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Pulmonary Mechanics
 Includes test of pulmonary mechanics
› FVC
› FEV (t)
› Forced Expiratory Flow (FEF) values
› Forced Inspiratory Flow (FIF) rates
› Maximum Voluntary Ventilation
 Measuring pulmonary mechanics is
assessing the ability of the lungs to move
large volumes of air quickly through the
airways to identify airway obstruction

INDICATIONS:
 Detect the presence or absence of
lung dysfunction suggested by
history or physical signs and
symptoms and/or the presence of
other abnormal diagnostic tests
(e.g., chest radiograph, ABGs)
 Quantify the severity of known lung
disease
 Assess the change in lung function
over time or following administration
of or change in therapy

INDICATIONS:
 Assess the potential effects or response
to environmental or occupational
exposure
 Assess the risk for surgical procedures
known to affect lung function
 Assess impairment and/or disability (e.g.,
for rehabilitation, legal reasons, military)

CONTRAINDICATIONS
 Circumstances listed here could affect the
reliability of spirometry measurements. In
addition, forced expiratory maneuvers may
aggravate these conditions, which may
make test postponement necessary until the
medical condition resolves. The following are
some relative contraindications to
performing spirometry:
› Hemoptysis of unknown origin (forced expiratory
maneuver may aggravate the underlying
condition)
› Pneumothorax
› Unstable cardiovascular status (forced expiratory
maneuver may worsen angina or cause changes
in blood pressure) or recent myocardial infarction
or pulmonary embolus

CONTRAINDICATIONS
 Thoracic, abdominal, or cerebral
aneurysms (danger of rupture resulting
from increased thoracic pressure)
 Recent eye surgery (e.g., cataract)
 Presence of an acute disease process
that might interfere with test
performance (e.g., nausea, vomiting)
 Recent surgery of thorax or abdomen

Hazards and complications
 Although spirometry is a safe procedure,
untoward reactions may occur, and the
value of the test should be weight against
potential hazard . The following have been
reported anecdotaly:
› Pneumothorax
› Paroxysmal coughing
› Increased intracanial pressure
› Contraction of nosocomial infection
› Syncope, dizziness, or lightheadedness
› O2 desaturation resulting from interruption of O2
therapy
› Chest pain
› Bronchospasm

Assessment of Test Quality
 Spirometry performed for the listed
indications is valid only if the spirometer
functions acceptably and the subject is
able to perform the maneuvers in an
acceptable and reproducible fashion.
All reports should contain a statement
about the technician’s assessment of
test quality and specify which
acceptability criteria were not met

Quality Control
 Volume verification (calibration)
› At least daily before testing
› Use a calibrated known-volume syringe
with a volume of at least 3L to ascertain
that the spirometer reads a known
volume accurately. The known volume
should be injected or withdrawn at least
3 times, at flows that may vary between
2 L/sec and 12 L/sec
› The tolerance limits for an acceptable
calibration are ±3% of a known volume

Quality Control
 Leak test
› Volume displacement spirometers must
be evaluated for leaks daily
 A spirometry procedure manual
should be maintained
 A log that documents daily
instrument calibration, problem
encountered, corrective action
required, system hardware or
software changes should be
maintained

Quality Control
 Computer software for measurement and
computer calculations should be checked
against manual calculations, if possible
 The known-volume syringe should be
checked for accuracy at least quarterly
using a 2nd known-volume syringe
 For water-sealed spirometers, water level
and paper tracing speed should be
checked daily. The entire range of volume
should be checked quarterly

Quality Assurance
 Each laboratory or testing site should
develop, establish, and implement
quality assurance indicators for
equipment calibration and
maintenance and patient preparation
 Methods should be devised and
implemented to monitor technician
performance (with appropriate
feedback) while obtaining, recognizing,
and documenting acceptability criteria

