Attenuated inspiratory muscle metaboreflex in endurance-trained individuals

The inspiratory metaboreflex is activated during loaded breathing to task failure and induces sympathetic activation and peripheral vasoconstriction that may limit exercise performance. Inspiratory muscle training appears to attenuate the inspiratory metaboreflex in healthy subjects. Since whole body aerobic exercise training improves breathing endurance and inspiratory muscle strength, we hypothesized that endurance-trained individuals would demonstrate a blunted inspiratory muscle metaboreflex in comparison to sedentary individuals. We studied 9 runners (23±0.7 years; maximal oxygen uptake [VO2 max] = 53 ± 4 ml kg(-1) min(-1)) and 9 sedentary healthy volunteers (24±0.7 years; VO2 max = 37 ±2 ml kg(-1) min(-1)). The inspiratory muscle metaboreflex was induced by breathing against an inspiratory load of 60% of maximal inspiratory pressure (MIP), with prolonged duty cycle. Arterial pressure, popliteal blood flow, and heart rate were measured throughout the protocol. Loaded breathing to task failure increased mean arterial pressure in both sedentary and endurance-trained individuals (96±3 to 100±4 mmHg and 101±3 to 110±5 mmHg). Popliteal blood flow decreased in sedentary but not in trained individuals (0.179±0.01 to 0.141±0.01 cm/s, and 0.211±0.02 to 0.214±0.02 cm/s). Similarly, popliteal vascular resistance increased in sedentary but not in trained individuals (559±35 to 757±56 mmHg s/cm, and 528±69 to 558±64 mmHg s/cm). These data demonstrate that endurance-trained individuals have an attenuated inspiratory muscle metaboreflex.     

Pulmonary adaptations to swim and inspiratory muscle training

Because the anomalous respiratory characteristics of competitive swimmers have been suggested to be due to inspiratory muscle work, the respiratory muscle and pulmonary function of 30 competitively trained swimmers was assessed at the beginning and end of an intensive 12-week swim training (ST) program. Swimmers (n = 10) combined ST with either inspiratory muscle training (IMT) set at 80% sustained maximal inspiratory pressure (SMIP) with progressively increased work-rest ratios until task failure for 3-days per week (ST + IMT) or ST with sham-IMT (ST + SHAM-IMT, n = 10), or acted as controls (ST only, ST, n = 10). Measures of respiratory and pulmonary function were assessed at the beginning and end of the 12 week study period. There were no significant differences (P > 0.05) in respiratory and pulmonary function between groups (ST + IMT, ST + SHAM-IMT and ST) at baseline and at the end of the 12 week study period. However, within all groups significant increases (P < 0.05) were observed in a number of respiratory and pulmonary function variables at the end of the 12 week study, such as maximal inspiratory and expiratory pressure, inspiratory power output, forced vital capacity, forced expiratory and inspiratory volume in 1-s, total lung capacity and diffusion capacity of the lung. This study has demonstrated that there are no appreciable differences in terms of respiratory changes between elite swimmers undergoing a competitive ST program and those undergoing respiratory muscle training using the flow-resistive IMT device employed in the present study; as yet, the causal mechanisms involved are undefined.     

The influence of inspiratory muscle work history and specific inspiratory muscle training upon human limb muscle fatigue

