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Limitations of oximetry and respiratory polygraphy
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Techniques for optimising total sleep time range from removing estimated sleep/
wake periods, making start and stop times of the recording more accurate, using
position sensors, actigraphy, sleep diaries and event markers. Diagnostic accuracy for
estimating sleep time includes removing periods of probable wakefulness based on
heart rate, breathing patterns, movements, oximetry and activity, or a combination of
these parameters.
Absence of the EEG also does not allow for accurate identification of sleep stages,
most importantly REM sleep during which SDB can worsen.
Finally, the term AHI should not be used to describe the summary of breathing events
acquired using type III devices. Studies have shown that the AHI diers by up to 20%
between type III studies and PSG, with considerable under- or over-estimation of the
degree of SDB recorded. This has considerable diagnostic, management, financial and
other implications for the sleep service and the patient. It is particularly misleading in
research. More suitable terms include one of the following:
• Apnoeas+hypopnoeas per estimated hours asleep
• Respiratory events index (per estimated hours asleep)
• Apnoeas+hypopnoeas per estimated hours of monitoring time
Oximetry
As discussed in chapter 6.1 of this Handbook, ‘Methods of dierent sleep tests’,
oximetry can be useful as a screening tool in sleep disorders for monitoring the natural
history of a disease aecting nocturnal ventilation and for assessing eectiveness of
treatment (e.g. with CPAP).
However, there are significant limitations that must be considered. These include:
• What defines a normal or an abnormal ODI in general and in a particular patient?
• What constitutes pathological ODI and at what percentage (e.g. 3% desaturations,
4% desaturations)?
• Lack of accurate measurement of actual sleep time or estimation of sleep time.
• Problems with blood flow, haemoglobinopathies, tissue optics in the very obese,
ethnicity, and the presence of gel nails or opaque nail polish when the finger is
being used for monitoring.
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Figure 1. Oximetry trace of sleep apnoea.
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ERS Handbook: Respiratory Sleep Medicine
Limitations of oximetry and respiratory polygraphy
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Figure 2. Oximetry trace of CSR.
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• Presence of movement artefact.
• Measurement inaccuracies greater than ±2% can aect accuracy.
• The presence of apnoeas of short duration with no or minor desaturation will not
be detected.
• Device-specific signal processing which can dier across devices and automated
systems, and which may not be reliable (there are no basic technological
specifications set for any equipment used in sleep monitoring).
• The presence of comorbidities other than SDB, or causes of hypoventilation and
oxygen desaturations other than SDB.
• Diculties with keeping the monitor in situ in unattended settings, and the
discomfort of the probe.
Finally, understanding the morphology of the overall oximetry trace requires a great
deal of experience and training to prevent misinterpretation of the results and can
never be anything but an assumption due to the lack of any other form of physiological
monitoring (figures 1 and 2). For an overview of the uses and limitation of oximetry in
numerous settings, the reader is referred to Pretto et al. (2014).
Further reading
• Álvarez D, et al. (2022). Oximetry indices in the management of sleep apnea: from overnight
minimum saturation to the novel hypoxemia measures. Adv Exp Med Biol; 1384: 219–239.
• Pretto JJ, et al. (2014). Clinical use of pulse oximetry: ocial guidelines from the Thoracic
Society of Australia and New Zealand. Respirology; 19: 38–46.
• Riha RL, et al. (2023). ERS technical standards for using type III devices (limited channel
studies) in the diagnosis of sleep disordered breathing in adults and children. Eur Respir J; 61:
2200422.
135ERS Handbook: Respiratory Sleep Medicine
Nocturnal capnography
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Francesco Fanfulla
The third edition of the ICSD defines sleep hypoventilation as an increase during sleep
in the P
(≥10 mmHg) increase in P
supine value) to a value >6.7 kPa (>50 mmHg) for ≥10 min.
