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Clinical Measurements > Respiratory Measurements > Flashcards

Respiratory Measurements Flashcards

1
Q

Define anatomical dead space

A

Parts not taking part in gas exchange (everything apart from the alveoli)

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2
Q

What happens during inspiration?

A
  • 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
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3
Q

What happens during expiration?

A
  • 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
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4
Q

What happens in heavy breathing? (inspiration)

A

Accessory inspiratory muscles also play a part, not just intercostal muscles

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5
Q

What happens in forced expiration?

A
  • internal intercostals contract, accessory expiratory muscles aid expiration
  • heavy breathing
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6
Q

How is lung connected to chest wall?

A
  • 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
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7
Q

What happens to the intrapulmonary pressure in breathing?

A
  • 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
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8
Q

What is intrapleural pressure vs. intrapulmonary pressure?

A 
Intrapulmonary = in lungs
Intrapleural = Outside lungs between visceral and parietal pleura
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9
Q

What are the different lung measurements?

A

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

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10
Q

What do lung measurements change with?

A

Age, sex, height

Females = 20-35% lower

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11
Q

3 basic types of pulmonary testing measurements

A
  • ventilation
  • distribution
  • diffusion
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12
Q

Ventilation Testing

A
  • how body acts as an air pump (move air and speed)

- tool example is a spirometer for volume displacement

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13
Q

Distribution testing

A
  • where flow goes in lungs (i.e. are there restrictions)

- tool example is nitrogen washout procedure

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14
Q

Diffusion Testing

A
  • ability to exchange gas (rate of exchange)

- tool example is lung diffusion test user CO

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15
Q

Examples of pulmonary function tests/techniques

A

Spirometry
Helium Dilution Technique
Nitrogen Washout Technique

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16
Q

What is a spirometer?

A
  • Calibrated container which collects gas

- Measures volumes and capacity

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17
Q

What is a pneumotachometer?

A

Determines flow rate (FEV. FEV1)

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18
Q

What does a whole body plethysmography measure?

A

TLC
FRC
RV

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19
Q

How does a spirometer work?

A
  • 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
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20
Q

Pros of spirometry

A 
No hysteresis 
Low inertia
Low resistance
Linear response
Highly accurate
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21
Q

Cons of spirometry

A

Bulky, not portable = not modern devices though

High maintenance

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22
Q

What are the modern spirometers like?

A
  • record data electronically
  • measure flow rate (integrate for volume)
  • good accuracy
  • no resistance or momentum
  • good for hygiene as disposable air tubes used
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23
Q

What graphs are produced from spirometry? How does this work?

A
  • 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
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24
Q

Important features on volume flow curve

A
  • 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)
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25
Q

Pneumatochometers

A
  • 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
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26
Q

2 main types of pneumatochometer

A

Type Fleish = series of parallel capillaries

Type Lilly = membrane with known resistance

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27
Q

Difference between 2 types of pneumatochometer

A
  • 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
28
Q

Cons of pneumatochometer

A
  • very sensitive to temperature, humidity and atmospheric pressure of surrounding air
  • must be calibrated very often (pass known gas through it)
29
Q

Turbine pneumotachometers

A
  • 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
30
Q

Peak Flow Meter

A
  • 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
31
Q

Electronic Peak Flow Meter

A
  • 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)
32
Q

What does spirometry not allow us to obtain?

A

Functional Residual Volume

33
Q

How do we obtain functional residual volume?

A
  • whole body plethysmography
  • gas dilution techniques
  • imaging techniques
34
Q

Define restrictive disorders

A

Reduced TLC and VC

- decreased lung volume volume

35
Q

Define obstructive disorders

A
  • increased functional residual capacity and potentially a huge residual volume as they progress
  • VC is lost
36
Q

Ordinary TV

A

0.5L

37
Q

Anatomic dead space normal

A

0.15L (air in trachea and mouth)

= cannot contribute to gas exchange in lungs

38
Q

Normal respiratory reserve

A

2L

39
Q

What is whole body plethysmography?

A
  • 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
40
Q

Advantages of whole body plethysmography

A
  • 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!)
41
Q

Disadvantage of plethysmography

A
  • more difficult to set up as needs larger area for equipment
  • big box with person sitting in it
42
Q

How does whole body plethysmography work?

A
  • 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
43
Q

What is Boyle’s Law?

A

At a constant temperature:
- the pressure is inversely proportional to volume
P = 1/V

44
Q

How is whole body plethysmography related to Boyle’s Law?

A
  • 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
45
Q

What do we need to know to calculate lung volume in whole body plethysmography?

A
  • box volume

- calculate lung volume

46
Q

Why can a capacitive pressure transducer be used in whole body plethysmography?

A
  • even though large volume changes (1L for example), this equates to very small pressure changes (0.16kPa) is 600L in all the box
47
Q

Why is Boyle’s Law a bit different in practice?

A
  • patient keeps breathing
  • so mouth and box pressure keep changing
  • compressed/decompressed
48
Q

How do we calibrate whole body plethysmography?

A
  • 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
49
Q

Flow plethysmography- How it works and why do we need it?

A
  • 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
50
Q

What does a gas dilution technique involve?

A
  • 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
51
Q

Why is helium commonly used for gas dilution technique?

A
  • analyser is relatively accurate, very robust and cheap
52
Q

When is helium rebreathing used?

A
  • diagnosing restrictive disease patterns
  • routine lung volume measurements
  • enable differentiation between restrictive and obstructive
  • response to therapeutics
  • evaluate and monitor patients
53
Q

What are the steps of helium rebreathing?

A
  • 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
54
Q

Why is patient asked to breathe out at the beginning of helium rebreathing?

A
  • what is left within respiratory system is purely functional residual capacity
  • can then determine lung volume from change in volume of helium in box
55
Q

How do we know equilibrium is reached in helium rebreathing?

A

When change in concentration is <0.02% for 30 secs

- then disconnect patient from system

56
Q

Requirements of helium rebreathing

A
  • 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)
57
Q

How is FRC calculated?

A
  • compare conc. of helium at initial concentration (C1) and final concentration (C2)
  • see formula
58
Q

Why does care need to be taken with gas dilution?

A
  • need to remove CO2 with absorber
  • add O2 at rate at which it is used
  • must monitor temperature
  • must be totally sealed
59
Q

How is it easy to create errors?

A
  • 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
60
Q

Nitrogen washout Method

A
  • 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%
61
Q

How does Fowler method work?

A
  • 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
62
Q

Single breath test

A
  • 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
63
Q

Multiple breath test

A
  • 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
64
Q

Why is there potential for large error in nitrogen washout?

A
  • 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
65
Q

Define physiological anatomical dead space

A

Respiratory bronchioles downwards

Poorly perfused alveoli

Clinical Measurements (11 decks)

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