Monitoring of Respiration
BY
AHMAD YOUNES
PROFESSOR OF THORACIC MEDICINE
Mansoura Faculty Of Medicine
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,
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.
The setup will consist of a flow head
(pneumotachometer) and a transducer
which will integrate volume from flow.
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).
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.
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.
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
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.
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. A–C,
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).
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.
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).
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.
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.
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.
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 effort–related arousals (RERAs) is
most accurately performed with esophageal pressure
manometry.
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.
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.
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.
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 crosssectional 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.
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.
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 non–rapid 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.
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.
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.
Apneas are followed by arterial oxygen
desaturations. Longer apneas are
associated with more severe desaturation.
SpO2 = pulse oximetry.
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).
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.
MEASUREMENT OF PaCO2 DURING SLEEP
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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.
PETCO2
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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
(Patm—47 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
(PaCO2—PETCO2) 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.
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 2to 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
PETCO2
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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.
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.
The exhaled CO2 tracing shows continued
deflections during “inspiratory apnea” due to
small exhaled puffs of air rich in CO2
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.
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.
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.”
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.
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.
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