The American Academy of Sleep Medicine (AASM) recommends, during diagnostic
PSG for the detection of sleep hypoventilation, the use of P
carbon dioxide tension (P
PAP titration or during NIV, the AASM recommends the use of P
Arterial blood gas analysis is considered the gold standard for the assessment of
alveolar ventilation and arterial oxygenation. This test, although widely available
and simple, requires an arterial puncture that is painful and not risk-free. Since
repetitive arterial blood samples usually awaken patients, increasing their discomfort
and producing important diagnostic biases, the placement of an indwelling arterial
(or surrogate) to a value >7.3 kPa (>55 mmHg) for ≥10 min or a ≥1.3 kPa
aCO
2
(or surrogate) during sleep (in comparison to an awake
aCO
2
, P
2
tcCO
or P
2
or end-tidal
2
only.
tcCO
2
). However, for the detection of hypoventilation during
ETCO
2
aCO
aCO
Key points
• Diagnosis of sleep-related hypoventilation requires the measurement of P
or monitoring of P
• Two technologies are widely used to monitor P
and transcutaneous measurement (P
• Capnometry can be performed using mainstream (intubated patients) or
sidestream measurements. Sidestream capnometry is preferentially used as a
complementary sensor to detect apnoea.
• P
monitoring is oen used for diagnosis of sleep-related hypoventilation
tcCO
2
and for monitoring alveolar ventilation patients with stable chronic respiratory
failure.
• P
measurement may be biased by: operating temperature, sensor
tcCO
2
placement, sensor dri, perfusion, skin thickness and diusivity.
• The possibility of simultaneous recording of P
without recalibration and application during mechanical ventilation makes
the P
mechanical ventilation.
measurement is useful for titration or long-term monitoring of
tcCO
2
136
tcCO
or P
2
ETCO
aCO
.
2
capnography/capnometry
CO
2
).
tcCO
2
and S
tcCO
2
ERS Handbook: Respiratory Sleep Medicine
over 8-h periods
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2
2
Nocturnal capnography
P
https://t.me/medicina_free
catheter is suggested when there is a clinical need for repetitive arterial blood gas
assessments. Nonetheless, this setting permits only discrete monitoring of blood
gases, since a blood sample should be obtained and quickly analysed every time a new
assessment of gas exchange is required, with additional logistic overload. However,
this invasive approach is inappropriate in a diagnostic sleep setting as well as in the
management of stable chronic patients.
Two technologies are widely used to monitor carbon dioxide tension (P
graphy/capnometry and electrochemical sensors such as the Stow–Severinghaus
electrode for transcutaneous measurement (P
used extensively in acute and chronic settings and in childhood or adulthood.
). Both these methods have been
tcCO
2
): capno-
CO
2
Capnometry/capnography
The term ‘airway capnometry’ refers to the measurement of the amount of carbon
dioxide (CO2) in exhaled air. More properly, the term ‘capnometry’ refers to the
measurement of the peak P
plot of a capnogram: P
CO
2
, while the term ‘capnography’ usually refers to the
ETCO
2
in the exhaled air as a function of time (figure 1).
Two distinct modalities exist for airway capnometry: mainstream measurement and
sidestream measurement.
Mainstream capnometry is usually employed in intubated patients or in those
receiving NIV by means of a nonvented mask (the sensor is placed proximally before
the exhalation port) or by a specifically designed facial mask (Kohden, Tokyo, Japan).
The CO2 sensor is positioned on the main airway, so that the entirety of breathed
airflow is forced through. This technique yields instantaneous P
and a real-time capnogram plot.
measurement
ETCO
2
Sidestream capnometry consists of a small-diameter sampling tube (similar to a nasal
cannula or oronasal cannula), which continuously aspirates a fraction of the patient’s
breathed air. The sample is then conveyed to the main unit for analysis. This system can
also be used in ventilated patients (invasively or noninvasively), by means of appropriate
connectors. Furthermore, this technique can be used for monitoring patients requiring
2
CO
Figure 1. The dierent phases of a capnogram. Inspiration (I), and expiration: dead space
volume (II), mixed dead space and alveolar air (III), and alveolar air (IV), P
the end of the plateau (P
(2021) with permission.
ETCO
2
IIIIII
IV P
ETCO
2
Time
value reached at
CO
), and the two angles α and β. Reproduced from Dervieux et al.