Monitoring
 The following should be evaluated during the
performance of spirometric measurements to
ascertain the validity of the results
› Acceptability maneuver and reproducibility of FVC
and FEV1
› Level of effort and cooperation by the subject
› Equipment function or malfunction ( calibration)
 Final report should contain a statement about
test quality
 Spirometry results should be subject to ongoing
review by a supervisor, with feedback to the
technologist
 Quality assurance or quality improvement
programs should be designed to monitor
technician competency initially and in an
ongoing fashion

Forced Vital Capacity
(FVC)
 The most commonly
performed test of
pulmonary
mechanics
› Many measurements
are made while the
patient is performing
the FVC maneuver
 Measuring FVC often
occurs under
baseline or
untreated conditions

 The maximum volume of gas that the
subject can exhale as forcefully and as
quickly as possible

 Is an effort-dependent maneuver that
requires careful patient instruction,
understanding ,coordination, and
cooperation
 Spirometry standards for FVC maneuver
specify that patients must be instructed in
the FVC maneuver, that the appropriate
technique be demonstrated, and that
enthusiastic coaching occur. When
measuring FVC, the RT needs to coach
the preceding IC as enthusiastically as the
FVC.

 FVC may be measured
on a spirometer that
measures volume or
flow, that presents a
graph of volume and
time or flow and volume,
that is mechanical or
electronic, and that has
a calculator or
computer
 The forced expiratory VC
is sometimes followed by
forced inspiratory VC to
produce a complete
image of forced
breathing called flow-
volume loop
Flow-Volume loop showing successful FVC maneuver. Positive values represent expiration, negative values represent inspiration. The trace moves clockwise for expiration followed by inspiration. (Note the FEV1, FEV1/2 and FEV3 values are arbitrary in this graph and just shown for illustrative purposes, they must be recorded as part of the experiment).

 Nose clips are encouraged but not required,
and patients may be tested in a sitting or
standing position. Although standing usually
produces a larger FVC compared with
sitting, sitting is considered safer in case of
light-headedness
 It is recommended that the position be
consistent for repeat testing of the same
patient.
 FVC should be converted to body
temperature conditions and reported as
liters under BTPS condition

Forced Vital Capacity ( FVC )
 For baseline testing, patients should
temporarily abstain from bronchodilator
medications.
 Short-acting inhaled drugs( β2agonist
salbutamol, anti-cholinergic agent
ipratropium bromide) should not be used
4 hours before baseline spirometry
 Long acting β-agonist bronchodilators
(salmeterol / formoterol) and oral
therapy with β-agonistor slow-release
should be stopped for 12 hours prior to
test
 Smoking should be stopped 1 hour prior
to testing

 When a patient’s baseline result shows
airway obstruction, performing FVC after
treatment (salbutamol aerosol or MDI)
can help determine if the treatment is
effective
 The FVC maneuver is also performed
repeatedly during bronchial
provocation testing

Procedure in the performance
of FVC maneuver
1. Check the spirometer calibration
2. Explain the test
3. Prepare the subject
› Ask about smoking , recent illness,
medication used etc.
› Measure weight and height without shoes
4. Wash hands

of FVC maneuver
5. Instruct and demonstrate the test to the
subject, to include:
› Correct posture with head slightly elevated
› Inhale rapidly and completely
› Position of the mouthpiece
› Exhale with maximum force

of FVC maneuver
6. Perform the maneuver (closed circuit method)
 Have the subject assume the correct posture
 Attach nose clip, place mouthpiece in mouth and
close lips around the mouthpiece
 Inhale completely and rapidly with a pause of <1 s
at TLC
 Exhale maximally until no more air can be expelled
while maintaining an upright posture
 Repeat instructions as necessary, coaching
vigorously
 Repeat for a minimum of three(3) maneuvers, no
more than eight (8) are usually required
 Check test repeatability and perform more
maneuvers as necessary

of FVC maneuver
6. Perform the maneuver (open circuit method)
 Have the subject assume the correct posture
 Attach nose clip
 Inhale completely and rapidly with a pause of <1 s at TLC
 Place mouthpiece in mouth and close lips around the
mouthpiece
 Exhale maximally until no more air can be expelled while
maintaining an upright posture
 Repeat instructions as necessary, coaching vigorously
 Repeat for a minimum of three(3) maneuvers, no more
than eight (8) are usually required
 Check test repeatability and perform more maneuvers
as necessary

