What are flow-volume loops? Draw flow-volume loops for a healthy subject and patients with COPD, restrictive lung disease, fixed obstruction of the upper airway, variable extrathoracic obstruction, and variable intrathoracic obstruction.
Answer:
Flow-volume loops provide a graphic analysis of flow at various lung volumes. To perform the test, subjects are asked to inhale maximally (to TLC) and then exhale as forcefully and maximally as possible (to RV). This cycle is repeated. The flow (liters per second) is plotted on the y-axis and volume (liters) on the x-axis (Fig. 1.2A). The test requires a compliant patient for accurate results—the majority of the test is effort dependent, including the entire inspiratory curve and both ends of the expiratory curve (near TLC and RV). The forced expiratory flow during 25% to 75% of VC (FEF25%-75%) is reflective of the small- to medium-sized airways and is considered a relatively effort-independent value.
Normal flow-volume loop (Fig. 1.2A). Inspiratory limb of loop is symmetric and convex. Expiratory limb is linear. Airflow at the midpoint of IC and airflow at the midpoint of expiratory capacity are often measured and compared. Maximal inspiratory flow (MIF) at 50% FVC is greater than maximal expiratory flow (MEF) at 50% FVC because dynamic compression of the airways occurs during exhalation.
Obstructive disorder (e.g., emphysema, asthma) (Fig. 1.2B). Although all airflow is diminished, expiratory prolongation predominates, and MEF < MIF. Peak expiratory flow (PEF) is sometimes used to estimate degree of airway obstruction but depends on patient effort.
Restrictive disorder (e.g., interstitial lung disease, kyphoscoliosis) (Fig. 1.2C). The loop is narrowed because of diminished lung volumes. Airflow is greater than normal at comparable lung volumes because the increased elastic recoil of lungs holds the airways open.
Fixed obstruction of the upper airway (e.g., tracheal stenosis, goiter) (Fig. 1.2D). The top and bottom of the loops are flattened so that the configuration approaches that of a rectangle. Fixed obstruction limits flow equally during inspiration and expiration, and MEF = MIF.
Variable extrathoracic obstruction (e.g., unilateral vocal cord paralysis, vocal cord dysfunction) (Fig. 1.2E). When a single vocal cord is paralyzed, it moves passively with pressure gradients across the glottis. During forced inspiration, it is drawn inward, resulting in a plateau of decreased inspiratory flow. During forced expiration, it is passively blown aside, and expiratory flow is unimpaired. Therefore, MIF 50% FVC < MEF 50% FVC.
Variable intrathoracic obstruction (e.g., tracheomalacia) (Fig. 1.2F). During a forced inspiration, negative pleural pressure holds the floppy trachea open. With forced expiration, loss of structural support results in tracheal narrowing and a plateau of diminished flow. Airflow is maintained briefly before airway compression occurs.
Reference(s):
- Barash PG, Cullen BF, Stoelting RK, et al, eds. Clinical Anesthesia. 8th ed. Philadelphia, PA: Wolters Kluwer; 2017:361-383.
- Jameson JL, Fauci AS, Kasper DL, et al, eds. Harrison’s Principles of Internal Medicine. 20th ed. New York: McGraw-Hill; 2018:1945-1950.
- The Merck Manual: Professional Edition. http://www.merckmanuals.com/professional/pulmonary_disorders/tests_of_pulmonary_function_pft/airflow_lung_volumes_and_flow-volume_loop.html?qt=flow%20volume%20loops&alt=sh. Accessed December 21, 2018.