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Monitoring of Respiration BYAHMAD
YOUNESPROFESSOR OF THORACIC MEDICINE Mansoura
Faculty Of Medicine
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The three major components of respiratory
monitoring during sleep include

• 1-Measurement of airflow,
• 2-Measurement of respiratory effort,
• 3-Measurement of arterial oxygen saturation
  (SaO2).
• Ancillary monitoring may include detection of
  snoring and recording surrogates of the arterial
  partial pressure of carbon dioxide (PaCO2)
  including end-tidal partial pressure of carbon
  dioxide (PETCO2) and transcutaneous partial
  pressure of carbon dioxide (TcPCO2).
• The AASM scoring manual recommends specific
  sensor types and techniques to be used for
  recording respiration during sleep,

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TECHNIQUES TO MEASURE AIRFLOW OR TIDAL VOLUME

• The pneumotachograph (PNT) is the most accurate
  method to measure airflow during sleep studies ,
• This device quantifies airflow by measurement of
  the pressure drop across a linear (constant)
  resistance (usually a wire screen). The
  relationship between the pressure change, flow
  rate, and resistance is given by the following
  equation Pressure change Flow X Resistance
• The PNT is worn in a mask covering the nose and
  mouth.
• Although the PNT is commonly used to measure
  airflow during sleep research, this device is
  rarely used during clinical sleep studies.

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The setup will consist of a flow head
(pneumotachometer) and a transducer which will
integrate volume from flow.
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Thermal devices were the first to be used to
monitor airflow during clinical sleep studies.

• These devices actually detect changes in
  temperature induced by airflow (cooler inspired
  air, warmer exhaled air).
• The changes in device temperature result in
  changes in voltage output (thermocouples) or
  resistance (thermistors).
• Thermal sensors are generally adequate to detect
  an absence of airflow (apnea), but their signal
  does not vary in proportion to airflow.
  Therefore, thermal sensors are not an ideal means
  of detecting a reduction in airflow (hypopnea).

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Thermal signals and PNT flow are equal at 1
L/sec. However, as airflow decreases, the thermal
signals overestimate flow.

• This illustrates that thermal sensors are not
  ideal sensors to detect hypopneas (reductions in
  flow).
• The thermal sensor signal decreases when the
  nostrils are large or the thermal sensor is
  further from the nares.
• Thermal devices composed of polyvinylidine
  fluoride (PVDF) film may offer a better estimate
  of flow.
• Nasal-oral thermal sensors usually have a portion
  of the device placed within or just outside the
  nostrils with another portion over the mouth
  (detection of oral flow).
• A major advantage of thermal sensors is that they
  can detect both nasal and oral airflow without
  the need for a cumbersome mask covering the face.

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Measurement of nasal pressure (NP) provides an
estimate of nasal airflow that is more accurate
than one obtained with most thermal sensors.

• NP is measured using a nasal cannula connected to
  an accurate pressure transducer .
• Because the cannula tips are inside the nares and
  the other side of the pressure transducer is open
  to the atmosphere, the pressure being measured is
  actually the pressure drop across the resistance
  of the nasal inlet associated with nasal airflow.
  
• The relationship of NP and flow is given by
  Equations
• NP K1(Flow)2 K1 constant
• Flow K2 NP. K2
  constant
• Changes in cannula position, periods of partial
  oral flow, and obstruction of the cannula by
  nasal secretions make the linearized NP signal a
  less accurate measure of flow over the entire
  night.

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The nasal prongs signal decreases more than that
of the pneumotachograph during a reduction in
airflow. The linearized nasal prongs signal is
very similar to that of the pneumotachograph
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The AHI values from both the NP and the
linearized NP signals showed excellent agreement
with the AHI values determined from the PNT

• The AHI values detected by the NP signal tended
  to be slightly higher than the linearized NP
  signal, but the differences were usually small.
• The inter-measurement agreements (kappa) between
  NP and PNT and linearized NP and PNT signals were
  both excellent and essentially identical.
• During normal unobstructed flow, the inspiratory
  shape (contour) of the NP signal is round ,
  whereas during airflow limitation , the shape of
  the PNT and NP signals is flattened .
• Airflow limitation is characteristically present
  during obstructive reductions in airflow
  (hypopnea) or snoring.
• In contrast, when reductions in airflow are
  simply due to a fall in inspiratory effort, the
  NP signal amplitude is reduced but the shape is
  round.
• The most important limitation of the NP technique
  is that approximately 10 of patients are mouth
  breathers and the NP signal may be misleading.

