The effect of learning on ventilatory responses to inspiratory threshold loading in COPD

Background: Progressive threshold loading (PTL) is a common test of respiratory muscle endurance. Healthy naïve subjects improve endurance with successive exposures to PTL by altering their breathing responses, thus necessitating a familiarization period before reproducible measures can be obtained. This study sought to determine whether a similar "learning effect" is evident in patients with COPD, and what the mechanism of any such effect may be. Methods: Ten subjects with COPD (FEV₁ 34±13% predicted underwent PTL on four occasions (>24h apart). During PTL measurements were obtained of breathing pattern and maximum threshold pressure (P_thmax) achieved. Maximum inspiratory pressure (P_Imax) was measured on each occasion. Results: Over the four tests P_Imax improved by 21±16% (SD) (P<0.05) and P_thmax by 32±21% (P<0.05) with a plateau in these measures achieved by test three. P_thmax/ P_Imax was unchanged, being 61±11% at test one and 67±12% at test four. In contrast to healthy subjects, PTL was not associated with increased expiratory time or decreased end-expiratory lung volume. Conclusions: In contrast to P_Imax and P_thmax, which changed with successive tests, a single measure of the ratio P_thmax/P_Imax may present a useful guide to the endurance capacity of the respiratory muscles in patients with COPD.