Lung Volumes and Gas Distribution

1
Lung Volumes and Gas Distribution

• RET 2414
• Pulmonary Function Testing
• Module 3.0

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Lung Volumes / Gas Distribution

• Objectives
• Describe the measurement of lung volume using
  direct and indirect spirometry
• Explain two advantages of measuring lung volumes
  using the body plethysmograph

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Lung Volumes / Gas Distribution

• Objectives
• Calculate residual volume and total lung capacity
  from FRC and the subdivisions of VC
• Identify restriction from measuring lung volumes

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Lung Volumes / Gas Distribution

• Direct Spirometry
• Used to measure all volumes and capacities EXCEPT
  for RV, FRC and TLC

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Lung Volumes / Gas Distribution

• Indirect Spirometry
• Required for the determination of RV, FRC and TLC
• Most often, indirect spirometry is performed to
  measure FRC volume
• FRC is the most reproducible lung volume and it
  provides a consistent baseline for measurement

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Lung Volumes / Gas Distribution

• Indirect Spirometry
• Two basic approaches
• Gas dilution
• Body plethysmography
• Measurements are in Liter or Milliliters
• Reported at BTPS

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Lung Volumes / Gas Distribution

• Gas dilution techniques
• All operate on a principle SIMILAR to Boyles Law
  (P1 V1 P2 V2), which states,
• In isothermic conditions, the volume of a gas
  varies inversely with its pressure
• Fractional concentration of a known gas is used
  instead of its partial pressure
• C1 V1 C2 V2

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Lung Volumes / Gas Distribution

• Gas dilution techniques
• By having a known (or measured) gas concentration
  at the start and end of the study and a single
  known volume, the unknown volume can be
  determined. For example
• V1 C2 V2
• C1

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Lung Volumes / Gas Distribution

• Gas dilution techniques
• Can only measure lung volumes in communication
  with conducting airways !!!

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Lung Volumes / Gas Distribution

• Gas dilution techniques
• Obstruction or bullous disease can have trapped,
  noncommunicating air within the lungs
• FRC may be measured as being less than its actual
  volume

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Lung Volumes / Gas Distribution

• Open-Circuit Nitrogen Washout
• The natural volume of nitrogen in the subjects
  lungs at FRC is washed out and diluted with 100
  oxygen
• Test must be carefully initiated from the FRC
  baseline level

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Lung Volumes / Gas Distribution

• Open-Circuit Nitrogen Washout
• All exhaled gas is collected in a Tissot (large
  volume) spirometer for measurement of its volume
• Analyzer in the breathing circuit monitors
  nitrogen concentrations

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Lung Volumes / Gas Distribution

• Open-Circuit Nitrogen Washout

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Lung Volumes / Gas Distribution

• Open-Circuit Nitrogen Washout

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Lung Volumes / Gas Distribution

• Open-Circuit Nitrogen Washout
• Approximately 3-7 minutes of breathing 100 O2 to
  wash out N2 from the lungs
• If oxygen-induced hypoventilation is a documented
  problem (as in COPD), a different method of FRC
  determination is needed

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Lung Volumes / Gas Distribution

• Open-Circuit Nitrogen Washout
• Test is successfully completed when the N2 levels
  decrease to become less than 1.5 for at least 3
  successive breaths (subjects without obstructive
  disorders)
• Premature discontinuation may occur due to
• System leak
• Patient unable to continue
• Tissot spirometer is full

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Lung Volumes / Gas Distribution

• Open-Circuit Nitrogen Washout
• The FRC has a N2 concentration of approximately
  0.75, based on the atmospheric nitrogen minus CO2
  and water vapor at BTPS
• (CAlvN2) 0.75

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Lung Volumes / Gas Distribution

• Open-Circuit Nitrogen Washout
• The final collected volume of exhaled gas in the
  Tissot spirometer
• (VExh)
• Has a measurable concentration of N2
• (CExhN2)

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Lung Volumes / Gas Distribution

• Open-Circuit Nitrogen Washout
• FRC determination is based on the following
  equation
• VFRC (CExhN2)(VExh)
• CAlvN2

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Lung Volumes / Gas Distribution

• Open-Circuit Nitrogen Washout
• In the actual FRC determination by this method,
  the calculation is more complex
• Do not get scared !
• You will not be asked to do the calculation!