2
137ERS Handbook: Respiratory Sleep Medicine
Nocturnal capnography
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simultaneous oxygen therapy using a specific nasal cannula. The sidestream technique
presents some technical issues related to the delay in recording, due to movement
of gases from the airway to the main unit, obstruction in the sampling tube, and
water vapour pressure changes that may aect CO2 concentration. The peak expired
carbon dioxide (P
are usually 0.27–0.67 kPa (2–5 mmHg) lower than P
becomes poor in cases of elevated physiological dead space or ventilation/perfusion
) is well correlated to P
ETCO
2
in patients in stable conditions: values
aCO
2
. However, this correlation
aCO
2
mismatch seen in COPD patients or in those with HF or OHS.
New technology has been introduced to the market recently, the so-called
‘microstream’. This monitor uses laser-based technology (molecular correlation
spectroscopy) as a CO2-specific infrared source. This system requires a low sample
flow rate (50 mL·min−1) and is particularly useful in younger patients. Nonetheless,
the measurement of P
to detect apnoea.
is currently preferentially used as a complementary sensor
ETCO
2
Assessment of P
The P
in the management of chronic conditions: continuous monitoring of P
measurement has several advantages in the field of sleep medicine and
tcCO
2
tcCO
2
night avoids sleep disruption and the need for arterial punctures, and connection
with the most common PSG devices available on the market. However, it is quite
expensive, requires the intervention of highly qualified healthcare professionals and
may sometimes overestimate the level of P
This system allows the measurement of CO2 that diuses through the skin.
The measurement is obtained by the application of a specific sensor, the Stow–
Severinghaus electrode, which is heated above body temperature (usually 40–44°C),
to obtain a local arterialisation. This sensor is separated from the skin by a membrane
permeable to CO2. Once applied, the CO2 diuses from the skin across the membrane,
modifying the pH around the electrode.
However, CO2 is also produced by the keratinocytes at the base of the epidermis, so
the P
is generally higher in the skin than in the blood. The amplitude of dierences
CO
2
depends on skin thickness including the stratum corneum, environmental and skin
temperature, vascularity and metabolic CO2 production.
The agreement between P
of studies, performed in dierent settings and involving dierent populations. The
dierence between P
significant in middle-aged or elderly subjects, probably related to the thickness of
the skin, with an increasing reduction in tissue permeability for gas exchange.
Other important sources of bias are the site of placement of the sensor, the
operating temperature and the type of monitor. For instance, the sensor of devices
that simultaneously monitor P
be placed in the earlobe with a set temperature of 42°C. When monitoring on the
earlobe is not possible, the sensor temperature should be set at >42°C. Finally, in the
presence of hyperoxaemia (P
overestimation of P
tcCO
Another technical issue that should be considered is sensor dri. This phenomenon
can be the result of environmental contamination, vibration, extreme temperature or
exposure to air. New devices that make a correction for dri increase the accuracy and
precision of measurement. However, in the presence of a wide dierence between
measured and dri-corrected P
138
tcCO
2
.
CO
.
CO
2
and P
tcCO
2
and P
2
aCO
tcCO
>13.3 kPa (>100 mmHg)) there is the possibility of an
aO
2
aCO
has been investigated in a large number
aCO
2
is lower in young adults, but becomes more
2
and peripheral oxygen saturation (S
2
values, the test should be repeated.
2
ERS Handbook: Respiratory Sleep Medicine
during the
2
) should
pO
2
Nocturnal capnography
a)
b)
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Despite these technical issues, the monitoring of P
method for diagnosis of sleep-related hypoventilation and for monitoring alveolar
is currently the most used
tcCO
2
ventilation patients with stable chronic respiratory failure. This has been particularly
true since the availability of new devices that allow the simultaneous recording of
P
and S
tcCO
2
without significant development of local discomfort. Importantly, these monitors can
, and continuous recording over 8-h periods without recalibration and
pO
2
be used during mechanical ventilation, independently of the PAP mode (including
CPAP), without changes in the level of agreement between P
consequence, overnight P
mechanical ventilation both during the titration procedure as well as for long-
measurement is suggested in patients requiring
tcCO
2
tcCO
and P
2
aCO
. As a
2
term monitoring. Figure 2a shows the baseline diagnostic study of patients with
sleep-related hypoventilation due to diaphragmatic paralysis, as well as corrected
values during NIV treatment (figure 2b).