The purpose of this study was to assess the influence of the work history of the inspiratory muscles upon the fatigue characteristics of the plantar flexors (PF). We hypothesized that under conditions where the inspiratory muscle metaboreflex has been elicited, PF fatigue would be hastened due to peripheral vasoconstriction. Eight volunteers undertook seven test conditions, two of which followed 4 week of inspiratory muscle training (IMT). The inspiratory metaboreflex was induced by inspiring against a calibrated flow resistor. We measured torque and EMG during isometric PF exercise at 85% of maximal voluntary contraction (MVC) torque. Supramaximal twitches were superimposed upon MVC efforts at 1 min intervals (MVC_TI); twitch interpolation assessed the level of central activation. PF was terminated ( T _lim) when MVC_TI was <50% of baseline MVC. PF T _lim was significantly shorter than control (9.93 ± 1.95 min) in the presence of a leg cuff inflated to 140 mmHg (4.89 ± 1.78 min; P = 0.006), as well as when PF was preceded immediately by fatiguing inspiratory muscle work (6.28 ± 2.24 min; P = 0.009). Resting the inspiratory muscles for 30 min restored the PF T _lim to control. After 4 weeks, IMT, inspiratory muscle work at the same absolute intensity did not influence PF T _lim, but T _lim was significantly shorter at the same relative intensity. The data are the first to provide evidence that the inspiratory muscle metaboreflex accelerates the rate of calf fatigue during PF, and that IMT attenuates this effect.     

Effects of preoperative inspiratory muscle training in obese women undergoing open bariatric surgery: respiratory muscle strength, lung volumes, and diaphragmatic excursion

OBJECTIVE:

To determine whether preoperative inspiratory muscle training is able to attenuate the impact of surgical trauma on the respiratory muscle strength, in the lung volumes, and diaphragmatic excursion in obese women undergoing open bariatric surgery.

DESIGN:

Randomized controlled trial.

SETTING:

Meridional Hospital, Cariacica/ES, Brazil.

SUBJECTS:

Thirty-two obese women undergoing elective open bariatric surgery were randomly assigned to receive preoperative inspiratory muscle training (inspiratory muscle training group) or usual care (control group).

MAIN MEASURES:

Respiratory muscle strength (maximal static respiratory pressure – maximal inspiratory pressure and maximal expiratory pressure), lung volumes, and diaphragmatic excursion.

RESULTS:

After training, there was a significant increase only in the maximal inspiratory pressure in the inspiratory muscle training group. The maximal expiratory pressure, the lung volumes and the diaphragmatic excursion did not show any significant change with training. In the postoperative period there was a significant decrease in maximal inspiratory pressure in both the groups. However, there was a decrease of 28% in the inspiratory muscle training group, whereas it was 47% in the control group. The decrease in maximal expiratory pressure and in lung volumes in the postoperative period was similar between the groups. There was a significant reduction in the measures of diaphragmatic excursion in both the groups.

CONCLUSION:

The preoperative inspiratory muscle training increased the inspiratory muscle strength (maximal inspiratory pressure) and attenuated the negative postoperative effects of open bariatric surgery in obese women for this variable, though not influencing the lung volumes and the diaphragmatic excursion.     

The effects of different inspiratory muscle training intensities on exercising heart rate and perceived exertion

This study investigated the relationship between the intensity of an inspiratory muscle training programme and its effect on respiratory muscle strength, exercising heart rate, and ratings of perceived exertion. A total of 66 subjects were randomly assigned to one of three groups. One group trained at 100% of maximum inspiratory pressure (MIP) for 6 weeks (MAX, n=22). A second group performed 6 weeks of inspiratory muscle training at 80% of MIP (SUB, n=21) and a third control group received no inspiratory training (CON, n=23). Both the MAX and SUB training groups improved MIP relative to the control group [32 (19) cmH₂O, P=0.01; 37 (25) cmH₂O, P=0.001, respectively]. A significant decrease in heart rate [–6 (9) beats min^–1, P=0.02] and rating of perceived exertion [–0.5 (1.4), P=0.04] was observed for the MAX group only. It is concluded that 6 weeks of both MAX and SUB training were sufficient to improve inspiratory muscle strength. However, exercising heart rate and perceived exertion decreased with MAX training only.     

