Other Tests of Pulmonary Mechanics

Airway Resistance (Raw)

• Pressure change per unit flow as gas flows into and out of lungs

• measure in cmH2O /L / sec

Raw=\frac{Pressure}{Flow}

Raw influenced by

• size of airway

• number of airways

• elastic recoil

• the lung volume at which it is measured

  • sRaw: specific airways resistance

  • sRaw = Raw x VTG ( resistance at VTG)

  • sRaw is stable across changing lung volumes

Raw during insp

• during quiet breathing most of the resistance is in the trachea and large airways

• during inspiration intrathoracic airways increase in diameter and resistance decreases

• The opposite happens on exhalation

Resistance is distributed as follow

• nose, mouth, and upper airways = 50%

• Trachea and bronchi = 30%

• small airways = 20% (<2 mm in dia)

Airway Conductance (Gaw)

• Conductance is the reciprocal of airway resistance ( 1/Raw)

• flow generated per unit of pressure drop across the airways

• Measure in L/sec/CmH2O

sGaw=\frac{Gaw}{VTG}

NOTES: may see increase in sGaw as an indication for positive response response to bronchodilator even if there’s no obvious improvement FEV1. Therefore, very good objective test for bronchodilator test for bronchodilator response testing

Produce

• place patient in body box if doing an VTG, close door

• Ask patient to perform normal tidal breathing

• Once rhythmic, ask patient to pant with small VT rate 1.5 - 2.5Hz… Prove graphic plot V/P_Box

• once plot is complete, close shutter and ask patient to continue panitng against the losed shutter at rate of ~1.0 Hz.

• If performing sRaw or SGaw, then will need to make sure FRC is calculated from each of the tests and sGaw is calculated with values from the same test

• Report the mean of 3, or more, acceptable tests (test should be within 10%).

Interpretation of results

• good tests?

• are individual Raw and sGaw values within 10%?

• if Raw > 2.4 cmH2O/L/sec & / or Gaw < 0.15L/ sec /cmH2O/L suspect OBSTRUCTION

• distinguish large verus small airway obstruction by correlasting with other spirometry finding.

Resistance Measurement via FOT

Forced Oscillation Technique

• Useful method to measure airway resistance in patients unable to perform spirometry or a panting maneuver

• the patient only require to perform quiet breathing throughout the test

what is FOT

• technique applies oscillating gas flow, at wide range of frequencies (5 -35hz) to the respiratory system

• Oscillations are create by built-in loudspeaker or vibrating mesh

• Resulting pressure from oscillating flow is measured at the mouth

• complex magthematchicals operations are performed to calculate the resistance of the re

what type of tracing is this

Normal tracing

what type of tracing is this

Thermal drift

what type is this

Panting too fast with small volumes

what type is this

slow panting or large volumes

is this high inspiratory resistance or low

high

high or low expiratory resistanace

High exp resistance

which type fo dx tracing

HIgh inspiratory & expiratory resistance

what is this tracing

Overall increase resistance asthma

FOT procedure

• describe the procedure to the patient

• have the patient breathe normally for 30-60s

  • neck should be slightly extended

  • hands should be placed on patient’s checks

• report the mean of at least 3 acceptable test

  • adult tests be within 10% mean

  • peds tests should be within 15% of mean

compliance (C_L)

• volume change per unit of pressure change for the lung, the thorax or the lung thorax together

• measurement of elastic resistance (elastane or elastic recoil)

  • elastic recoil pressure of the lungs = transpulomnary pressure (Ptp)

• other major factor determining maximal airflow (elastic recoil)

• measures in L/cmH2O

Lung compliance (C_L)

• amount of transpulmonary pressure change required to produce a change lung volume

• static lung compliance measurements can be mdae in the plethysmorgraph

• this is not commonly performed test

P_tp = P_A - P_PL

Technique

• patient inhales to TLC and then exhales exhales to stabilized lung compliance

• inhale again to TLC and then slowly exhales to FRC

• shutter closes during exhalation at various points for 1 to 2s (zero flow)