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At A, the flow is rounded, whereas at C, the
flattened airflow contour is associated with an
increase in pressure drop across the upper airway
and airflow limitation. AC,
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The AASM scoring manual recommends nasal-oral
thermal sensors for detection of apnea and NP
sensors (with or without square root
transformation of the signal) for detection of
hypopnea

• Simultaneous use of both NP and nasal-oral
  thermal sensors is recommended and has the
  additional advantage of having a backup sensor if
  the other airflow detection device fails .
• The AASM scoring manual notes that if the
  recommended sensor signal is not reliable, the
  alternative sensor can be used.
• In adults, the alternative airflow sensor for
  apnea detection is the NP signal. The alternative
  sensors for hypopnea detections are oronasal
  thermal flow and respiratory inductance
  plethysmography (RIP).

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The nasal pressure signal shows an absence of
airflow, whereas the nasal-oral thermal sensor
shows continued airflow. This pattern of airflow
is due to oral breathing.
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RIP is another method that can be used to detect
apnea and hypopnea

• The signals from rib cage (RC) and abdominal
  bands (AB) sensors can be summed in an
  uncalibrated manner (RIPsum RC AB) or as a
  calibrated signal (RIPsum a RC b AB) as
  an estimate of tidal volume (not airflow).
• Here, RC and AB are signals from bands around the
  rib cage and abdomen and a and b are
  calibration factors determined during a
  calibration procedure.
• If one takes the time derivative of the RIPsum
  signal, the result is an estimate of airflow
  (RIPflow).
• The RIPsum during apnea has minimal deflections
  (approximately zero tidal volume) and during
  hypopnea reduced deflections (reduced tidal
  volume).

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In the case of an obstructive apnea , the RC and
AB deflections must nearly exactly cancel each
other (paradox).In the case of hypopnea, there
is a reduction in the RIPsum signal (low tidal
volume) as well as both the RC and the AB
signals.

• In the case of obstructive hypopnea, there may
  also be paradox with chest and abdomen moving in
  opposite directions .
• If a RIPsum signal is not available, one can
  detect hypopnea by a reduction in the RC and AB
  RIP signals.
• The AHI values obtained from the RIPsum and time
  derivative of the RIPsum signals showed good
  agreement with AHI values from the PNT signal.

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Obstructive apnea Respiratory inductance
plethysmography signals from the rib cage (RC)
and abdominal bands (AB) are summed (RIPsum). The
RIPsum is an estimate of tidal volume.
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Obstructive hypopnea Respiratory inductance
plethysmography signals from the rib cage (RC)
and abdominal bands (AB) are summed (RIPsum). The
RIPsum is an estimate of tidal volume.
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MEASURING RESPIRATORY EFFORT

• Determination of respiratory effort is essential
  to classify apneas as obstructive (continued
  respiratory effort), central (absent effort), or
  mixed (central followed by obstructive portions).
  
• The most sensitive and accurate method of
  detecting respiratory effort is by measurement of
  esophageal pressure.
• Changes in esophageal pressure are estimates of
  changes in pleural pressure that occur during
  respiration (negative intrathoracic pressure
  during inspiration).
• Esophageal pressure monitoring can detect rather
  feeble respiratory efforts even when RC and AB
  movements are minimal. In addition, the size of
  the pressure deflections provides an estimate of
  the magnitude of respiratory effort.
• Detection of respiratory effortrelated arousals
  (RERAs) is most accurately performed with
  esophageal pressure manometry.

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MEASURING RESPIRATORY EFFORT

• Measurement of esophageal pressure can be
  performed using air-filled balloons, fluid-filled
  catheters, or catheters with pressure transducers
  on their tips.
• The technique does require special equipment and
  expertise and is routinely performed in only a
  few sleep centers.
• Some research sleep studies measure supraglottic
  pressure instead of esophageal pressure using a
  transducer tip placed just below the tongue base.
  This allows measurement of the pressure drop
  across the upper airway . Because this site is
  below the area of upper airway closure or
  narrowing in obstructive respiratory events,
  supraglottic pressure can also be used to detect
  respiratory effort.

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Esophageal pressure deflections increase during
an obstructive apnea that might at first glance
appear to be central apnea (absent inspiratory
effort).The chest and abdomen effort belt signals
showed minimal deflections during obstructive
apnea.
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The most common method for detecting respiratory
effort in clinical sleep studies utilized
piezoelectric (PE) sensors connected to bands
around the RC and AB.

• Changes in the tension on the PE transducer as
  the RC and AB expand and contract produce a
  voltage that can be measured .The signal from
  these devices depends on the degree of tension on
  the transducer.
• The PE belts are adequate for detection of
  respiratory effort in most patients but do not
  really quantify the changes in RC or AB volume.
• Although relatively inexpensive compared with RIP
  effort belts, the PE effort belts may provide
  misleading information (false absence of
  respiratory effort), especially if not properly
  positioned and tensioned.