(mmHg)
CO
2
S
(%)
pO
2
averaging:
S
pO
2
5 sec/6 sec
ORI: 4 sec
Mean: 94
1.12 min
Desat/h 0
Range: auto
Grid: auto
PR
(bpm)
Mean: 1
Range: auto
Grid: auto
70
60.0
50.0
40.0
30.0
20.0
10
100
96.5
93.0
89.5
86.0
82.5
79
40
38.3
86.7
35.0
33.3
31.7
30
P
Baseline
43.1
Mean: 49.9
Time >55.0 mmHg:
0 sec
Range: manual
Grid: auto
– (mmHg)
CO
2
S
(%)
pO
2
S
averaging:
pO
2
5 sec/6 sec
ORI: 4 sec
Mean: 93
Time <88%
0 sec
Desat/h 2
Range: auto
Grid: auto
PR
(bpm)
Mean: 45
Range: auto
Grid: auto
70
60.0
–
50.0
40.0
30.0
20.0
10
100
97.5
95.0
92.5
90.0
87.5
85
50
47.0
44.0
41.0
38.0
35.0
32
P
Dri corrected
Baseline
35.2
Mean: 45.1
Time >55.0 mmHg:
0 sec
Range: manual
Grid: auto
Figure 2. a) A 6-h overnight combined P
paralysis showing a stepper increase in CO2 during REM-phase sleep (*). Sleep eciency was
reduced and REM sleep was particularly fragmented in this patient. b) A 7-h monitoring in the
same patient during NIV.
tcCO
and S
2
monitoring in a patient with diaphragm
pO
2
139ERS Handbook: Respiratory Sleep Medicine
Nocturnal capnography
https://t.me/medicina_free
Future developments
Neither of the systems described for CO2 monitoring is specifically intended for longterm home use. Indeed, they are quite expensive, requiring costly consumables and
intervention of qualified personnel for calibration or sampling, and are relatively
bulky. For these reasons, home use is limited to very selected and generally highly
dependent patients.
There are research lines that are focused on the engineering of miniaturised wearable
devices that potentially can be used by the patients themselves for continuous
monitoring of blood gases in a remote manner, with the possibility to send data
directly to healthcare centres.
There are two dierent kinds of technologies under investigation currently: optical
transcutaneous CO2 sensors based on luminescent materials and/or nondispersive
infrared sensors (NDIR). Both systems seem to be insensitive to humidity, which is one
of the technical issues that must be addressed in this field, together with intersubject
variability in skin diusivity. They are promising tools.
Further reading
• Aarrestad S, et al. (2016). Validity of transcutaneous P
treated with non-invasive ventilation. Respir Med; 112: 112–118.
• American Academy of Sleep Medicine (AASM) (2014). International Classification of Sleep
Disorders. 3rd Edn. Darien, AASM.
• Ammadeo A, et al. (2016). Oxygen and carbon dioxide monitoring during sleep. Paediatr Respir
Rev; 20: 42–44.
• Berry RB, et al. (2020). The AASM Manual for the Scoring of Sleep and Associated Events.
Version 2.6. Darien, AASM.
• Cascales JP, et al. (2022). A patient-ready wearable transcutaneous CO2 sensor. Biosensors;
12: 333.
• Conway A, et al. (2019). Accuracy and precision of transcutaneous carbon dioxide monitoring:
a systematic review and meta-analysis. Thorax; 74: 157–163.
• Dervieux E, et al. (2021). Carbon dioxide sensing – biomedical applications to human subjects.
Sensors; 22: 188.
• Janssens JP, et al. (2010). Nocturnal monitoring of home non-invasive ventilation: the
contribution of simple tools such as pulse oximetry, capnography, built-in ventilator soware
and autonomic markers of sleep fragmentation. Thorax; 66: 438–445.
• Nassar BS, et al. (2017). Estimating arterial partial pressure of carbon dioxide in ventilated
patients: how valid are surrogate measures? Ann Am Thorac Soc; 14: 1005–1014.
