleepand can be affected by changes in alveolar ventilation as well as cardiac output, tissue perfusionand metabolism. Carbon dioxide levels are 0 Torr on inspiration and rise through the expiratoryphase to reach a plateau level at the end of expiration. This end-expiratory carbon dioxide levelcorrelates well with Pa,CO2 in most situations, unless altered by one of the factors previouslymentioned. Each of the methods described below have significant limitations and carbon dioxidemonitoring is currently not used routinely in most clinical sleep laboratories.

End-tidal carbon dioxide

End-tidal carbon dioxide monitoring involves sampling drawing a stream of air into a chamberand measuring light absorption through the gas chamber. This technique provides an accuratereflection of Pa,CO2 in most patients, although it may underestimate Pa,CO2 in patients withobesity, COPD, congestive heart failure or neuromuscular disease.

It is also commonly used in the assessment of paediatric sleep-related breathing disorders ( fig. 4).End-tidal carbon dioxide monitoring is limited to diagnostic PSG as it is not compatible with theapplication of positive airway pressure (PAP) treatment.

32

NNNNNNNNNNNNNNNNN

94 948996959394949292919394959393 88

6 3 46 52

4 2

Time s

30 33 46 40 16 2 42 48 52 3 14

N

93

Figure 4. End-tidal carbon dioxide monitoring. A 90-s snapshot of polysomnography in a paediatric patientwith sleep-disordered breathing. The end-tidal carbon dioxide monitor is depicted on the bottom row.

Hypoventilation, including a hypopnoea, is most notable towards the end of the snapshot, as evidenced byelevated end-tidal carbon dioxide and oxygen desaturation.

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Transcutaneous carbon dioxide

The transcutaneous method of measuring carbon dioxide is also noninvasive but again has itslimitations. The technology is comprised of a glass electrochemical sensor with a thermostaticallycontrolled heater unit. Raising the skin temperature increases capillary blood flow and skinpermeability to carbon dioxide diffusion. Measurements must be corrected for the falsely elevatedcarbon dioxide levels due to the increased skin temperature. If not properly adjusted,

transcutaneous carbon dioxide monitoring has the potential to overestimate Pa,CO2. It shouldalso be noted that the response time for transcutaneous carbon dioxide monitoring is slower(,50 s) than oximetry (,3 s), which limits its ability to assess ventilation on a breath-to-breathbasis.

Arterial blood gas

Although this is the most accurate method of determination ofPa,CO2, it is also the most invasiveand again rarely used in clinical polysomnography.

Assessment of snoring

The evaluation of sleep-related breathing disorders with PSG generally includes the use of amicrophone to detect snoring. In conjunction with subjective reports of the sleep technicianmonitoring the patient, some sense of frequency and volume of snoring can be ascertained, whichmay be useful in patients without significant OSA (nonapnoeic snoring). The snore monitor mayalso serve as an adjunct in optimising pressure settings during a positive pressure titration study inthe treatment of OSA.

Despite the fact that snoring is often the initial or primary symptom that leads to evaluation in thesleep laboratory, there is no widely accepted, validated instrument to quantify snoring for either

diagnostic or therapeutic purposes at this point in time. The measurement of snoring in thecurrent format described above is heavily dependent on head/body position, origin and type ofsnoring, location and placement of the recording device, and characteristics of the room.

Assessing cardiovascular function

Since nocturnal cardiac events are relatively common in the OSA population, attended type I PSGincludes electrocardiographic monitoring. A modified ECG lead II is recommended withplacement of one electrode just inferior to the right clavicle and the other over the left lateralseventh rib (as opposed to the classic lead II placement on the extremities).

Cardiac parameters are routinely included in a sleep study interpretation and may correlate withthe nature and severity of a patients sleep-related breathing disorder. Prior studies suggest thatmean sleep heart rate and cardiac variability are important signals that may affect morbidity.Nevertheless, the prognostic value of sleep heart rate is limited, particularly in adults withcoexisting medical conditions and medications that impact heart rate and its variability.

Detection of atrial fibrillation and other dysrhythmias occurs with predictable frequency in OSApatients. However, again caution must be taken in diagnosing specific cardiac pathology based ona single modified lead [34, 35]. If clinically indicated, a full 12-lead ECG and/or cardiologyconsultation may be warranted to further evaluate the PSG findings.

Blood pressure is also commonly measured during PSG and can have clinical implications. Bloodpressure may be measured continuously or as a single measurement before and after the sleepstudy, with the latter being the most common method in routine clinical practice. An automatedblood pressure cuff measurement immediately prior to and after the study provides an easy,noninvasive, consistent assessment.

