6 - Lung Volumes & Gas Distribution

measures of lung function

• total lung capacity (TLC)

• functional residual capacity (FRC)

• expiratory reserve volume (ERV)

• residual volume (RV)

total lung capacity (TLC)

amount of air in lungs after maximal inspiration

• normal % predicted: 80-120%

• increased in obstructive disease due to air trapping

  • __ = SVC + RV

  • __ = FRC + IC

• < 80% predicted = restrictive disease

functional residual capacity (FRC)

gas left in lungs after normal expiration

expiratory reserve volume (ERV)

maximum air in lungs exhaled from V_T

residual volume (RV)

gas left in lungs after complete exhalation

• obtained from TLC studies

  • FRC − ERV = __

• increased in air trapping

static lung volumes

assessment of air trapping and hyperinflation in obstructive disease

• increased RV and FRC, maybe increased TLC

static lung volumes for hyperinflation

• ↑RV with ↑TLC

• RV/TLC < 35%

ex: emphysema

static lung volumes for air trapping

• ↑RV with normal TLC

• RV/TLC > 35%

ex: asthma

3 methods to measure FRC

• closed-circuit helium dilution (multi-breath)

• open-circuit nitrogen washout (multi-breath)

• body plethysmography

closed-circuit helium dilution

method of measuring FRC

• spirometer filled with known volume of air

  • approx. 10-14% __

    • __ analyzer: thermal conductivity

  • approx. 21% O₂

• CO₂ is removed

• uses rebreathing system

technique for closed-circuit helium dilution

1. record initial helium concentration

2. start test at FRC

3. continue until equilibrium is reached between patient & spirometer (3-7 minutes)

4. record final concentration of helium

calculations for closed-circuit helium dilution

SV = He added ÷ % He_initial

FRC = [(% He_initial − % He_final) × SV] ÷ % He_final

SV = system volume; volume of spirometer, circuit, and valves

acceptability criteria for closed-circuit helium dilution

• stable % helium before starting test; no leaks

• regular breathing

• continue until % helium changes < 0.02% in 30 seconds

• O₂ added is appropriate for normal tidal breathing

• smooth regular fall of helium on curve

• if multiple measurements:

  • wait 5-10 minutes in between attempts

  • FRCs should be within 10% of each other

  • average FRC reported

open-circuit nitrogen washout

method of measuring FRC

• requires minimal effort and learning

• patient breathes 100% O₂ to “washout” __

• _₂ analyser

  • emission (Geissler tube) or mass spectrometry

technique for open-circuit nitrogen washout

1. measure initial N₂ %

2. at FRC, breath 100% O₂

3. N₂ volume and % measured with each exhalation

4. continue until F_eN₂ is ≤ 1.5% for 3 breaths (≥ 7 minutes)

5. measure ending N₂ % and total volume washed out

(F_eN₂ = fraction of expired N₂)

calculations for open-circuit nitrogen washout

• starting F_eN₂: 75-80%

• ending F_eN₂: 1-1.5%

• formula:

FRC = (F_eN_2.final × expired volume − N_2.tissue) ÷ (F_AN_2.initial − F_AN_2.final)

(N_2.tissue = volume of N₂ washed out of tissue [blood])

acceptability criteria for open-circuit nitrogen washout

• indication of continually falling N₂

  • continuous until N₂ concentration fall to 1.5%

• time should be appropriate for lung disease

  • washout time reported

  • normal washout time 3-4 minutes; ≥ 7 minutes significant

• ≥ 1 acceptable FRC needed

  • if needing to repeat, wait 15 minutes

  • multiple measures within 10% of each other

  • mean FRC reported

troubleshooting for nitrogen washout: leak in N₂ system

increases in either inhaled/exhaled F_eN₂ by 1% or larger indicates leak

• stop test and repeat

• leaks overestimate volume

• use nose clips to prevent leaks

calculating RV from FRC

if FRC is known and IC and ERV are obtained by spirometry:

• __ = FRC − ERV

notes on dilution techniques

• RV is function of measured TLC

  • resting and end-expiratory levels are less patient dependent

• must measure end-expiratory levels and ERV accurately

• accuracy depends on distribution of ventilation

  • poorly-ventilated areas/air trapping can underestimate lung volumes

  • moderate-to-severe obstruction

• calibrate gas analyzers

body plethysmography (“body box”)

lung volume technique

• measures total gas volume in thorax

  • total gas volume (TGV) when not referenced to a starting point

  • FRC_pleth when measured from normal end-expiratory level

• pneumotachometer with pressure transducer

  • measures pressure change at mouth

• plethysmograph has sensitive pressure transducer

  • measures volume changes

• uses Boyle’s Law

  • volume of gas varies inversely to pressure if temperature is constant

  • P₁V₁ = P₂V₂

body plethysmography technique

1. Patient is acclimated to box (temperature regulation).

2. Patient breathes quietly with nose clips (holds cheeks).

   1. Monitor tidal breathing

3. Patient pants with shutter open (~1 Hz).

4. Shutter closes; patient continues panting with no airflow.

   1. Measurement taken.

5. Test begins/ends with spirometry.

body box calculations

• change in volume = change in body box volume when panting

• change in pressure = change in alveolar pressure when panting

FRC = (V₂ × P_B) ÷ P₁

acceptability criteria for body box

• panting shows closed loop without drift or artifact

• pressure changes are within calibration range

• panting frequency: 0.5-1 Hz

• 3-5 successful panting maneuvers recorded

• FRC average within 5%

• report average from 3 acceptable maneuvers

variables that affect FRC

• body size

• body position

• time of day (diurnal variations)

  • best done mid-afternoon, worse during morning

• race/ethnicity

body box pathologies

• increased FRC

• pathological

• >120% predicted = air trapping

  • chronic COPD/asthma

  • compensation for lung tissue removal/thoracic deformity

  • leads to mechanical/muscular inefficiency

body box pathologies

• increased RV

• usually results in a decrease in VC

  • acute asthma (reversible)

  • emphysema and bronchial obstruction

  • can result in air trapping

• usually increases as FRC increases

body box pathologies

• normal/increased TLC

• indicates obstruction

• 2 patterns:

  • increased RV + normal __, VC reduced

    • air trapping

  • increased RV + increased __, VC preserved

    • hyperinflation

body box pathologies

• decreased FRC, RV, and TLC

indicates restriction

• examples:

  • ILDs

  • chest wall and neuromuscular disorders, including obesity

  • when many alveoli are occluded (pneumonia, CHF)

body box pathologies

• reduced VC with normal/increased FEV₁/FVC

• pure restrictive disease

• proportional decreases in most lung volumes

• reduced TLC needed to confirm

body box pathologies

• decreased TLC

any process that occupies space in lungs

• examples:

  • edema, pulmonary congestion, pleural effusion, atelectasis, pneumothorax, tumors/lesions, thoracic deformities

body box pathologies

• RV/TLC ratio

• normal: 20% (young)—35% (old)

• if <35%, check RV and TLC

• air trapping

  • increased RV, normal TLC

  • high ratio (>35%)

• hyperinflation

  • increased RV and TLC