(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.