 To ensure validity, each patient must
perform a minimum of three
acceptable FVC maneuvers.
 To ensure reliability, the largest FVC
and second largest FVC from
acceptable trials should not vary by
more than 0.150 L
 To perform an FVC trial, the patient
should inhale rapidly and completely
to TLC from the resting FRC level

 The forced exhalation should begin
abruptly and without hesitation
 A satisfactory start of expiration is defined
as an extrapolated volume at the zero
time point less than 5% of the FVC or
0.150 L, whichever is greater
› The volume exhaled before the zero time
point is called the extrapolated volume

 A cough, an inspiration, a Valsalva
manuever, a leak, or an obstructed
mouthpiece while an FVC maneuver is
performed disqualifies the trial.
 FVC must be completely exhaled or an
exhalation time of at least 6 secs must occur
for adults and children older than10 y/o
(longer times are needed for obstructive
patients)
 3 Secs exhalation is acceptable for children
younger than 10 y/o

FORCED VITAL CAPACITY
 FVC (should be within 150 ml of VC)

 FVC: Criteria for Acceptability
1. Maximal effort; no cough or glottic closure during the
first second; no leaks or obstruction of the mouthpiece.
2. Good start-of-test; back extrapolated volume <5% of
FVC or 150 ml, whichever is greater

3. Tracing shows 6 seconds of exhalation or an obvious
plateau (<0.025L for ≥1s); no early termination or cutoff;
or subject cannot or should not continue to exhale

 FVC: Selection Criteria
The largest FVC and largest FEV1 (BTPS) should be
reported, even if they do not come from the same curve

4. Three acceptable spirograms obtained; two largest FVC values
within 150 ml; two largest FEV1 values within 150 ml

SUMMARY OF ACCEPTABILITY CRITERIA
 With-in maneouver criteria
› Individual spirograms are acceptable if
 They are free from artefacts
 Cough during the first second of exhalation
 Glottic closure that influences the measurement
 Early termination or cut-off
 Effort that is not maximal througghout
 Leak
 Obstructed mouthpiece
 They have good starts
 Extrapolated volume <5% of FVC or 0.150 L whichever is
greater
 They show satisfactory exhalation
 Duration of ≥6 sec (3 s for children or a plateau in the
volume-time curve or if the subject cannot or should not
continue to exhale

SUMMARY OF REPRODUCIBILITY CRITERIA
 Between maneuver criteria
› After 3 acceptable spirograms have been
obtained, apply the following test
 The 2 largest values of FVC must be within 0.15L of
each other
 The 2 largest values of FEV1 must be within 0.15L of
each other
› If both of these criteria are met, the test session may
be concluded
› If both of these criteria are not met, continue testing
until:
 Both of the criteria are met with analysis of additional
spirogram or
 A total of 8 test have been performed or
 The patient/subject cannot or should not continue

PFT Reports
o When performing PFT’s three values are
reported:
o Actual – what the patient performed
o Predicted – what the patient should have
performed based on Age, Height, Sex, Weight,
and Ethnicity
o % Predicted – a comparison of the actual value
to the predicted value

PFT Reports
 Example
Actual Predicted %Predicted
VC 4.0 5.0 80%

Significance
 A reduced FVC may occur with obstructive
or restrictive impairments.
 The primary difference between the curve
in the restricted patient is the slope of the
tracing; obstructive disease produce
flattened slopes and smaller FEV1
 In FV loop, the shapes are different, the
obstructive diseases produces lower peaks
and lower flow rates at all lung volumes
 FIF are sometimes useful in identifying
extrathoracic airway obstruction

