With spirometric measurements volumes are plotted on the horizontal (X) axis and flows on the vertical (Y) axis = flow-volume loop. The flow-volune loop allows the assessment of additional flows such as FEFmax, FEF50%, FEF75% and FIF50%. The decrease of mid-expiratory flow rates is a typical finding in COPD patients and is an expression of flow limitation. Two mechanisms explain expiratory flow limitation in COPD patients: an intrinsic mechanism, i.e., decrease of the caliber of the small airways (inter diameter of < 2mm) by inflammation, fibrosis, and mucus plugging; and an extrinsic mechanism, one due to lack of parenchymal support of the peripheral airways as a consequence of disruption of the elastic network of the lung. An enhanced collapsibility of the flow-limiting airways.
If that made sense to you, congratulations. Because I don't hardly understand word one. Stupid pulmonologists...This woman talked for an hour and a half, and we left dumber for it.
To understand these shape variations of the flow-volume loop, it is important to consider the pressure gradient across the walls of the airway during inspiration and expiration. First, breathing through an external orifice of 6mm reduces peak flows and produces plateaus on both inspiration and expiration. During inspiation the extrathoracic airway has a transmural pressure favouring narrowing because intraluminal pressure is subatmospheric while extraluminal pressure is approximately atmospheric. In variable extrathoracic lesions, during expiration intraluminal pressure is positive relative to extraluminal pressure, thus tending to dilate the airway and obscure the presence of the lesion. When a variable lesion is intrathoracic in location, during inspiration extraluminal pressure (equivalent to pleural pressure) is (-) relative to intraluminal pressure so that transmural pressure favors airway dilatation. During expiration extraluminal pressure is positive relative to intraluminal pressure so that airway narrowing occurs.