Respiratory Measurements Flashcards
Define anatomical dead space
Parts not taking part in gas exchange (everything apart from the alveoli)
What happens during inspiration?
- Enlargement of thorax/chest by diaphragm contraction (pulls downwards)
- External intercostals elevate ribs
- Results in enlargement of lungs
- Pressure in lung drops below atmospheric pressure as volume increases = air is sucked in
What happens during expiration?
- passive during quiet breathing
- elastic recoil when diaphragm and intercostal muscles relax
- diaphragm relaxes = volume in thorax/lungs decreases = pressure increases
- ribs are lowered
- air is released
What happens in heavy breathing? (inspiration)
Accessory inspiratory muscles also play a part, not just intercostal muscles
What happens in forced expiration?
- internal intercostals contract, accessory expiratory muscles aid expiration
- heavy breathing
How is lung connected to chest wall?
- friction free movement
- 2 pleural linings
- visceral and parietal
- smooth serous membrane
- make up pleural cavity/space
- pleural fluid (5ml) in cavity = lubrication between membranes and maintains pressure gradient
- 10-20 micrometres pleural cavity width
What happens to the intrapulmonary pressure in breathing?
- air comes into lungs due to intrapulmonary pressure being lower than atmospheric
- air flows out when intrapulmonary pressure is higher than atmospheric
- creates pressure gradients
What is intrapleural pressure vs. intrapulmonary pressure?
Intrapulmonary = in lungs Intrapleural = Outside lungs between visceral and parietal pleura
What are the different lung measurements?
RV = volume left in lungs after forced expiration, cannot be expired
VC = sum of all volumes apart from RV, max amount of air that can be breathed in after max amount breathed out in forced expiration
IC = max volume than can be inspired (forced)
TV = normal amount of inspiration and expiration occurring during normal quiet breathing
FRC = volume in lungs after normal expiration, reservoir for air
IRV = max amount of air that can be inspired after inspiring TV
ERV = max amount of air that can be expired after expiring TV
Alevolar ventilation = volume of fresh air entering alveoli in each breath, approx the breathing rate
What do lung measurements change with?
Age, sex, height
Females = 20-35% lower
3 basic types of pulmonary testing measurements
- ventilation
- distribution
- diffusion
Ventilation Testing
- how body acts as an air pump (move air and speed)
- tool example is a spirometer for volume displacement
Distribution testing
- where flow goes in lungs (i.e. are there restrictions)
- tool example is nitrogen washout procedure
Diffusion Testing
- ability to exchange gas (rate of exchange)
- tool example is lung diffusion test user CO
Examples of pulmonary function tests/techniques
Spirometry
Helium Dilution Technique
Nitrogen Washout Technique
What is a spirometer?
- Calibrated container which collects gas
- Measures volumes and capacity
What is a pneumotachometer?
Determines flow rate (FEV. FEV1)
What does a whole body plethysmography measure?
TLC
FRC
RV
How does a spirometer work?
- original involved displacement of water when breathe in and out, now use volume sensing spirometer based on same concept
- use linear potentiometers attachment
- movement of bell proportional to TV
- if using within rebreathing experiment = need to add soda lime canister inside bell to prevent CO2 build up
- used to measure slow respiratory rates not rapid breathing in exercise or after anaesthetic
Pros of spirometry
No hysteresis Low inertia Low resistance Linear response Highly accurate
Cons of spirometry
Bulky, not portable = not modern devices though
High maintenance
What are the modern spirometers like?
- record data electronically
- measure flow rate (integrate for volume)
- good accuracy
- no resistance or momentum
- good for hygiene as disposable air tubes used
What graphs are produced from spirometry? How does this work?
