The flow-volume loop is a diagnostic tool used in pulmonary function testing. It visually represents how quickly air moves in and out of the lungs, helping healthcare professionals identify various respiratory conditions.
Understanding the Flow Volume Loop
A flow-volume loop plots airflow against lung volume during forced inspiration and expiration. Airflow, measured in liters per second, is represented on the vertical (y) axis, while lung volume, in liters, is shown on the horizontal (x) axis. The loop consists of two main parts: the expiratory limb and the inspiratory limb.
The expiratory limb, appearing above the x-axis, illustrates the air forcefully exhaled from the lungs, starting from total lung capacity (TLC) and ending at residual volume (RV). The peak expiratory flow (PEF) is the highest point on this limb, indicating the maximum speed of exhalation. The inspiratory limb, below the x-axis, shows the air forcefully inhaled back into the lungs.
How the Loop is Generated
The flow-volume loop is created during a spirometry test, a common pulmonary function test. During this procedure, a patient sits upright with a nose clip to ensure all breathing occurs through the mouth. The patient then breathes into a mouthpiece connected to a spirometer, a device that measures airflow and volume.
The process begins with the patient taking a maximal deep breath to fill their lungs completely, reaching total lung capacity. Next, they exhale as forcefully and completely as possible into the spirometer until their lungs are emptied to residual volume. Immediately following this forceful exhalation, the patient inhales forcefully and completely back to total lung capacity. This sequence of actions generates the characteristic flow-volume loop.
Interpreting Flow Volume Loop Patterns
A normal flow-volume loop has a characteristic appearance. The expiratory limb typically rises rapidly to a peak flow rate and then declines in a relatively linear or slightly convex manner as air is exhaled. The inspiratory limb generally appears symmetrical and rounded or saddle-shaped.
In obstructive lung diseases, such as asthma or chronic obstructive pulmonary disease (COPD), the expiratory limb often appears “scooped out” or concave. This pattern indicates reduced airflow, particularly during the latter part of exhalation, due to increased airway resistance. The overall loop may also show a lower peak expiratory flow.
Conversely, restrictive lung diseases, like pulmonary fibrosis, typically result in a proportionally smaller flow-volume loop that otherwise maintains a relatively normal shape. This smaller size reflects reduced lung volumes, meaning the patient cannot inhale or exhale as much air. Despite the reduced volume, the airflow rates relative to the lung volume may be normal or even higher due to increased elastic recoil.
Fixed airway obstructions, such as tracheal stenosis, cause a flattening of both the inspiratory and expiratory limbs of the loop. This happens because the obstruction consistently limits airflow during both inhalation and exhalation. The loop takes on a box-like or squared appearance, reflecting a constant reduction in flow regardless of the respiratory phase.
Variable airway obstructions affect either the inspiratory or expiratory limb, depending on their location relative to the chest cavity. An extrathoracic (outside the chest) variable obstruction, like vocal cord paralysis, primarily flattens the inspiratory limb. This occurs because negative pressure during inhalation causes the already narrowed airway to collapse further. In contrast, a variable intrathoracic (inside the chest) obstruction, such as tracheomalacia, mainly flattens the expiratory limb. During exhalation, positive intrathoracic pressure can compress the weakened airway, limiting airflow.
Clinical Importance of Flow Volume Loops
Flow-volume loops provide healthcare professionals with a detailed assessment of lung mechanics. They help in diagnosing various respiratory diseases by providing a visual distinction between obstructive and restrictive lung conditions.
Beyond initial diagnosis, these loops are used to monitor disease progression over time. Changes in the loop’s shape and size can indicate whether a condition is worsening or stabilizing. They also aid in assessing a patient’s response to treatment, such as bronchodilators for asthma or COPD, by showing improvements in airflow limitations after medication. Flow-volume loops are also effective at detecting upper airway obstructions, which might be missed by standard spirometry measurements.