Lung Function Tests - Physiology

Lung Function Tests

·        Definitions:

o   PEFR = peak expiratory flow rate

o   FEV1.0 = maximum volume of air that can be exhaled during the first second of a Forced Vital Capacity (FVC= maximum expired volume = FV). May use SVC if its higher

§  Normally 80% of FVC

§  < 75% Þ obstructive lung disease

§  > 75% Þ restrictive lung disease, eg lung fibrosis, neuro-muscular disease, chronic PEs (® scarring) and heart failure

§  But this can lead you astray. Eg if bronchodilator ® ­FVC by more than FEV, then the ratio drops

o   FEV 25-75%: Mean forced expiratory flow from 25 to 75% of vital capacity = Mean Mid-expiratory Flow (MMEF). Gets rid of peak flow which is very effort dependent ® more reliable measure

·        VC = vital capacity:

o   FVC: forced vital capacity

o   SVC: slow vital capacity

·        FIF50%: Forced inspiratory flow at 50% of vital capacity

·        PIFR: Peak inspiratory flow rate

·        Patterns:

·        Diffusing Capacity: DL CO: Diffusing capacity of the lung for Carbon Monoxide (needs adjustment for Hb – eg anaemia, polycythaemia)

·        Performing Lung Function Tests:

o  Requires at least 3 manoeuvres, the best two having FEV1.0‟s and FVCs within 5% of each other

o  Biggest problems with serial reproducibility are obtaining maximum inspiration first, and getting maximum effort

o  All are reported at BTPS (body temperature, saturated)

o  If assessing broncho-dilator responsiveness, should have no inhaler for 2 hours before hand. Maximum responsiveness usually 20 minutes after dosing

o  Predicted values are based on age, height and sex. Different racial groups and people with varying proportions (eg long legs/short torso) may be very poorly approximated by predicted values

o  Flow-volume Loop gives very valuable information. Eg in emphysema, ¯¯expiratory flows but inspiratory flows normal

·        Patterns of disease: 

o  Obstruction: ¯flow

o  Restrictive: ¯volume

·        Other lung function tests:

o  Asthma provocation: for pilots, divers, police, etc 

o  Cardiopulmonary exercise testing – finds how much of shortness of breath is due to pulmonary causes and how much to cardiac causes