Static Lung Volume Measurement

Question 1

Why do we measure?

Answer 1

TLC and subdivisions tells us the functional status of the lungs determined by elastic properties.

Results aid in diagnosis

• restrictive
• obstructive
• normal

Monitoring of LF progression

Pre-Op Assessment

Effect of Therapy

Question 2

What is elastic recoil?

Answer 2

Elastic Recoil – the rebound of the lungs after stretching/inspiration

Question 3

What happens in normal lungs in terms of recoil?

Answer 3

Lungs return to FRC.

Question 4

How might a patient develop airflow obstruction?

Answer 4

• Loss of elastic recoil
• Leads to airway collapse/airflow limitation
• Gas trapping and hyperinflation
• Increases seen in FRC/RV and TLC

Question 5

What happens in restrictive disease?

Answer 5

• Increases elastic inward recoil of the lung (e.g Pulmonary fibrosis)
• Reduction in TLC

Question 6

What is Lung Compliance

Answer 6

• Compliance –> ability of the lungs to stretch and the change in lung volume per change in pressure gradient

Question 7

What is lung compliance inversely related to?

Answer 7

Elastic recoil.

Question 8

Restrictive patients compliance?

Answer 8

Decreased.

Question 9

Obstructive patients compliance?

Answer 9

Increased compliance.

Question 10

Compliance pathway?

Answer 10

Decreased compliance -> Stiff Lungs -> Inc Collagen and Dec Elastin -> Inc inwards elastic recoil -> Dec FRC

Question 11

Which subdivisions cannot be measured directly?

Answer 11

TLC/FRC/RV

Question 12

DEF: TLC

Answer 12

Total Lung Capacity: The Volume of Gas In the lungs and airways at a position of full inspiration.

Question 13

DEF: RV

Answer 13

Residual Volume: The volume of gas remaining in the lungs and airways at the position of full expiration.

Question 14

DEF: FRC

Answer 14

Functional Residual Capacity: The volume of gas remaining in the lungs and airways at the position of full expiration.

Question 15

DEF: TV

Answer 15

Tidal Volume: The Volume of gas inspired or expired during one breathing cycle.

Question 16

DEF: ERV

Answer 16

Expiratory Reserve Volume: The maximum volume of gas which can be expired from the position of FRC.

Question 17

DEF: VC

Answer 17

The maximal volume of gas which can be expired from the lungs during a relaxed expiration from a position if full inspiration.

Question 18

DEF: IC

Answer 18

The maximum volume of gas which can be inspired from the position of FRC.

Question 19

DEF: TGV

Answer 19

Absolute volume gas in the lungs at any point in time (or at point of airway occlusion).

Question 20

What are all units of measurements in?

Answer 20

All volumes and capacities are expressed in litres, corrected to BTPS.

Question 21

Why do we need special techniques?

Answer 21

• RV/TLC/FRC cannot be measured directly
• Once FRC is known then you can calculate RV and TLC if a VC manoeuvre is also performed
• FRC - ERC = RV
• RV + VC = TLC

Question 22

What is Helium Dilution?

Answer 22

• He is inert and not very soluble
• If a gas mixture with a known He is breathed in it will dilute as it mixes with the air in the lungs
• Tidal breathing pattern/FRC
• You monitor how much HE is breathed out until fully diluted in lungs

Question 23

What is Nitrogen Washout?

Answer 23

Based on the washing out of N2 from the lungs.

Normal levels (78%)

Patient breathes 100% O2

Initial alveolar N2 and the amount of N2 washed out can be used to calculate the lung volume at the start of the washout (FRC)

Question 24

Sources of Error

Answer 24

• Position of FRC for switching in to 100% O2
• System Leaks at mouth/nose can easily be detected by a spike in the N2 -> test should be discarded and repeated
• test duration can be increased in airflow obstruction. should be terminated after 10 minutes

Question 25

What is Body Plethysmography?

Answer 25

• Thoracic Gas Volume TGV not FRC
• Based on Boyles Law
• Patient enclosed in an airtight box, with pressure transducers
• Airflow measured at mouth
  (Pneumotachograph or Ultrasonic Flow Sensor)
• Shutter is closed to occlude the airway
• Mouth pressure = Alveolar Pressure
• Continue to breathe/gently pant against closed shutter
• Changes in mouth and box pressure
• Compression of gas in thorax (volume) is reflected by changes in box pressure
• Pressure changes can be used to measure TGV
• VC Manoeuvre performed
• Calculation of RV and TLC

Question 26

What are criteria for box?

Answer 26

minimum of 3 technically acceptable measurements of FRC (within 5%) are obtained and the mean values reported

Question 27

Why is box useful?

Answer 27

• Unable to perform spirometry
• Contraindications to spirometry
• Young
• Assessment of bronchodilator response
• Bronchial challenge testing

Question 28

What are the sources of error?

Answer 28

• Patient Leak
• Panting Manoeuvres
• Intra-Abdominal gas
• Thermal drift due to temperature changes inside the box
• Reporting of results

Question 29

What is Boyles Law?

Answer 29

PV = Constant(k)

P1.V1 = P2.V2

Question 30

Advantages of Helium Dilution?

Answer 30

Easy to Perform

Question 31

Advantages of Nitrogen Washout?

Answer 31

Easy to Perform

Can be used to measure additional parameters - LCI

Question 32

Advantages of Body Plethysmography?

Answer 32

Rapid

Multiple measurements

Can measure additional parameters including airways resistance (Raw) and conductance (SGaw)

Measures poorly/unventilated lung areas

Question 33

Disadvantages of Helium Dilution?

Answer 33

May underestimate in airflow obstruction and poorly/unventilated lungs

Can be time consuming in patients with airflow obstruction

Question 34

Disadvantages of Nitrogen Washout?

Answer 34

May underestimate in airflow obstruction and poorly/unventilated lungs

Can be time consuming in patients with airflow obstruction, especially if repeat measurements are required

Question 35

Disadvantages of Body Plethysmography?

Answer 35

Patients claustrophobia

Relatively expensive

Not suitable for wheelchair users

May overestimate in airflow obstruction (mouth pressure does not equal alveolar pressure during panting)