• Randerath W, et al. (2017). Definition, discrimination, diagnosis and treatment of central
breathing disturbances during sleep. Eur Respir J; 49: 1600959.
• Siobal MS (2016). Monitoring exhaled carbon dioxide. Respir Care; 61: 1397–1416.
• Tipparaju VV, et al. (2021). Wearable transcutaneous CO2 monitor based on miniaturized
nondispersive infrared sensor. IEEE Sens J; 21: 17327–17334.
• Umeda A, et al. (2021). Recent insights into the measurement of carbon dioxide concentrations
for clinical practice in respiratory medicine. Sensors; 21: 5636.
in monitoring chronic hypoventilation
CO
2
140
ERS Handbook: Respiratory Sleep Medicine
Assessment of excessive
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daytime sleepiness
Francesco Fanfulla
EDS is the inability to stay awake and alert during the day, resulting in periods in which
the subject experiences an irrepressible need for sleep or unintentionally lapses into
drowsiness or sleep. The assessment of sleepiness is an important component of the
clinical and diagnostic evaluation of subjects with sleep disorders. Several dierent
methods are used for the subjective measurement (e.g. self-assessment scales, such
as the Stanford Sleepiness Scale (SSS) and the ESS) and objective measurement (e.g.
pupillometry, performance tasks and laboratory tests that measure sleep latency in a
specific setting, such as the MSLT and the maintenance of wakefulness test (MWT)) of
sleepiness. These methods are presented shown in table 1.
Subjective measurement of sleepiness is limited by individual variability in reporting
or perceiving symptoms. These measures may also be influenced by conditions in
which an assessment of sleepiness is required to evaluate fitness to drive or fitness to
perform specific work-related activities.
Two laboratory tests for the assessment of EDS have been standardised and the
execution protocol for both tests has recently been updated by the American Academy
of Sleep Medicine (AASM). In this section we will discuss the two laboratory methods –
the MSLT and the MWT.
Key points
• Tools that have been developed to identify and quantify sleepiness include
behaviour measures, subjective scales, performance tests and objective
polysomnographic measures.
• The MWT is used to measure the subject’s ability to stay awake in
unstimulating conditions for a defined period of time.
• The OSLER has been proposed as an alternative to the MWT because it allows
the use of unassisted portable techniques to measure the subject’s ability to
maintain wakefulness.
• Rigorous protocols should be in place to ensure that specific procedures are
applied before and during test execution.
• A diary of sleep–wake schedules should be kept for 2 weeks before the test.
• Concomitant sleep disturbances and particular medications/substances may
interfere with the results of these tests.
141ERS Handbook: Respiratory Sleep Medicine
Assessment of EDS
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Table 1. Tools used to assess sleepiness, listed according to sleepiness state and sleepiness trait
Tool State Trait
Behaviour measures Eye blink
Subjective scales SSS
KSS
Performance tests PVT
Drive simulator
OSLER
Polysomnographic tests MSLT
MWT
KSS: Karolinska Sleepiness Scale; PVT: psychomotor vigilance test; FOSQ-10: short version of the
Functional Outcomes of Sleep Questionnaire.
ESS
Barcelona Sleepiness Index
FOSQ-10
The MSLT
The MSLT is used to measure a subject’s physiological propensity to fall asleep under
standardised conditions, in the absence of external alerting factors. The shorter the
mean sleep latency (MSL), the greater the level of sleepiness. This test is used for the
diagnosis of narcolepsy types 1 and 2 and idiopathic hypersomnia; it is also used in
patients with other previously diagnosed sleep disorders who continue to experience
EDS despite optimal treatment. MSLT can also be used to assess the persistence of
sleepiness aer treatment in patients with sleep disorders such as OSA.
The MSLT requires the application of rigorous protocol to ensure specific procedures
are applied before and during test execution, as reported in table 2. Adherence
to previously prescribed therapy in patients with other sleep disorders should be
monitored and quantified before the test. Timing of the MSLT’s execution is important
and should be individualised in a specific population. In shi workers, the test should
be performed when the subject has a consistent sleep–wake schedule. The test times
should coincide with the patient’s typical waking period in long-sleepers or subjects
with the delayed sleep-phase disorder.
