A flow volume loop is a specialized graphic representation of air movement during breathing, produced as part of a pulmonary function test known as spirometry. This test helps physicians assess the mechanical function of the lungs and airways by plotting the rate of airflow against the total volume of air inspired and expired. By visually analyzing this unique shape, healthcare providers can gain insight into the presence and type of many respiratory conditions. The loop provides a way to determine if a patient’s airflow is appropriate for their lung volume. This non-invasive diagnostic tool is widely used to distinguish between different categories of breathing problems, guiding diagnosis and treatment plans.
Creating the Flow Volume Loop
The process of generating a flow volume loop requires the patient to follow a specific, maximum effort breathing sequence through a device called a spirometer. To begin the maneuver, the patient first takes a deep, maximal breath, filling their lungs completely to their total lung capacity (TLC).
The patient is then instructed to exhale as forcefully and rapidly as possible until their lungs are emptied down to the residual volume (RV), the air remaining after a maximal forced exhalation. This forceful expiration is the most critical part of the test and must be performed with maximum effort to achieve accurate results. Immediately following this maximal exhalation, the patient takes another deep, rapid breath back up to TLC to complete the loop.
The spirometer continuously measures the speed of air moving in and out of the lungs (flow) and the total amount of air moved (volume) throughout this entire cycle. Because the resulting loop is entirely dependent on the patient’s cooperation and maximum effort, a technician must coach the patient to ensure the test’s validity.
Deconstructing the Healthy Loop
A healthy flow volume loop has a distinct and recognizable shape, which serves as the baseline for interpreting abnormal results. The graph is structured with flow rate, measured in liters per second (L/sec), on the vertical axis and lung volume, measured in liters (L), on the horizontal axis. This plotting creates a closed figure composed of two main sections: an upper limb for expiration and a lower limb for inspiration.
The upper portion, known as the expiratory limb, is characterized by a rapid, sharp rise to a single peak, followed by a gradual, roughly linear decline. The sharpest point, the maximum speed of air out of the lungs, is the peak expiratory flow (PEF). After the PEF, the rate of flow becomes less dependent on muscle effort and more on the mechanical properties of the lungs and airways.
The total width of the loop along the volume axis represents the forced vital capacity (FVC), the total volume of air exhaled from maximal inspiration. A healthy expiratory curve is convex, indicating a sustained flow rate throughout the maneuver.
The lower portion of the loop, the inspiratory limb, represents the air rapidly breathed back into the lungs. This curve is typically smooth, symmetrical, and rounded or “saddle-shaped,” reflecting an even rate of airflow during maximal inhalation. Since inspiration is primarily an active muscular process, its flow rate is effort-dependent throughout the entire maneuver, unlike the later stages of expiration.
Visual Signatures of Lung Disease
Changes in the mechanical properties of the lungs and airways produce characteristic visual alterations in the flow volume loop, allowing for the differentiation of disease patterns. In obstructive lung diseases, such as chronic obstructive pulmonary disease (COPD) or asthma, the primary issue is resistance to airflow, particularly during exhalation. This manifests as a distinct “scooping out” or concave appearance of the expiratory limb, where the flow rate drops off sharply and prematurely in the middle and later parts of expiration.
The shape change reflects the collapse of small airways due to pressure changes during forced exhalation, which severely limits the speed at which air can be expelled. While the total volume exhaled (FVC) may sometimes appear normal, the volume exhaled in the first second (FEV1) is greatly reduced, resulting in a low FEV1/FVC ratio. The inspiratory limb in these conditions often remains relatively normal, as the problem is usually confined to the small conducting airways.
Restrictive lung diseases, like pulmonary fibrosis or conditions affecting the chest wall, are defined by a reduction in total lung volume, making it difficult to fully inflate the lungs. On the flow volume loop, this appears as a loop that is tall and narrow, maintaining a generally normal contour but being significantly smaller overall. The reduced width along the volume axis directly reflects the diminished FVC.
The peak flow rate in restrictive disease may be preserved or even appear higher than predicted because the stiffened, fibrotic lung tissue has increased elastic recoil that snaps the air out quickly. However, because the total lung capacity is decreased, the overall size of the loop is reduced.
A fixed upper airway obstruction, caused by conditions like tracheal stenosis, presents a unique and easily recognizable pattern. Because the obstruction is constant and rigid, it limits airflow equally during both inspiration and expiration. This results in a flattening of both the expiratory and inspiratory limbs, creating a box-like or square appearance. This visual signature indicates that a constant choke point is severely restricting the maximum attainable flow rate.