Respiratory muscle overloading and dyspnoea during bronchoconstriction in asthma: Protective effects of fenoterol

Whether, and to what extent, β₂-agonists protect against respiratory muscle overloading and breathlessness during bronchoconstriction remains to be defined in patients with asthma. In a double blind placebo-controlled study, 100 μg of fenoterol were administered to six stable asthmatics before a bronchial provocation test, performed by inhaling doubling concentrations of histamine from a Devilbiss 646 nebulizer. We recorded breathing pattern (tidal volume V(T), inspiratory time T(I), total time of the respiratory cycle T(TOT)), inspiratory capacity (IC), dynamic pleural pressure swing (P(plsw)), total lung resistance (R(L)) and FEV₁. V(T) was expressed both in actual values and as % of IC. Changes in V(T) (%IC) during histamine inhalation reflected changes in dynamic end-inspiratory lung volume (EILV). P(plsw) was expressed as % of maximal (the most negative in sign) pleural pressure, obtained under control conditions during a sniff manoeuvre (P(plsn)). P(plsw)(%P(plsn)) is an index of inspiratory muscle effort. The test ended when the concentration of histamine which caused a decrease in FEV₁ of ≥ 40% post-saline was reached. Dyspnoea rating was scored by a modified Borg scale. At the ultimate degree of bronchoconstriction (UDB) with histamine: (i) decrease in FEV₁ was similar after placebo and fenoterol, while increase in R(L) was lower after fenoterol (P <0.005); (ii) V(T)(%IC) increased less after fenoterol (P <0.027); (iii) increases in P(plsw)(%P(plsw)) was lower after fenoterol (P <0.001); (iv) ΔBorg (from saline) was lower (P <0.01) after fenoterol; (v) differences in ΔBorg, from placebo to fenoterol, related to concurrent changes in V(T)(%IC) (r² = 0.67). In conclusion, at UDB 100 μg of fenoterol produced a beneficial effect on the degree of inspiratory muscle loading and breathlessness, an effect greater than it would be expected from measuring FEV₁ alone.