Lecture 17- Clinical Assessment of Lung FUnction Flashcards
What aspects of pulmonary function are we testing?
- Ventilation
- Gas-exchange
- Perfusion (quality and quantity)
- Respiratory Control
Why do we perform these tests?
To assess:
- Impact of a pathology or ageing process
- To track the progress of a disease and/or treatment regimen
- Assess degree of interventional risk or risk associated with specific activity
Common tests
- Spirometry (dynamic volumes)
- Static Lung volumes
- DLCO (diffusion capacity): how effective the alveolar-capillary membrane is at exchanging gas
- Bronchodilator Response (bad validity)
- Bronchoprovocation Tests (try to illicit bronchospasm, measure it)
- Cardiopulmonary Exercise test (CPET)
Dynamic Lung Volumes are done by?
Spirometry/ FVC
Spirometry is? What does it indicate?
Concurrent Measurement of flow and volume during a maximal effort expiration followed by a maximal effort inspiration
Indicates:
- obstructive ventilatory defect (establish or confirm)
- Assess effects of intervention
- Preoperative evaluation
- Assessment of ‘fitness’ to participate in various recreational or work related activities
- Assess the impact of work place exposure on airway/lung function
PEF is never used… why?
Only used as an indicator of repeatability of effort
What is measured from FVC
FVC
FEF
instantaneous flows at % of expired volumes
PEF
FEV at a specific time periods (FEV1, FEV0.5)
How do spirometry values c
These change with age, you get a lowering on the FEV1 graphs at
Female: 20-25 yrs
Males: 25-30yrs
and in FEV1/FVC graph decline
~25mLs per year
Kink in the FEV1/FVC graph is due to
differential rate of airway maturation in comparison to the lungs.
Teens-young adults: disproportionally larger lung volumes to airways
Obstructive
Obstructive + gas trapping
Restrictive: lungs stiff
Variable extrathoracic large airway obstruction
Variable intrathoracic large airway obstruction
Fixed large airway obstruction, compressed airway (large goita)
Static Lung Volumes and TLC. What are they and what do they indicate?
Measure all static volumes and capacities of the lung
- Plethysmography
- He dilution
- N2 washout
*importantly residual volume, that cannot be measured with spirometry
Indicates: establish or confirms diagnosis of ‘restrictive’ ventilatory defect.
Plethysmography
The principle of measuring thoracic gas volume (V TG) using a body plethysmograph is based on boyle’s law. Patient sits in box and pants.
Panting creates a positive pressure around the lungs, it will reduce in volume, if we know the total P we can use:
P1 x V1 = P2 x V2
so V1 = [changeV/ changeP] x P1
Allows you to measure unknown volumes in lungs. Only takes 30s.
Then: we do a TLC manoever, and then out fully to measure Residual volume.
Helium Dilution
Method for testing FRC (and TLC), uses the ‘conservation of mass’ principle to measure the total volume of gas in the lung
C1 x V1 = C2 x V2
reservoir with a known conc of Helium is connected via tube to the test subject, who breathes quickly and deeply. Once the equilibrium between He in reservoir and He in lungs is reached (no change)
Where [He] spirometer = [He] lung,
Vreservoir x CHe initial = Vreservoir+FRC x CHe final
Limitations: Doesn’t measure collapsed airways, only the upper airways
Nitrogen Wash-out
Inhale: 100% O2
Exhale/Collect: the nitrogen out of your lungs, till nearly 0%.
We know the composition of N2 in airways is the same as in alveolar = 79%. If we know the V of N2 that comes out and the 79% at the start, then we know the original volume
VN2/ 0.79 = VFRC
Limitations: Doesn’t always measure total volume (only upper airways
if diseased takes a long time to wash gas out.
From the dynamic lung volume to zero gives us…
The Residual Volume
Single-Breath Carbon monoxide diffusing capacity (D LCO)
Alveolar-capillary Diffusion Assessment, that evaluates the transfer of gas from the airspaces into the pulmonary capillaries
indications:
- Evaluation and followup panrenchymal lung disease
- Differenctiatinf among chronic bronchitis, emphysema(decr. D LCO) and asthma
- Evaluation of pulmonary involvement in systemic diseases
- Prediction of arterial desaturation during exercise in some patients with lung disease
what is (D LCO)
Fick Equation.
Rate of diffusion = rate CO is taken up.
Looking at effect of thickness and SA of alveolar membrane
(D LCO) = DA/dx
(D LCO) = ventilation CO / (PACO - PcCO)
How is (DLCO) test done?
From RV (using mouthpiece) you inhale a gas containing CO and CH4 in trace amounts. Methane will sit in your lungs, doesn’t undergo gas exchange. SO if you measure methane at the end of the test, its the same conc of methane at the start of the test. Therefore we also know the equal CO value at the start of the test, and cn calculate the mean driving force.By knowing the time and volume, we can measure the rate of the alveolar membrane uptake. If diseased, lines will get closer together (middle line), less gas-exchange.
Why is CO used when its such a dangerous gas?
1) Not dangerous in such trace amounts
2) Co follows the same diffusion path as O2 (similar molecular weight)
3) the rate of CO diffusion is much lower than O2 or CO2, so we can measure.
4) Amount of CO transported across the alveolar capillary membrane is DIFFUSION LIMITED and during the test CO never equilibriates with pulmonary capillary plasma.
5) As haemoglobin binds avidly to CO the concentration of CO in the pulmonary capillary plasma can be considered to be zero
Broncho dilator Response
Do spirometer, give highest amount possible of salbutomal, then FEV1 is asessed, if it increases >200mL and/or 12-15% then we say that you have a postivie bronchodilator response. (You could have asthma).
BronchoProvocation Testing
Direct (pharmacological agents) and indirect (hypertonic challenges).
Direct: ACh, metacholine , histamine etc (gaining disfavour)
Indirect: exercise, eucpnic hyperventilation, 4.5% saline, manitol, cold air (helpful with treatments)
If you do demonstrate airway hyperresponsiveness, the degree to which your test is positive is associated with the severity of your asthma
Sensitivity and specificity
Sensitivity: THe % of asthmatics who test positive to a specific test
Specificity: THe % of patients who dont have the disease who test negative for specific test
NO bronchoprovocation test has 100%, most about 90% for both, therefore NONE of these tests can diagnose asthma in isolation
Progressive Exercise Test (CPET)
CPET- involves the assessment of cardio-pulmonary function during incremental exercise and combines
- ECG
- Power output
- BP
- exhaled gas analysis (flow, volume, composition)
- arterial haemoglobin saturation
Measuring responses of O2, CO2 expired ventilation and many other CP variable >> evaluation of physiological stress on the cardiopulmonary system and/or their limitations
CPET indicates..
- Execise capacity
- cause of any exercise impairment
- Evaluation of abnormal responses
- evaluation of treatment
- pre-operative evaluation
- selection for cardiac tranplant patients
- evaluating unexplained dyspnoea
Spirometry looks at ventilation
Rapid desaturation during CPET could be due to
Shunt: patent fossa ovalis
