Dissociation between diaphragmatic and rib cage muscle fatigue

Altering magnetic field strength impacts the assessment of diaphragmatic function using cervical magnetic stimulation

Quantifying diaphragm neuromuscular function using cervical magnetic stimulation (CMS) typically uses only a single stimulator (1-Stim) which may be inadequate to maximally stimulate the phrenic nerves. We questioned if using two stimulators (2-Stim) together alters diaphragm neuromuscular function at baseline and following inspiratory pressure threshold loading. Six (n = 3 female) healthy young participants were instrumented with esophageal and gastric balloon tipped catheters and electrodes over the 7-8th intercostal space. With either 1-Stim or 2-Stim an incremental protocol, where the stimulator intensity was progressively increased was completed prior to a series of potentiated twitches. The inspiratory threshold loading test consisted of loaded breathing to failure. Compared to 1-Stim, 2-Stim resulted in significantly greater unpotentiated Pditw and M-waves during the incremental protocol (both p < 0.01). Similarly, 2-Stim resulted in greater potentiated Pditw (31 ± 8 vs. 41 ± 9 cmH2O; p = 0.02) and M-waves (6.4 ± 2.9 vs. 8.6 ± 2.4 V; p = 0.02). Our findings suggest that CMS using 1-Stim is unlikely to generate a sufficient magnetic field to maximally stimulate the phrenic nerves and may underestimate diaphragm function.

Changes in electromyographic activity, mechanical power, and relaxation rates following inspiratory ribcage muscle fatigue

Muscle fatigue is a complex phenomenon enclosing various mechanisms. Despite technological advances, these mechanisms are still not fully understood in vivo. Here, simultaneous measurements of pressure, volume, and ribcage inspiratory muscle activity were performed non-invasively during fatigue (inspiratory threshold valve set at 70% of maximal inspiratory pressure) and recovery to verify if inspiratory ribcage muscle fatigue (1) leads to slowing of contraction and relaxation properties of ribcage muscles and (2) alters median frequency and high-to-low frequency ratio (H/L). During the fatigue protocol, sternocleidomastoid showed the fastest decrease in median frequency and slowest decrease in H/L. Fatigue was also characterized by a reduction in the relative power of the high-frequency and increase of the low-frequency. During recovery, changes in mechanical power were due to changes in shortening velocity with long-lasting reduction in pressure generation, and slowing of relaxation [i.e., tau (τ), half-relaxation time (½RT), and maximum relaxation rate (MRR)] was observed with no significant changes in contractile properties. Recovery of median frequency was faster than H/L, and relaxation rates correlated with shortening velocity and mechanical power of inspiratory ribcage muscles; however, with different time courses. Time constant of the inspiratory ribcage muscles during fatigue and recovery is not uniform (i.e., different inspiratory muscles may have different underlying mechanisms of fatigue), and MRR, ½RT, and τ are not only useful predictors of inspiratory ribcage muscle recovery but may also share common underlying mechanisms with shortening velocity.

Ventilation and respiratory mechanics

During dynamic exercise, the healthy pulmonary system faces several major challenges, including decreases in mixed venous oxygen content and increases in mixed venous carbon dioxide. As such, the ventilatory demand is increased, while the rising cardiac output means that blood will have considerably less time in the pulmonary capillaries to accomplish gas exchange. Blood gas homeostasis must be accomplished by precise regulation of alveolar ventilation via medullary neural networks and sensory reflex mechanisms. It is equally important that cardiovascular and pulmonary system responses to exercise be precisely matched to the increase in metabolic requirements, and that the substantial gas transport needs of both respiratory and locomotor muscles be considered. Our article addresses each of these topics with emphasis on the healthy, young adult exercising in normoxia. We review recent evidence concerning how exercise hyperpnea influences sympathetic vasoconstrictor outflow and the effect this might have on the ability to perform muscular work. We also review sex-based differences in lung mechanics.

Role of the diaphragm in trunk rotation in humans

The objectives of the present study were to test the hypothesis that the costal diaphragm contracts during ipsilateral rotation of the trunk and that such trunk rotation increases the motor output of the muscle during inspiration. Monopolar electrodes were inserted in the right costal hemidiaphragm in six subjects, and electromyographic (EMG) recordings were made during isometric rotation efforts of the trunk to the right ("ipsilateral rotation") and to the left ("contralateral rotation"). EMG activity was simultaneously recorded from the parasternal intercostal muscles on the right side. The parasternal intercostals were consistently active during ipsilateral rotation but silent during contralateral rotation. In contrast, the diaphragm was silent in the majority of rotations in either direction, and whenever diaphragm activity was recorded, it involved very few motor units. In addition, whereas parasternal inspiratory activity substantially increased during ipsilateral rotation and decreased during contralateral rotation, inspiratory activity in the diaphragm was essentially unaltered and the discharge frequency of single motor units in the muscle remained at 13-14 Hz in the different postures. It is concluded that 1) the diaphragm makes no significant contribution to trunk rotation and 2) even though the diaphragm and parasternal intercostals contract in a coordinated manner during resting breathing, the inspiratory output of the two muscles is affected differently by voluntary drive during trunk rotation.

Putative protective effect of inspiratory threshold loading against exercise-induced supraspinal diaphragm fatigue

The present investigation was intended to assess the consequences of an inspiratory load on the diaphragm central component of fatigue during exercise. We recorded the motor potential evoked (MEP) by transcranial magnetic stimulation of the motor cortex in 10 subjects. The diaphragm and rectus femoris were studied before and 10, 20, and 40 min after two 16-min cycling exercise (E) trials requiring 55% of maximal oxygen uptake: 1) one with an inspiratory threshold load (E + ITL), corresponding to 10% of maximal inspiratory pressure; and 2) the other without the load (E). Dyspnea, heart rate, electromyographic activity of the sternocleidomastoid, and diaphragm work were significantly higher in E + ITL than in E. Neither trial affected the response to phrenic magnetic stimulation, which was performed 15 and 25 min postexercise, or the maximal inspiratory pressure (116 and 120 cm H(2)O before E and E + ITL, respectively, and 110 and 114 cm H(2)O at 30 min postexercise). Whereas the amplitude of the diaphragm MEP was unaffected by E + ITL (+2.1 +/- 29.4%), a significant decrease was observed 10 min after E compared with baseline (-37.1 +/- 22.3%) and compared with E + ITL. The MEP amplitude of rectus femoris remained unchanged with E and E + ITL. The recruitment of synergistic agonists during E + ITL may have normalized the major ventilatory stress and reset up the excitability of the diaphragm pathway.

Respiratory muscle testing

Respiratory muscles are a vital component of the respiratory system. Compromise in the function of respiratory muscles is assessed with a careful history, determination of the breathing frequency, and observation of the pattern of thoracoabdominal movements in the upright and supine positions. The most important diagnostic tools include measurement of lung volumes, FVC, oximetry, Pi(max) and Pe(max), and SNIP. Measurement of Pes and Pga pressures, Tw Pmo, phrenic nerve stimulation, magnetic stimulation, and EMG add new dimensions to the study of these muscles. These tools are helpful in understanding patients with acute and chronic disease that affects the respiratory muscles.

Effect of expiratory resistive loading on the noninvasive tension-time index in COPD

Expiratory resistive loading (ERL) is used by chronic obstructive pulmonary disease (COPD) patients to improve respiratory function. We, therefore, used a noninvasive tension-time index of the inspiratory muscles (TT(mus) = I/PI(max) x TI/TT, where I is mean inspiratory pressure estimated from the mouth occlusion pressure, PI(max) is maximal inspiratory pressure, TI is inspiratory time, and TT is total respiratory cycle time) to better define the effect of ERL on COPD patients. To accomplish this, we measured airway pressures, mouth occlusion pressure, respiratory cycle flow rates, and functional residual capacity (FRC) in 14 COPD patients and 10 normal subjects with and without the application of ERL. TT(mus) was then calculated and found to drop in both COPD and normal subjects (P<0.05). The decline in TT(mus) in both groups resulted solely from a prolongation of expiratory time with ERL (P<0.001 for COPD, P<0.05 for normal subjects). In contrast to the COPD patients, normal subjects had an elevation in I and FRC, thus minimizing the decline in TT(mus). In conclusion, ERL reduces the potential for inspiratory muscle fatigue in COPD by reducing TI/TT without affecting FRC and I.

