Perception of Acute or Progressive Resistive Loads in Normal and Asthmatic Subjects

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Abstract
To determine if the rapidity of increase in airway resistance influences its perception, we looked at the perception of acute and progressive expiratory resistive loads in 9 controls (CN) and 9 asthmatics (AS). Each had 4 single-blind tests in a randomized order, during which resistances were increased from 0 to 24 cm H2O/l/s in 1,3 or 6 steps or decreased in 6 steps. Dyspnea scores were recorded on a modified Borg scale (0-10). FEV1 and lung volumes were measured in all subjects initially and after the last resistance applied during tests C and D. Tidal volume, respiratory rate, the ratio of inspiratory time over total breathing time (Ti/Ttot) and minute ventilation were measured throughout each test. Borg scores were not significantly different from one test to the other. Overall, AS tended to have a higher perception of resistive loads than CN, although it did not reach statistical significance. FEV1 did not change significantly in both groups between the tests and before and after application of resistances. Functional residual capacity was not significantly different between AS and CN or before and after the tests. Residual volume was higher in AS (mean of the 4 tests: 1.6 ± 0.05 1) compared to CN (1.1 ± 0.14 1) (p = 0.003) but was unchanged after the applied resistances. Ti/Ttot ratio was similar for AS and CN and decreased significantly from 0 to 24 cm H2O/l/s in both groups (all tests, p 2O/l/s in AS or CN. Tidal volume was significantly greater in AS than in CN (24 cm H2O/l/s: AS 0.68 ± 0.101, CN 0.54 ± 0.091, p = 0.027) and minute ventilation was higher in CN than in AS (24 cm H2O/l/s: AS 7.9 ± 1.3 1/ min, CN 12.9 ± 1.61/min, p = 0.001). In conclusion, in AS compared to CN, application of external resistive loads was associated with a lower minute ventilation, an increased tidal volume and a trend towards a keener perception. Perception of maximal resistive loads was similar whenever they were applied slowly or acutely.

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