FVC
 FVC: Significance and Pathophysiology
› FVC equals VC in healthy individuals
› FVC is often lower in patients with obstructive
disease

› Healthy adults can exhale their FVC within 4 – 6
seconds
› Patients with severe obstruction (e.g., emphysema)
may require 20 seconds, however, exhalation times
>15 seconds will rarely change clinical decisions

› FVC can be reduced by:
 Mucus plugging
 Bronchiolar narrowing
 Chronic or acute asthma
 Bronchiectasis
 Cystic fibrosis
 Trachea or mainstem bronchi obstruction

› FVC is also decreased in restrictive lung
disease
 Pulmonary fibrosis
 dusts/toxins/drugs/radiation
 Congestion of pulmonary blood flow
 pneumonia/pulmonary hypertension/PE
 Space occupying lesions
 tumors/pleural effusion

› FVC is also decreased in restrictive lung
disease
 Neuromuscular disorders, e.g,
 myasthenia gravis, Guillain-Barre
 Chest deformities, e.g,
 scoliosis/kyphoscoliosis
 Obesity or pregnancy

Significance
 In moderate and severe obstructive lung
disease, the FVC lungs is reduced if
weakened bronchioles collapse and
trap air in the lungs creating an
increased RV because the patient’s
inhaled volume is reduced.
 Some laboratories compare the SVC
and FVC to identify air trapping
 VC is reduced in restrictive lung diseases

Forced Expiratory Volume in 1
Second (FEV1)
 During FVC testing, several other
measurements are also made.
 The FEV1 is a measurement of the volume
exhaled in the first second of FVC
 To ensure validity of FEV1, the
measurement must originate from a set of
acceptable FVC trials

Second (FEV1)
 The volume expired over the first
second of an FVC maneuver

Forced Expiratory Volume in
1 Second (FEV1)
 The first second of forced exhalation
begins at zero-time point
 To ensure reliability of FEV1, the largest
and second largest FEV1 from
acceptable trials should not vary by
more than 0.150 L.
 Sometimes the largest FEV1 comes from
a different trial than the largest FVC

Second (FEV1)
 May be reduced in obstructive or restrictive
patterns, or poor patient effort

Significance
 FEV 0.5 is an indicator of patient effort
during the initial phase of the FVC
maneuver.
 A patient should exhale at least 50% of
his or her VC in the initial half of a
second

Significance
 FEV1 may be reduced with obstructive
or restrictive impairments.
 For patients with airway obstruction,
the FEV1 measures the severity of the
airway obstruction.
 For restrictive patients, the FEV1 may
be reduced when the patient’s FVC is
smaller than the predicted FEV1

Forced Expiratory Volume (FEV1)
 In obstructive disease, FEV1 may be
decreased because of:
› Airway narrowing during forced expiration
 emphysema
› Mucus secretions
› Bronchospasm
› Inflammation (asthma/bronchitis)
› Large airway obstruction
 tumors/foreign bodies

 FEV1 may be reduced in restrictive lung
processes
› Fibrosis
› Edema
› Space-occupying lesions
› Neuromuscular diseases
› Obesity
› Chest wall deformity

 FEV1 is the most widely used spirometric
parameter, particularly for assessment of
airway obstruction

 FEV1 is used in conjunction with FVC
for:
› Simple screening
› Response to bronchodilator therapy
› Response to bronchoprovocation
› Detection of exercise-induced bronchospasm

Forced expiratory volume in 1 sec to
vital capacity ratio (% FEV1/FVC)
 Percent of the measured forced vital
capacity that can be exhaled in 1 sec
 Is calculated by dividing the patient’s
largest FEV1 by the patient’s largest FVC
and converting it to a percentage
 The two values do not have to come
from the same trial

