Figure 2: Typical dose-response curves in normals (right curve), mild asthmatics (middle curve) and moderate to severe asthmatics (left curve). Although responsiveness can be measured in normals, the curve does not reach the 20% decline in forced expiratory volume in 1 s (FEV1). Therefore, these individuals do not have a PD20; i.e. a provocative dose that decreases FEV1 by 20%. However, several other indices can be used to assess their level of airway responsiveness. The position of the curves is a measure of sensitivity. A leftward displacement of the curve indicates hypersensitivity, i.e. that responsiveness can be measured at lower doses. The slope of the curves is a measure of airway reactivity. A greater dose-response slope (DRS) indicates a greater airway reactivity, i.e. that the magnitude of the response above the threshold point is greater. The plateau is a measure of the maximal achievable response. A higher plateau indicates a greater maximal achievable response. The dose-response curve in moderate to severe asthmatics is oftentimes characterized by the absence of a plateau.
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