Body Plethysmography

Question 1

Draw the subdivisions of the lungs graph?

Answer 1

Whiteboard x3

Question 2

What does plethysmography actually measure?

Answer 2

Static Lung Volume

Question 3

What can box measure directly?

Answer 3

• FRC and Raw (airways resistance)

Question 4

What is Boyles Law?

Answer 4

Pressure inversely proportional to volume if temperature is constant.

• PV=K
• P1V1 = P2V2

Question 5

Boyles law in terms of pressure and Volume (x2)

Answer 5

• Increasing volume and decreased pressure

- Decreased volume and increased pressure

Question 6

Describe the science behind box

Answer 6

Waiting

Question 7

Method for Box (FRC)

Answer 7

• Box is all sealed except for the pressure transducers
• A shutter mechanism is within a box opposite the mouth so flow can be occluded enabling mouth pressure to be measured instead of alveolar pressure
• Seperate transducers measure box pressure and mouth pressure
• Box pressure change can be used to give indirect measurement of volume change in the lung
• The Pneumotachograph measured flow which integrated gives volumes
• Air temp rises with subject inside so box pressure rises
• When there is no further increase in Box P the test can start
• Subject breathes normally and then breathes in against closed shutter (activated at FRC)
• Mouth Pressure/Alv falls and box pressure rises
• Relationship displayed as line and tangent calculated
• FRC = (change in box/change in mouth) x PBar

Question 8

RV formula

Answer 8

RV = FRC - ERV

Question 9

TLC =

Answer 9

RV + VC

Question 10

Resistance =

Answer 10

Pressure/Flow

Question 11

What is airway resistance?

Answer 11

Pressure difference between alveoli and the mouth divided by a pre-determined flow rate

Question 12

Units for Airway resistance

Answer 12

kPa/L/s

Question 13

What can we not assume?

Answer 13

Mouth pressure is equivalent to alveolar pressure as we need to measure flow

Question 14

Method for Box (Airway Resistance)

Answer 14

• Patient breathes at tidal volume but increased freq (panting)
• Box pressure and flow measured with shutter open (related 1)
• Shutter closes and patient tries to breathe in against shutter (FRC) and thus mouth pressure and box pressure changes (related 2)
• slope of 2 relations multiplied (PBox/flow) x (Pmouth/Pbox)
• Relationship between box pressure and flow is displayed as line and tangent is measured
• Raw is then calculated as (box pressure/flow)/FRC and measured on inspiratory

Question 15

Raw =

Answer 15

(BP/Flow) / FRC

Question 16

sRaw =

Answer 16

Raw x FRC

Question 17

Where is the main site of Raw?

Answer 17

• central airway
• larynx
• nose

Question 18

What is specific conductance?

Answer 18

Resistance per lite of lung volume

Question 19

sGaw =

Answer 19

1/sRaw

Question 20

What suggests airflow limitation?

Answer 20

Increase in sRaw and decrease in sGaw

Question 21

What factors determine Raw?

Answer 21

• as lung volume decreases raw rises

- contraction of bronchial smooth muscle narrows the airways and increases airways resistance significantly

Question 22

Acceptability for Box

Answer 22

• Slopes should be uneuivocal to ensure accuracy
• 3 reproducible slopes with related ERV/IVC
• 3 FRCs within 5%
• 3 reproducible slopes Air

Question 23

Quality control for Box

Answer 23

• Pressure transducers checked for drift, linearity and accuracy
• Calibrated daily using sinusoidal pump for box pressure
• Decay curves to check for leaks
• Calibration syringe
• BioControls

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Question 24

Potential Box problems?

Answer 24

• Mouth pressure does not reflect the exact change in alv pressure resulting in overestimated FRC
• Incorrecr allowance for gut gas
• Expensive