Effects of concurrent inspiratory and expiratory muscle training on respiratory and exercise performance in competitive swimmers

The efficiency of the respiratory system presents significant limitations on the bodys ability to perform exercise due to the effects of the increased work of breathing, respiratory muscle fatigue, and dyspnoea. Respiratory muscle training is an intervention that may be able to address these limitations, but the impact of respiratory muscle training on exercise performance remains controversial. Therefore, in this study we evaluated the effects of a 12-week (10 sessions week^–1) concurrent inspiratory and expiratory muscle training (CRMT) program in 34 adolescent competitive swimmers. The CRMT program consisted of 6 weeks during which the experimental group (E, n=17) performed CRMT and the sham group (S, n=17) performed sham CRMT, followed by 6 weeks when the E and S groups performed CRMT of differing intensities. CRMT training resulted in a significant improvement in forced inspiratory volume in 1 s (FIV_1.0) (P=0.050) and forced expiratory volume in 1 s (FEV_1.0) (P=0.045) in the E group, which exceeded the S groups results. Significant improvements in pulmonary function, breathing power, and chemoreflex ventilation threshold were observed in both groups, and there was a trend toward an improvement in swimming critical speed after 12 weeks of training (P=0.08). We concluded that although swim training results in attenuation of the ventilatory response to hypercapnia and in improvements in pulmonary function and sustainable breathing power, supplemental respiratory muscle training has no additional effect except on dynamic pulmonary function variables.     

Increased inspiratory and expiratory muscle strength following respiratory muscle strength training (RMST) in two patients with late-onset Pompe disease

Respiratory muscle strength training (RMST) is an exercise-based intervention which targets respiratory muscle weakness. We implemented RMST in two patients with late-onset Pompe disease (LOPD), both who had received long-term enzyme replacement therapy and had severe respiratory weakness. Over 16–32 weeks, inspiratory muscle strength increased by 73–74%. Expiratory muscle strength increased 31–48% over 12–22 weeks. These findings suggest that RMST may increase respiratory muscle strength, even in the setting of LOPD and severe baseline weakness.     

Mouth pressure twitches induced by cervical magnetic stimulation to assess inspiratory muscle fatigue

This study aimed at determining whether twitch mouth pressure (TwPmo) induced by cervical magnetic stimulation (CMS) was sensitive to inspiratory muscle fatigue produced by whole body exercise (WBE) in normal subjects. Twenty subjects performed one or two of the following protocols: (i). cycling at 85% V(O(2),max) until exhaustion; (ii). inspiratory resistive load (IRL) breathing at 62% of maximal inspiratory pressure until task failure. In eight subjects, oesophageal (TwPoes), gastric (TwPga) and transdiaphragmatic (TwPdi) pressures were recorded. The TwPmo was significantly reduced (P<0.05) 20 min after both WBE and IRL, from 17.5+/-4.4 to 15.9+/-3.9 cmH(2)O and from 19.4+/-4.9 to 17.7+/-4.5 cmH(2)O, respectively. Subsequently to IRL, the TwPdi decrease was associated with a reduction in TwPoes/TwPga ratio; not after WBE. Independently of the mode of ventilatory loading, inspiratory muscle fatigue was detected. Thus, inspiratory muscle fatigue after WBE can be assessed in normal subjects with a noninvasive technique.     

Factors contributing to thixotropy of inspiratory muscles

Thixotropy is a passive property of the skeletal muscle dependent on the muscle's immediate history of contraction and length change. Thixotropic properties of inspiratory muscles, introduced by forceful muscle contraction at an inflated lung volume, cause an increased end-expiratory position (EEP) of the rib cage. We searched for factors contributing to the development of inspiratory muscle thixotropy in nine healthy subjects. Using induction plethysmography, we examined aftereffects on EEP of the duration of inspiratory muscle contraction and subsequent muscle relaxation. We also studied effects of inspiratory effort intensity measured by mouth pressure at different lung volumes. EEP elevation was noted subsequent to 5-s contraction followed by 2-s relaxation and was enhanced when conditioned at higher lung volumes with a strong inspiratory effort. Our results suggest four factors that influence inspiratory muscle thixotropy: (1) intensity of muscle contraction, (2) lung volume when contraction occurs, (3) duration of contraction, and (4) muscle relaxation.     