Significance
 The FEV1/FVC ratio separates patients
with airway obstruction from individuals
with normal pulmonary function and
from patients with restrictive impairments
 Generally, individuals without airway
obstruction are able to exhale 70% of
their VC in the first second and
individuals with airway obstruction
exhale less than 70% of their VC in the
first second

Forced Expiratory Volume Ratio
(FEVT%)
 FEVT% = FEVT/FVC x 100
› Useful in distinguishing between obstructive and
restrictive causes of reduced FEV1 values
 A decrease FEV1/FVC ratio is the “hallmark” of
obstructive disease
› FEV1/FVC <70%

Forced Expiratory Volume
Ratio (FEVT%)
 Patients with restrictive disease often have
normal or increased FEVT% values
› FEV1 and FVC are usually reduced in equal
proportions
 The presence of a restrictive disorder may by
suggested by a reduced FVC and a normal or
increased FEV1/FVC ration

Other measurements
 Except for PEF rate, all other
measurements that originate from FVC
come from the “best curve”
› FEF 200-1200
› FEF25-75
› FEF 75 – 85
› Instantaneous FEF25%, FEF 50%n and FEF75%
› The best curve is defined as the trial that
meets acceptability criteria and gives the
largest sum of FVC plus FEV1

FEF200-1200 of the FVC
 FEF200-1200 and FEF 25%-75% of the FVC represents
average flow rates that occur during specific
intervals of FVC.
 Both measurements can be made on a
volume-time spirogram as the slope of a line
connecting the two points in their subscripts
 For FEF200-1200 the 200 ml point and the 1200 ml
point are identified, a straight line is drawn
connecting these two points, and a line is
extended to intersect two vertical time lines
one second apart
 The volume measured between the two time
lines is FEF200-1200 in L/sec

FEF 25%-75% of the FVC
 Is a measurement of the flow during the
middle portion of the FVC, or the time
necessary to exhale the middle 50%
 The VC of the best curve is multiplied by
25% and 75%, and t points are identified on
the tracing. A , a straight line is drawn
connecting these two points, and a line is
extended to intersect two vertical time lines
one second apart
 The volume measured between the two
time lines is FEF 25%-75% in L/sec
 Volume measured is corrected to BTPS

Forced Expiratory Flow 25%-75%
 Also known as maximum mid-expiratory flow
› FEF 25%-75% is measured from a segment of the FVC
that includes flow from medium and small airways
 Normal values: 4 – 5 L/sec

Forced Expiratory Flow 25% - 75%
 In the presence of a borderline
value for FEV1/FVC, a low FEF 25%-
75% may help confirm airway
obstruction

Flow – Volume Curve
 Flow – Volume Curve
› AKA: Flow–Volume Loop
(FVL)
The maximum expiratory
flow-volume (MEFV) curve
shows flow as the patient
exhales from maximal
inspiration (TLC) to maximal
expiration (RV)
› FVC followed by FIVC

 X axis: Volume
 Y axis: Flow
› PEF (Peak Expiratory Flow)
› PIF (Peak Inspiratory Flow)
.
› Vmax 75 or FEF 25%
FVC Remaining or Percentage FVC exhaled
.
.
FEF 25% or Vmax 75
FEF 75% or Vmax 25%

FEF 75%-85%
 Measure of the average expiratory flow
during the end of the FVC

FEF 25%
 Forced expiratory flow at 25%
 Vmax 25
 Maximum expiratory flow after 25% of
the FVC has been expired

FEF 50%
 Vmax 50

FEF 75%
 Vmax 75

FIF50%
 Forced inspiratory flow at 50% of the FVC
 Maximum inspiratory flow after 50% of
the forced vital capacity has been
inspired

 Significant decreases in flow or volume are easily
detected from a single graphic display