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RIP belts provide a more accurate method of
detecting changes in RC and AB motion during
respiration than PE belts.

• The inductance of coils in bands around the RC
  and AB changes during respiration as the RC and
  AB expand and contract.
• The band inductance varies proportionately to the
  cross-sectional area the band encircles.
• An oscillator is applied to each circuit and
  changes in inductance are converted into a
  voltage output.
• The RIP bands consist of wires attached to a
  cloth band in a zig-zag pattern. This produces a
  larger change in inductance for a given change in
  band circumference.
• Recall that if the RIP signals are calibrated,
  the RIPsum signal aX RC bX AB is an estimate
  of tidal volume. Here ,the constants a and b are
  determined during a calibration procedure.

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The accuracy of the RIPsum signal can deteriorate
if body position changes or the positions of the
bands change during sleep.

• Studied patients with sleep apnea using both
  calibrated RIP belts and esophageal pressure
  monitoring showed that only 9 of patients were
  obstructive apneas sometimes misclassified as
  central apneas by the RIP belts. In these
  instances, esophageal pressure deflections were
  present when there was no detectable change in
  the RIP belt signals.
• Thus, RIP effort belts are not 100 sensitive for
  detecting respiratory effort. However, if the
  bands are properly positioned and tensioned
  (sized) , they will detect respiratory effort (if
  present) in most patients.
• The vast majority of sleep centers do not perform
  RIP belt calibration. It should be noted that the
  deflections of uncalibrated RIP bands do not
  always accurately reflect the magnitude of
  inspiratory effort or always show paradox during
  obstructive apnea and hypopnea.

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Surface diaphragm EMG recording utilizes two
electrodes about 2 cm apart horizontally in the
seventh and eighth intercostal spaces in the
right anterior axillary line.

• The right side of the body is used to reduce ECG
  artifact.
• Intercostal EMG recording often uses the right
  parasternal area (second and third intercostal
  spaces in the midaxillary line).
• Inspiratory EMG activity is noted in the
  intercostal muscles and the diaphragm during
  nonrapid eye movement (NREM) sleep.
• During rapid eye movement (REM) sleep, the
  intercostal activity is inhibited but
  diaphragmatic activity persists, although often
  diminished in amplitude during bursts of eye
  movements.
• The AASM scoring manual recommends use of
  esophageal manometry or calibrated or
  uncalibrated RIP belts for detection of
  respiratory effort during sleep studies in adults
  and children . The measurement of respiratory
  muscle EMG is listed as an alternative method of
  detecting respiratory effort in adults.

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An obstructive apnea with respiratory effort
monitored by both chest and abdominal respiratory
inductance plethysmography (RIP) bands and right
intercostal electromyogram (EMG). The right
intercostal EMG signal shows bursts coincident
with inspiratory effort (and movement of chest
and abdomen).A blow up of one EMG burst is shown
at the bottom of the figure in a raw form and
with the electrocardiogram (ECG) artifact
minimized.
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OXYGEN SATURATION

• Pulse oximetry. In this method SaO2 is determined
  by the passage of two wavelengths of light (650
  nm and 805 nm) through a pulsating vascular bed
  from one sensor to another. The light is
  partially absorbed by the oxygen-carrying
  molecule, hemoglobin, depending on the percent of
  the hemoglobin saturated with oxygen. A processor
  calculates absorption at the two wavelengths and
  computes the proportion of hemoglobin that is
  oxygenated, giving it a numerical value.
• A thin anatomic pulse site (such as the finger
  tip, ear lobe, nose, or toe) is required, as is
  proper alignment of the sensors.
• With movement in sleep, the device can become
  dislodged.
• The readings can also be affected by anemia,
  hemoglobinopathies, a high carboxyhemoglobin
  level, elevated methemoglobin level, anatomic
  abnormalities/previous injury to the site tested,
  sluggish arterial flow (due to hypovolemia or
  vasoconstriction),and the use of nail polish.

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Apneas are followed by arterial oxygen
desaturations. Longer apneas are associated with
more severe desaturation. SpO2 pulse oximetry.
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OXYGEN SATURATION

• In sleep monitoring, an arterial oxygen
  desaturation is usually defined as a decrease in
  the SpO2 of 3 or 4 or more from baseline.
• The nadir in SaO2 commonly follows apnea
  (hypopnea) termination by approximately 6 to 8
  seconds (longer in severe desaturations) . This
  delay is secondary to circulation time and
  instrumental delay (the oximeter averages over
  several cycles before producing a reading).