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Lung Volumes / Gas Distribution

• Open-Circuit Nitrogen Washout
• The small final concentration of alveolar N2
  remaining in the lung needs to be subtracted from
  the original CalvN2
• Deep breath of O2 at the end of the test and
  slowly exhaled. The end-expiratory CN2 is used
  as the CFN2
• (This volume should not be exhaled into the
  spirometer)

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Lung Volumes / Gas Distribution

• Open-Circuit Nitrogen Washout
• The second correction is the volume of nitrogen
  released from the body tissues during the washout
  procedure (body tissue N2 factor or BTN2)
• Rages from 30 50 ml/minute of the washout
  procedure (TTest)

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Lung Volumes / Gas Distribution

• Open-Circuit Nitrogen Washout
• Final Calculation
• VFRC (CExhN2 X (VExh VD) ) - BTN2 Factor X
  TTest
• CAlvN2 CFN2
• Must be BTPS converted
• Test can be repeated after 15 minutes (longer if
  COPD)

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Lung Volumes / Gas Distribution

• Open-Circuit Nitrogen Washout
• Modern computer-operated pneumotachometer systems
  do not require collection of total VExh or
  measurement of the CExhN2
• Breath-by-breath CExhN2 and VExh measurements are
  made

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Lung Volumes / Gas Distribution

• Open-Circuit Nitrogen Washout

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Lung Volumes / Gas Distribution

• Open-Circuit Nitrogen Washout

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Lung Volumes / Gas Distribution

• Open-Circuit Nitrogen Washout
• Leak

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Lung Volumes / Gas Distribution

• Open-Circuit Nitrogen Washout
• Criteria for Acceptability
• The washout tracing/display should indicate a
  continually falling concentration of alveolar N2
• The test should be continued until the N2
  concentration falls to lt1.5 for 3 consecutive
  breaths

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Lung Volumes / Gas Distribution

• Open-Circuit Nitrogen Washout
• Criteria for Acceptability
• Washout times should be appropriate for the
  subject tested. Healthy subjects should washout
  N2 completely in 3-4 minutes
• The washout time should be reported. Failure to
  wash out N2 within 7 minutes should be noted

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Lung Volumes / Gas Distribution

• Open-Circuit Nitrogen Washout
• Criteria for Acceptability
• Multiple measurements should agree within 10
• Average FRC from acceptable trials should be used
  to calculate lung volumes
• At least 15 minutes of room-air breathing should
  elapse between repeated trials, gt1 hour for
  patients with severe obstructive or bullous
  disease

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Lung Volumes / Gas Distribution

• Closed-Circuit Helium Dilution
• FRC is calculated indirectly by diluting the gas
  in the lungs at the end-expiration level with a
  known concentration of helium (an inert gas)

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Lung Volumes / Gas Distribution

• Closed-Circuit Helium Dilution

FRC
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Lung Volumes / Gas Distribution

• Closed-Circuit Helium Dilution

• Procedure
• Spirometer is filled with a known volume of air
  with added oxygen of 25 30
• A volume of He is added so that a concentration
  of approximately 10 is achieved
• System volume (spirometer, tubing) and He
  concentration are measured

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Lung Volumes / Gas Distribution

• Closed-Circuit Helium Dilution
• C1 V1 C2 V2
• (C1 initial He concentration)(V1 system volume)

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Lung Volumes / Gas Distribution

• Closed-Circuit Helium Dilution

• Procedure
• The patient breathes through a free-breathing
  valve that allows either connection to both room
  air or the rebreathing system
• The patient is switched into the rebreathing
  system at end-expiration level (FRC)
• The patient rebreathes the gas in the spirometer,
  until the He concentration falls to a stable level

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Lung Volumes / Gas Distribution

• Closed-Circuit Helium Dilution

O2 Added
CO2 Absorbed
H2O Absorbed
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Lung Volumes / Gas Distribution

• Closed-Circuit Helium Dilution

He Concentration
System Volume
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Lung Volumes / Gas Distribution

• Closed-Circuit Helium Dilution

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Lung Volumes / Gas Distribution

• Closed-Circuit Helium Dilution

• Procedure
• Once the He reaches equilibrium between the
  spirometer and the patient, the final
  concentration of He is recorded
• The FRC can then be calculated

FRC
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Lung Volumes / Gas Distribution

• Closed-Circuit Helium Dilution
• C1 V1 C2 V2

(CIHe)(SV)
(CFHe)
(FRC)
FRC (HeInitial - HeFinal) x System
volume HeFinal
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Lung Volumes / Gas Distribution

• Closed-Circuit Helium Dilution
• Volume Corrections
• A volume of 100 ml is sometimes subtracted from
  the FRC to correct loss of He to the blood
• The dead space volume of the breathing valve and
  filter should be subtracted from the FRC

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Lung Volumes / Gas Distribution

• Closed-Circuit Helium Dilution
• Criteria for Acceptability
• Spirometer tracing should indicate no leaks
  (detected by a sudden decrease in He), which
  would cause an overestimation of FRC
• Test is successfully completed when He readings
  change by less than 0.02 in 30 seconds or until
  10 minutes has elapsed

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Lung Volumes / Gas Distribution

• Closed-Circuit Helium Dilution
• Criteria for Acceptability
• Multiple measurements of FRC should agree within
  10
• The average of acceptable multiple measurements
  should be reported

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Lung Volumes / Gas Distribution

• Body Plethysmography (BP)
• Measurement of FRC by body plethysmograph is
  based on an application of Boyles law
• P1V1 P2V2
• or
• V1 P2V2
• P1

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Lung Volumes / Gas Distribution

