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Flow rate and lung volume measurements can be used to differentiate obstructive from restrictive pulmonary disorders, to characterize disease severity, and to measure responses to therapy. Measurements are typically reported as absolute flows and volumes and as percentages of predicted values derived from large populations of people presumed to have normal lung function. Variables that help predict normal values include age, sex, ethnicity, and height.

Flow rates: Quantitative measures of inspiratory and expiratory flow are obtained by forced spirometry. Nose clips are used to occlude the nares.

In assessments of expiratory flow, the patient inhales as deeply as possible, seals his lips around a mouthpiece, and exhales 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 sec [FEV₁]—see Fig. 1: Tests of Pulmonary Function (PFT): Normal spirogram.). Newer instruments measure flow and integrate time in order to estimate volumes. In assessments of inspiratory flow and volume, the patient exhales as completely as possible, then forcibly inhales. These maneuvers provide several measures. The FVC is the maximal amount of air that the patient can forcibly exhale after taking a maximal inhalation. The FEV₁ is the most reproducible flow parameter and is especially useful in diagnosing and monitoring patients with obstructive pulmonary disease.

Fig. 1
Normal spirogram. FEV1 = forced expiratory volume in the 1st second of forced vital capacity maneuver; FEF25–75% = forced expiratory flow during expiration of 25 to 75% of the FVC; FVC = forced vital capacity (the maximum amount of air forcibly expired after maximum inspiration).

The forced expiratory flow measured during exhalation of 25 to 75% of the FVC may be a more sensitive marker of mild small airway obstruction than the FEV₁, but reproducibility is poor. The peak expiratory flow (PEF) is the peak flow occurring during exhalation and is used primarily for home monitoring of patients with asthma and for determining diurnal variations in airflow.

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, and absence of early termination of expiration (eg, minimum exhalation time of 6 sec with no change in volume for the last 1 sec). 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: Lung volumes (see Fig. 2: Tests of Pulmonary Function (PFT): Normal lung volumes.) are measured by determining functional residual capacity (FRC) and with spirometry.

Fig. 2
Normal lung volumes. TLC = total lung capacity; VT = tidal volume; ERV = expiratory reserve volume; IRV = inspiratory reserve volume; FRC = functional residual capacity; IC = inspiratory capacity; VC = vital capacity; RV = residual volume; FRC = RV + ERV; IC = VT + IRV; VC = VT + IRV + ERV.

FRC is measured using gas dilution techniques or body-box plethysmography. Gas dilution techniques include nitrogen washout and helium equilibration. With nitrogen washout, the patient exhales to FRC and then breathes from a spirometer containing 100% O₂. 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 connects to a closed system containing known volumes of helium and O₂. 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 is estimated by helium dilution. Both of these techniques may underestimate FRC because they measure only the lung volume that communicates with the upper airways, and in patients with severe airflow limitation, a considerable volume of trapped gas may communicate very poorly or not at all.

Body-box 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 a calculation of the change in box volume, which must equal the change in lung volume.

Knowing FRC allows the lung to be divided into subvolumes that are either measured with spirometry or calculated (see Fig. 2: Tests of Pulmonary Function (PFT): Normal lung volumes.). Normally the FRC represents about 40% of total lung capacity (TLC).

Flow-volume loop: In contrast to the spirogram, which displays flow (in L) over time (in sec), the flow-volume loop (see Fig. 3: Tests of Pulmonary Function (PFT): Flow-volume loops.) displays flow (in L/sec) 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 flows are appropriate for a particular lung volume. For example, flow is normally slower at low lung volumes. Because patients with pulmonary fibrosis have low lung volumes, flow appears to be decreased if measured alone. However, when flow is measured against lung volumes, it becomes apparent that flow is actually higher than normal because of the increased elastic recoil characteristic of fibrotic lungs.

Fig. 3
Flow-volume loops. This table is presented as a PDF and requires the free Adobe PDF reader. Get Adobe Reader

Flow-volume loops require that absolute lung volumes be measured. Unfortunately, many laboratories simply plot flow against the FVC; the flow-FVC loop does not have an inspiratory limb and therefore does not provide as much information.

Patterns of Abnormalities

Most common respiratory disorders can be categorized as obstructive or restrictive on the basis of flow rates and lung volumes (see Table 1: Tests of Pulmonary Function (PFT): Characteristic Physiologic Changes Associated With Pulmonary Disorders).

Table 1
Characteristic Physiologic Changes Associated With Pulmonary Disorders Measure Obstructive Disorders Restrictive Disorders Mixed Disorders FEV1/FVC Decreased Normal or increased Decreased FEV1 Decreased Decreased, normal, or increased Decreased FVC Decreased or normal Decreased Decreased TLC Normal or increased Decreased Decreased RV Normal or increased Decreased Decreased, normal, or increased FEV1 = forced expiratory volume in 1 sec; FVC = forced vital capacity; TLC = total lung capacity; RV = residual volume.

Obstructive disease: Obstructive disease is a reduction in flow rates, particularly the FEV₁ and the FEV₁ as a percentage of the FVC (FEV₁/FVC). The reduction in FEV₁ determines the degree of the obstructive defect (see Table 2: Tests of Pulmonary Function (PFT): Severity of Obstructive and Restrictive Lung Diseases). Obstructive defects are caused by increased resistance to flow from abnormalities within the airway lumen (eg, tumors, secretions, mucosal thickening); changes in the wall of the airway (eg, contraction of smooth muscle, edema); or elastic recoil (eg, the parenchymal destruction that occurs in emphysema). With decreased flow rates, expiratory times are longer than usual, and air may become trapped in the lungs from incomplete emptying and increased lung volumes (eg, TLC, RV).

Table 2
Severity of Obstructive and Restrictive Lung Diseases Obstructive Restrictive Severity*   FEV1/FVC (% predicted)           FEV1      (% predicted)        TLC (% Predicted) Normal ≥ 70 ≥ 80 ≥ 80 Mild < 70 ≥ 80 70–79 Moderate < 70 50 ≤ FEV1 < 80 50–69 Severe < 70 30 ≤ FEV1 < 50 < 50 Very severe < 70 < 30 or < 50 with chronic respiratory failure — *Criteria vary by guideline. FEV1 = forced expiratory volume in 1 sec.

Improvement of FEV₁ and FEV₁/FVC by ≥ 12% or 200 mL with the administration of a bronchodilator confirms the diagnosis of asthma or airway hyperresponsiveness. However, some patients with asthma can have normal pulmonary function and normal spirometric parameters between exacerbations. When suspicion of asthma remains high despite normal spirometry, provocative testing with methacholine, a synthetic analog of acetylcholine that is a nonspecific bronchial irritant, is indicated to detect or exclude bronchoconstriction. In a methacholine challenge test, spirometric parameters are measured at baseline and after inhalation of increasing concentrations of methacholine. Laboratories have different definitions of airway hyperreactivity, but in general a provocative concentration of methacholine that causes a 20% drop in FEV₁ from baseline (PC₂₀) of < 1 mg/mL is considered diagnostic of asthma, whereas a PC₂₀ > 16 mg/mL excludes the diagnosis. PC₂₀ values between 1 and 16 mg/mL are inconclusive.

Exercise testing may be used to detect exercise-induced bronchoconstriction but is less sensitive than methacholine challenge testing for detecting general airway hyperresponsiveness. The patient performs a constant level of work on a treadmill or cycle ergometer for 6 to 8 min at an intensity selected to produce a heart rate of 80% of predicted maximum heart rate. The FEV₁ and FVC are measured before and 5, 15, and 30 min after exercise. Exercise-induced bronchospasm reduces FEV₁ or FVC ≥ 15% after exercise.

Restrictive disease: Restrictive disease is a reduction in lung volume, specifically, a TLC < 80% of the predicted value. The decrease in TLC determines the severity of restriction (see Table 2: Tests of Pulmonary Function (PFT): Severity of Obstructive and Restrictive Lung Diseases). The decrease in lung volumes produces a decrease in flow rates (reduced FEV₁ and FVC—see Fig. 3: Tests of Pulmonary Function (PFT): Flow-volume loops.B). However, the airflow relative to the specific volume is increased, so the FEV₁/FVC ratio is normal or increased. Restrictive defects are caused by a loss in lung volume (eg, lobectomy), abnormalities of structures surrounding the lung (eg, pleural disease, kyphosis, obesity), weakness of the inspiratory muscles of respiration (eg, neuromuscular disease), or abnormalities of the lung parenchyma (eg, pulmonary fibrosis). The feature common to all is a decrease in the compliance of the lungs, the chest wall, or both.

Last full review/revision November 2005

Content last modified November 2005