We compared the extent and origin of muscle fatigue induced by short-pulse-low-frequency [conventional (CONV)] and wide-pulse-high-frequency (WPHF) neuromuscular electrical stimulation. We expected CONV contractions to mainly originate from depolarization of axonal terminal branches (spatially determined muscle fiber recruitment) and WPHF contractions to be partly produced via a central pathway (motor unit recruitment according to size principle). Greater neuromuscular fatigue was, therefore, expected following CONV compared with WPHF. Fourteen healthy subjects underwent 20 WPHF (1 ms-100 Hz) and CONV (50 μs-25 Hz) evoked isometric triceps surae contractions (work/rest periods 20:40 s) at an initial target of 10% of maximal voluntary contraction (MVC) force. Force-time integral of the 20 evoked contractions (FTI) was used as main index of muscle fatigue; MVC force loss was also quantified. Central and peripheral fatigue were assessed by voluntary activation level and paired stimulation amplitudes, respectively. FTI in WPHF was significantly lower than in CONV (21,717 ± 11,541 vs. 37,958 ± 9,898 N·s P<0,001). The reductions in MVC force (WPHF: -7.0 ± 2.7%; CONV: -6.2 ± 2.5%; P < 0.01) and paired stimulation amplitude (WPHF: -8.0 ± 4.0%; CONV: -7.4 ± 6.1%; P < 0.001) were similar between conditions, whereas no change was observed for voluntary activation level (P > 0.05). Overall, our results showed a different motor unit recruitment pattern between the two neuromuscular electrical stimulation modalities with a lower FTI indicating greater muscle fatigue for WPHF, possibly limiting the presumed benefits for rehabilitation programs.
We examined the respiratory activity of the posterior cricoarytenoid muscle (PCA) simultaneously with the movements of the vocal cords during tidal breathing and panting in four normal seated subjects. A bipolar electrode was constructed to record the surface electromyogram (EMG) of the PCA. The glottis was visualized with a fiberoptic bronchoscope, and the glottic image was recorded simultaneously with tidal volume and a digital time marker on video tape. During quiet breathing the integrated EMG signal (EPCA) showed consistent phasic variations in each subject. The inspiratory onset of EPCA in the four subjects preceded inspiratory flow by 170 +/- 80, 650 +/- 310, 130 +/- 80, and 130 +/- 90 ms (mean +/- SD), respectively. This lead time of the PCA was similar to that between the onset of glottic widening and inspiration in each subject. The proportion of each cycle during which EPCA increased (the duty cycle) was 31 +/- 3% (mean +/- SE), whereas the inspiratory portion of the respiratory cycle constituted 37 +/- 2% (mean +/- SE), respectively. The duty cycle of the PCA remained relatively constant in the same subject on different days. During panting at functional residual capacity, the EPCA increased to 142 +/- 11% of the peak activity recorded during the preceding control breaths. This was accompanied by a sustained increase in the glottic width to 91 +/- 9% of the peak value in the preceding breaths. These results confirm the role of the PCA as a principal abductor of the vocal cords and indicate a temporal relationship between PCA activation and the inspiratory phase of the respiratory cycle during tidal breathing in humans.
A retrospective review of all patients presenting to a tertiary referral center with acute nontraumatic upper limb ischemia between January 1992 and June 1997 was undertaken to examine the role of intraarterial thrombolysis in the management of such cases. Twenty-one patients were identified in the radiology and vascular surgery departments' registers. Twenty (95%) underwent angiography, demonstrating subclavian artery occlusion in four, axillary in two, brachial in 13, and one at the digital level. Intraarterial thrombolysis was attempted in 12 patients. There were three technical failures, all requiring embolectomy. Six had complete lysis and resolution of their symptoms. One patient had partial lysis but experienced no further rest pain. Thrombolysis was unsuccessful in two cases with one subsequently requiring embolectomy and the other surgical bypass. Three patients had surgical intervention as their primary procedure with two favorable outcomes and one ending in above-elbow amputation. Five patients were treated conservatively with heparin, resulting in three partial and two full recoveries. One patient experienced complete resolution of symptoms with an intravenous prostacyclin infusion. Both electrocardiograms (ECG) and echocardiograms (ECHO) were of limited diagnostic aid, and long-term warfarin anticoagulation was prescribed to all patients. There was no recurrence of upper limb ischemia at a median follow up of 18 months. Intraarterial thrombolysis is an effective first line treatment for acute nontraumatic upper limb ischemia in selected cases.
We examined the effect of increasing work rate, without a corresponding increase in the pressure-time product, on energy cost and inspiratory muscle endurance (Tlim) in five normal subjects during inspiratory resistive breathing. Tidal volume, mean inspiratory mouth pressure, duty cycle, and hence the pressure-time product were kept constant, whereas work rate was varied by changing the frequency of breathing. There was a linear decrease in Tlim of -2.1 +/- 0.5 s.J-1.min-1 (r = 0.87 +/- 0.06) with increasing work rate. The data satisfied a model of energy balance during fatiguing runs (Monod and Scherrer. Ergonomics 8: 329-337, 1965) and were consistent with the hypothesis that the rate of energy supply, or respiratory muscle blood flow, is fixed when the pressure-time product is constant. Our results indicate that during inspiratory resistive breathing against fatiguing loads, work rate determines endurance independently of the pressure-time product. On the basis of the model, our results lead to estimates of respiratory muscle blood flow and available energy stores under the conditions of our experiment.
In six normal male subjects we compared the O2 cost of resistive breathing (VO2 resp) between equivalent external inspiratory (IRL) and expiratory loads (ERL) studied separately. Each subject performed four pairs of runs matched for tidal volume, breathing frequency, flow rates, lung volume, pressure-time product, and work rate. Basal O2 uptake, measured before and after pairs of loaded runs, was subtracted from that measured during resistive breathing to obtain VO2 resp. For an equivalent load, the VO2 resp during ERL (184 +/- 17 ml O2/min) was nearly twice that obtained during IRL (97 +/- 9 ml O2/min). This twofold difference in efficiency between inspiratory and expiratory resistive breathing may reflect the relatively lower mechanical advantage of the expiratory muscles in overcoming respiratory loads. Variable recruitment of expiratory muscles may explain the large variation of results obtained in studies of respiratory muscle efficiency in normal subjects.
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