Static Lung Volume Measurement Flashcards
Why do we measure?
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
What is elastic recoil?
Elastic Recoil – the rebound of the lungs after stretching/inspiration
What happens in normal lungs in terms of recoil?
Lungs return to FRC.
How might a patient develop airflow obstruction?
- Loss of elastic recoil
- Leads to airway collapse/airflow limitation
- Gas trapping and hyperinflation
- Increases seen in FRC/RV and TLC
What happens in restrictive disease?
- Increases elastic inward recoil of the lung (e.g Pulmonary fibrosis)
- Reduction in TLC
What is Lung Compliance
- Compliance –> ability of the lungs to stretch and the change in lung volume per change in pressure gradient
What is lung compliance inversely related to?
Elastic recoil.
Restrictive patients compliance?
Decreased.
Obstructive patients compliance?
Increased compliance.
Compliance pathway?
Decreased compliance -> Stiff Lungs -> Inc Collagen and Dec Elastin -> Inc inwards elastic recoil -> Dec FRC
Which subdivisions cannot be measured directly?
TLC/FRC/RV
DEF: TLC
Total Lung Capacity: The Volume of Gas In the lungs and airways at a position of full inspiration.
DEF: RV
Residual Volume: The volume of gas remaining in the lungs and airways at the position of full expiration.
DEF: FRC
Functional Residual Capacity: The volume of gas remaining in the lungs and airways at the position of full expiration.
DEF: TV
Tidal Volume: The Volume of gas inspired or expired during one breathing cycle.
DEF: ERV
Expiratory Reserve Volume: The maximum volume of gas which can be expired from the position of FRC.
DEF: VC
The maximal volume of gas which can be expired from the lungs during a relaxed expiration from a position if full inspiration.
DEF: IC
The maximum volume of gas which can be inspired from the position of FRC.
DEF: TGV
Absolute volume gas in the lungs at any point in time (or at point of airway occlusion).
What are all units of measurements in?
All volumes and capacities are expressed in litres, corrected to BTPS.
Why do we need special techniques?
- 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
What is Helium Dilution?
- 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
What is Nitrogen Washout?
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)
Sources of Error
- 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
What is Body Plethysmography?
- 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
What are criteria for box?
minimum of 3 technically acceptable measurements of FRC (within 5%) are obtained and the mean values reported
Why is box useful?
- Unable to perform spirometry
- Contraindications to spirometry
- Young
- Assessment of bronchodilator response
- Bronchial challenge testing
What are the sources of error?
- Patient Leak
- Panting Manoeuvres
- Intra-Abdominal gas
- Thermal drift due to temperature changes inside the box
- Reporting of results
What is Boyles Law?
PV = Constant(k)
P1.V1 = P2.V2
Advantages of Helium Dilution?
Easy to Perform
Advantages of Nitrogen Washout?
Easy to Perform
Can be used to measure additional parameters - LCI
Advantages of Body Plethysmography?
Rapid
Multiple measurements
Can measure additional parameters including airways resistance (Raw) and conductance (SGaw)
Measures poorly/unventilated lung areas
Disadvantages of Helium Dilution?
May underestimate in airflow obstruction and poorly/unventilated lungs
Can be time consuming in patients with airflow obstruction
Disadvantages of Nitrogen Washout?
May underestimate in airflow obstruction and poorly/unventilated lungs
Can be time consuming in patients with airflow obstruction, especially if repeat measurements are required
Disadvantages of Body Plethysmography?
Patients claustrophobia
Relatively expensive
Not suitable for wheelchair users
May overestimate in airflow obstruction (mouth pressure does not equal alveolar pressure during panting)
