Resp Function Tests Flashcards
Spirometry Aim
Diagnostic tool to assess lung disease, quantify lung impairment, monitor the effects of occupational/environmental exposure + determine effects of medications
Spirometry Description
Measure expired + inspired air
Volume, time and flow
Measure FEV1 and VC
Hering-Breuer Reflex
Reflex triggered to prevent over-inflation of the lung
Obstructive defect
FEV1 more reduced than VC
FEV1/VC <75%
Asthma
COPD
Restrictive defect
FEV1 reduced no more than VC
FEV1/VC ratio >75% and may be 100%
VC reduced
Peak flow measurement
Only valid if done at TLC and with max effort
Peak flow reduction
Large airway obstruction
Upper airway obstruction
Asthma
Peak flow less affected in
COPD
Small airway disease
Interpreting Spirometry data
In adults, age, height, sex + race main determinants
Reduced inspiratory limb flow with normal expiratory flow
flow vol loop
Extra-thoracic obstruction
Equal reduction in both flows
flow vol loop
Intrathoracic obstruction
e.g. retrosternal goitre
Obstructive Flow vol loop
Shifts to left
Scoop in top bit, and smaller top bit
Same bottom bit
Restrictive flow vol loop
Shifts to right
Exact same shape as normal but smaller
FEV1/FVC ratio
Ratio of Forced Expiratory volume in 1 sec + forced vital capacity
FEV1/FVC ratio incorrect in
Asian communities
Total lung capacity measurement
Inspiration of gas mixture which includes helium, with rebreathing and VC manoeuvre for mixing
Dilution of helium + decrease in concentration x VC gives TLC
Alveolar volume + gas transfer calculation
Single large breath of air + CO + helium
Hold breath for 10 seconds
TLCO equation
KCO x Ca
Decreased TLCO
Decrease perfusion
Decrease ventilation
V/Q mismatch
Anaemia
Increased TLCO
Increased CO
Polycythaemia
Alveolar haemorrhage
The Fick principle
the volume of gas per unit time which diffuses across a tissue sheet is proportional to:
- proportional to area of sheet
- inversely proportional to thickness
- proportional to difference in pressure on 2 sides
- dependent upon permeability coefficient for that gas
Fick equation
Volume/time = area/thickness x pressure (P2-P1) x diffusion constant
Diffusion constant
Dependent on solubility + molecular weight of gas
More soluble, easier to pass
Graham’s law
Rate of diffusion of a gas is inversely proportional to the square root of molecular weight
Bigger things diffuse more slowly
