A flow volume curve is a graph of a person’s breathing, showing how much air they can exhale and how quickly. Generated during a procedure called spirometry, this test provides a snapshot of lung function. The resulting graph helps clinicians identify potential issues with airflow or lung capacity.
The Spirometry Test and Its Graph
To generate a flow volume curve, a patient undergoes a spirometry test. The patient is seated, often with a clip on their nose to ensure all air is expelled through the mouth. They then breathe into a mouthpiece connected to a spirometer, which measures the amount and speed of air inhaled and exhaled.
The core of the test is a specific breathing maneuver. The patient is instructed to take the deepest breath possible, filling their lungs completely. They then blast the air out as forcefully and rapidly as they can, continuing to exhale until their lungs feel empty. This entire effort is recorded by the spirometer.
This data is plotted on a graph to create the flow volume curve. The vertical axis (Y-axis) represents the rate of airflow in liters per second. The horizontal axis (X-axis) shows the total volume of air exhaled in liters. Plotting the speed of air against the volume creates a visual representation of lung performance during a forced exhalation.
Anatomy of a Normal Curve
A normal flow volume curve has a distinct shape. The graph begins with a sudden, sharp increase in flow, reaching a maximum point almost immediately. This peak is followed by a steady, almost straight-line decline as the lungs empty. The entire shape resembles a triangle with a rapid ascent and a longer, more gradual descent.
Several measurements are derived from this graph. The highest point on the curve is the Peak Expiratory Flow (PEF), which represents the fastest rate a person can exhale. The total distance the curve travels along the horizontal axis represents the Forced Vital Capacity (FVC). The FVC is the total amount of air that can be forcefully exhaled after a full inspiration.
Another measurement is the Forced Expiratory Volume in 1 second (FEV1), the volume of air expelled in the first second of exhalation. These values—PEF, FVC, and FEV1—are compared against predicted normal values. These norms are calculated based on a person’s age, height, sex, and ethnicity, providing a baseline for comparison.
Recognizing Abnormal Patterns
Deviations from the normal triangular shape can indicate different lung conditions. These abnormalities fall into two main categories: obstructive and restrictive patterns. Each pattern has a distinct visual appearance on the graph that provides clues about the underlying issue.
An obstructive pattern suggests difficulty getting air out of the lungs, seen in conditions like asthma or Chronic Obstructive Pulmonary Disease (COPD). The flow volume curve has a characteristic “scooped-out” or concave appearance. The descending part of the curve bows inward, indicating that airflow slows down more than it should. This happens because the airways may narrow or collapse during forced exhalation, impeding airflow.
A restrictive pattern indicates a problem with the lungs’ ability to hold a normal amount of air. This can be caused by conditions like pulmonary fibrosis, where lung tissue is stiff, or by issues with the chest wall that limit lung expansion. The flow volume curve looks like a miniature version of a normal curve. Its triangular shape is preserved but is significantly smaller in size, indicating the total volume of air is reduced.
In an obstructive pattern, the FEV1 is reduced more significantly than the FVC. In a restrictive pattern, both the FEV1 and FVC are proportionally reduced. This distinction, combined with the shape of the curve, helps clinicians differentiate between the two types of lung disease.
Clinical Use in Diagnosis and Management
The flow volume curve is a practical tool for diagnosis and the ongoing management of lung conditions. Clinicians use the results to track the course of a respiratory illness and to evaluate the effectiveness of treatments. This provides objective measurements of lung function for patient care over time.
One use of serial testing is to monitor disease progression. By comparing tests taken over months or years, a physician can determine if a patient’s lung condition is improving, worsening, or stable. This information is used for making decisions about treatment plans.
The test is also used to assess how a patient is responding to medication. For example, a person with asthma may perform a test before and after using a bronchodilator inhaler. An improvement in the FEV1 or a change in the curve’s shape after the medication indicates the treatment is effective at opening the airways. This pre- and post-medication testing helps tailor treatment to the individual.
The flow volume curve can also be part of a pre-surgical evaluation. For patients undergoing major surgery, assessing lung fitness helps in understanding and mitigating potential post-operative risks. The test provides a baseline of respiratory function that helps the surgical team prepare for and manage the patient’s care.