measuring lung function Flashcards
how is spirometry done
Done slowly or forced
Most often as forced expiration
Fvc or ‘tiffeneau’ manoeuvre
Deep breath in, don’t hold breath, lips round outside of tube, blow out as hard and long as possible
how is spirometry interpreted
FEV1/FVC ratio
For all results compare to set of predicted values
Vary by age, height and gender
Abnormal is <80% of normal predicted value
FEV1/FVC ratio should exhale >70% of FVC in first second, less than indicates obstruction
what is a flow volume loop
plot of flow x time
what does a flow volume loop indicate
where obstruction is located in tracheobronchial tree
how does early airflow obstruction present on a flow volume loop
Peak flow many be normal, mid-expiratory flow rates are usually more affected
how does an extra-thoracic obstruction present on a flow volume loop
increase of flow rate less than expected, plateaus on expiration with slight decrease
top chopped off usual graph
why is peak flow rate used
Easy to perform
Easy to maintain device
what is peak flow rate useful for
Asthma diagnosis, not for COPD Monitoring day to day variation Picking up exacerbations Assessing response to treatment Mandatory for patients on nebulised Rx
how is peak flow seen as abnormal
Peak flow shouldn’t vary over a day
Compared to predicted values
what are causes of obstruction
Recall flow = pressure gradient / raw
Decrease radius of an airway – mucous, bronchoconstriction, compression (a mass)
how can lung volume be measured
Cannot be obtained from spirometry
Method of measurement – helium dilution, plethysmography “body box’
what do low lung volumes diagnose
restrictive lung disease Reduced TLC, FRC and RV Preserved tidal volume Reduced IRV Reduced vital capacity (FVC and SVC)
what is TLC, FRC, RV, TV, IRV, FVC, SVC
TLC total lung capacity FRC functional residual capacity RV residual volume TV tidal volume IRV inspiratory reserve volume FVC forced vital capacity SVC slow vital capacity
what are causes of restriction and decrease lung volumes
Alveolar filling process eg pneumonia lung disease eg fibrotic pleural disease eg pneumothorax, large pleural effusion, fibrosis of pleural tissue (trapped lung) chest wall disease eg kyphoscoliosis weakness (nerve and muscle disease)
how does hyperinflation present
elevated TLC (in emphysema loss of elastic recoil – CI curve plateaus at higher volume (breathing at high volume)
