From the Guidelines
Flow-volume loops are moderately sensitive but highly specific for diagnosing variable upper airway obstruction, with a sensitivity ranging from 60-70% and specificity approaching 90-95%. When interpreting flow-volume loops for variable upper airway obstruction, it is essential to look for characteristic patterns, such as a flattened inspiratory limb in variable extrathoracic obstruction and a flattened expiratory limb in variable intrathoracic obstruction 1. The physiological basis for these patterns relates to the dynamics of airway pressure during breathing, where extrathoracic airways narrow during inspiration due to negative intraluminal pressure, and intrathoracic airways narrow during expiration due to positive pressure 1. Key features to identify include a "box-like" or "plateau" appearance of the affected limb, reduced peak flow rates, and normal vital capacity. To optimize diagnostic yield, proper patient technique is crucial, and comparing results to baseline values when available is recommended. It is also important to note that direct visualization techniques like bronchoscopy remain the gold standard for confirmation when flow-volume loops suggest upper airway obstruction. Some of the key points to consider when evaluating flow-volume loops include:
- The presence of a repeatable plateau of forced inspiratory flow, with or without a forced expiratory plateau, which suggests a variable extrathoracic central or upper airway obstruction 1
- The pattern of a repeatable plateau of forced expiratory flow, along with the lack of a forced inspiratory plateau, which suggests a variable intrathoracic central or upper airway obstruction 1
- The effects of anatomical or functional lesions on maximum flows, which depend on the site of the obstruction, kind of lesion, and extent of anatomical obstruction 1.
From the Research
Sensitivity and Specificity of Flow-Volume Loop
- The sensitivity and specificity of a flow-volume loop in diagnosing variable upper airway obstruction are reported in several studies 2, 3.
- According to a study published in Respiratory Care in 2009, the aggregate sensitivity for detecting upper airway obstruction using a flow-volume loop was 69.4% 2.
- The same study found that the individual quantitative criteria had poor diagnostic performance, but the aggregate criterion performed better with an area under the curve of 0.605 2.
- Another study published in The Journal of Emergency Medicine in 2013, described the use of flow-volume loops in diagnosing bilateral vocal cord paralysis, which can produce severe airway obstruction 3.
- However, the specificity of the flow-volume loop in diagnosing variable upper airway obstruction is not explicitly reported in the provided studies.
Diagnostic Value of Flow-Volume Loop
- The flow-volume loop is a useful tool in diagnosing upper airway obstruction, as it can provide information about the location of the obstruction and differentiate between obstructive pulmonary disease and upper airway obstruction 4, 5, 6.
- The shape of the flow-volume loop can be particularly helpful in diagnosing and localizing upper airway obstruction 5.
- Flow-volume loops can also be used to identify bronchodilator response to treatment and characterize lung disorders such as obstructive or restrictive lung diseases 5.