Tension-time index of inspiratory muscles in COPD patients: role of airway obstruction

Inspiratory muscle function has been shown to be related to general muscle weakness, weight loss, blood gas tensions, airway obstruction and hyperinflation. The aim of this study was to define (1) the factor that is the main determinant of the tension-time index of the inspiratory muscles (TTmus), and which this increases the risk of inspiratory muscle fatigue; and (2) whether a breathing strategy is adopted to avoid inspiratory muscle fatigue. Twenty-seven normal volunteers and 35 stable COPD outpatients (FEV1% predicted, range: 21-89%; and FRC/TLC, range: 49-77%) were studied. The TTmus was determined as follows: TTmus = PI/PImax.TI/Ttot, where Pi is the mean inspiratory pressure calculated from the mouth occlusion pressure (P0.1), PImax is the maximal inspiratory pressure, TI is the inspiratory time, and Ttot is the total time of the breathing cycle. COPD patients showed significantly lower PImax and higher P0.1, PI, PI/PImax, and TTmus than normal subjects. No patient had a TTmus value higher than the inspiratory muscle fatigue threshold of 0.33. The FEV1 was significantly correlated with TTmus and all its components in the patients. The FRC/TLC was also correlated with all components except PI. Body weight was only correlated with PImax. In a forward and backward stepwise regression analysis, FEV1 appeared to be the only significant factor explaining the variance of log (PI/PImax) and log (TTmus), whereas FRC/TLC was the principal determinant of PImax. In COPD patients, a non-linear relationship was found between TI and P0.1. A negative linear relationship was found between TI/Ttot and PI/PImax. In conclusion, although hyperinflation predominantly affected inspiratory muscle strength in a group of stable COPD patients with a wide range of severity, airway obstruction was the principal factor determining the magnitude of TTmus. In addition, in order to remain below the inspiratory muscle fatigue threshold, as the severity of airway obstruction increased, patients adopted a breathing strategy characterized by decreased TI/Ttot as inspiratory pressure demand increased.

Cervical magnetic stimulation as a method to discriminate between diaphragm and rib cage muscle fatigue

Inspiratory muscle fatigue can probably determine hypercapnic respiratory failure. Diaphragm fatigue is detected by electrical phrenic stimulation (ELS), but there is no simple tool to assess rib cage muscle (RCM) fatigue. Cervical magnetic stimulation (CMS) costimulates the phrenic nerves and RCM. We reasoned that changes in transdiaphragmatic pressure twitch (Pdi,tw) with CMS and ELS should be different after selective diaphragm vs. RCM fatigue. Five volunteers performed inspiratory resistive tasks while voluntarily uncoupling diaphragm and RCM. Baseline Pdi,twELS and Pdi,twCMS were 28.57 +/- 1.68 and 32.83 +/- 2.92 cmH2O. After selective diaphragm loading, Pdi,twELS and Pdi,twCMS were reduced by 39 and 26%, with comparable decreases in gastric pressure twitch (Pga,tw). Esophageal pressure twitch (Pes,tw) was better preserved with CMS. Therefore Pes,tw/Pga,tw was lower with ELS than CMS (-1.24 +/- 0.16 vs. -1.73 +/- 0.11, P = 0.05). After selective RCM loading, there was no diaphragm fatigue, but Pes,twCMS was significantly reduced (-30%). These findings support the role of rib cage stiffening by CMS-related RCM contraction in the ELS-CMS differences and suggest that CMS can be used to assess RCM fatigue.

Changes in breathing pattern and respiratory muscle performance parameters during difficult weaning

OBJECTIVE

This study examined, using noninvasive means, the changes in breathing pattern and inspiratory muscle pressure-time indices during difficult progressive withdrawal of pressure-support ventilation.

DESIGN

A prospective analysis of the temporal evolution of several respiratory variables in difficult-to-wean patients.

SETTING

A university hospital intensive care unit.

PATIENTS

A heterogeneous group of 17 patients receiving prolonged mechanical ventilation.