Resistive inspiratory muscle training and exercise performance in COPD patients. A comparative study with conventional breathing retraining

Twenty patients with stable COPD (mean age 67.8 yr; mean FEV1 1.08 1), all limited by ventilation at maximum exercise, were randomly allocated after a four week control period, to an eight week programme of either inspiratory resistive training (IRT), with a P Flex device, or conventional breathing retraining (BR). Exercise performance was evaluated every four weeks, using a 12-min walking test, an incremental progressive exercise on a cycle ergometer and a cycle endurance test. Inspiratory muscle endurance was measured as the highest tolerated resistance for 10 min on a P Flex device. IRT produced a significant (p less than 0.05) increase in the highest tolerated resistance, but IRT and BR failed to improve lung function or exercise performance. The present study shows that in COPD patients with ventilatory limitation on exercise an IRT programme may fail to improve exercise performance, in spite of an efficient training effect on the endurance of the inspiratory muscles.     

The influence of inspiratory and expiratory muscle training upon rowing performance

We investigated the effect of 4 week of inspiratory (IMT) or expiratory muscle training (EMT), as well as the effect of a subsequent 6 week period of combined IMT/EMT on rowing performance in club-level oarsmen. Seventeen male rowers were allocated to either an IMT (n = 10) or EMT (n = 7) group. The groups underwent a 4 week IMT or EMT program; after interim testing, both groups subsequently performed a 6 week program of combined IMT/EMT. Exercise performance and physiological responses to exercise were measured at 4 and 10 week during an incremental rowing ergometer ‘step-test’ and a 6 min all-out (6MAO) effort. Pressure threshold respiratory muscle training was undertaken at the 30 repetition maximum load (∼50% of the peak inspiratory and expiratory mouth pressure, P _Imax or P _Emax, respectively). P _Imax increased during the IMT phase of the training in the IMT group (26%, P < 0.001) and was accompanied by an improvement in mean power during the 6MAO (2.7%, P = 0.015). Despite an increase in P _Emax by the end of the intervention (31%, P = 0.03), the EMT group showed no significant changes in any performance parameters during either the ‘step-test’ or 6MAO. There were no significant changes in breathing pattern or the metabolic response to the 6MAO test in either group, but the IMT group showed a small decrease in HR (2–5%, P = 0.001). We conclude that there were no significant additional changes following combined IMT/EMT. IMT improved rowing performance, but EMT and subsequent combined IMT/EMT did not.     

Effect of inspiratory muscle work on peripheral fatigue of locomotor muscles in healthy humans

The work of breathing required during maximal exercise compromises blood flow to limb locomotor muscles and reduces exercise performance. We asked if force output of the inspiratory muscles affected exercise-induced peripheral fatigue of locomotor muscles. Eight male cyclists exercised at ≥ 90% peak O₂ uptake to exhaustion (CTRL). On a separate occasion, subjects exercised for the same duration and power output as CTRL (13.2 ± 0.9 min, 292 W), but force output of the inspiratory muscles was reduced (−56% versus CTRL) using a proportional assist ventilator (PAV). Subjects also exercised to exhaustion (7.9 ± 0.6 min, 292 W) while force output of the inspiratory muscles was increased (+80% versus CTRL) via inspiratory resistive loads (IRLs), and again for the same duration and power output with breathing unimpeded (IRL-CTRL). Quadriceps twitch force ( Q _tw), in response to supramaximal paired magnetic stimuli of the femoral nerve (1–100 Hz), was assessed pre- and at 2.5 through to 70 min postexercise. Immediately after CTRL exercise, Q _tw was reduced −28 ± 5% below pre-exercise baseline and this reduction was attenuated following PAV exercise (−20 ± 5%; P < 0.05). Conversely, increasing the force output of the inspiratory muscles (IRL) exacerbated exercise-induced quadriceps muscle fatigue ( Q _tw=−12 ± 8% IRL-CTRL versus −20 ± 7% IRL; P < 0.05). Repeat studies between days showed that the effects of exercise per se , and of superimposed inspiratory muscle loading on quadriceps fatigue were highly reproducible. In conclusion, peripheral fatigue of locomotor muscles resulting from high-intensity sustained exercise is, in part, due to the accompanying high levels of respiratory muscle work.     