• during shutter closure pressure and volume change are measured

• data is plotted on volume/pressure curve

• values from calculation are taken at FRC and at +0.5L above FRC

C_L formula

decrease compliance

• kyphoscoliosis

• rheumatoid spondylitis

• severe obesity

increase compliance

obstructing lung disease.. ex emphysema, bronchiolitis obliterans

Review: distribution of tidal ventilation

• breathing from FRC

  • At FRC, most gas are the Apices

  • with tidal breathing, most gases preferentially ventilate the bases

• When breathing TLC

  • alveoli in the apices and bases are about the same size they are are fully open

• when breathing at RV

  • airways at the bases are compressed and closure occurs

  • When breath is inhaled from RV the air will preferentially go to the apices first as teh based are closed

single breath nitrogen wash washout is AKA

Fowler’s test

Single breath nitrogen washout

• measure the distribution of ventilation

  • helps explain abnormal GE in patient with normal PFT’s

  • may detect small airway disease

• use similar equipment as those used for the Open circuit Nitrogen washout FRC test

technique of single breath

1. have patient exhale to RV

2. then place patient onto the system and instruct patient to inspire a VC breath of 100% oxygen to TLC

3. immediately ask patient to exhale slowly and evenly at 0,3 - 0.5 L/sec back to RV (*MAKE SURE PATIENT DOESN’T DO A BREATH HOLD BEFORE EXHALING)

technical ntoes

• N2 conc. is measured by an N2 analyzer

• exhale volume measured by a spirometer

• volume expired is plotted against N2 conc. on a graph

Acceptability Criteria

• inspired and expired VC within 5% or 200ml

• VC from SBN₂ within 200 ml or 5% of VC from spirometry

• expiratory flow between 0.3 - 0.5L/sec

• washout tracing shows minimal cardiac oscillations

interpretation and significance

• evenness of distribution is assess by 2 parameters

• % change of N2 between 750ml – 1250 ml (∆

  %N_{2 750-1250})

  • 1.5% or less in healthy young adults up to 3.0% for older adults

  • ncreased = uneven distribution of gas on inspiration or emptying rates on exhalation. May be > 10% in emphysematous patients.

• Slope of Phase III of the washout curve (best fit line from point where 30% VC remains above RV to the onset of phase IV)

  • 0.5% - 1.0%, with wide variability depending on lung

    volumes

    Accuracy of this phase is affected if the expiratory flow is too slow which causes oscillation in the tracing

  • patient with small VC may also not exhale enough gas to make phase III meaningful

  • slope steeper with poor distribution of ventilation

What else is measured with SBN₂

Closing volume (CV) = the part of then vital capacity that can be exhaled from the lungs after lower airway closure (beginning of phase IV to RV)

Can also measure closing capacity(CC) the sum of CV and RV

Significance of SBN₂ measurements

• ∆%N2 750-1250 < 1.5% in healthy, ~3% in elderly, in severe emphysema may be > 10%

• Slope of Phase III = 0.5 – 1.0% N2/L, with wide variability.

• Onset of Phase IV = beginning of airway closure (Normal = 80 -90% of VC), therefore, normal CC ~30% TLC

• CV/VC% > 20% = suspect small airway abnormalities, but must correlate this with clinical findings

Calculating RV from from SBN2 test

FEN2 = mean expired N2 conc. determined by integration of

the area under the curve

FAN2 = N2 conc. in the lungs at the beginning of inspiration,

~0.75-0.79

FRC or Thoracic Gas Volume

An indirectly measured value

Varies with body size, position, and time of day

Increase represents air-trapping

Decrease typically seen in restrictive diseases

RV

• a calculated value

• FRC - ERV

• increase in obstructive disease states, air trapping

• RV and FRC usually increase together

TLC

• calculated value TLC = RV + VC OR TLC = FRC + IC

• decrease in restrictive diseases

• may increase in obstructive diseases called hyperinflation

RV?TLC ratio (RV/TLC%)

