pulmonary disorders
Airflow, Lung Volumes, and Flow Volume Loop
Flow volume loop
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). (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<MIF. Peak expiratory flow is sometimes used to estimate degree of airway obstruction but depends on patient effort. (C) Restrictive disorder(eg, interstitial lung disease, kyphoscoliosis). 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. (D) Fixed obstruction of the upper airway(eg, tracheal stenosis, goiter). 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. (E) Variable extrathoracic obstruction(eg, unilateral vocal cord paralysis, vocal cord dysfunction). 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. (F) Variable intrathoracic obstruction(eg, tracheomalacia). 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. FVC = forced vital capacity; MEF = maximal expiratory flow; MIF = maximal inspiratory flow; PEF = peak expiratory flow; RV = residual volume; TLC = total lung capacity. 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. 