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

Pressure measurement characteristics of a micro-transducer and balloon catheters

Respiratory pressure responses to cervical magnetic stimulation are important measurements in monitoring the mechanical function of the respiratory muscles. Pressures can be measured using balloon catheters or a catheter containing integrated micro‐transducers. However, no research has provided a comprehensive analysis of their pressure measurement characteristics. Accordingly, the aim of this study was to provide a comparative analysis of these characteristics in two separate experiments: (1) in vitro with a reference pressure transducer following a controlled pressurization; and (2) in vivo following cervical magnetic stimulations. In vitro the micro‐transducer catheter recorded pressure amplitudes and areas which were in closer agreement to the reference pressure transducer than the balloon catheter. In vivo there was a main effect for stimulation power and catheter for esophageal (P_es), gastric (P_ga), and transdiaphragmatic (P_di) pressure amplitudes (p < 0.001) with the micro‐transducer catheter recording larger pressure amplitudes. There was a main effect of stimulation power (p < 0.001) and no main effect of catheter for esophageal (p = 0.481), gastric (p = 0.923), and transdiaphragmatic (p = 0.964) pressure areas. At 100% stimulator power agreement between catheters for P_di amplitude (bias =6.9 cmH₂O and LOA −0.61 to 14.27 cmH₂O) and pressure areas (bias = −0.05 cmH₂O·s and LOA −1.22 to 1.11 cmH₂O·s) were assessed. At 100% stimulator power, and compared to the balloon catheters, the micro‐transducer catheter displayed a shorter 10–90% rise time, contraction time, latency, and half‐relaxation time, alongside greater maximal rates of change in pressure for esophageal, gastric, and transdiaphragmatic pressure amplitudes (p < 0.05). These results suggest that caution is warranted if comparing pressure amplitude results utilizing different catheter systems, or if micro‐transducers are used in clinical settings while applying balloon catheter‐derived normative values. However, pressure areas could be used as an alternative point of comparison between catheter systems.

Influence of Upper-Body Exercise on the Fatigability of Human Respiratory Muscles

Purpose

Diaphragm and abdominal muscles are susceptible to contractile fatigue in response to high-intensity, whole-body exercise. This study assessed whether the ventilatory and mechanical loads imposed by high-intensity, upper-body exercise would be sufficient to elicit respiratory muscle fatigue.

Methods

Seven healthy men (mean ± SD; age = 24 ± 4 yr, peak O₂ uptake [V˙O_2peak] = 31.9 ± 5.3 mL·kg⁻¹·min⁻¹) performed asynchronous arm-crank exercise to exhaustion at work rates equivalent to 30% (heavy) and 60% (severe) of the difference between gas exchange threshold and V˙O_2peak. Contractile fatigue of the diaphragm and abdominal muscles was assessed by measuring pre- to postexercise changes in potentiated transdiaphragmatic and gastric twitch pressures (P_di,tw and P_ga,tw) evoked by supramaximal magnetic stimulation of the cervical and thoracic nerves, respectively.

Results

Exercise time was 24.5 ± 5.8 min for heavy exercise and 9.8 ± 1.8 min for severe exercise. Ventilation over the final minute of heavy exercise was 73 ± 20 L·min⁻¹ (39% ± 11% maximum voluntary ventilation) and 99 ± 19 L·min⁻¹ (53% ± 11% maximum voluntary ventilation) for severe exercise. Mean P_di,tw did not differ pre- to postexercise at either intensity (P > 0.05). Immediately (5–15 min) after severe exercise, mean P_ga,tw was significantly lower than pre-exercise values (41 ± 13 vs 53 ± 15 cm H₂O, P < 0.05), with the difference no longer significant after 25–35 min. Abdominal muscle fatigue (defined as ≥15% reduction in P_ga,tw) occurred in 1/7 subjects after heavy exercise and 5/7 subjects after severe exercise.

