27 - Respiratory function tests

Question 1

What do the normal respiratory function figures depend on?

Answer 1

Age, sex, race, height

Question 2

What are limitations of testing

Answer 2

= Need to understand the orders
= Need to breathe out from TLC
= Maximum effort
= Mouthpiece leak
= Ability to carry out orders ( IPD/CVA)

Question 3

What is the normal respiratory rate

Answer 3

12-25

but measurements, mouthpieces and equipment changes the rate

Question 4

What is spirometry

Answer 4

Use volume /time plot
Measures FEV1, VC
Can compare over time and against normal values

Question 5

What classifies is an obstruction defect

Answer 5

FEV1/VC is <75%
The volume in 1 second is reduced more than the vital capacity
Asthma, COPD

Question 6

What classifies is a restrictive defect?

Answer 6

The FEV 1 is reduced no more than the VC
VC is reduced
FEV1/VC ratio is >75% and may be 100%
Restrictive lung disease - fibrosis, muscle disease

Question 7

Peak Flow

Answer 7

Valid if TLC and max effort
Easy to use
Cheap - patients have one
Used in large airway obstruction less in COPD

Question 8

Peak flow over time

Answer 8

Lowest in the morning

Saw tooth pattern in asthma

Question 9

What pattern of peak flow characterises asthma

Answer 9

> 20% diurnal variation for .3days /week for 2 weeks on PEFR diary = asthma

Question 10

Response to bronchodilators

Answer 10

Increase in FEV1 or PEFR in response to bronchodilators suggests reversibility of obstruction
FEV1 >15% and 200ml after 400microgram salbutamol by spacer or 2.5mg via nebuliser or 30mg/day of steroids for 14 days

Question 11

Flow volume loop

Answer 11

Measures small airway function

Question 12

How can you measure lung volume

Answer 12

VC - spirometry
TLC - measure all the air in the chest
Inspire gas (helium) rebreathing
Dilution of Helium and decrease in concentration x vital capacity gives TLC

Question 13

What is TLCO

Answer 13

TLCO = KCO x Va

Transfer capacity of the lung for carbon monoxide

Question 14

Causes of decreased TLCO

Answer 14

o Decreased Perfusion
o Decreased Ventilation
o V/Q mismatch
o Anaemia

Question 15

Causes of increased TLCO

Answer 15

Increased Cardiac Output
Polycythaemia
Alveolar Haemorrhage

Question 16

Fick’s principle

Answer 16

Vol/time= Area/thickness x pressure ( P2-P1) x diffusion constant

Question 17

What does diffusion constant depend on

Answer 17

solubility and molecular weight of the gas ( more soluble easier to pass)

Question 18

Graham’s law

Answer 18

The rate of diffusion of a gas is inversely proportional to the square root of molecular weight ( big things diffuse slower)

Question 19

What disease reduces SA (and so gas transfer)

Answer 19

pneumonectomy, lobectomy or reduced ventilation from airway obstruction or reduced effective area with emphysaema or increased dead space

Question 20

What disease reduces membrane thickness (and so gas transfer)

Answer 20

pulmonary fibrosis, alveolar proteinosis, and acute lung injury

Question 21

Alveolar-arterial gradient

Answer 21

PAO2= FiO2 (Patm- PH20) – PaCO2/R

Question 22

What is airway resistance

Answer 22

pressure difference between the alveolae and the mouth.

Question 23

How can you measure airway resistance

Answer 23

body plethysmograph or by direct measurement of intrathoracic oesophageal pressures

Question 24

What is airway conductance

Answer 24

reciprocal of resistance

Question 25

Where does the most resistance to air flow occur?

Answer 25

in the first 5 generations of airways