This subject coughed during the forced expiration. Typically the glottis is nearly closed (flow drops to slightly above 0), removing the flow-limiting segment from the intrathoracic airways. The glottis is subsequently opened rapidly, giving rise to a new peak flow and re-establishment of a flow-limiting segment. Neither the FVC nor the FEV1 can be reliably measured during cough, so that recorded data should not be interpreted. The fact that the maneuver induces coughing is medically relevant: in healthy subjects this is rare, but it is often observed in subjects with hyperreactive airways.
See also: Flow-volume curve without cough