Monday, December 7, 2009

Spirometry


Bronchial provocation testing was our topic for last month and this month lets talk about spirometry
Spirometry is a measure of airflow and lung volumes during a forced expiratory manoeuvre from full inspiration. The measurements made during spirometry are sometimes referred to as ‘dynamic lung volumes’. Although the simplest of all respiratory function tests correct interpretation requires that it be performed correctly

Spirogram
A diagrammatic representation of lung volumes and capacities based on a simple spirogram which we see every day. Relationships between the subdivisions and relative sizes as compared with TLC are shown. Resting expiratory level is used as a starting point for FRC determinations because it remains more stable than other identifiable points during repeated measurements

Volume-Time Curve
Spirometry measures airflow obstruction, airflow reactivity, lung restriction and normal lung function and these can be graphed in a volume-time or flow-volume format.
The volume-time curve produces FVC & FEV1. We can then calculate the ratio of these to obtain FEV1/FVC or FER. Accurate measurement of FEV1 requires an acceptable spirometer, preferably one that allows inspection of the volume-time curve and back-extrapolation. Additionally we can measure mid expiratory flow which gives an indication of small airway function & PEF which tells us how much force the patient is able (or willing) to generate

Mathematically these can be converted on some equipment to flow-volume curves and many conditions have typical curves. A flow-volume curve usually records flow in liters per second and the volume is recorded in liters, BTPS

Flow-Volume Loop
Expiratory and inspiratory flow-volume curves constitute a flow-volume loop and most modern spirometers can produce flow-volume loops. Visual pattern recognition from repeatable maximal flow volume loops, if available will help decide whether the measurement was performed correctly, as well as confirming the presence of abnormality

Of all the pulmonary function tests performed, spirometry remains the most widely used test. Although the simplest pulmonary function test, special emphasis must be placed on the performance of each test and the technician must have a sound understanding of the criteria for judging the acceptability and repeatability of test data based on the most recent guidelines published by the American Thoracic Society/European Respiratory Society (ATS/ERS) Task Force on Standardization of Lung Function Testing
Vanessa

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