From the Guidelines
The shape of the flow-volume loop suggesting variable extrathoracic pressure is indicative of upper airway obstruction, which can be caused by conditions such as vocal cord dysfunction, and is characterized by a flattened inspiratory limb with a normal expiratory limb. This pattern is a result of the negative intrathoracic pressure created during inspiration, which causes the extrathoracic airway to narrow in the presence of structural abnormalities. According to the expert panel report 3 (EPR-3) guidelines for the diagnosis and management of asthma, variable flattening of the inspiratory flow loop on spirometry is strongly suggestive of vocal cord dysfunction 1. The standardisation of spirometry guidelines also highlight the importance of the shape of the maximum flow–volume loop in detecting upper airway obstruction, although they note that numerical indices from this loop have limited clinical utility compared to other measures such as FEV1, FVC, and PEF 1.
Key Points to Consider
- The flattened inspiratory limb with a normal expiratory limb on the flow-volume loop is characteristic of variable extrathoracic obstruction
- Vocal cord dysfunction is a common cause of this pattern and should be considered in the differential diagnosis, especially in patients with atypical or difficult-to-treat asthma symptoms 1
- Additional diagnostic tests such as laryngoscopy or bronchoscopy may be necessary to identify the specific cause of the obstruction
- Treatment will depend on the underlying condition, with options including speech therapy for vocal cord dysfunction or surgical intervention for other causes of obstruction.
Clinical Implications
The identification of variable extrathoracic pressure on a flow-volume loop has significant implications for patient management, as it suggests the presence of an upper airway obstruction that may require specific treatment. By recognizing this pattern and considering the potential causes, clinicians can provide more targeted and effective care for their patients.
From the Research
Flow-Volume Loop Analysis
The shape of the flow-volume loop can provide valuable information about lung function and potential airway obstruction.
- The flow-volume loop is a graphical representation of the relationship between airflow and lung volume during inhalation and exhalation.
- Abnormalities in the flow-volume loop can indicate various types of airway obstruction, including extrathoracic and intrathoracic obstruction.
Extrathoracic Airway Obstruction
Extrathoracic airway obstruction can be variable, meaning that it can change with different respiratory maneuvers.
- A study published in 1994 found that the FEF 50%/FIF 50% ratio was a useful indicator of fixed and variable extrathoracic upper airway obstruction 2.
- Another study published in 2009 found that an abnormal inspiratory curve on the flow-volume loop can be indicative of extrathoracic upper airway obstruction, and that evaluation of the inspiratory curve is important for detecting this type of obstruction 3.
- A 2020 study highlighted the use of the flow-volume curve as a diagnostic tool in extrathoracic airway obstruction, demonstrating its ability to evaluate the severity and progression of various upper-airway conditions 4.
Flow-Volume Loop Patterns
Different patterns on the flow-volume loop can indicate various types of lung disease.
- A study published in 2013 found that patients with tracheobronchomalacia often had characteristic flow-volume loop patterns, including low maximum forced expiratory flow, biphasic expiratory curve, flow oscillations, and notching 5.
- A 2017 review article discussed the clinical use of pulmonary function tests, including the flow-volume loop, and highlighted the importance of interpreting these tests in conjunction with chest imaging and other ancillary data 6.