Conclusions

High-intensity, upper-body exercise elicits significant abdominal, but not diaphragm, muscle fatigue in healthy men. The increased magnitude and prevalence of fatigue during severe-intensity exercise is likely due to additional (nonrespiratory) loading of the thorax.

Effect of acute hypoxia on respiratory muscle fatigue in healthy humans

Background

Greater diaphragm fatigue has been reported after hypoxic versus normoxic exercise, but whether this is due to increased ventilation and therefore work of breathing or reduced blood oxygenation per se remains unclear. Hence, we assessed the effect of different blood oxygenation level on isolated hyperpnoea-induced inspiratory and expiratory muscle fatigue.

Methods

Twelve healthy males performed three 15-min isocapnic hyperpnoea tests (85% of maximum voluntary ventilation with controlled breathing pattern) in normoxic, hypoxic (SpO₂ = 80%) and hyperoxic (FiO₂ = 0.60) conditions, in a random order. Before, immediately after and 30 min after hyperpnoea, transdiaphragmatic pressure (P_di,tw ) was measured during cervical magnetic stimulation to assess diaphragm contractility, and gastric pressure (P_ga,tw ) was measured during thoracic magnetic stimulation to assess abdominal muscle contractility. Two-way analysis of variance (time x condition) was used to compare hyperpnoea-induced respiratory muscle fatigue between conditions.

Results

Hypoxia enhanced hyperpnoea-induced P_di,tw and P_ga,tw reductions both immediately after hyperpnoea (P_di,tw : normoxia -22 ± 7% vs hypoxia -34 ± 8% vs hyperoxia -21 ± 8%; P_ga,tw : normoxia -17 ± 7% vs hypoxia -26 ± 10% vs hyperoxia -16 ± 11%; all P < 0.05) and after 30 min of recovery (P_di,tw : normoxia -10 ± 7% vs hypoxia -16 ± 8% vs hyperoxia -8 ± 7%; P_ga,tw : normoxia -13 ± 6% vs hypoxia -21 ± 9% vs hyperoxia -12 ± 12%; all P < 0.05). No significant difference in P_di,tw or P_ga,tw reductions was observed between normoxic and hyperoxic conditions. Also, heart rate and blood lactate concentration during hyperpnoea were higher in hypoxia compared to normoxia and hyperoxia.

Conclusions

These results demonstrate that hypoxia exacerbates both diaphragm and abdominal muscle fatigability. These results emphasize the potential role of respiratory muscle fatigue in exercise performance limitation under conditions coupling increased work of breathing and reduced O₂ transport as during exercise in altitude or in hypoxemic patients.

Non-invasive ventilation applied for recovery from exercise-induced diaphragmatic fatigue

Background:

Exercise-induced diaphragmatic fatigue (DF) is conventionally considered to reflect impaired diaphragm function resulting from load imposed on the diaphragm during exercise and is known to be reduced by the application of non-invasive ventilation (NIV) during heavy-intensity exercise testing (HEET). On that physiological condition NIV applied for diaphragm unloading during recovery from exercise should be capable of accelerating recovery from DF and therewith prolonging exercise time to exhaustion and limiting the development of DF during a subsequent HEET compared to recovery during spontaneous breathing.

Methods:

Seven highly-trained subjects (V’O_2max 62.7±7.8 ml/kg/min) performed four HEET at 85% V’O_2max with 60 min of recovery during I spontaneous breathing and II NIV between two HEET.

Results:

Twitch transdiaphragmatic pressure (TwPdi) during supramaximal magnetic phrenic nerve stimulation decreased (p<0.04) following first HEET and subsequently completely recovered (p>0.2) during I and II. Following second HEET TwPdi comparably decreased (I 0.24±0.21 vs II 0.32±0.29 kPa; p=0.17). Exercise time to exhaustion during second HEET was equal during I and II (I 514±49 vs II 511±92 s; p=0.88).

