Vocal cord dysfunction (VCD) is a condition where the vocal cords close when they should open. This abnormal movement can obstruct the airway, making it difficult to breathe. A primary tool to investigate such breathing problems is a pulmonary function test that generates a graph called a flow volume loop. This graph provides a visual representation of airflow, offering clues that can point toward a diagnosis of VCD.
Understanding a Normal Flow Volume Loop
A flow volume loop is a graph produced during a spirometry test, which measures lung function. The graph plots the speed of airflow on the vertical axis against the volume of air in the lungs on the horizontal axis. This provides a detailed picture of airflow capacity and can reveal obstructions that might not be apparent from numerical data alone.
In a healthy individual with no airway obstruction, the flow volume loop has a distinct shape. The top portion of the loop, representing exhalation, shows a rapid burst of airflow that peaks quickly and then declines steadily in a near-straight line as the lungs empty.
The bottom portion of the loop graphs the process of inhalation. A normal inspiratory curve is a symmetrical, semi-circular shape. This smooth, rounded curve indicates that air is moving into the lungs without any impediment.
The VCD Pattern on a Flow Volume Loop
Vocal Cord Dysfunction creates a specific alteration to the shape of the flow volume loop. The characteristic sign of VCD is a flattening of the inspiratory part of the loop. This means the bottom, semi-circular curve fails to achieve its normal rounded peak, appearing blunted or cut off. This pattern graphically represents an upper airway obstruction that is variable and located outside the chest cavity (extrathoracic).
This flattening occurs because VCD causes the vocal cords to close during inhalation. As a person tries to breathe in, the narrowed opening at the vocal cords limits how fast air can enter the lungs, no matter how much effort is applied. The spirometer detects this limited airflow speed and plots it as a flattened curve. While inspiratory flattening is the primary indicator, the expiratory part of the loop may also be affected in some cases.
Comparing VCD and Asthma Flow Volume Loops
Distinguishing Vocal Cord Dysfunction from asthma is a challenge, as both can cause shortness of breath and wheezing, leading to frequent misdiagnosis. The flow volume loop is a tool for differentiating between the two conditions because they produce very different graphical patterns. The key to telling them apart lies in which part of the breathing cycle is most affected.
The VCD pattern primarily shows a flattening of the inspiratory loop due to the obstruction at the voice box during inhalation. In contrast, the flow volume loop for an individual with asthma displays a “coved” or “scooped-out” appearance in the expiratory portion of the loop. This happens because asthma causes the smaller airways deep within the lungs to narrow. During a forced exhalation, these narrowed airways are prone to collapse, which slows down the airflow as the person breathes out.
This difference in shape points to the location of the problem. VCD is an upper airway issue affecting air coming in, while asthma is primarily a lower airway issue affecting air going out.
Role of the Flow Volume Loop in VCD Diagnosis
While the flow volume loop is a useful tool for suggesting Vocal Cord Dysfunction, it is not a definitive diagnosis. A flattened inspiratory loop is an indicator, but the test’s utility is highest when the patient is symptomatic. The flow volume loop may appear normal if the person’s vocal cords are functioning properly at the moment of testing, as the patterns are most reliable when symptoms are active.
Because of this variability, the definitive diagnosis of VCD requires direct visualization of the vocal cords. The standard procedure is a flexible laryngoscopy, where a thin scope with a camera is passed through the nose to view the larynx. A clinician can then observe the vocal cords’ movement during breathing.
To confirm the diagnosis, this procedure is often performed as a provocation test where the patient exercises to trigger symptoms. Observing the vocal cords closing during inspiration while the patient is symptomatic provides conclusive evidence of VCD.