INTERVENTIONS

Daily measurements of breathing pattern and respiratory muscle performance parameters in difficult-to-wean patients.

MEASUREMENTS AND MAIN RESULTS

We examined breathing pattern variables, rapid shallow breathing (respiratory rate/tidal volume), tracheal occlusion pressure, maximal inspiratory pressure (P(I)max), and the tension-time index of the inspiratory muscles (TTmus = P(I)/P(I)max x Ti/Ttot) (where Ti/Ttot is inspiratory fraction of the cycle). All measurements were repeated at 24-hr intervals throughout the difficult weaning period. The patients were extubated on satisfying ten of 12 classical weaning criteria. Eleven patients were successfully weaned from mechanical ventilation while six patients were not. Weaning failure was associated with the following: a) longer periods of mechanical ventilation before weaning; b) high values of tracheal occlusion pressure, respiratory rate, minute ventilation, and effective impedance maintained throughout the difficult weaning period; and c) persistent high PaCO2 and intrinsic positive end-expiratory pressure values. As the weaning failure patients' inspiratory muscles confronted an increasing inspiratory load, values of the tension-time index of the inspiratory muscles entered or remained in the fatigue zone. In contrast, weaning success patients normalized their breathing pattern and decreased their tracheal occlusion pressure, effective impedance, and tension-time index values.

CONCLUSIONS

Breathing pattern alterations and respiratory muscle performance impairments lead to ventilator dependency after prolonged mechanical ventilation. The measurement of variables such as the noninvasive tracheal occlusion pressure, inspiratory power of breathing, and tension-time index of the inspiratory muscles facilitate the management of difficult-to-wean patients.

Determinants of the tension-time index of inspiratory muscles in children with cystic fibrosis

Nutritional status and chronic pulmonary hyperinflation can alter respiratory muscle function in cystic fibrosis (CF). This study investigated: 1) whether inspiratory muscle function is reduced in children with stable CF in comparison with healthy controls; and 2) the mechanisms leading to inspiratory muscle weakness, which probably predispose to respiratory muscle fatigue. We determined the tension-time index of the inspiratory muscles (TTMUS) noninvasively at rest in 16 children with mild to moderate CF (mean age, 11 +/- 2 years) and 10 healthy controls (mean age, 11 +/- 2 years). The TTMUS was determined as follows: TTMUS = TI/TTOT.PI/PIMAX, where PI is the mean inspiratory pressure estimated from the measure of mouth occlusion pressure (P0.1), PIMAX is the maximal inspiratory pressure, and TI/TOT is the duty cycle. The results showed similar nutritional status in both groups, as well as mild to moderate airway obstruction, hyperinflation, and trapped gas in the CF group. In this group only, a significant inverse relationship was found between TI/TOT and PI/PIMAX[TITTOT = 0.482 - (0.388PI/PIMAX), r = -0.53; p < 0.05]. These patients also had greater TTMUS (TTMUS = 0.087 +/- 0.030 in CF vs. 0.056 +/- 0.014 in controls, P < 0.01) that increased with decreasing lean body mass (r = -0.70, P < 0.005), with increasing percent predicted functional residual capacity (r = 0.70, P < 0.05), and increasing volumes of trapped gas (r = 0.77, P < 0.01). The multiple linear regression analysis for these factors was significant (R2 = 0.84, P < 0.01); however, the partial regression coefficient was significant only for lean body mass (r2 = 0.60, P < 0.05). Therefore, muscle mass appeared as the strongest determinant of TTMUS in CF. This study used a noninvasive method to assess the inspiratory muscle performance in children with CF. The results suggest impairment in inspiratory muscle function in these children despite good nutritional status and only mild to moderate alteration in pulmonary function tests. In addition, we were able to investigate some of the determinants of inspiratory muscle weakness, namely, muscle mass, hyperinflation, and trapped gas, and found that muscle mass played a predominant role.