Augmented peripheral chemoreflex in patients with heart failure and inspiratory muscle weakness

We hypothesized that heart failure patients with inspiratory muscle weakness (IMW) present greater peripheral chemoreflex responsiveness and augmented exercise ventilatory oscillation compared to patients with preserved inspiratory muscle strength. We studied 19 heart failure patients: 9 with IMW (maximal inspiratory pressure [PImax] < 70% of predicted) and 10 with preserved inspiratory muscle strength. Inspiratory muscle strength was measured via pressure transducer. Peripheral chemoreflex was evaluated by the single-breath CO₂ test. Exercise ventilatory oscillation was determined as the ratio between amplitude and mean of each oscillation during incremental exercise. Patients with IMW had greater peripheral chemoreflex response (0.11 ± 0.03 l min⁻¹ Torr⁻¹) than those with preserved inspiratory muscle strength (0.07 ± 0.03 l min⁻¹ Torr⁻¹, p = 0.02). Moreover, there was a significant and inverse correlation between PImax and peripheral chemoreflex response (r = −0.57, p = 0.01). Likewise, there was a significant and inverse correlation between PImax and ventilatory oscillations (r = −0.46, p = 0.04). Our findings indicate that IMW is linked to increased peripheral chemoreflex and augmented exercise ventilatory oscillation in patients with chronic heart failure.     

Influence of environmental temperature on exercise-induced inspiratory muscle fatigue

Exercise in the heat has detrimental effects on circulation that might negatively influence endurance performance. If blood is diverted away from the inspiratory muscles to the skin during exercise in the heat, exercise-induced inspiratory muscle fatigue might be exacerbated. Thus, we hypothesised that prolonged heavy endurance exercise in the heat would impair exercise performance and exacerbate inspiratory muscle fatigue compared to exercise in a thermo-neutral environment. Using a crossover design, seven male endurance trained subjects [mean (SEM) maximum oxygen uptake = 62.2 (1.5) ml·kg^–1·min^–1] were assigned at random to either a group that exercised in the heat at an ambient temperature of 35°C (H) or a group that exercised in the cool at 15°C (C). Maximum inspiratory mouth pressure at zero flow (P ₀), pressure normalised maximum relaxation rate (MRR/P ₀), time constant for the pressure decay (), and maximum inspiratory flow at 30% P ₀ ( ₃₀) were assessed immediately before and reassessed within 2, 30, and 60 min of completing a pre-loaded time trial [40 min at 65% peak power, plus ~30 min time trial] on a cycle ergometer . Group H completed the time trial 432 (135) s slower than group C [2,285 (180) vs 1,852 (122) s, respectively; =24 (8)%, P=0.0094]. Repeat measurements within 2 min post-exercise revealed significant declines in P ₀, MRR/P ₀, , and ₃₀ from baseline values, but no between-group differences were observed. In conclusion, heavy sustained exercise in the heat impaired subsequent time-trial performance but did not exacerbate inspiratory muscle fatigue in endurance-trained subjects.     

The effect of inspiratory muscle training upon maximum lactate steady-state and blood lactate concentration

