Airflow and lung volume measurements can be used to differentiate obstructive from restrictive pulmonary disorders, to characterize severity, and to measure responses to therapy. Measurements are typically reported as absolute flows and volumes and as percentages of predicted values using data derived from large populations of people presumed to have normal lung function. Variables used to predict normal values include age, sex, ethnicity, and height.
Whether to correct for ethnicity is controversial. For example, historical cohorts of primarily American and European adults showed lower lung volumes in multiple non-White ethnic groups [eg, Blacks, Hispanics, Asians] than in Whites. However, environmental and other nongenetic factors are thought by some to be equally or more important than genetic differences in explaining these differences. Studies have found that unadjusted flow and volume values are more predictive of patient outcomes than ethnically adjusted values, suggesting that these adjustments may result in under-recognition [and thus undertreatment] of true decreases in pulmonary function in non-White individuals.
Quantitative measures of inspiratory and expiratory flow are obtained by forced spirometry. Nose clips are used to occlude the nares.
In expiratory flow assessments, patients inhale as deeply as possible, seal their lips around a mouthpiece, and exhale as forcefully and
completely as possible into an apparatus that records the exhaled volume [forced vital capacity [FVC]] and the volume exhaled in the first second [the forced expiratory volume in 1 second [FEV1]—see figure
Normal spirogram
Normal spirogram
In inspiratory flow and volume assessments, patients exhale as completely as possible, then forcibly inhale.
These maneuvers provide several measures:
FVC: Maximal amount of air that the patient can forcibly exhale after taking a maximal inhalation
FEV1: Volume exhaled in the first second
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Peak expiratory flow [PEF]: Maximal speed of airflow as the patient exhales
FEV1 and FVC help differentiate obstructive and restrictive lung disorders. A normal FEV1 makes irreversible obstructive lung disease unlikely whereas a normal FVC makes restrictive disease unlikely.
Normal spirogram
FEF25–75% = forced expiratory flow during expiration of 25 to 75% of the FVC; FEV1 = forced expiratory volume in the first second of forced vital capacity maneuver; FVC = forced vital capacity [the maximum amount of air forcibly expired after maximum inspiration].
The forced expiratory flow averaged over the time during which 25 to 75% of the FVC is exhaled may be a more sensitive marker of mild, small airway airflow limitation than the FEV1, but the reproducibility of this variable is poor.
Interpretation of these measures depends on good patient effort, which is often improved by coaching during the actual maneuver. Acceptable spirograms demonstrate
Good test initiation [eg, a quick and forceful onset of exhalation]
No coughing
Smooth curves
Absence of early termination of expiration [eg, minimum exhalation time of 6 seconds with no change in volume for the last 1 second]
Reproducible efforts agree within 5% or 100 mL with other efforts. Results not meeting these minimum acceptable criteria should be interpreted with caution.
Lung volumes are measured by determining functional residual capacity [FRC]. FRC is the amount of air remaining in the lungs after normal exhalation. The total lung capacity [TLC] is the volume of gas that is contained in the lungs at the end of maximal inspiration.
Normal lung volumes
ERV = expiratory reserve volume; FRC = functional residual capacity; IC = inspiratory capacity; IRV = inspiratory reserve volume; RV = residual volume; TLC = total lung capacity; VC = vital capacity; VT= tidal volume.
FRC = RV + ERV; IC = VT + IRV; VC = VT+ IRV + ERV.
FRC is measured using gas dilution techniques or a plethysmograph [which is more accurate in patients who have airflow limitation and trapped gas].
Gas dilution techniques include
Nitrogen washout
Helium equilibration
With nitrogen washout, the patient exhales to FRC and then breathes from a spirometer containing 100% oxygen. The test ends when the exhaled nitrogen concentration is zero. The collected volume of exhaled nitrogen is equal to 81% of the initial FRC.
With helium equilibration, the patient exhales to FRC and then is connected to a closed system containing known volumes of helium and oxygen. Helium concentration is measured until it is the same on inhalation and exhalation, indicating it has equilibrated with the volume of gas in the lung, which can then be estimated from the change in helium concentration that has occurred.
Both of these techniques may underestimate FRC because they measure only the lung volume that communicates with the airways. In patients with severe airflow limitation, a considerable volume of trapped gas may communicate very poorly or not at all.
Body plethysmography uses Boyle’s law to measure the compressible gas volume within the thorax and is more accurate than gas dilution techniques. While sitting in an airtight box, the patient tries to inhale against a closed mouthpiece from FRC. As the chest wall expands, the pressure in the closed box rises. Knowing the pre-inspiratory box volume and the pressure in the box before and after the inspiratory effort allows for calculation of the change in box volume, which must equal the change in lung volume.
Knowing FRC allows the lungs to be divided into subvolumes that are either measured with spirometry or calculated [see figure
Normal lung volumes
Normal lung volumes
In contrast to the spirogram, which displays airflow [in L] over time [in seconds], the flow-volume loop displays airflow [in L/second] as it relates to lung volume [in L] during maximal inspiration from complete exhalation [residual volume [RV]] and during maximum expiration from complete inhalation [TLC]. The principal advantage of the flow-volume loop is that it can show whether airflow is appropriate for a particular lung volume. For example, airflow is normally slower at low lung volumes because elastic recoil is lower at lower lung volumes. Patients with pulmonary fibrosis have low lung volumes and their airflow appears to be decreased if measured alone. However, when airflow is presented as a function of lung volume, it becomes apparent that airflow is actually higher than normal [as a result of the increased elastic recoil characteristic of fibrotic lungs].
Flow-volume loops
[A] Normal. Inspiratory limb of loop is symmetric and convex. Expiratory limb is linear. Airflow at the midpoint of inspiratory capacity and airflow at the midpoint of expiratory capacity are often measured and compared. Maximal inspiratory airflow at 50% of forced vital capacity [MIF 50% FVC] is greater than maximal expiratory airflow at 50% FVC [MEF 50% FVC] because dynamic compression of the airways occurs during exhalation.
[B] Obstructive disorder [eg, emphysema, asthma]. Although all airflow is diminished, expiratory prolongation predominates, and MEF