Spirometry and Flow-Volume Curves - the Idiot’s Guide. Dr Rod Taylor Consultant Respiratory Physician (part-time). Bob Dylan at 70. Born on the 24 th May 1941. History of Spirometry. 1846 John Hutchinson Defined Vital Capacity Invented the Spirometer 1947 Tiffeneau: FEV 1
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Spirometry andFlow-Volume Curves- the Idiot’s Guide Dr Rod Taylor Consultant Respiratory Physician (part-time)
Bob Dylan at 70 Born on the 24th May 1941
History of Spirometry • 1846John Hutchinson • Defined Vital Capacity • Invented the Spirometer • 1947 Tiffeneau: FEV1 • VEMS: Vol. Expired Maximale, une Seconde • 1959 Wright: Peak Flow Meter
Dr. John Hutchinson 1811 – 1861 Findings are still valid today
VITAL CAPACITY Hutchinson 1846: “The greatest voluntary expiration, following the deepest inspiration” Simple modern definition: “The biggest breath out, after the biggest breath in”
Why no Women? “We do not know the vital capacity of women, nor is it easy to determine it, because of their tight dress.” Hutchinson 1852
“We never heard a woman acknowledge that she wore her clothes tight.”
Brompton Hospital 1852 “Waste of time trying to do spirometry in women and girls: they are simply incapable of doing it.” Of course I can’t blow into it – I’m a woman! I’m pretty useless, too!
ForcedExpiratoryVolume in1second = FEV1 • The volume of air • expelled in the first second • of a maximal forced expiration • starting from full inspiration • i.e. the first 1 second of a FVC
20% either side of Mean 80 – 120% of the mean predicted value For FVC ~ 1 litre either side of mean
Forced Expiratory Ratio 70% 100% FEV1 70% FVC Can blow out 70% of FVC in the first second
FEV1 and FVC vary with • Age • Gender • Height • Ethnic origin • predicted value
Absolute values • e.g. FEV1/FVC = 3.75/4.60 litres • should always be given • Percentage of predicted value • e.g. FEV1 = 67% predicted • Forced expiratory ratio • e.g. FEV1/FVC = 53%
Graphically • As volume-time curve: spirogram • As flow-volume curve • Examples will be shown
(Remember, VC = biggest breath out after biggest breath in) So, only TWOcauses of a reduced VC! • Can’t get the air in =RESTRICTIVE • Can’t get the air out =OBSTRUCTIVE
The lungs are small • FVC is small • so FEV1 is also reduced • But no obstruction to expiration • so forced expiratory ratio (FEV1/FVC) • is normal – or even increased • becauseelastic recoil increased
Obstructive Defect Definition • FEV1 significantly reduced • to < 80% of predicted value • Forced expiratory ratio 70% or less • FEV1/FVC < 70% NB: Severe airflow obstruction reduces FVC as well as FEV1 , and so increases FER
Forced Expiratory RatioFEV1/FVC • Depends on both FEV1and FVC • Is reduced if • FEV1 is reduced • But goes back up again • if FVC is reduced • by incomplete expiration • or by severe airflow obstruction
Severe airflow obstruction Moderate airflow obstruction Normal FVC FVC FEV1 FVC FEV1 FEV1
Volume-Time Curve 6 5 4 FEV1 = 2.9 FVC = 5.6 Volume (litres) 3 FVC 2 FEV1 1 0 1 2 3 4 5 6 7 8 9 10 Time (seconds)
The Flow-Volume Curve r h Volume = πr2h Flow
Flow can be • derived from vol-time curve • measured directly
Deriving Flow A tangent to the curve gives the flow at that point Flow = volume/time Volume Time
Deriving Flow Zero at start (TLC) Flow very fast at first Slows soon after this Gets steadily slower Zero when reach RV Volume Time
Measuring Flow Directly Beaufort Scale 1805 Rear Admiral Sir Francis Beaufort 1777-1857
Personal Airflow Meter Portable Readily available Cheap Easy to use Non-invasive No batteries needed Duplicated
There is no time-scale on a flow-volume curve. But some spirometers do have an in-built timer to mark the FEV1.
At TLC, flow = zero (by definition) At RV, flow = zero (by definition) Flow reaches a peak soon after start of expiration, then slows steadily
Expiratory FlowAt TLC: before start to blow Expiratory Flow Expiratory flow = Zero Expired Volume
Expiratory FlowSoon after start blowing Peak Flow
Expiratory FlowAs expiration progresses Flow slows progressively
Expiratory F-V Curve Peak Flow
Mild airflow obstruction Slightly concave throughout expiration Airflow is reduced at a given lung volume, because the airway is narrower. Sometimes called ‘volume dependent’ reduction in flow.
Bronchodilator Effect Peak flow Before Flow has increased throughout expiration, and peak flow slightly. In this example, there is no increase in FVC. After
High compliance: easy to stretch, not much recoil
Medium Compliance Harder to stretch Stronger recoil
Low Compliance Much harder to stretch Much stronger recoil
How the Lung Behaves I am really elastic! Stretch me and I spring back!
Airway, Lung and Pleura Airway Negative pressure ‘physiological glue’ Parietal pleura Elastic lung tissue Visceral pleura Negative pressure ‘glue’