 Severe Obstruction

 Bronchodilation

Peak Expiratory Flow Rate
(PEFR)
 Is difficult to identify on a volume time graph of FVC
 The peak flow is the slope of the tangent to the
steepest portion of the FVC curve
 In flow-volume graph, it is easy to identify as the
highest point on the graph
 It is sometimes measured independently of FVC with
a PF meter
 The validity of PEF rate is based on a preceding
inspiration to TLC and a maximum effort
 The reliability of the test – the 2 largest repeated
measurements should agree within 5%

Peak Expiratory Flow (PEF)
 The maximum flow obtained during a
FVC maneuver
› Measured from a FVL
› In laboratory, must perform a minimum of 3 PEF
maneuvers
› Largest 2 of 3 must be within 0.67 L/S (40 L/min)
› Primarily measures large airway function
› Many portable devices available

› When used to monitor asthmatics
 Establish best PEF over a 2-3 week period
 Should be measured twice daily (morning and
evening)
 Daily measurements are compared to personal
best

 The National Asthma Education Program suggests a zone
system
› Green: 80%-100% of personal best
 Routine treatment can be continued; consider
reducing medications
› Yellow: 50%-80% of personal best
 Acute exacerbation may be present
 Temporary increase in medication may be needed
 Maintenance therapy may need increases
› Red: Less than 50% of personal best
 Bronchodilators should be taken immediately; begin
oral steroids; clinician should be notified if PEF fails to
return to yellow or green within 2 – 4 hours

› PEF is a recognized means of
monitoring asthma
› Provides serial measurements
of PEF as a guide to treatment
› ATS Recommended Ranges
 60-400 L/min (children)
 100-850 L/min (adults)

Maximum Voluntary Ventilation
(MVV)
1. The volume of air exhaled in a specific
interval during rapid, forced breathing
2. Maximum volume of air in liters per
minute that a subject can breathe during
a 12 to 15 sec period.
3. Also known as maximum breathing
capacity

Maximum Voluntary
Ventilation
 Effort-dependent test for which the
patient is asked to breathe as deeply
and as rapidly as possible for at least 12
seconds.
 It reflects patients cooperation and
effort, the ability of the diaphragm and
thoracic muscles to expand the thorax
and lungs, and airway patency
 Because of the potential for acute
hyperventilation and fainting or
coughing, the patient should be seated

› RT demonstrate the expected breathing
pattern
› Patient is instructed to breathe as rapidly
and as deeply as possible for at least 12
seconds
› The patients breathing is measured on a
spirogram or electronically for the specific
number of seconds and the volume
breathed when the MVV is converted to
LPM

› The validity depends on the duration of the
maneuver, which should be at least 12 seconds,
and a breathing frequency of at least 90 per min.
And the average volume should be at least 50%
of FVC
› Patients should perform at least 2 MVV trials
› When the first trial does not exceed 80% of the
subjects FVE1x40, may indicate less than
maximum effort, or 80% of the predicted normal
value may indicate disease
› Reliability – less than 20% variability between the
two largest trials
› Reported in BTPS

MVV
 Rapid, deep breathing
 VT ~50% of VC
 For 12-15 seconds

MVV
 Tests overall function
of respiratory system
› Airway resistance
› Respiratory muscles
› Compliance of lungs/chest
wall
› Ventilatory control
mechanisms

MVV
 At least 2 acceptable maneuvers should be
performed
 Two largest should be within 10% of each
other
 Volumes extrapolated out to 60 seconds
and corrected to BTPS
 MVV is approximately equal to 35 time the
FEV1

MVV
Decreased in:
 Patients with moderate to severe obstructive lung
disease
 Patients who are weak or have decreased
endurance
 Patients with neurological deficits

MVV
Decreased in:
 Patients with paralysis or nerve damage
 A markedly reduced MVV correlates with
postoperative risk for patients having abdominal or
thoracic surgery

MVV
 Selection Criteria
› The highest MVV (L/min, BTPS) and MVV rate
(breaths / min) should be reported

SPIROMETRY
 Maximal Inspiratory Pressure (MIP)
› The lowest pressure developed during a
forceful inspiration against an occluded
airway
 Primarily measures inspiratory muscle strength

