Assessment of dyspnoea.

The recognition of dyspnoea or breathlessness and the assessment of its severity is clinically based on the intensity of functional activity, such as walking or stair-climbing, that is required to produce discomfort in the act of breathing. This approach is similar to the measurement of sensory intensity by determination of the smallest change in stimulus magnitude that an individual can reliably perceive, and then subsequently use in the summation of just noticeable differences (JND) in stimulus magnitude from the threshold of detection, to indicate magnitude. The criterion state in the assessment of breathlessness is obvious discomfort, where as with the Fechnerian approach the criterion state is a JND, which may be precisely defined. Both techniques are indirect, in that the sensory magnitude is inferred from the stimulus magnitude. The fundamental process involved in making any measurement is the "matching" of one continuum (number continuum being one specific example) to another, while conforming to preset rules. The rules and the independent continuum define a "scale". The types of scaling may be grouped into; nominal, to distinguish one object or event from another; ordinal, to rank objects or events in order of magnitude; interval, to determine equality and magnitude of differences in objects or events whilst preserving rank order; and ratio, to determine rdtio relationships. STEVENS [23-27] made the initially controversial observation that there is no fundamental reason why sensory intensity cannot be matched with preset rules and showed that sensory intensity can be directly scaled by "open magnitude scaling". He went on to show that sensations varying in intensity (sound, light, lifted weights etc.) can be matched by a variety of continuums (cross modality matching). whilst observing the preset rule of ratio relationships. The perceptual magnitude (Y) of specific sensations increases in a unique way relative to the physical magnitude of a stimulus (ST) as expressed by the equation: