The downstroke of the flow-volume loop

The downstroke follows the upstroke of the expiratory limb of the flow-volume loop.

The upstroke of the flow volume loop, which precedes the down stroke, is the period from full inflation to peak expiratory flow rate. This component is effort dependent; correctly performed, the respiratory muscles force air out of the lungs at great velocity.

Following the sharp rise to peak flow, you can see that suddenly the slope falls downwards; this reflects the slowing of air flow as the lungs empty. The longer the subject exhales, the slower the airflow becomes until the patient reaches residual volume, where airflow ceases.

Flow-volume loop

This downwards sloping curve reflects diminishing lung volume and increased airway resistance, particularly of the more peripheral airways. The positive pleural pressure generated by the contraction of expiratory muscles causes the peripheral airways to narrow – the peripheral airways are more prone to this as they have no cartilaginous support. This will further reduce the lumen of the airways, and increase airway resistance significantly.

Unlike the upstroke, however, the downstroke is effort independent. Rather than the driving force generated by the respiratory muscles, the lung parenchyma is now limiting airflow.

Any pathology which increases resistance within the airways, or reduces the elastic recoil of the lungs (reduced radial traction of the airways), such as COPD, will ultimately lead to reduced airflow.

This is evident by the downslope, with a typical scalloped (concave) pattern. In severe cases, a sharp rise to a peak flow is followed immediately by a sharp fall in flow rates and prolonged emptying of the lungs at very low flow rates.

Any pathology which increases resistance within the airways, or reduces the elastic recoil of the lungs (reduced radial traction of the airways), such as COPD, will ultimately lead to reduced airflow.

This is evident by the downslope, with a typical scalloped (concave) pattern. In severe cases, a sharp rise to a peak flow is followed immediately by a sharp fall in flow rates and prolonged emptying of the lungs at very low flow rates. This ‘steeple’ shaped flow-volume curve will be examined later in this course.

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