Spirometry & Related Testing
Equipment
 Respiratory Pressure Manometers
› Measures MIP and MEP

Spirometry & Related
Testing Equipment
 Maximal Inspiratory Pressure (MIP)
› The lowest pressure developed during a
forceful inspiration against an occluded
airway
 Primarily measures inspiratory muscle strength
› Usually measured at maximal expiration
(residual volume)
› Can be measured at FRC
› Recorded as a negative number in
cm H20 or mm Hg, e.g. (-60 cm H2O)

MIP
Significance and Pathophysiology
› Healthy adults > -60 cm H2O
› Decreased in patients with:
 Neuromuscular disease
 Diseases involving the diaphragm, intercostal, or
accessory muscles
 Hyperinflation (emphysema)
› Sometimes used to measure response to respiratory
muscle training
› Often used in the assessment of respiratory muscle
function in patients who need ventilatory support

Maximal Expiratory Pressure
(MEP)
› The highest pressure developed during a
forceful exhalation against an occluded airway
 Dependent upon function of the abdominal muscles,
accessory muscles of expiration, and elastic recoil of
lung and thorax
› Usually measured at maximal inspiration (total
lung capacity)
› Can be measured at FRC
› Recorded as a positive number in cm H20 or
mm Hg

Significance and Pathophysiology
› Healthy adults >80 to 100 cm H2O
› Decreased in:
 Neuromuscular disorders
 High cervical spine fractures
 Damage to nerves controlling abdominal and
accessory muscles of inspiration
› A low MEP is associated with inability to cough
 May complicate chronic bronchitis, cystic fibrosis, and
other diseases that result in excessive mucus production
MEP

Spirometry & Related Testing
Equipment
 MIP

SPIROMETRY
 Airway Resistance (Raw)
› The drive pressure required to create a
flow of air through a subject’s airway
› Recorded in cm H2O/L/sec
› When related to lung volume at the time
of measurement it is known as specific
airway resistance (SRaw)

SPIROMETRY
 Raw
› Measured in a
plethysmograph
as the patient
breathes through
a pneumo-
tachometer

SPIROMETRY
 Raw
› Criteria of Acceptability
 Mean of three or more acceptable efforts
should be reported; individual values
should be within 10% of mean

SPIROMETRY
Normal Adult Values
Raw 0.6 – 2.4 cm H2O/L/sec
SRaw 0.190 – 0.667 cm H2O/L/sec/L

SPIROMETRY
› May be increased in:
 Bronchospasm
 Inflammation
 Mucus secretion
 Airway collapse
 Lesions obstructing the larger airways
 Tumors, traumatic injuries, foreign bodies

SPIROMETRY
 Raw
Significance and Pathology
› Increased in acute asthmatic episodes
› Increased in advanced emphysema because of
airway narrowing and collapse
› Other obstructive disease, e.g., bronchitis may
cause increase in Raw proportionate to the
degree of obstruction in medium and small
airways

SPIROMETRY
 Airway Conductance (Gaw)
› A measure of flow that is generated from
the available drive pressure
› Recorded in L/sec/cm H2O
› Gaw is the inverse of Raw
› When related to lung volume at the time
of measurement it is known as specific
airway conductance (SGaw)

SPIROMETRY
 Gaw
› Measured in a
plethysmograph
as the patient
breathes through
a pneumo-
tachometer

SPIROMETRY
 Gaw
› Criteria of Acceptability
 Mean of three or more acceptable efforts
should be reported; individual values
should be within 10% of mean

SPIROMETRY
Significance and Pathology
 SGaw Values <0.15 – 0.20 L/sec/cm
H2O/L are consistent with airway
obstruction

SPIROMETRY
Normal Adult Values
Gaw 0.42 – 1.67 L/sec/cmH2O
SGaw 0.15 – 0.20 L/sec/cm H2O/L

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