Regional distribution of myosin heavy chain isoforms in rib cage muscles as a function of postnatal development

We studied the expression of myosin heavy chain (MHC) isoforms, utilizing electrophoretic methods, in rib cage (RC) muscles: the scalenus medius, the parasternal, cephalic, midthoracic, and caudal intercostal muscles; and in the diaphragm (DI) of rats during postnatal development and when mature. At day 1, all RC muscles and the DI expressed MHC neonatal/embryonic (69-92% of total MHC complement) with little MHC slow and 2A; the RC muscles alone expressed a small proportion of MHC 2B (2-4%). On day 4, MHC neonatal/embryonic expression still predominated (55-71%) but increased MHC 2A expression was observed in both the RC (11-21%) and DI (31%); MHC 2B (5-7%) was noted in the RC muscles but not the DI. By day 14, MHC neonatal/embryonic and 2A expression each comprised a third of the total MHC complement of the RC muscles, MHC 2X was first observed, and MHC 2B expression increased. The day 14 DI was comprised of equal proportions of MHC neonatal/embryonic, slow and 2A with little MHC 2X (11%). The adult and day 30 animals expressed comparable muscle-specific MHC phenotypes: the DI characterized by a proportional mixture of MHC slow, MHC 2A, and MHC 2X, with little MHC 2B, whereas the RC muscles expressed predominantly MHC 2B (40-62%). We conclude that the RC muscles and DI show comparable MHC phenotypes in the immediate newborn period but differ in their MHC expression during postnatal development and when mature. The RC muscles show only minor intermuscle variations in MHC phenotype during development, and when mature are characterized by fast MHC isoform expression, particularly MHC 2B.

Recruitment of some respiratory muscles during three maximal inspiratory manoeuvres

BACKGROUND

A study was undertaken to determine the level of recruitment of the muscles used in the generation of respiratory muscle force, and to ascertain whether maximal diaphragmatic force and maximal inspiratory muscle force need to be measured by separate tests. The level of activity of three inspiratory muscles and one expiratory muscle during three maximal respiratory manoeuvres was studied: (1) maximal inspiration against a closed airway (Muller manoeuvre or maximal inspiratory pressure (MIP)); (2) maximal inspired manoeuvre followed by a maximal expiratory effort (combined manoeuvre); and (3) maximal inspiratory sniff through the nose (sniff manoeuvre).

METHODS

All the manoeuvres were performed from functional residual capacity. The gastric (PGA) and oesophageal (POES) pressures and their difference, transdiaphragmatic pressure (PDI), and the integrated EMG activity of the diaphragm (EDI), the sternomastoid (ESTR), the intercostal parasternals (ERIC), and the rectus abdominis muscles (ERA) were recorded.

RESULTS

Mean (SD) PDI values for the Muller, combined, and sniff manoeuvres were: 127.6 (19.4), 162.7 (22.2), and 136.6 (24.8) cm H2O, respectively. The pattern of rib cage muscle recruitment (POES/PDI) was similar for the Muller and sniff manoeuvres (88% and 80% respectively), and was 58% in the combined manoeuvre, confirming data previously reported in the literature. Peak EDI amplitude was greater during the sniff manoeuvre in all subjects (100%) than during the combined (88.1%) and Muller (61.1%) manoeuvres. ESTR and EIC were more active in the Muller and the sniff manoeuvres. The contribution of the expiratory muscle (ERA) to the three manoeuvres was 100% in the combined, 26.1% for the sniff, and 11.5% for the Muller manoeuvre.

CONCLUSIONS

Each of these three manoeuvres results in different mechanisms of inspiratory and expiratory muscle activation and the intrathoracic and intra-abdominal pressures generated are a reflection of the interaction between the various muscle groups. The Muller and sniff manoeuvres reflect mainly the force of the inspiratory muscles and the combined manoeuvre that of the diaphragm.

