Lung volumes + Gas distribution tests ppt

TLC definition

Aka absolute lung volumes, being a volume of lung that cannot exceed a certain volume (aka being a maximum that other lung volumes are based upon)

Two types of ways to measure volumes + capacities

Indirect + direct spirometry

What does direct spirometry measure

All volumes + capacities EXCEPT for RV, FRC, and TLC

What does indirect spirometry measure?

RV, FRC, and TLC, being performed usually to measure FRC volume.

FRC is the MOST REPRODUCIBLE VOLUME + provides baseline

Types of ways TLC measures

1. Gas dilution techniques: nitrogen washout and helium dilution

2. Body plethysmography.

What can dilution techniques provide information about

gas distribution in lungs

How does gas dilution work?

Have a KNOWN gas concentration @ start (C1) AND end (C2) of study and a SINGLE KNOWN volume (V1) to determine unknown (V2).

To achieve this, use either N2 or He as gas (doesn’t cross AC membrane). If we know what concentration goes in and out of body, and the volume in and out of body, can learn what volume remains w/I body. Algebra!

Where can you only measure lung volumes in dilution techniques?

In conducting airways! Only works if gas is NOT trapped, as obstruction or bullies diseases can produce noncommunicating air w/I the lungs.

How can RV be measured?

Indirectly once FRC or TLC is determined like the body box.

RV = FRC - ERV

Nitrogen washout process

Individual inhales 100% O2 and then exhales into a N2 sampler (Tissot), measuring % of N2 exhaled and total volume of air exhaled. This uses O2 to washout N2, which takes several breaths due to the need to also remove N2 from RV as well.

Mathematical theory behind this includes:
C1 = N2 concentration in FRC @ start (79%)
C2 = N2 concentration in exhaled volume
V1 = FRC volume (value x)
V2 = total volume exhaled during test

How is N2 washout a open circuit method

Pt inhales from O2 source but DOES NOT REBREATHE a known gas concentration, as they breathe in 1 system + then breathe in. through a 2nd system. This is done through a + shaped tubing system that directs passageway of gas to prevent rebreathing: O2 from the left → pt. mouth up top → N2 out the right.

This allows N2 to be eliminated from lungs w/ pure O2, as FRC has a N2 concentration of approximately 0.75, due to BTPS mixes N2 w/ CO2 and H2O, causing decrease in N2 %.

Where should N2 washout test be initiated at?

Carefully from FRC baseline level.

How to establish end tidal point in N2 washout?

End tidal point, or where FRC begins, happens when pt breathes normally into system + breathes 100% O2,

When is the N2 washout test terminated and its contraindication?

1. Exhaled N2 is <1.5%

2. leaks detected

3. pt unable to continue

   Contraindication: COPD, as they rely on low O2 to prevent O2 from going to bad alveoli.

How long should pt be breathing in N2 washout?

3-7 minutes of 100% O2 breathing to washout N2 from lungs (30-40mL/min of N2 for total washout time)

N2 washout equipment

1. Tissot spirometer ( + shaped)

2. Newer system uses breath by breath N2 analyzer + flowmeter to plot exhaled volume.

Calculation of FRC and RV

FRC = (exhaled volume)(FEN2) / (FAN2ini - FAN2final)
V2 V1 C2 C1 C2

Exhaled volume = volume collected during test
FEN2 = %N2 collected
FAN2ini = %N2 in alveoli @ start (~75%)
FAN2final = %N2 in alveoli @ end

N2 washout acceptability

Graph should indicate falling concentration of alveolar N2 and should continue to fall until <1.5% for 3 consecutive breaths.

Disadvantages for N2 washout

1. beginning alveolar N2 is estimated @78%, but cannot be measured

2. N2 is more soluble in tissues, so it participates in gas exchange

3. Can underestimate lung volumes in obstructed pt b/c does NOT MEASURE TRAPPED GAS! So cannot measure RV.

He deletion theory

Normal person exhales 80% of TLC, so RV remains. This happens bc re-expansion of collapsed lung takes tremendous energy, so it is difficult to measure RV when you cannot exhale it.

How to indirectly measure RV

Start w/ known volume + concentration of inert gas that does NOT PARTICIPATE IN GAS EXCHANGE and then allow equilibrium of mixture.

He dilution system

CLOSED circuit system where spirometer is filled w/ mixture of He and O2 (cannot breathe pure N2 bc you’ll die). It is a closed circuit that prevents rebreathing due to use of CO2 scrubber to remove CO2.

He dilution process in regards to pt

1. Pt breathes in mixture through free-breathing valve, starting from FRC, which is the gas volume in lung after normal breath. This valve allows connection to both room air OR rebreathing system

2. Pt switched into rebreathing system @ end-exp level (FRC)

3. Pt rebreathes gas in spirometer

4. He then spreads into lungs and settles until STABLE level

5. Once He is @ equilibrium b/w spirometer + pt., final concentration is recorded. This then allows for calculation of C2 because of the difference of He from start to end, and FRC calculation as well.

He dilution process in regards to system

1. Spirometer is filled w/ known volume of AIR w/ 25-30% O2

2. Volume of He is added so concentration of ~10% is achieved

3. System volume + He concentration are then measured.

Values known/calculated in He dilution

C1 and V1 = amount of He in spirometer is known @ beginning of test
C2 = spread of He into lungs of patient
FRC = calculated due to no leak + constant He concentration.

He dilution calculaiton/equations

C1V1 = C2V2

(CIHe)(Sv) = (CFHe)(FRC)

FRC = (%Heini - %Hefin) x system volume

Volume corrections in He dilution

Volume of 100mL can be subtracted from FRC to correct loss of He to blood. DS volume of breathing valve + filter should be subtracted from FRC

Acceptability of He dilution and what is reported?

1. spirometer tracing indicates 0 leaks (sudden decrease in He), which would OVERESTIMATE FRC

2. Test is completed when He readings change AND IS SUSTAINED by <0.02% in 30s OR until 10min elapsed

3. Multiple FRC tests w/I 10%

Reported: AVERAGE of acceptable multiple measurements

Common problems w/ He dilution

1. Gas leaks (tubing, around mouth, ears)

2. Inadequate CO2 absorber (resp pattern is erratic)

3. excessive moisture (He meter becomes erratic)

4. Excess blower pressure/flow (He meter becomes erratic)

Limitations of He dilution

1. Variation in ERV in COPD pt (uncomfortable to exhale that much)

2. method ignores trapped gas

3. elevation of FRC

Significance of He dilution

1. Decreased FRC (restricitve disease, as lung volumes are equally reduced which preserves RV/TLC relationships

2. Pneumonia (occluded alveoli)

What method is best b/w dilution, body pleths and imaging?

1. dilutions: less coordination BUT underestimates lung volumes w/ obstruction + leaks

2. pleth: includes trapped air, not affected by quality of ventilation, rapid, repeatable BUT overestimates lung volumes if pt has poor technique

3. imaging: does not require PFT equipment BUT requires breath hold @ TLC + is expensive