• Body Plethysmography (BP)
• Unlike gas dilution tests, BP includes both air
  in communication with open airways as well as air
  trapped within noncommunicating thoracic
  compartments
• In patients with air trapping, plethysmography
  lung volumes are usually larger those measured
  with gas dilution methods
• Volume measured is referred to as thoracic gas
  volume (TGV or VTG)
• ATS is recommending term be dropped and changed
  to plethysmographic lung volume (VL, pleth),
  and FRC by body plethysmography or TGV at FRC
  (FRCpleth)

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Lung Volumes / Gas Distribution

• Body Plethysmography (BP)

• Procedure
• Patient is required to support cheeks with both
  hands and pant with an open glottis at a rate of
  0.5 - 1 Hz (30 60 breaths/min)
• BP shutter is suddenly closed at end-expiration
  prior to inspiration
• Panting is continued for several breaths against
  closed shutter (no air flow)

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Lung Volumes / Gas Distribution

• Body Plethysmography (BP)

• Procedure
• The thoracic-pulmonary volume changes during
  panting produce air volume changes within the BP
  cabinet
• Decreases in cabinet volume are an equal inverse
  response to thoracic volume increase (As thoracic
  volumes increase with panting inspiration, BP
  cabinet volume decreases and visa versa)

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Lung Volumes / Gas Distribution

• Body Plethysmography (BP)

• Criteria of Acceptability
• Panting maneuver shows a closed loop without
  drift
• Tracing does not go off the screen
• Panting is 0.5 1 Hz
• Tangents should be within 10
• At least 3 FRCpleth values should agree within 5
  and the mean reported

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Lung Volumes / Gas Distribution

• Body Plethysmography (BP)
• Airway Resistance (Raw) and Specific Airway
  Conductance (SGaw) can be measured simultaneously
  during open-shutter panting (1.5-2.5 Hz)
• Most plethysmographs have built-in
  pneumotachometers and allow VC maneuvers to be
  performed during the same testing session

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Lung Volumes / Gas Distribution

• Single-Breath Nitrogen Washout
• Measures Distribution of Ventilation
• Closing Volume
• Closing Capacity

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Lung Volumes / Gas Distribution

• SBN2 (SBO2)
• Equipment

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Lung Volumes / Gas Distribution

• SBN2
• Procedure
• Patient exhales to RV
• Inspires a VC breath of 100 O2
• Patient exhales slowly and evenly (0.3-0.5L/s)
• N2 concentration is plotted against volume

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Lung Volumes / Gas Distribution

• SBN2
• Phase I upper airway gas from anatomical dead
  space (VDanat), consisting of 100 O2
• Phase II mixed airway gas in which the relative
  concentrations of O2 and N2 change abrubtly as
  VDanat volume is expired

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Lung Volumes / Gas Distribution

• SBN2
• Phase III a plateau caused by the exhalation of
  alveolar gas in which relative O2 and N2
  concentrations change slowly and evenly
• Phase IV an abrupt increase in the concentration
  of N2 that continues until RV is reached

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Lung Volumes / Gas Distribution

• SBN2
• ? N2 750 1250
• Is 1.5 or less in healthy adults up to 3 in
  older adults
• Increased ? N2 750 1250 is found in diseases
  characterized by uneven distribution of gas
  during inspiration or unequal emptying rates
  during expiration.
• Patients with severe emphysema may exceed 10

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Lung Volumes / Gas Distribution

• SBN2
• Slope of Phase III
• Is an index of gas distribution
• Values in healthy adults range from 0.5 to 1.0
  N2/L of lung volume

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Lung Volumes / Gas Distribution

• SBN2
• Closing Volume
• The onset of Phase IV marks the lung volume at
  which airway closure begins
• In healthy adults, airways begin closing after
  80-90 of VC has been expired, which equates to
  30 of TLC
• Reported as a percentage of VC

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Lung Volumes / Gas Distribution

• SBN2
• Closing Capacity
• If RV has been determined, CV may added to it
  and expressed at Closing Capacity (CC)
• CC is recorded as a percentage of TLC

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Lung Volumes / Gas Distribution

• SBN2
• Normal Values for CC and CV
• ________________________________
• Male Female
• CV/VC 7.7 8.7
• CC/TLC 24.8 25.1

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Lung Volumes / Gas Distribution

• SBN2
• CV and CC may be increased, indicating earlier
  onset of airway closure in
• Elderly patients
• Smokers, early obstructive disease of small
  airways
• Restrictive disease patterns in which FRC becomes
  less than the CV
• Congestive heart failure when the caliber of the
  small airways is compromised by edema

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Lung Volumes / Gas Distribution

• SBN2
• Acceptability Criteria
• Inspired and expired VC should be within 5 or
  200 ml
• The VC during SBN2 should be within 200 ml of a
  previously determined VC
• Expiratory flows should be maintained between 0.3
  and 0.5 L/sec.
• The N2 tracing should show minimal cardiac
  oscillations