Following instrumentation (same as Experiment 1 plus abdominal electrodes and RIP bands), subjects undertook threshold loading. To track changes in EELV, subjects were required to perform one IC maneuver against no external load every minute during loading, and at task failure ( http://www.selleckchem.com/products/CP-690550.html Hussain et al., 2011). The purpose of this experiment, conducted in 8 subjects who sustained inspiratory threshold load to task failure, was: to determine whether contractile fatigue of respiratory muscles contributes to task failure and to identify determinants of contractile fatigue. Contractile fatigue was assessed by measuring the transdiaphragmatic
twitch pressures elicited by electrical stimulation (electrical-PdiTw) and magnetic stimulation of the phrenic nerves (magnetic-PdiTw) before and after loading (Laghi et al., 1996). The rationale for using both techniques was based on the Selleckchem Z VAD FMK observation that electrical-PdiTw selectively quantifies diaphragmatic contractility while magnetic-PdiTw is affected by both diaphragmatic and rib-cage muscle contractility (Similowski et al., 1998 and Mador et
al., 1996). After placement of all transducers (same as Experiment 1 plus electrodes to record CDAPs), maximal voluntary Pdi (Pdimax) was measured during at least five maximal Müller-expulsive efforts at EELV ( Laghi et al., 1998). Approximately 10 s following each Pdimax maneuver, electrical and magnetic phrenic-nerve stimulations were delivered at relaxed EELV in random order. This sequence was repeated at task failure and 20 and 40 min later. EAdi signals were processed using the methods of Sinderby et al. (1998). These signals were normalized to the maximum ΔEAdi recorded during IC maneuvers (Fig. 2) (Sinderby et al., 1998). Abdominal electromyographic (EMG) signals (Experiment 2) were rectified, moving-averaged and normalized to the maximum signal recorded during loading ( Strohl et al., 1981). No processing was required to measure surface CDAP amplitudes elicited by phrenic-nerve stimulations (Experiment 3) ( Laghi et al., 1996). Diaphragmatic neuromechanical coupling was assessed as the ratio of tidal change in Pdi to tidal change of the normalized EAdi (ΔPdi/ΔEAdi) (Druz
and Sharp, 1981 and Beck et al., 2009). Processed abdominal EMG signals were Phosphoprotein phosphatase marked at three points in time: the highest value during exhalation (maximal activity during neural exhalation), beginning of inhalation (onset of neural inhalation), and highest value during inhalation (maximal phasic activity during neural inhalation). Tension-time index of the diaphragm (TTdi) was quantified using standard formulae (Laghi et al., 1996). Relative contribution of different respiratory muscles to tidal breathing was assessed as ratio of tidal change in Pga to tidal change in Pes (ΔPga/ΔPes) (Hussain et al., 2011). Electrical-PdiTw and magnetic-PdiTw were measured as the difference between maximum Pdi displacement elicited by phrenic-nerve stimulations and the value immediately before stimulations.