Several medication types and substances may interfere with sleep architecture,
particularly those that aect REM sleep or NREM sleep latency. Most of these
substances/types of medication are frequently prescribed in the general population
as well as in patients with EDS. Table 3 lists the most common medications and those
requiring a prolonged wash-out period. This list is not exhaustive so it is important to
check whether any of the medication the patient regularly takes has a documented
eect on sleep architecture. In general, every medication that could interfere with
the results should be stopped according to its half-life (usually 2 weeks), to permit an
appropriate wash-out period. However, as this is not easily achieved in every clinical
condition, an eort to taper the dose should be made. This recommendation could be
extended to caeine consumption.
The night before the test, a full, standard PSG should be performed. A minimum of
6 h of total sleep time during a total recording time of ≥7 h is required. The duration
of PSG should be extended to avoid insucient sleep.
The mean latency of all five naps should be calculated. If REM sleep is noted, the
latency from sleep onset to REM should be also calculated. The occurrence of sleeponset REM periods (SOREMPs) during each trial should be reported. A MSL of <5 min
is considered abnormal, as MSL in healthy adult controls ranges 10–20 min.
142
ERS Handbook: Respiratory Sleep Medicine
Assessment of EDS
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Table 2. Summary of the procedures of the MSLT, the MWT and the OSLER
The MSLT
The test should be performed 1.5–3 h aer the subject has awoken from nocturnal
sleep and at the end of PSG
Five naps at 2-h intervals are required
For each nap, the subject should be lying in bed and should be asked to ‘lie quietly,
assume a comfortable position, keep your eyes closed and try to fall asleep’
Once the lights are turned o (the start of the test), the subject has 20 min to fall
asleep, which is defined as any sleep stage that lasts 30 s, including NREM sleep
stage 1 (sleep onset)
The nap trial stops if the subject does not fall asleep aer 20 min
If the subject falls asleep, recording should continue for 15 min in order to document
the potential occurrence of REM sleep (SOREMP)
The sleep room should be dark, quiet and at a comfortable temperature during testing
Stimulating activities, such as the use of electronic devices and cell phones, should end
≥30 min before each nap trial
The subject should avoid consuming stimulating substances, performing activities that
involve exposure to sunlight/bright artificial light, and physical activity
Sleep–wake schedules recorded using a sleep diary should be collected for 2 weeks
before the test; contemporary use of actigraphy is not mandatory but is suggested
The MWT
Four 40-min trials at 2-h intervals are required, with the first beginning 1.5–3 h aer
the usual wake-up time or PSG termination
The subject should either be seated in bed with their back and head supported by a
headrest or seated in a comfortable chair
The test should be performed in a quiet, dimly lit sleep room; the light source should
deliver an illuminance of 0.1–0.13 lux at corneal level (corresponding to a 7.5 W
nightlight)
At the beginning of each trial, the subject should be asked to ‘sit still and remain awake
for as long as possible; look directly ahead of you and not at the light’; extraordinary
measures to stay awake (such as moving, smiling or singing) are forbidden
If the subject remains awake aer 40 min or if the occurrence of sleep is unambiguous
(three consecutive periods of sleep stage 1 or a period of any other sleep stage) the
trial is interrupted
Sleep latency is calculated as the first period of any sleep stage
The OSLER
The subject is asked to respond by hitting a button each time a dim light flashes
The light flashes regularly for 1 s every 3 s
Both the light and the hand-controlled device should be connected to a personal
computer that is located in the adjacent control room; this will record the
response data
The subject is instructed to remain awake for a maximum testing time of 40 min
When the subject fails to respond for 21 s (i.e. seven consecutive illuminations), the
test ends and it is assumed that the patient has fallen asleep
The MWT
The MWT measures the subject’s ability to stay awake in unstimulating conditions
for a defined period of time. MSL, obtained by performing four single sleep latency
measurements at regular intervals across the day, is used to determine the subject’s
ability to stay awake.
143ERS Handbook: Respiratory Sleep Medicine
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