Effect of pressure and timing of contraction on human rib cage muscle fatigue

Breathing against inspiratory loads can be accomplished with different degrees of coupling between the diaphragm and the other muscles attached to the rib cage (RCM). Thus, the electromyographic signs of fatigue develop separately in each muscle group. While breathing with diaphragm emphasis, the occurrence of diaphragmatic fatigue was found to be related to the tension-time index TTdi (= Pdi/Pdimax x Ti/Ttot). Above the critical range of 0.15 to 0.18, the endurance of the diaphragm is less than 1 h and it is inversely related to the TTdi value. However, in most loaded breathing conditions, the spontaneous pattern of breathing is characterized by predominant activation of RCM. The tension-time conditions at which fatigue develops during breathing with RCM emphasis are not known. We assessed the critical tension-time value in four normal subjects breathing with RCM emphasis against inspiratory threshold loads. RCM predominance was achieved by developing negative abdominal pressure swings during inspiration, and it was characterized by the tension-time index TTrc (Ppl/Pplmax x Tl/Ttot), where Ppl is pleural pressure developed under this condition. Above a critical TTrc value of 0.30, endurance time was inversely related to TTrc, and it resulted from failure of the RCM rather than of the diaphragm. We conclude that the critical threshold, as assessed by TTrc, is higher for breathing patterns with RCM emphasis than previously described by TTdi for diaphragm emphasis. However, when predominantly recruited, as in breathing patterns commonly adopted in loaded conditions, the RCM fatigue earlier than the diaphragm.

Evidence of acute diaphragmatic fatigue in a "natural" condition. The diaphragm during labor

Acute diaphragmatic fatigue has been experimentally shown to occur in normal healthy subjects and in patients with chronic obstructive pulmonary disease by asking them to modify their pattern of breathing or to breathe against high inspiratory resistances. During the expulsive period of labor women are asked periodically to make strong expulsive efforts and to sustain them isometrically for many seconds; this is likely to produce "natural" diaphragmatic fatigue. To investigate whether this was the case, six women were studied in the delivery room from the moment of the rupture of the amnion until delivery of the infant occurred. The development of diaphragmatic fatigue was assessed both by measuring the static maximal inspiratory pressure (MIP) and by analyzing the electromyographic power spectrum of the diaphragm (H/L ratio). The majority of contractions were concentrated in the expulsive period of labor. MIP, recorded in the supine position, significantly decreased from 103.2 +/- 17.2 cm H2O (at the beginning of the expulsive period) to 73.8 +/- 10.1 cm H2O (after the delivery). The H/L ratio fell progressively during the expulsive period; after the delivery, it was 59.2 +/- 15.7% of the value recorded at the beginning of the expulsive period. This study demonstrates that (1) the diaphragm is active in the expulsive efforts during labor and (2) the tension developed and the time each contraction is maintained may lead to the development of diaphragmatic fatigue. Therefore, we provide evidence of acute diaphragmatic fatigue in a natural condition.

The effect of inspiratory muscle fatigue on breathing pattern and ventilatory response to CO2

1. The effects of inducing inspiratory muscle fatigue on the subsequent breathing pattern were examined during resting unstimulated breathing and during CO2 rebreathing. In addition, we examined whether induction of inspiratory muscle fatigue alters CO2 responsiveness. 2. Global inspiratory muscle fatigue and diaphragmatic fatigue were achieved by having subjects breathe against an inspiratory resistive load while generating a predetermined fraction of either their maximal mouth pressure or maximal transdiaphragmatic pressure until they were unable to generate the target pressure. 3. Induction of inspiratory muscle fatigue had no effect on the subsequent breathing pattern during either unstimulated breathing or during CO2 rebreathing. 4. Following induction of inspiratory muscle fatigue, the slope of the ventilatory response to CO2 was significantly decreased from 18.8 +/- 3.3 during control to 13.8 +/- 2.1 l min-1 (% end-tidal CO2 concentration)-1 with fatigue (P < 0.02).

High intensity exercise training-induced metabolic alterations in respiratory muscles

