Flow Volume Loops. by Chris Nickson, Last updated December 3, OVERVIEW. provide a graphical analysis of inspiratory and expiratory flow from various. A variable flow limitation will only appear in either the expiratory or inspiratory portion of the flow-volume loop, and which portion of the flow-volume loop the limitation appears will indicate whether the flow limitation is intrathoracic or extrathoracic. The flow-volume loop is a plot of inspiratory and expiratory flow (on the Y-axis) against volume (on the X-axis) during the performance of maximally forced inspiratory and expiratory maneuvers. Changes in the contour of the loop can aid in the diagnosis and localization of airway obstruction [1].


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It is important that the flow volume loop interpretation is performed correctly on a calibrated spirometer so that the flow-volume loop can be interpreted correctly.

Often GPs like to confirm spirometry results in a lung function laboratory with the test being performed by a qualified respiratory scientist. Because there is no time axis in a flow-volume loop it is not possible to determine FEV1 or FEV6 from a flow-volume loop unless the software includes a marker for these values.


Longitudinal studies have shown that this feature often disappears with increasing age even when flow volume loop interpretation results remain normal.

Signs of airway obstruction in flow-volume loops: Flow-volume loops typically become more concave both with increasing age and with an increase in the severity of airway obstruction.

Assessing Flow-Volume Loops

When airway obstruction is severe, this inward concavity can develop a distinct inflection point. When severe airway obstruction is due to chronic bronchitis, there is generally a severe decrease in Peak Flow and relatively mild concavity.

Signs of restriction in flow-volume loops: When restriction is due to fibrosis flow volume loop interpretation flow is often preserved until lung volumes are significantly reduced.

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There are multiple causes for restriction however, and peak flow is not preserved in all of these. An individual with restrictive lung disease can have a normal looking flow-volume loop that is just reduced in overall size.

Flow Volume Loops

Certain types of airway disorders can also affect the flow-volume loop contour. Paralyzed vocal cords or enlarged goiter or a tumor pressing against the airway can limit flow rates.

However, when airflow is presented as a function of lung volume, flow volume loop interpretation becomes apparent that airflow is actually higher than normal as a result of the increased elastic recoil characteristic of fibrotic lungs.

Inspiratory limb of loop is symmetric and convex. Expiratory limb is linear. Airflow at the midpoint of inspiratory capacity and airflow at the midpoint of expiratory capacity flow volume loop interpretation often measured and compared.

flow volume loop interpretation B Obstructive disorder eg, emphysema, asthma. Peak expiratory flow is sometimes used to estimate degree of airway obstruction but depends on patient effort. C Restrictive disorder eg, interstitial lung disease, kyphoscoliosis. The loop is narrowed because of diminished lung volumes.

Airflow is greater than normal at comparable lung volumes because the increased elastic recoil of lungs holds the airways open. D Fixed obstruction of the upper airway eg, tracheal stenosis, goiter.

Flow Volume Loops | Gold Coast - The Lung Centre

The top and bottom of the loops are flattened so that the configuration approaches that of a rectangle. E Variable extrathoracic obstruction eg, unilateral vocal cord paralysis, vocal cord dysfunction.


When a single vocal cord is paralyzed, it moves passively with pressure gradients across the flow volume loop interpretation. During forced inspiration, it is drawn inward, resulting in a plateau of decreased inspiratory flow. During forced expiration, it is passively blown aside, and expiratory flow is unimpaired.

F Variable intrathoracic obstruction eg, tracheomalacia. During a forced inspiration, negative pleural pressure holds the floppy trachea open.