Background and objective: Thixotropy conditioning of inspiratory muscles changes the end-expiratory position of the respiratory system during resting breathing. We examined the immediate effects of thixotropy conditioning of inspiratory muscles on chest wall inflation induced by CPAP.
Methods: A cross-over design study was performed in 13 healthy men. Operating chest wall volume (Vcw) was measured by respiratory inductive plethysmography. Conditioning consisting of a 5 s inspiratory effort at a mask pressure of -20 cm H(2)O was performed under CPAP (10 cm H(2)O) at three Vcw values to change the muscle length at which conditioning occurred: (i) FRC at a CPAP of 10 cm H(2)O (FRC(10)); (ii) FRC at a CPAP of 0 cm H(2)O (FRC(0)); and (iii) RV at a CPAP of 10 cm H(2)O (RV(10)).
Results: CPAP (10 cm H(2)O) increased FRC by 0.60-0.70 L. Decreases in operating Vcw were noted after conditioning at RV(10), which decreased end-expiratory Vcw by 0.16 +/- 0.12 L and end-inspiratory Vcw by 0.20 +/- 0.14 L at 180 s (P < 0.01). Inspiratory capacity (1.59 +/- 0.45 L) and inspiratory reserve volume (1.18 +/- 0.40 L) were greater than preconditioning values (1.44 +/- 0.49 L and 0.99 +/- 0.41 L, respectively; P < 0.01). Conditioning at FRC(0) also decreased operating Vcw significantly, but conditioning at FRC(10) did not change operating Vcw.
Conclusions: Thixotropy conditioning of inspiratory muscles performed below inflated FRC decreased the level of CPAP-induced inflation.