Several studies have reported that improvements in endurance performance following respiratory muscle training (RMT) are associated with a decrease in blood lactate concentration ([Lac]_B). The present study examined whether pressure threshold inspiratory muscle training (IMT) elicits an increase in the cycling power output corresponding to the maximum lactate steady state (MLSS). Using a double-blind, placebo-controlled design, 12 healthy, non-endurance-trained male participants were assigned in equal numbers to an experimental (IMT) or sham training control (placebo) group. Cycling power output at MLSS was initially identified using a lactate minimum protocol followed by a series of constant power output rides (2.5% increments) of 29.5 min duration; MLSS was reassessed following six weeks of IMT or sham IMT. Maximum inspiratory mouth pressure increased significantly (26%) in the IMT group, but remained unchanged in the placebo group. The cycling power output corresponding to MLSS remained unchanged in both groups after the intervention. After IMT, [Lac]_B decreased significantly at MLSS power in the IMT group [–1.17 (1.01) mmol l^–1 after 29.5 min of cycling; mean (SD)], but remained unchanged in the placebo group [+0.37 (1.66) mmol l^–1]. These data support previous observations that IMT results in a decrease in [Lac]_B at a given intensity of exercise. That such a decrease in [Lac]_B was not associated with a substantial (>2.5%) increase in MLSS power is a new finding suggesting that RMT-induced increases in exercise tolerance and reductions in [Lac]_B are not ascribable to a substantial increase in the lactate threshold.     

Respiratory muscle performance with stretch-shortening cycle manoeuvres: maximal inspiratory pressure-flow curves

AIM: To test the hypothesis that the maximal inspiratory muscle (IM) performance, as assessed by the maximal IM pressure-flow relationship, is enhanced with the stretch-shortening cycle (SSC). METHODS: Maximal inspiratory flow-pressure curves were measured in 12 healthy volunteers (35 +/- 6 years) during maximal single efforts through a range of graded resistors (4-, 6-, and 8-mm diameter orifices), against an occluded airway, and with a minimal load (wide-open resistor). Maximal inspiratory efforts were initiated at a volume near residual lung volume (RV). The subjects exhaled to RV using slow (S) or fast (F) manoeuvres. With the S manoeuvre, they exhaled slowly to RV and held the breath at RV for about 4 s prior to maximal inspiration. With the F manoeuvre, they exhaled rapidly to RV and immediately inhaled maximally without a post-expiratory hold; a strategy designed to enhance inspiratory pressure via the SSC. RESULTS: The maximal inspiratory pressure-flow relationship was linear with the S and F manoeuvres (r2 = 0.88 for S and r2 = 0.88 for F manoeuvre, P < 0.0005 in all subjects). With the F manoeuvre, the pressure-flow relationship shifted to the right in a parallel fashion and the calculated maximal power increased by approximately 10% (P < 0.05) over that calculated with the S manoeuvre. CONCLUSION: The maximal inspiratory pressure-flow capacity can be enhanced with SSC manoeuvres in a manner analogous to increases in the force-velocity relationship with SSC reported for skeletal muscles.     

Sternocleidomastoid muscle deoxygenation in response to incremental inspiratory threshold loading measured by near infrared spectroscopy

This study investigated the pattern of changes in muscle oxygenation, deoxygenation and blood volume in the sternocleidomastoid (SCM) in comparison with the parasternal (PS) and intercostal (IC) muscles during a bout of incremental inspiratory threshold loading (ITL) in healthy subjects using near-infrared spectroscopy. During progressive loading, the PS and IC showed a significant increase in oxygenated hemoglobin (5.9 ± 2.3 and 6.8 ± 2.4 μM, P<0.05) and the SCM showed an increase in deoxygenated hemoglobin (17.3 ± 3.8 μM, P<0.05). Total hemoglobin also steadily increased in the SCM whereas it decreased in the quiescent vastus lateralis muscle (20.7 ± 6.1μM vs. -6.6 ± 2.4 μM, P<0.05), which was used as the control muscle during the ITL. Our data suggests that the SCM is recruited progressively during progressive ITL and is accompanied by an increased blood volume and maintenance of O(2)Hb. Blood redistribution away from the nonactive limb muscles during the ITL may provide one source of maintaining inspiratory muscle blood volume and oxygenation during high respiratory motor output.     