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OXYGEN SATURATION

• The assessment of the severity of desaturation
  include the number of desaturations, the average
  minimum SpO2 during desaturations, the time below
  80, 85, 90,as well as the mean SaO2 and the
  minimum saturation during NREM and REM sleep.
• The time with an SpO2 88 is also commonly
  determined.
• Oximeters may vary considerably in the number of
  desaturations they detect and their ability to
  discard movement artifact.
• Using long averaging times may dramatically
  decrease the detection of desaturations.
• The ability of oximeters to detect desaturations
  is especially important in light of the
  definitions of hypopnea that depend on an
  associated desaturation.
• The AASM scoring manual recommends a maximum
  averaging time of 3 seconds at a heart rate of 80
  bpm. In patients with a slow heart rate, a
  slightly longer averaging time (at least a 3-beat
  average) may be needed.

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MEASUREMENT OF PaCO2 DURING SLEEP

• Documentation of hypoventilation during sleep
  requires measurement (or estimate) of the PaCO2.
• The AASM scoring manual defines sleep-related
  hypoventilation in adults as an increase in the
  PaCO2 during sleep 10 mm Hg compared with an
  awake supine value.
• Continuous ABG monitoring during polysomnography
  to determine the PaCO2 is not practical. An ABG
  sample sometimes is performed at the start or the
  end of the study. The sample can be used to
  validate a surrogate measure of PaCO2 such as
  PETCO2 or TcPCO2.
• If an ABG sample is taken just at awakening, it
  may be used to infer hypoventilation.

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PETCO2

• Capnography consists of the continuous
  measurement of the fraction of CO2 in exhaled
  gas.
• This is usually performed using an infrared
  sensor and, less commonly, a mass
  spectrophotometer.
• The PCO2 is determined by multiplication of the
  fraction of CO2 by (Patm47 mm Hg). Here, the
  Patm is the atmospheric pressure (760 mm Hg at
  sea level )and 47 mm Hg is the partial pressure
  of H2O in exhaled gas at body temperature.
• During initial exhalation, the dead space (PCO2
  0) reaches the sensor (phase 1), then a mixture
  of dead space and alveolar gas (phase 2),and
  finally, alveolar gas (phase 3). The alveolar
  plateau occurs because the PCO2 in the air from
  the different alveoli differs slightly.
• The differences are larger (slope of alveolar
  plateau steeper) in patients with lung disease.
• The PETCO2 is an estimate of the mean alveolar
  PCO2 (and, therefore, an estimate of the PaCO2).
• Of note, there is a gradient between the PaCO2
  and the PETCO2 (PaCO2PETCO2) with the PaCO2
  being typically 2 to 5 mm Hg higher than the
  PETCO2 in normal individuals.
• In lung disease, the gradient can be much larger.
  
• In general, the PETCO2 is a valid estimate of
  PaCO2 only if an alveolar plateau is present.

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Nonstructural risk factors

• Some nonstructural risk factors include obesity,
  age, male sex, postmenopausal state, and habitual
  snoring with daytime somnolence.
• Familial factors Relatives of patients with SDB
  have a 2- to 4-fold increased risk of OSA
  compared with control subjects.
• Environmental exposures include smoke,
  environmental irritants or allergens, and alcohol
  and hypnotic-sedative medications.
• Both hypothyroidism and acromegaly are associated
  with macroglossia and increased soft tissue mass
  in the pharyngeal region. They are associated
  with an increased risk of OSA. Hypothyroidism is
  also associated with myopathy that may contribute
  to UA dysfunction

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PETCO2

• In the mainstream method, the sensor is located
  directly in the path of exhaled gas.
• In the side stream method, gas is continually
  suctioned through a tube to a more remote sensor
  (in the instrument at bedside).
• In the side stream approach, nasal cannulas are
  used to suction exhaled gas from the nares . When
  no CO2 is exhaled (during inspiration or apnea),
  the nasal cannula suctions room air (PCO2 0).
• In the side stream method, there is a delay in
  exhaled gas reaching the sensor so the CO2
  tracing is delayed compared with the exhaled
  airflow .
• The exhaled CO2 tracing is sometimes used to
  indicate apnea (absence of exhaled PCO2).
  However, this is not recommended for two reasons.
  First, gas sampled by the nasal cannula may not
  detect mouth breathing, and second, small
  expiratory puffs rich in CO2 may still produce
  deflections in the exhaled CO2 trace.
• Capnography is used much more frequently during
  pediatric than in adult sleep studies.

