Module 4: Physiology of the Respiratory System - Definition of Spirometry .

Definition of Spirometry:

Spirometry is the measurement of lung volumes and flow rates during a forced expiration.

The three essential measurements usually made and recorded in spirometry are:

• FVC: Forced vital capacity = The difference between maximal inspiratory capacity and maximal expiratory capacity (residual volume) in a forced expiration. (ie the total amount of air that can be exhaled).
• FEV1: Forced expiratory volume in one second = The amount of air exhaled in the first second of a forced expiration.
• Ratio of FEV1/FVC expressed as a percentage.

A flow-volume loop further provides a measure of the Peak Expiratory Flow Rate (PEFR) and a variety of measures of mid-expiratory flow, such as the Forced Expiratory Flow at 50% of Vital Capacity (FEF50). These measures are consider to have greater intra-individual variation in repeat spirometric measurements and are therefore less "robust" measurements than FEV1 and FVC. The FEV6, is another measure that is gaining popularity. In this instance the Forced Expiratory Volume at 6 seconds is measured. The reason for favouring this measure is that one of the commonest errors in spirometry is premature termination of effort, particularly in instances where there is airflow obstruction. This leads to an under-estimation of FVC, which may then lead to under-recognition of airflow obstruction.