- volume flow curves (volume on x, flow on y)
- bottom is inspiration, top is expiration
- take a deep breathe before start
- connect to spirometer
- breathe out as forcefully as you can
- take a deep breathe back in to return to the start
Important features on volume flow curve
- peak expiratory flow rate on top peak
- forced expiratory volume (at 0.5, 1, 2, 3 seconds)
- forced vital capacity at right end of x axis
- forced expiratory flow (at discrete intervals)
Pneumatochometers
- measure of expiratory flow only
- measures pressure drop when patient blows in the device
- use venturi principle (gas accelerates through constricted central region and the static pressure drops)
- calculate flow rate from pressure change
2 main types of pneumatochometer
Type Fleish = series of parallel capillaries
Type Lilly = membrane with known resistance
Difference between 2 types of pneumatochometer
- Fleish more reliable as allows laminar flow at much higher flow rate, if flow was turbulent pressure drop may not be proportional to flow rate
Cons of pneumatochometer
- very sensitive to temperature, humidity and atmospheric pressure of surrounding air
- must be calibrated very often (pass known gas through it)
Turbine pneumotachometers
- turbine measures expiratory flow
- harder the patient blows, faster the turbine rotates
- rotations are measured (usually infrared)
- results are reliable and reproducible
- no influence of pressure humidity on the results
- no calibration needed
- no thermostat needed if made of carbon or kevlar
- disposable = single patient use
Peak Flow Meter
- spring or electronic
- simple single measure of peak value
- baffle pushed forward uncoiling a spring to expose the exhaust orifice
- the area of the orifice exposed is proportional to the peak flow
- or bending piece of metal, this opens up the area of the orifice
- dependent on gas density, affected by marked changes in atmospheric pressure such as at high altitude
- weather variations in barometric pressure and temperature are not important
Electronic Peak Flow Meter
- gas flow is directed tangentially to strike a rotating vane in the gas pathway
- a light source and photodetector positioned across the vane count its rotation
- tendency to under-read at low flows (because of friction) and to over-read at high flows (because of momentum)
- much easier to record FEV1 (can record flow throughout forced expiration)
What does spirometry not allow us to obtain?
Functional Residual Volume
How do we obtain functional residual volume?
- whole body plethysmography
- gas dilution techniques
- imaging techniques
Define restrictive disorders
Reduced TLC and VC
- decreased lung volume volume
Define obstructive disorders
- increased functional residual capacity and potentially a huge residual volume as they progress
- VC is lost
Ordinary TV
0.5L
Anatomic dead space normal
0.15L (air in trachea and mouth)
= cannot contribute to gas exchange in lungs
Normal respiratory reserve
2L
What is whole body plethysmography?
- a special case of spirometry
- able to measure lung volume and airway resistance
- device is a volumetric displacement box
- displacements are measured
- completely sealed box
- record what patient is breathing in and out
- small controlled known leak to stabilise internal pressures
- uses pressure transducers
- respiratory flow rate recorded by pneumotochograph
- measures absolute volume of lungs and alveolar pressure and airway resistance
Advantages of whole body plethysmography
- easily attain functional residual capacity quickly
- any unventilated regions of the lungs (asthma) can be detected
- can be done with unconscious patient but much easier with conscious patient
- highly sensitive = can detect lung pathology that might be missed with conventional pulmonary function tests
- can measure absolute volume of air within lungs more accurately than gas techniques (due to Boyle’s Law!)
Disadvantage of plethysmography
- more difficult to set up as needs larger area for equipment
- big box with person sitting in it
How does whole body plethysmography work?
- rigid airtight box which patient sits in
- patient breathes in and out of spirometer
- measure small pressure changes in box as patient breathes
- relate to volume changes
What is Boyle’s Law?
At a constant temperature:
- the pressure is inversely proportional to volume
P = 1/V
How is whole body plethysmography related to Boyle’s Law?
- subject tries to breathe in airtight box
- expands gas in lungs
- increases lung volume and decreased pressure
- box gas volume decreases
- box pressure rises
- initial pressure of box X box volume = pressure after expansion X new unknown box volume
- volume change in box = volume change in chest
- original volume - new volume = change in volume within lungs as well
What do we need to know to calculate lung volume in whole body plethysmography?
- box volume
- calculate lung volume
Why can a capacitive pressure transducer be used in whole body plethysmography?
- even though large volume changes (1L for example), this equates to very small pressure changes (0.16kPa) is 600L in all the box
Why is Boyle’s Law a bit different in practice?
- patient keeps breathing
- so mouth and box pressure keep changing
- compressed/decompressed
How do we calibrate whole body plethysmography?
- 30ml sinusoidal pump
- 30ml volume stroke in and out changes box pressure signal
- calibrate pressure changes against a known volume
- defines relationship between change in box volume and change in box pressure so can then determine box to mouth measurements
Flow plethysmography- How it works and why do we need it?
- measure airway resistance from this
- patient asked to make rapid shallow breaths
- end up with change in lung pressure taken at mouth when shutter closed, everything in equilibrium so same pressure in lungs as alveoli
then open shutter to allow immediate inspiration - inspiration = alveolar gas expands = box pressure rises
- during immediate opening we measure Q using pneumotacometer
- this provides a measure of driving pressure used to move air into the lungs
- can then measure airways resistance from this
What does a gas dilution technique involve?
