Abstract
The volumes assessed by optoelectronic plethysmography (OEP) and based on a three-compartmental model provide an accurate breath-by-breath index of expiratory and inspiratory (ribcage muscles and diaphragm) muscle length. Thus, after performing thixotropic maneuvers, OEP may also provide evidence regarding the history-dependent properties of these muscles. We studied the after-effects of different thixotropic conditionings on chest wall (CW) and compartmental operational volumes of 28 healthy subjects (25.5 ± 2.2 years, FVC%pred 94.8 ± 5.5, and FEV1%pred 95.5 ± 8.9) using OEP. Conditionings were composed of inspiratory or expiratory contractions performed from total lung capacity (TLC) or residual volume (RV). The study protocol was composed of three consecutive contractions of the same maneuver, with 60 s of spontaneous breathing in between, and after-effects were studied in the first seven respiratory cycles of each contraction. Cumulative effects were also assessed by comparing the after-effects of each thixotropic maneuver. Inspiratory contractions performed from both TLC and RV acutely increased end-inspiratory (EIV) CW volumes (all p < 0.0001), mainly on both upper and lower ribcage compartments (i.e., non-diaphragmatic inspiratory muscles and diaphragm, respectively); while, expiratory contractions from RV decreased CW volumes (p < 0.0001) by reducing the upper ribcage and abdominal volumes (all p < 0.0001). The response of the thixotropic maneuvers did not present a cumulative effect. In healthy, the use of the three-compartmental model through OEP allows a detailed assessment of the diaphragm, inspiratory and expiratory muscle thixotropy. Furthermore, specific conditioning maneuvers led to thixotropy of the inspiratory ribcage, diaphragm, and expiratory muscles.
Highlights
Skeletal muscle fibers possess complex biophysical properties that make the stiffness and resting tension at a given muscle length dependent on the previous history of movements and contractions
Regarding EEVCW, no significant differences were observed when compared with quiet spontaneous breathing (QB) values
The main findings of this study were that Inspiratory contraction from TLC (ICoTLC) and inspiratory contraction from RV (ICoRV) thixotropic conditionings affect the operational volumes of healthy subjects; i.e., increases EIVCW
Summary
Skeletal muscle fibers possess complex biophysical properties that make the stiffness and resting tension at a given muscle length dependent on the previous history of movements and contractions. These history-dependent properties, referred in the biomedical literature to as thixotropy (Lakie et al, 1984; Hagbarth et al, 1985; Gregory et al, 1998), have several possible explanations. According to Homma and Hagbarth (2000), the rib cage respiratory muscles possess thixotropic properties These authors proposed that the history-dependent passive properties of inspiratory muscles are an important component of expiration because expiratory movements need to stretch inspiratory muscles to reduce lung volume. A series of studies were performed in both healthy (Izumizaki et al, 2004, 2006c) and COPD patients (Izumizaki et al, 2006b, 2008b) in order to understand the aftereffects of different thixotropic conditioning maneuvers on EEV, a measure of functional residual capacity
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