ATS Pulmonary Function Laboratory Manual

ATS Pulmonary Function Laboratory Management & Procedure Manual | 3rd Edition

Effective Date: 2014 Version #1

Chapter 6

Procedure Name: Spirometry

Purpose or Principle Spirometry is a physiological test that measures inhaled and exhaled volumes of air as a function of time. The pri- mary signal measured in spirometry is volume or flow. Spirometry measures the vital capacity (VC), the largest volume of air that can either be inspired or expired from the lungs. VC measured from a maximal forced exhala- tion is called the forced vital capacity (FVC). The most commonly measured parameters from the FVC maneuver are the FVC itself and the forced expiratory volume in one second (FEV 1 ) (1). Various flows can be measured in conjunction with an FVC maneuver, either at specific points (e.g., FEF 75% ) or across specific intervals (FEF 25-75% ), but have not been shown to increase diagnostic information or contribute usefully to clinical decision making over and above information from FEV 1 , FVC, and FEV 1 /FVC ratio (2, 3). Many of the flow measurements from the forced expiration can also be obtained during forced inspiration (4). The forced inspiratory vital capacity (FIVC) is often measured in conjunction with the FVC, and like the FVC, vari- ous inspiratory flows can be measured (e.g., peak inspiratory flow [PIF] and forced inspiratory flow when 50% of the FIVC has been inhaled [FIF 50% ]). When the VC is measured in an unforced manner it is called the slow vital capacity (SVC). SVC maneuvers are considered unforced; however, a maximal inspiratory and expiratory effort are still required. Table 6.1 lists common spirometric terms and measurements (5). Volumes are reported in L (BTPS), and flows are reported in L/s (BTPS) (6). An additional test, sometimes considered part of spirometry, is the maximal voluntary ventilation (MVV), which is recorded while the patient breathes rapidly and deeply for 12 or 15 seconds. The MVV is reported in L/min (BTPS) (6). The graphic display of the FVC maneuver (spirogram) is necessary during testing for assessing patient effort and cooperation. The spirogram is also helpful for assessing the quality of the maneuver and for interpretative purposes. Two common spirogram displays are used: volume–time and flow–volume; both should be available to the techni- cian and interpreter. Since the FVC maneuver includes measurements of both volume and flow, it is useful in the assessment of both restrictive and obstructive diseases. Diseases that interfere with the bellows action of the chest wall or lungs result in a reduction of vital capacity. A reduced FVC or VC, without a disproportionately greater reduction in flows, sug- gests a restrictive disorder (7). This “restrictive” pattern predicts a reduced total lung capacity (TLC) only about half the time; the absence of the restrictive pattern strongly predicts a normal TLC (8, 9). Measurement of additional lung volumes, such as TLC, may be required to confirm the presence of a restrictive pattern.

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