Limited information exists concerning the effects of high intensity interval exercise training (HIET) on metabolic alterations in both inspiratory and expiratory muscles. To test the hypothesis that HIET will improve the oxidative capacity of the diaphragm and major expiratory muscles, we examined Krebs cycle and beta oxidation enzyme activities in the diaphragm and three groups of expiratory (abdominal) muscles in rats subjected to 12 weeks (5 days.wk-1) of treadmill exercise. Two groups of female Sprague-Dawley rats (age ca 120 days) were studied: (1) HIET group (n = 10; animals performed 6 x ca 5-min running intervals.day-1 at ca 90-95% VO2max); (2) sedentary control group (n = 7). When compared to controls, HIET resulted in significantly elevated (P less than 0.05) activities of 3-hydroxy-acyl-Co-A dehydrogenase (HADH) and citrate synthase (CS) in the costal diaphragm, rectus abdominus, external obliques, and the plantaris muscles. In contrast, training did not increase (P greater than 0.05) the activities of CS or HADH in the crural diaphragm or the internal obliques/transversus abdominus muscles. By comparison, the training-induced increases in oxidative capacity (e.g., CS activity) in the costal diaphragm, rectus abdominus, and external obliques were relatively small (ca 23, 10, 12%, respectively) when contrasted to the exercise-induced increase in CS activity in the plantaris muscle (ca 47%). We conclude that HIET results in small but significant improvements in the oxidative and beta oxidation capacities of the costal diaphragm and at least two abdominal expiratory muscles.

Dyspnea-limited response in chronic obstructive pulmonary disease: reduced unsupported arm activities

Dyspnea, the sensation of uncomfortable breathing, is the primary activity-limiting symptom leading to reduced functional ability in chronic obstructive pulmonary disease (COPD). Patients with severe COPD report a marked increase in the sensation of dyspnea with routine tasks that require arm use, especially activities necessitating unsupported arm elevation. Dyspnea is associated with alterations in respiratory muscle function, such as an increase in muscle force requirement, a reduction in respiratory muscle strength and endurance, and an increase in the recruitment of the rib cage and accessory muscles. Unsupported arm exercise (UAE) further compromises respiratory muscle capacity for ventilation because it requires the muscles' concomitant recruitment in the maintenance of chest wall stabilization. This article presents respiratory muscle mechanisms leading to reduced UAE, methods of measuring unsupported arm endurance, and treatment strategies to improve unsupported arm activity endurance in patients with COPD.

Respiratory muscle fatigue and breathing pattern

The major determinants of VO2 resp and, thus, diaphragmatic endurance are the tension-time index, work rate and lung volume. Changes in breathing pattern that adversely affect any of these factors can impair diaphragmatic endurance. A TTdi above 0.15-0.18 indicates a potentially fatiguing pattern of contraction. However, fatigue may occur at a TTdi below 0.15 if the work rate is sufficiently high or the Pdimax is not measured at the prevailing lung volume. Inspiratory muscle fatigue usually elicits an increase in minute ventilation and respiratory rate and to a lesser degree a reduction in tidal volume. However, fatigue may not always be accompanied by changes in breathing pattern. Furthermore, fatigue may sometimes result in a reduction rather than an increase in motor outflow to the respiratory muscles. Finally, abdominal paradox initially considered to be characteristic clinical sign of inspiratory muscle fatigue appears to be due to increases in respiratory load rather than to muscle fatigue.

Maximal relaxation rate of inspiratory muscle can be effort-dependent and reflect the activation of fast-twitch fibers

We have measured the normalized maximal relaxation rate (MRR, % pressure loss/10 ms) of esophageal and transdiaphragmatic pressures in five normal subjects who performed unoccluded shifts from FRC, with the peak pressure varying between 10 and 100% of each subject's maximum. MRR was computed as the maximal rate of decay of pressure divided by the peak pressure, with units of %pressure loss/10 ms. We observed that MRR became progressively faster as sniff peak pressure increased in amplitude above 10% maximum. In four subjects this trend was most marked for sniffs of less than 40% maximal pressure, with little change as peak pressure increased further. In a fifth subject this trend continued across the full range of pressure. Thus, MRR may be an effort-dependent variable during voluntary inspiratory maneuvers. We postulate that sniff MRR becomes faster with increasing peak pressure because of progressive activation of fast-twitch type II muscle fibers. The findings of this study suggest that erroneous conclusions about the significance of slowing of sniff MRR with fatigue may be made if MRR is determined from voluntary efforts with a peak pressure of less than 60% of control maximum, as may occur with fatigue.