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In a normal healthy population, blood pressure generally drops slightly in the morning comparedwith the pre-study afternoon measurement. However, the lack of a morning dip must beinterpreted with caution as numerous factors can influence these blood pressure measurements.Outside of a rigorous research environment, it is difficult to draw substantive conclusions aboutthe prognostic significance of an absence of nocturnal blood pressure dipping.

Assessing body position

PSG usually includes a report of body position, as sleep-related breathing disorders can havepositional variability in some patients. Body position is commonly measured objectively by apiezoelectric sensor in one of the respiratory effort belts. However, the most reliable assessment ofbody position is achieved by video monitoring and sleep technician observation.

The data on body position must be interpreted with caution as the effects on sleepdisorderedbreathing may be confounded by the position of the head and neck, not only the orientation of thetorso. Furthermore, it is difficult to translate the effects of body position into practical, measurableoutpatient therapy for sleep apnoea.

Assessing limb and body movements

Two EMG surface electrodes are typically placed over each anterior tibialis muscle symmetricallyand longitudinally so they are 23 cm apart. Specific criteria for the scoring of leg movements,periodic leg movement series and other movement disorders have been defined and published bythe AASM [18]. Separate monitoring of each leg is recommended. Additional muscles may bemonitored in a similar fashion if clinically indicated.

Future directions and emerging technology

There are a number of areas where further development and opportunities exist in the laboratoryevaluation of OSA. These include the standardisation of defining sleep-related respiratory events,new applications of sleep laboratory testing, and the deployment of new technology in the in-laboratory setting.

The AASM is in the process of convening an international workshop and consensus conference todefine sleep-disordered breathing events both in the attended (in-laboratory) and unattendedsetting. As the field has evolved, the definitions of hypopnoea, apnoea/hypopnoea index andrespiratory disturbance index have also evolved. This has created considerable difficulty in thedevelopment of practice parameters and standards. For example, the new AASM scoring manual

suggests that two different definitions of hypopnoeas could be used for scoring, incorporatingeither 3 or 4% desaturations. Standardising the nomenclature for defining events would be helpfulfor communication to third party payers, i.e. private or government-based health insurancecompanies, and outcomes for research studies.

Out-of-laboratory diagnosis and treatment pathways for OSA may require some limitedinteractions with technical laboratory staff to help introduce PAP in a best-practice approach.One possibility is to incorporate an accommodation visit for PAP in which a controllededucational and behavioural intervention, in the form of a brief daytime modified treatmentstudy, is employed for selected patients. In a pilot study by KRAKOW et al. [36], the authorseffectively utilised a PAP NAP to help accommodate OSA patients with comorbid insomnia toPAP therapy.

New technology utilising advanced signal processing and additional signal acquisition, such as high-density EEG, will undoubtedly be more routinely deployed in the sleep laboratory [37, 38].Techniques using spectral analysis of EEG and ECG can deconstruct the respective signals and iden-tify useful information relating to the depth of sleep and status of sympathomimetic balance [39].

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These data have the potential to further characterise and possibly individualise treatment in selectedpatients based on their unique physiological signature(s).

Conclusion

Laboratory testing for OSA has evolved since the initial description by GASTAUTet al.[7] almost ahalf century ago. Methods for staging and scoring sleep, characterising sleep-disordered breathing

events and the acute physiological consequences have been refined. Further standardisation ofdefinitions will require input from clinicians and investigators from throughout the world. Thisprocess is currently underway. New sleep laboratory technologies may help further phenotypeOSA patients with comorbid conditions and serve to personalise care in an attempt to facilitatemeaningful outcomes.

Despite the burgeoning expansion of unattended diagnostic and therapeutic strategies forevaluating and managing OSA outside the laboratory, it is unlikely that attended PSG willdisappear from the practice of sleep medicine. Selected patients will need attended studies toclarify diagnostic uncertainty arising from indeterminate portable tests in situations such as device

failure, patient difficulty with the home testing environment, and the presence of significantcomorbid medical conditions, i.e. advanced heart failure, neuromuscular disease and/or COPD,which may preclude testing in the home.

Statement of Interest

P. Strollo is currently a co-investigator on four NIH grants and two foundation grants (WillRogers Foundation and ResMed Foundation). He is the principal investigator on one industrygrant funded by ResMed, Corp. He has pending support as a principal site investigator for twogrants from Philips-Respironics and the Canadian Institutes of Health.

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