ATS Pulmonary Function Laboratory Manual

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

Table 6.1

Spirometric terms and measurements

Term Name

Description

VC

Vital capacity

The volume change between the position of full inspiration and complete expiration. The maximal volume of air inhaled slowly from the point of maximal exhalation achieved by a slow expiration from end- tidal inspiration. The maximal volume of air exhaled slowly from the point of maximal inhalation. The maximal volume of air exhaled with maximally forced effort from a position of maximal inspiration. The maximal volume of air exhaled with maximally forced effort in t seconds; 1 and 6 seconds are the most common. The flow measured during a forceful expiration when x% of the FVC has been exhaled; FEF 25% , FEF 50% , and FEF 75% are commonly reported. The average flow measured over the middle 50% of an FVC maneuver. The maximal expiratory flow generated during an FVC maneuver. The maximum volume of air one can ventilate over a specified period of time (e.g., 12 seconds). The time required for the FVC to be expired. The ratio of FEV t to FVC expressed as a percentage; FEV 1 / FVC is the most commonly used measure.

IVC

Inspiratory vital capacity

EVC

Expiratory vital capacity

FVC

Forced vital capacity

Forced expiratory volume

FEV t

/FVC Forced expiratory volume in t

FEV t

seconds to forced vital capacity ratio

Forced expiratory flow

FEF x%

Forced mid-expiratory flow

FEF 25-75%

FET PEF

Forced expiratory time Peak expiratory flow

MVV Maximum voluntary ventilation

Spirometry is the diagnostic tool for evaluation of obstructive lung disease. Maximal air flow depends primarily on the elastic recoil of the lungs and the compliance and caliber of the airways (10). During forced expiration from TLC, airflow limitation begins in the large airways (trachea and mainstem bronchi) with the development of turbulent flow. As the forced expiration continues, the site of air flow limitation moves to smaller airways where flow is laminar. Flow is limited by the compression of the airways downstream from the “equal pressure point.” As the lungs empty, the equal pressure point moves into smaller airways. Measurements of flow at different lung volumes allows assessment of the status of the airways distal to the “equal pressure point” (11). Loss of elastic recoil (as in emphysema) results in increased compression of the airways and markedly reduced flows at all lung volumes. A decrease in the caliber of the airways (as in asthma or bronchitis) directly limits flow developed for a given driving pressure. The pattern of flow reduction can be used to assess the site of flow limitation. Large airway obstructions, such as tumors, usually limit flow across a wide range of lung volumes (12). By measuring maximal flows during both inspiration and expiration, the nature of central airway obstruction can often be identified (i.e., fixed versus vari- able, intrathoracic versus extrathoracic). Measured flows are compared to those of healthy individuals to determine the severity of airway obstruction.

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