Inspiratory and expiratory resistive loading as a model of dyspnea in asthma

20 ambulatory asthmatics were questioned regarding their perception of the dyspnea of acute asthmatic attacks; in particular, its relationship to the phase of respiration. 19 (95%) stated that inspiration was more difficult than expiration although 5 (25%) reported that they had been taught that the reverse was 'correct'. In order to explore further these observations, we applied psychophysical methods to determine the phase relationship of the conscious perception of resistive loads in healthy volunteers. 14 subjects of similar age and sex distribution to the asthmatics estimated numerically the magnitude of resistive loads applied in random sequence to either the inspiratory or expiratory arm of a low-resistance breathing circuit. The relationship between perceived magnitude (psi) and physical magnitude (phi) was described by Steven's law: psi = k phi n, where k and n are constants. The mean exponent (n) for inspiratory resistances was 0.69 +/- 0.28 (+/- SD) and for expiratory resistances was 0.51 +/- 0.27 (p less than 0.01 by two-tailed, paired t test). There was a positive correlation (r = 0.82) between inspiratory and expiratory exponents within individuals. In 12 of 14 subjects, the perceived magnitude of inspiratory loads was greater than the perceived magnitude of expiratory loads for all resistances greater than 10 cm H2O/l/s. At a load of 25 cm H2O/l/s, the inspiratory sensation of load exceeded the expiratory sensation in all subjects. Our findings that normal subjects scaled greater breathlessness with inspiratory versus expiratory flow resistive loads were consistent with the clinical observation that inspiratory, rather than expiratory difficulty contributed more to the dyspnea of asthma.