The word spirometry originally referred to any breathing test that could be performed solely with a spirometer but it has evolved to become an interchangeable term for the Forced Vital Capacity (FVC) test. Performing a spirometry (FVC) maneuver starts with the subject sitting upright with a noseclip attached. The subject then inhales rapidly from Functional Residual Capacity (FRC) to Total Lung Capacity (TLC) and then exhales maximally through a mouthpiece into a spirometer until no more air can be exhaled. Results from this test are used to assess lung capacity and the functional status of the airways.
Spirometry was initially represented graphically by a volume-time curve:
And later with as flow-volume loop:
The most common measurements that are made from a spirometry maneuver are:
FVC (Forced Vital Capacity)
The maximum volume of air that an individual can exhale with a maximally forced effort, in liters BTPS.
FEV1 (Forced Expired Volume, 1 Second)
The maximal volume of air exhaled in the first second of a forced exhalation starting at TLC, in liters BTPS.
FEV1/FVC Ratio
The ratio between the FEV1 and FVC, usually expressed as percent (i.e. 78 not 0.78).
PEF (Peak Expiratory Flow)
The maximum flow rate of air exhaled during an FVC maneuver. Usually reported in Liters/Second when derived from an FVC maneuver and in Liters/Minute when measured with a peak flow meter.
Other measurements with varying degrees of utility include:
FEF25-75
The average flow rate between 25% and 75% of the exhaled volume. It was originally used as an index to airflow in the smaller airways but it has since been realized that the FEF25-75 is poorly reproducible, highly influenced by the FVC volume and usually only reduced when the FEV1 is also reduced.
FEV0.5 (Forced Expired Volume, 0.5 seconds)
The maximal volume of air exhaled at 0.5 seconds of a forced exhalation starting at TLC, in liters BTPS. This measurement is most often reported for infants and children and only rarely in adults.
FEV3 (Forced Expired Volume, 3 seconds)
The maximal volume of air exhaled by the 3rd second of a forced exhalation starting at TLC, in liters BTPS. This value has been advocated as an index to airflow in the smaller airways and as a substitute for the FEF25-75.
FEV6 (Forced Expired Volume, 6 seconds)
The maximal volume of air exhaled by the 6th second of a forced exhalation starting at TLC, in liters BTPS. This value has been advocated as a substitute for FVC.
FEV1/FEV6 Ratio
The ratio between the FEV1 and FEV6, usually expressed as percent (i.e. 78 not 0.78).
MEF75%, MEF50%, MEF25%
The expiratory flow rate found at 75%, 50% and 25% of the remaining exhaled vital capacity. Also known as the FEF75%, FEF50% and FEF25%.
MIF75%, MIF50%, MIF25%
The inspiratory flow rate found at 25%, 50% and 75% of the inhaled vital capacity, usually referenced from TLC. Also known as the FEF75%, FEF50% and FEF25%.
It should be noted that because expiratory and inspiratory vital capacities aren’t necessarily the same, that depending on whether the MIF25%, MIF50% and MEF75% are referenced to maximal inhalation or to maximal exhalation, that they may not match the volumes at which the MEF25%, MEF50% and MEF75% are measured.
Recommendation:
The FVC, FEV1, FEV1/FVC ratio and PEF are the most commonly reported spirometry values, however some test systems only report the FEV6 and the FEV1/FEV6 ratio. Unless one or more of the remaining values are an integral part of a reviewer’s interpretation algorithm there is little value in reporting them.
PFT Interpretation by Richard Johnston is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License