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Imaging Studies

• Modalities available for identifying the site of
  obstruction include lateral cephalometry,
  endoscopy, fluoroscopy, CT scanning, MRI.
• At present, UA imaging is used primarily as a
  research tool. Routine radiographic imaging of
  the UA in the initial evaluation of SDB patients
  is of uncertain benefit and should not be
  performed unless a specific indication is
  present.

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The exhaled CO2 tracing shows continued
deflections during inspiratory apnea due to
small exhaled puffs of air rich in CO2
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TcPCO2 MONITORING

• Measurement of TcPCO2 depends on the fact that
  heating of capillaries in the skin causes
  increased capillary blood flow and makes the skin
  permeable to the diffusion of CO2.
• The CO2 in the capillaries diffuses through the
  skin and is measured by an electrode at the skin
  surface.
• The measured value is corrected for the fact that
  heat increases the skin CO2 production as the
  measured value exceeds the PaCO2 measured at 37C.
• Typically, TcCO2 electrodes are calibrated with a
  reference gas.
• A thermostat controls the heating of the
  membrane-skin interface.
• It is usually recommended that the probe of most
  TcPCO2 monitoring devices be moved every 3 to 4
  hours to avoid skin irritation/damage.
• The response time of newer TcPCO2 units has
  improved, but in general, the measured PCO2 may
  not increase rapidly enough to correlate with
  short respiratory events. However,TcPCO2 can be a
  good instrument for documenting trends in the
  PCO2 during the night.

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Trends in the SpO2 and TcPCO2 during the night.
Note the simultaneous increase in transcutaneous
PCO2 and the decrease in SpO2 during episodes of
REM sleep.
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ACCURACY OF PETCO2 AND TcPCO2

• The measurement of PETCO2 and TcPCO2 was not
  found to be accurate for determining changes in
  PaCO2 during sleep .
• PETCO2 was especially inaccurate during
  simultaneous administration of supplemental
  oxygen or during positive airway pressure
  treatment as exhaled gas was sampled from a mask
  rather than using a nasal cannula.
• The AASM scoring manual in the respiratory
  scoring rules for adults states that there was
  insufficient evidence to recommend a specific
  method for detecting hypoventilation during
  sleep. However, it was stated that PETCO2 or
  TcPCO2 may be acceptable if validated and
  calibrated. In the pediatric rules, it states
  that acceptable methods for assessing alveolar
  hypoventilation are either transcutaneous or
  end-tidal PCO2 monitoring.

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ACCURACY OF PETCO2 AND TcPCO2

• Concerning PETCO2 monitoring, the clinician
  should review the exhaled CO2 tracings to
  determine whether an alveolar plateau is present.
  
• If an alveolar plateau is not present, the PETCO2
  value may not be an accurate estimate of PaCO2.
• Other problems for exhaled PCO2 monitoring
  include oral breathing and occlusion of the nasal
  cannula with secretions.
• If tidal volumes are very small, a true alveolar
  sample may never reach the sensor. So PETCO2
  value will likely be much lower than the PaCO2.
• Concerning TcPCO2 measurement, the actual
  tracings should also be carefully reviewed. If an
  abrupt change (offset) in the TcPCO2 tracing is
  noted, this suggests a measurement artifact is
  present.
• Most TcPCO2 devices require calibration at the
  start of monitoring.
• Poor application of the sensor or dislodgment of
  the sensor during sleep can cause a measurement
  artifact.
• There is some advantage to the simultaneous use
  of both PETCO2 and TcPCO2 if tolerated. If the
  values show reasonable agreement during periods
  of stable breathing, this increases confidence in
  their validity. Of course, the goal standard to
  validate the accuracy of their measurements is a
  simultaneous ABG measurement.

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SNORING SENSORS

• Snoring is a sound produced by vibration of upper
  airway structures.
• When snoring is present, upper airway narrowing
  of some degree can be inferred.
• Snore sensors are usually microphones or PE
  transducers that are usually applied to the neck
  near the trachea.
• Microphones can also be attached to the upper
  chest area or the face.
• Snoring can also be seen in the NP signal as a
  rapid oscillation in the pressure tracing if an
  appropriate high frequency filter setting (100
  Hz) is used and the transducer is sufficiently
  sensitive.
• The AASM scoring manual does not provide guidance
  on use of snoring sensors and the signal is not
  part of the scoring criteria for respiratory
  events in adults.
• Snoring is mentioned in scoring of RERAs in
  children.

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Snoring noted both in the snore sensor (applied
to the neck near the trachea) and as a vibration
(oscillation) in the nasal pressure signal. Note
that the nasal pressure signal also has a
flattened shape. SpO2 pulse oximetry
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