The Flow-Volume loop

The traditional way of evaluating the FVC manoeuvre is by displaying the volume change against time (volume-time trace) where the FVC and FEV1 can be directly measured.

If this slope (in L/s) is plotted against exhaled volume (in L), then we get a flow-volume trace, from which flow at any volume can be directly measured. Notice how the time as an independent variable is lost.

The flow-volume loop

On the x-axis is volume, which is measured in Litres, and on the y-axis is flow, which is measured in litres per second. Remember these values are attained from the same spirometry manoeuvre that gives us the volume-time trace. The volumes measured will range from 3L in a small female to around 7L in a tall male. Maximal flow rates will reach 5L/s and 12L/s, respectively.

The flow-volume loop provides a graphical illustration of a patient’s spirometric values. It also adds further data to the dynamic volumes, such as the peak expiratory flow (PEF) and the mid-flow rates (MEF’s).

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