Conclusions:

In conclusion, NIV applied for diaphragm unloading during recovery following HEET does neither affect recovery from DF nor subsequent exercise performance thereby providing further evidence that DF might reflect post-exercise diaphragm shielding rather than impaired diaphragm function.

Increased fatigue resistance of respiratory muscles during exercise after respiratory muscle endurance training

Respiratory muscle fatigue develops during exhaustive exercise and can limit exercise performance. Respiratory muscle training, in turn, can increase exercise performance. We investigated whether respiratory muscle endurance training (RMT) reduces exercise-induced inspiratory and expiratory muscle fatigue. Twenty-one healthy, male volunteers performed twenty 30-min sessions of either normocapnic hyperpnoea ( n = 13) or sham training (CON, n = 8) over 4–5 wk. Before and after training, subjects performed a constant-load cycling test at 85% maximal power output to exhaustion (PREEXH, POSTEXH). A further posttraining test was stopped at the pretraining duration (POSTISO) i.e., isotime. Before and after cycling, transdiaphragmatic pressure was measured during cervical magnetic stimulation to assess diaphragm contractility, and gastric pressure was measured during thoracic magnetic stimulation to assess abdominal muscle contractility. Overall, RMT did not reduce respiratory muscle fatigue. However, in subjects who developed >10% of diaphragm or abdominal muscle fatigue in PREEXH, fatigue was significantly reduced after RMT in POSTISO(inspiratory: −17 ± 6% vs. −9 ± 10%, P = 0.038, n = 9; abdominal: −19 ± 10% vs. −11 ± 11%, P = 0.038, n = 9), while sham training had no significant effect. Similarly, cycling endurance in POSTEXHdid not improve after RMT ( P = 0.071), while a significant improvement was seen in the subgroup with >10% of diaphragm fatigue after PREEXH( P = 0.017), but not in the sham training group ( P = 0.674). However, changes in cycling endurance did not correlate with changes in respiratory muscle fatigue. In conclusion, RMT decreased the development of respiratory muscle fatigue during intensive exercise, but this change did not seem to improve cycling endurance.

Dyspnea as a Noxious Sensation: Inspiratory Threshold Loading May Trigger Diffuse Noxious Inhibitory Controls in Humans

Dyspnea, a leading respiratory symptom, shares many clinical, physiological, and psychological features with pain. Both activate similar brain areas. The neural mechanisms of dyspnea are less well described than those of pain. The present research tested the hypothesis of common pathways between the two sensations. Six healthy men (age 30–40 yr) were studied. The spinal nociceptive flexion reflex (RIII) was first established in response to electrical sural stimulation. Dyspnea was then induced through inspiratory threshold loading, forcing the subjects to develop 70% of their maximal inspiratory pressure to inhale. This led to progressive inhibition of the RIII reflex that reached 50 ± 12% during the fifth minute of loading ( P < 0.001), was correlated to the intensity of the self-evaluated respiratory discomfort, and had recovered 5 min after removal of the load. The myotatic H-reflex was not inhibited by inspiratory loading, arguing against postsynaptic alpha motoneuron inhibition. Dyspnea, like pain, thus induced counterirritation, possibly indicating a C-fiber stimulation and activation of diffuse noxious inhibitory descending controls known to project onto spinal dorsal horn wide dynamic range neurons. This confirms the noxious nature of certain types of breathlessness, thus opening new physiological and perhaps therapeutic perspectives.

