Magnitude scaling of externally added loads to breathing.

Externally added resistive and elastic loads to breathing elicit a sensation the psychological magnitude of which was described by a power function: psi = k theta n. The perceived magnitude (psi) was related to the physical magnitude (theta) by the exponential term (n). The exponent described the increase in sensation for an increase in load. In normal subjects, the mean exponent for added resistive loads was 0.80 +/- 0.35 SD, and for elastic loads was 0.99 +/- 0.45. Intrasubject variability (mean coefficient of variation = 7%) was much less than intersubject variability (coefficient of variation = 230%). The exponents for elastance and resistance were highly correlated (r = 0.92). In these studies, the subjects freely adopted their tidal volume and flow rate. During the addition of resistive loads, the subjects tended to reduce their flow rate, especially at the highest loads, and during the elastic loads, they tended to reduce their tidal volume. Fixing tidal volume during the addition of elastic loads and flow rates during the addition of resistive loads caused an increase in exponent. This finding implied that the tidal volume and flow rate, or factors related to them such as muscular effort, are important attributes of the sensation elicited by loaded breathing.