Airflow, Lung Volumes, and Flow-Volume Loop
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.
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, 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 sec [FEV1]—see Figure: Normal spirogram.). Most currently used devices measure only airflow and integrate time to estimate the expired volume.
In assessments of inspiratory flow and volume, patients exhale as completely as possible, then forcibly inhale. 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 FEV1is the most reproducible flow parameter and is especially useful in diagnosing and monitoring patients with obstructive pulmonary disorders (eg, asthma, COPD). 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.
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. The peak expiratory flow (PEF) is the peak flow occurring during exhalation. This variable 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 (see Figure: Normal lung volumes.) are measured by determining functional residual capacity (FRC) and with spirometry.
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
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 is connected 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 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.). Normally the FRC represents about 40% of total lung capacity (TLC).
In contrast to the spirogram, which displays airflow (in L) over time (in sec), the flow-volume loop (see Figure: Flow-volume loops.) displays airflow (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 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 require that absolute lung volumes be measured. Unfortunately, many laboratories simply plot airflow against the FVC; the flow-FVC loop does not have an inspiratory limb and therefore does not provide as much information.
Most common respiratory disorders can be categorized as obstructive or restrictive on the basis of airflow and lung volumes ( Characteristic Physiologic Changes Associated With Pulmonary Disorders).
Characteristic Physiologic Changes Associated With Pulmonary Disorders
Obstructive disorders are characterized by a reduction in airflow, particularly the FEV1 and the FEV1 expressed as a percentage of the FVC (FEV1/FVC). The degree of reduction in FEV1 compared with predicted values determines the degree of the obstructive defect ( Severity of Obstructive and Restrictive Lung Disorders*). Obstructive defects are caused by
With decreased airflow, expiratory times are longer than usual, and air may become trapped in the lungs due to incomplete emptying, thereby increasing lung volumes (eg, TLC, RV).
Severity of Obstructive and Restrictive Lung Disorders*
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 results, 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 patients showing at least a 20% drop in FEV1 from baseline (PC20) when the concentration of inhaled methacholine is < 1 mg/mL is considered diagnostic of increased bronchial reactivity, 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 does 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. Eucapnic voluntary hyperventilation (EVH) may also be used to diagnose exercise-induce bronchoconstriction and is the method accepted by the International Olympic Committee. EVH involves hyperventilation of a gas mixture of 5% CO2 and 21% O2 at 85% of maximum voluntary ventilation for 6 min. FEV1 is then measured at specified intervals after the test. As with other bronchial provocation tests, the drop in FEV1 that is diagnostic of exercise-induced bronchospasm varies by laboratory.
Restrictive disorders are characterized by a reduction in lung volume, specifically a TLC < 80% of the predicted value. The decrease in TLC determines the severity of restriction ( Severity of Obstructive and Restrictive Lung Disorders*). The decrease in lung volumes causes a decrease in airflow (reduced FEV1—see Figure: Flow-volume loops. B). However, airflow relative to lung volume is increased, so the FEV1/FVC ratio is normal or increased.
Restrictive defects are caused by the following:
The feature common to all is a decrease in the compliance of the lungs, the chest wall, or both.