- patient keeps rebreathing a gas mix containing a poorly soluble tracer gas
- tracer mixes with gas in lungs
- estimate lung volume using initial tracer conc, final tracer conc and initial air volume
- see how much expired tracer gas is diluted by non tracer gas
- most commonly helium used
Why is helium commonly used for gas dilution technique?
- analyser is relatively accurate, very robust and cheap
When is helium rebreathing used?
- diagnosing restrictive disease patterns
- routine lung volume measurements
- enable differentiation between restrictive and obstructive
- response to therapeutics
- evaluate and monitor patients
What are the steps of helium rebreathing?
- patient breathes out then connects to the system before next inspiration
- avoid leaks as need closed system
- breathes tracer gas from known volume box
- normal breaths before process so get used to it using mouthpiece
- 3 minutes for equilibrium to occur
- have constant flow of 100% oxygen (3-4ml/kg/minute) but depends on patient so needs to be adjusted accordingly
- check Helium conc. every 15 seconds
- once at equilibrium, disconnect patient from system and see how much helium remains in the box
- should take max 10 minutes
Why is patient asked to breathe out at the beginning of helium rebreathing?
- what is left within respiratory system is purely functional residual capacity
- can then determine lung volume from change in volume of helium in box
How do we know equilibrium is reached in helium rebreathing?
When change in concentration is <0.02% for 30 secs
- then disconnect patient from system
Requirements of helium rebreathing
- spirometer needs to be around 7-10L
- 3% static accuracy spirometer
- resolution of spirometer at least 25ml
- CO2 absorbed needed (soda lime cannister)
- CO2 absorber has to be changes and regularly checked
- mixing fan = mix gases sufficiently
- steady flow maintained (50L/minute)
- 0-10% helium
- monitor temperature (change from expired gas temp to room temperature and other temp changes)
How is FRC calculated?
- compare conc. of helium at initial concentration (C1) and final concentration (C2)
- see formula
Why does care need to be taken with gas dilution?
- need to remove CO2 with absorber
- add O2 at rate at which it is used
- must monitor temperature
- must be totally sealed
How is it easy to create errors?
- must always connect patient at same point in breathing cycle
- just before inspiration means volume measured will be FRC
- must continue until gas concentration appears to have reached equilibrium
Nitrogen washout Method
- fowler method
- often used in intensive care
- indirect interpretation of FRC and lung volume
- specifically can calculate dead space
- nitrogen washed out of lungs used oxygen
- amount of nitrogen collected used to determine lung volume
- relies on nitrogen levels in lungs being constant and known (room air is 78%)
- need a sensitive nitrogen sensor = at beginning should be at around 78%
How does Fowler method work?
- subject breathes 100% oxygen which floods respiratory tract and displace other gases
- this means anatomical dead space is oxygen filled at end of inspiration
- one way valve only
- pneumotacograph
- nitrogen meter
- sometimes patient starts breathing air mix with known nitrogen conc. to equilibrate
- after ordinary expiration, patient breathes pure O2
- then either single breath test or continual breathing test
- expired gas monitored to watch nitrogen level changes
Single breath test
- 100% oxygen held for a few seconds
- expiration event (forceful)
- 3 phases of expiration event
- INITIAL = 100% oxygen, 0% nitrogen (undiluted oxygen from anatomical dead space)
- TRANSITIONAL = N2 rises, transitional part of conducting airways and respiratory bronchioles
- FINAL = alveolar plateau, 100% oxygen has reached alveoli has fully mixed with N2
- midpoint of transitional phase gives volume of anatomic dead space
- final concentration of nitrogen gives functional residual volume
Multiple breath test
- measures how long it takes to expel N2 from lungs
- 7 minutes in healthy patients
- continue to breathe in 100% oxygen and measure N2 out until N2 reaches set value (2%)
- gas collected in a bag to measure expired nitrogen sometimes
- more accurate measure of FRC
- monitoring reducing rate also provide information on gas distribution in lungs
Why is there potential for large error in nitrogen washout?
- significant nitrogen in body stores which is also washed out with breathing
- but is reasonably well understood so can be deleted from total amounts
- release from body stores is time dependent (high at start of O2 breathing)
- depends on sex, age, height and weight
- relatively safe but not recommended for patients with heart conditions
Define physiological anatomical dead space
Respiratory bronchioles downwards
Poorly perfused alveoli
