Thixotropy of rib cage respiratory muscles in normal subjects

In this study, we searched for signs of thixotropic behavior in human rib cage respiratory muscles. If rib cage respiratory muscles possess thixotropic properties similar to those seen in other skeletal muscles in animals and humans, we expect resting rib cage circumference would be temporarily changed after deep rib cage inflations or deflations and that these aftereffects would be particularly pronounced in trials that combine conditioning deep inflations or deflations with forceful isometric contractions of the respiratory muscles. We used induction plethysmography to obtain a continuous relative measure of rib cage circumference changes during quiet breathing in 12 healthy subjects. Rib cage position at the end of the expiratory phase (EEP) was used as an index of resting rib cage circumference. Comparisons were made between EEP values of five spontaneous breaths immediately before and after six types of conditioning maneuvers: deep inspiration (DI); deep expiration (DE); DI combined with forceful effort to inspire (FID or expire (FEI); and DE combined with forceful effort to inspire (FIE) or expire (FEE), both with temporary airway occlusion. The aftereffects of the conditioning maneuvers on EEP values were consistent with the supposition that human respiratory muscles possess thixotropic properties. EEP values were significantly enhanced after all conditioning maneuvers involving DI, and the aftereffects were particularly pronounced in the FII and FEI trials. In contrast, EEP values were reduced after DE maneuvers. The aftereffects were statistically significant for the FEE and FIE, but not DE, trials. It is suggested that respiratory muscle thixotropy may contribute to the pulmonary hyperinflation seen in patients with chronic obstructive pulmonary disease.