Dynamic Hyperinflation: Is It Worth Measuring?

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A reduced exercise capacity is an important determinant of health status and an independent prognostic marker in patients with chronic obstructive pulmonary disease. The inability to increase expiratory flow at the resting end-expiratory lung volume in the face of expiratory-flow limitation means that end-expiratory lung volume must increase if gas exchange is to be maintained near normal values. This phenomenon is usually referred to as dynamic hyperinflation. The change in operating lung volumes during exercise is related to the intensity of breathlessness. Treatments such as bronchodilators that increase inspiratory capacity or supplemental oxygen, which reduces ventilatory demand, decrease the degree of dynamic hyperinflation at any external workload. However, dynamic hyperinflation is not seen universally in patients with chronic obstructive pulmonary disease as some adopt different breathing patterns when they exercise, or respond to inhaled bronchodilators by changing their pattern of abdominal muscle activation, a behavior that can be counterproductive. Finally, dynamic hyperinflation can be reduced when, for example, breathing oxygen after exercise without changes in dyspnea, as other factors are more important determinants of this symptom in these circumstances. Dynamic hyperinflation can be reliably measured from the inspiratory capacity maneuver in many laboratories. Although knowledge about this variable gives great insight into the mechanisms of therapy, its routine measurement cannot currently be recommended as it does not appear to add additional clinical data beyond those available in present laboratory exercise testing protocols.

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