• normal 20 -30%

• increase RV/TLC ratio and increased TLC indicates hyperinflation

• increase RV/TLC ratio and normal TLC indicated air trapping

Diffusion

• occurs between the alveoli and the pulmonary capillary blood

• oxygen and carbon dioxide pas through two barriers; A/C membrane and the blood plasa-red blood cell barrier

• diffusion across these membranes depends on the pressure gradients

diffusion

Rate of diffusion is affect by

• surface area

• distance the gas molecules must travel

• solubility coefficient of the gases

• partial pressure gradient

• density of the gas

why do diffusion capacity test?

• measures the ability of the lungs to transfer gas fro the alveoli to pulmonary capillary bed

• also referred to as “gas transfer factor”

diffusion capacity

• measure the gas transfer factor, you need a gas that can:

  • diffuse across the AC membrane

  • able to bind with Hb

• CO meet both these criteria

Diffusion Capacity

rate diffusion of CO form the alveoli to the Hb

CO diffusing capacity

4 steps occur during diffusion of CO

• diffusion across the AC membrane

• transfer to the RBC

• passage through the RBC membrane

• CO and Hb combining

SIngle breath Diffusion

Test gas used is CO (.3%) and an inert gas such as

Helium(10%) , Ne or CH4, oxygen(21%) and

nitrogen(balance)

• CO is the diffusing gas

• The inert gas is a tracer gas, used for calculating VA (STPD)

• Tracer gas used depends on type of gas analyzer used

Single Breath DLCO technique patient preparation

1. No smoking on day of test, note time of last cigarette

2. No alcohol on day of test

3. No exercise immediately before, seated for 5 minutes prior to test

4. No O2 for 10 minutes prior to test, if tolerated

5. Careful instruction and demonstration

Procedure/technique

1. Establish normal breathing pattern, then ask pt, to exhale to RV within 6 seconds

2. Once at RV, ask pt. to rapidly inhale to TLC (should be within 4 seconds), inspired volume (VI) ≥ 85% of VC

3. Once at TLC, ask pt. to hold breath for 8 – 12 seconds (make sure pt. doesn’t do a Valsalva or Muller maneuver); breath hold technique most used is known as the Jones-Meade

4. Discard the VD washout volume of 0.75 – 1.0 L (0.5 if VC < 2.0 L); washout should be < 4 seconds

5. Collect 0.5 – 1.0 L of alveolar sample (less if VC < 1.0 L), collected in 3 seconds

4 min. should elapse between tests

Breath hold technique most used is known as

Jones-Meade

Acceptability Criteria

1. Normal breaths prior to exhaling to RV, within 6 seconds

2. Rapid inspiration to TLC from RV, < 4 seconds

3. Volume inspired >85% of VC

4. Relaxed breath hold for 8 – 12 seconds

5. Rapid exhalation but not forced, 4 or less seconds

6. Dead space washout 0.75 – 1.0 L, 0.5 if VC< 2.0L

7. Alveolar volume 0.5 – 1.0L (< 0.5 L if VC <1.0 L), collected in < 3 seconds

8. 4 minutes between tests

Factors Affecting DLCO

• Hb & Hct ( increased Hb or Hct → increase

DLCO ~7%/g)

COHb% (increased COHb -decreased DLCO ~1%/1% COHb)

Pulmonary Capillary Blood Volume (increase -

increase DLCO)

Changes in body position (supine - increased

DLCO)

Alveolar volume (decrease - decreased DLCO)

Age and sex (DLCO decreases with increased

age, higher in males)

Altitude above sea level(direct relationship,

increasing DLCO

• When pulmonary capillary blood volume increases

• Exercise, left heart failure, supine position

• Polycythemia

• Pulmonary hemorrhage

Decreasing DLCO examples

Emphysema

Pulmonary fibrosis, sarcoidosis, asbestosis

Pulmonary embolism, pulmonary hypertension

Anemia, cigarette smoking