Lecture 17- Clinical Assessment of Lung FUnction

Question 1

What aspects of pulmonary function are we testing?

Answer 1

• Ventilation
• Gas-exchange
• Perfusion (quality and quantity)
• Respiratory Control

Question 2

Why do we perform these tests?

Answer 2

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

Question 3

Common tests

Answer 3

• 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)

Question 4

Dynamic Lung Volumes are done by?

Answer 4

Spirometry/ FVC

Question 5

Spirometry is? What does it indicate?

Answer 5

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

Question 6

PEF is never used… why?

Answer 6

Only used as an indicator of repeatability of effort

Question 7

What is measured from FVC

Answer 7

FVC
FEF
instantaneous flows at % of expired volumes
PEF
FEV at a specific time periods (FEV1, FEV0.5)

Question 8

How do spirometry values c

Answer 8

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

Question 9

Kink in the FEV1/FVC graph is due to

Answer 9

differential rate of airway maturation in comparison to the lungs.

Teens-young adults: disproportionally larger lung volumes to airways

Question 10

Answer 10

Obstructive

Question 11

Answer 11

Obstructive + gas trapping

Question 12

Answer 12

Restrictive: lungs stiff

Question 13

Answer 13

Variable extrathoracic large airway obstruction

Question 14

Answer 14

Variable intrathoracic large airway obstruction

Question 15

Answer 15

Fixed large airway obstruction, compressed airway (large goita)

Question 16

Static Lung Volumes and TLC. What are they and what do they indicate?

Answer 16

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.

Question 17

Plethysmography

Answer 17

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.

Question 18

Helium Dilution

Answer 18

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

Question 19

Nitrogen Wash-out

Answer 19

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.

Question 20

From the dynamic lung volume to zero gives us…

Answer 20

The Residual Volume

Question 21

Single-Breath Carbon monoxide diffusing capacity (D LCO)

Answer 21

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

Question 22

what is (D LCO)

Answer 22

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)

Question 23

How is (DLCO) test done?

Answer 23

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.

Question 24

Why is CO used when its such a dangerous gas?

Answer 24

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

Question 25

Broncho dilator Response

Answer 25

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).

Question 26

BronchoProvocation Testing

Answer 26

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

Question 27

Sensitivity and specificity

Answer 27

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

Question 28

Progressive Exercise Test (CPET)

Answer 28

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

Question 29

CPET indicates..

Answer 29

• 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

Question 30

Answer 30

Spirometry looks at ventilation

Question 31

Rapid desaturation during CPET could be due to

Answer 31

Shunt: patent fossa ovalis