Effect of respiratory muscle fatigue on breathing pattern during incremental exercise

We examined the breathing pattern during incremental exercise before and after induction of inspiratory muscle fatigue. Our aim was to determine whether induction of fatigue alters the ventilatory response to exercise and in particular whether such changes are most apparent at high levels of exercise when minute ventilation and thus inspiratory load are greatest. A group of 10 healthy subjects was studied on a cycle ergometer. Fatigue was achieved by having the subject breathe against an inspiratory threshold load that required the subject to generate 80% of the predetermined maximal mouth pressure to initiate airflow. Breathing pattern, oxygen consumption (VO2), mouth occlusion pressure (P0.1), and a visual analog scale (VAS) for respiratory effort were obtained for 3 min at rest and at 25, 50, 75, and 100% of the subject's maximal work load (Wmax) as determined by preliminary testing. Exercise was performed on two separate occasions, once immediately after induction of fatigue and the other as a control. Induction of fatigue had no effect on resting breathing and only minimal effects at the lower work loads (25 and 50% Wmax). At the higher work loads (75 and 100% Wmax) induction of fatigue significantly altered the pattern of breathing during exercise. At 75% of Wmax the respiratory frequency (f) increased from 22.5 +/- 4.4 (SD) during control to 27.0 +/- 6.7 breaths/min (p less than 0.02) following induction of fatigue; tidal volume was not significantly altered, 2.15 +/- 0.65 versus 2.24 +/- 0.74 L during control. The increase in f was due to reductions in both inspiratory and expiratory time because fractional inspiratory time remained unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)

Muscle fatigue in acute respiratory failure

Fatigue of the respiratory muscles is now well documented but still remains a process that cannot be assessed easily. Several methods are available to detect fatigue; they represent windows allowing different viewpoints on the same phenomenon. The definition and determinants of respiratory muscle fatigue and some of the methods used to detect it are reviewed. On the basis of the available evidence provided by each of these methods, the role played by muscle fatigue in acute respiratory failure (ARF) is discussed.

Activation of human respiratory muscles during different voluntary manoeuvres

1. This study used three techniques (bilateral phrenic nerve stimulation, motor cortex stimulation and quantitative electromyography) to assess the degree of activation of the diaphragm, intercostal-accessory muscles and abdominal muscles during postural tasks and respiratory manoeuvres. They included maximal inspiratory and expulsive efforts. 2. Bilateral phrenic nerve stimuli at supramaximal levels produced an average change in transdiaphragmatic pressure (Pdi) of 28 cmH2O during relaxation. During maximal inspiratory or expulsive efforts, all subjects were able to activate the diaphragm fully at functional residual capacity as judged by the failure of stimuli delivered during the voluntary efforts to increase Pdi. Peak voluntary Pdi was about 30% less for inspiratory than expulsive manoeuvres. 3. By contrast, transcranial activation of motor cortical output to the diaphragm and abdominal muscles produced an increment in abdominal pressure of 25 +/- 7 cmH2O during maximal voluntary expulsive efforts. Given the lack of response to phrenic nerve stimulation at similar voluntary pressures, this suggests that abdominal muscles, and not the diaphragm, fail to generate their full contractile force during maximal voluntary expulsive manoeuvres. 4. Motor cortical stimulation during weak inspiratory efforts produced a small reduction in oesophageal pressure (i.e. increase in net inspiratory force) of 7-14 cmH2O. This response could not be extinguished during maximal voluntary inspiratory efforts in two of three subjects. This occurred despite the cortical co-activation of 'antagonist' muscles in the chest wall and abdomen, and passive transmission of pressure from the abdominal to thoracic compartments. 5. Integrated electromyographic activity (EMG) recorded from abdominal muscles (rectus abdominis, external oblique) was greater during trunk flexion than during maximal expulsive efforts. Similarly, integrated EMG of the intercostal-accessory muscles (sternomastoid, scalenes, parasternal intercostals) was greater during tasks requiring head and/or neck flexion than during the maximal inspiratory efforts. 6. These data show that the diaphragm can be fully activated by the central nervous system during voluntary respiratory tasks but that other agonist 'respiratory' muscles need not be activated fully. Given the complex actions of 'inseries' respiratory muscles revealed here, it is argued that differences in the transdiaphragmatic pressure during various postural and respiratory tasks do not necessarily imply variation in the level of diaphragmatic neural drive.