Inspiratory muscle adaptations following pressure or flow training in humans

Skeletal muscle adapts differently to training with high forces or with high velocities. The effects of these disparate training protocols on the inspiratory muscles were investigated in ten healthy volunteers. Five subjects trained using high force (pressure) loads (pressure trainers) and five trained using high velocity (flow) loads (flow trainers). Pressure training entailed performing 30 maximal static inspiratory efforts against a closed airway. Flow training entailed performing 30 sets of three maximal dynamic inspiratory efforts against a minimal resistance. Training was supervised and carried out 5 days a week for 6 weeks. Inspiratory flow rates and oesophageal pressure-time curves were measured before and after training. Peak inspiratory pressures during maximal static and dynamic efforts and peak flows during the maximal dynamic efforts were calculated. The time-to-peak pressure and rate of rise in peak pressure during maximal static and dynamic manoeuvres were also calculated before and following training. Maximal static pressure increased in the pressure training group and maximal dynamic pressure increased in the flow training group. Both groups increased the rate of pressure production (dP/dt) during their respective maximal efforts. The post-training decrease in time-to-peak pressure was proportionately greater in the flow trainers than in the pressure trainers. The differences in time-to-peak pressure between the two groups were consistent with the different effects of force and velocity training on the time-to-peak tension of skeletal muscle.     

Trend of maximal inspiratory pressure in mechanically ventilated patients: predictors

INTRODUCTION: It is known that mechanical ventilation and many of its features may affect the evolution of inspiratory muscle strength during ventilation. However, this evolution has not been described, nor have its predictors been studied. In addition, a probable parallel between inspiratory and limb muscle strength evolution has not been investigated. OBJECTIVE: To describe the variation over time of maximal inspiratory pressure during mechanical ventilation and its predictors. We also studied the possible relationship between the evolution of maximal inspiratory pressure and limb muscle strength. METHODS: A prospective observational study was performed in consecutive patients submitted to mechanical ventilation for > 72 hours. The maximal inspiratory pressure trend was evaluated by the linear regression of the daily maximal inspiratory pressure and a logistic regression analysis was used to look for independent maximal inspiratory pressure trend predictors. Limb muscle strength was evaluated using the Medical Research Council score. RESULTS: One hundred and sixteen patients were studied, forty-four of whom (37.9%) presented a decrease in maximal inspiratory pressure over time. The members of the group in which maximal inspiratory pressure decreased underwent deeper sedation, spent less time in pressure support ventilation and were extubated less frequently. The only independent predictor of the maximal inspiratory pressure trend was the level of sedation (OR=1.55, 95% CI 1.003 - 2.408; p = 0.049). There was no relationship between the maximal inspiratory pressure trend and limb muscle strength. CONCLUSIONS: Around forty percent of the mechanically ventilated patients had a decreased maximal inspiratory pressure during mechanical ventilation, which was independently associated with deeper levels of sedation. There was no relationship between the evolution of maximal inspiratory pressure and the muscular strength of the limb.     

Recovery of inspiratory intercostal muscle activity following high cervical hemisection

Anatomical and neurophysiological evidence indicates that thoracic interneurons can serve a commissural function and activate contralateral motoneurons. Accordingly, we hypothesized that respiratory-related intercostal (IC) muscle electromyogram (EMG) activity would be only modestly impaired by a unilateral cervical spinal cord injury. Inspiratory tidal volume (VT) was recorded using pneumotachography and EMG activity was recorded bilaterally from the 1st to 2nd intercostal space in anesthetized, spontaneously breathing rats. Studies were conducted at 1-3 days, 2 wks or 8 wks following C2 spinal cord hemisection (C2HS). Data were collected during baseline breathing and a brief respiratory challenge (7% CO(2)). A substantial reduction in inspiratory intercostal EMG bursting ipsilateral to the lesion was observed at 1-3 days post-C2HS. However, a time-dependent return of activity occurred such that by 2 wks post-injury inspiratory intercostal EMG bursts ipsilateral to the lesion were similar to age-matched, uninjured controls. The increases in ipsilateral intercostal EMG activity occurred in parallel with increases in VT following the injury (R=0.55; P<0.001). We conclude that plasticity occurring within a "crossed-intercostal" circuitry enables a robust, spontaneous recovery of ipsilateral intercostal activity following C2HS in rats.