Spirometry is an essential tool for the diagnosis and long-term monitoring of chronic obstructive pulmonary disease (COPD). It is the only means to accurately assess the severity of airflow obstruction and is helpful in planning treatment and its response in COPD. Spirometry can separate obstructive lung conditions from restrictive diseases and is of great value in the investigation of breathlessness. In addition, abnormal spirometric tests can act as a marker for increased mortality and risk in coronary artery disease, stroke and lung cancer .
Basically, spirometry measures airflow from fully inflated lungs together with the total volume of air that can be exhaled. The three indices that are clinically important are:
• Forced vital capacity (FVC) —the volume of air that can be exhaled from fully inflated lungs
• Forced expiratory volume in 1s (FEV1) —the volume of air that can be expired with maximal effort from fully inflated lungs in one second.
• The ratio of FEV1/FVC as a percentage. The normal range lies above 70%. FEV1 tends to be an index of airflow and FVC of lung volume. The FEV1/FVC ratio when reduced below 70% indicates airflow obstruction.
Historically, spirometry was expressed as a curve of exhaled volume versus time. With the development of flow transducers, many spirometers also produce a representation of the exhaled manoeuvre as a flow-volume curve. Normal volume-time and flow-volume curves are shown in Fig. 4.1.
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