Influence of diaphragm and rib cage muscle fatigue on breathing during endurance exercise

Inspiratory muscle fatigue (IMF) can develop during exhaustive exercise and cause tachypnea or rapid shallow breathing. We assessed the effects of rib cage muscle (RCM-F) and diaphragm fatigue (DIA-F) on breathing pattern and respiratory mechanics during high-intensity endurance exercise. Twelve healthy subjects performed a constant-load (85% maximal power) cycling test to exhaustion with prior IMF and a cycling test of similar intensity and duration without prior IMF (control). IMF was induced by resistive breathing and assessed by oesophageal and gastric twitch pressure measurements during cervical magnetic stimulation. Both RCM-F and DIA-F increased RCM and abdominal muscle force production during exercise compared to control. With RCM-F, tidal volume decreased while it increased with DIA-F. RCM-F was associated with a smaller increase in end-expiratory oesophageal pressure (i.e. decrease in lung volume) than DIA-F. These results suggest that RCM-F and not DIA-F is associated with rapid shallow breathing and that lowering the operating lung volume with DIA-F may help to preserve diaphragmatic function.

Fatigue and respiratory disorders

OBJECTIVES

To analyze the factors at the origin of fatigue in respiratory disorders. To assess fatigue and its functional impact on patients affected from respiratory diseases. To evaluate the results of comprehensive care on fatigue and functional capacity.

MATERIALS AND METHODS

We systematically reviewed the literature in Medline and the Cochrane Library, using the following keywords: fatigue, respiratory disorders, questionnaire, evaluation, assessment, randomized controlled trial, meta-analysis.

RESULTS

Fatigue is a high frequency symptom (90%) and takes an important place, as much as dyspnea, in the genesis of the respiratory induced handicap. Its assessment is varied, according to the studies. It originates from multiple causes, as shown from clinical and experimental studies. The main treatment consists in rehabilitation, using physical exercises. Its efficacy is demonstrated on physical endurance, but is not clear in terms of general fatigue.

CONCLUSION

Although fatigue is very frequent complaint, along with a major disabling condition, the comprehensive assessment of fatigue, in respiratory disorders, including its physical and cognitive components, is not still really codified. Rehabilitation is the main treatment. Its efficiency has been demonstrated on the physical and functional components of fatigue. Its results on perceived fatigue remains to be evaluated.

Optimal arrangement of magnetic coils for functional magnetic stimulation of the inspiratory muscles in dogs

In an attempt to maximize inspiratory pressure and volume, the optimal position of a single or of dual magnetic coils during functional magnetic stimulation (FMS) of the inspiratory muscles was evaluated in twenty-three dogs. Unilateral phrenic magnetic stimulation (UPMS) or bilateral phrenic magnetic stimulation (BPMS), posterior cervical magnetic stimulation (PCMS), anterior cervical magnetic stimulation (ACMS) as well as a combination of PCMS and ACMS were performed. Trans-diaphragmatic pressure (Pdi), flow, and lung volume changes with an open airway were measured. Transdiaphragmatic pressure was also measured with an occluded airway. Changes in inspiratory parameters during FMS were compared with 1) electrical stimulation of surgically exposed bilateral phrenic nerves (BPES) and 2) ventral root electrical stimulation at C5-C7 (VRES C5-C7). Relative to the Pdi generated by BPES of 36.3 +/- 4.5 cm H2O (Mean +/- SEM), occluded Pdi(s) produced by UPMS, BPMS, PCMS, ACMS, and a combined PCMS + ACMS were 51.7%, 61.5%, 22.4%, 100.3%, and 104.5% of the maximal Pdi, respectively. Pdi(s) produced by UPMS, BPMS, PCMS, ACMS, and combined ACMS + PCMS were 38.0%, 45.2%, 16.5%, 73.8%, and 76.8%, respectively, of the Pdi induced by VRES (C5-C7) (48.0 +/- 3.9 cm H2O). The maximal Pdi(s) generated during ACMS and combined PCMS + ACMS were higher than the maximal Pdi(s) generated during UPMS, BPMS, or PCMS (p < 0.05). ACMS alone induced 129.8% of the inspiratory flow (73.0 +/- 9.4 L/ min) and 77.5% of the volume (626 +/- 556 ml) induced by BPES. ACMS and combined PCMS + ACMS produce a greater inspiratory pressure than UPMS, BPMS or PCMS. ACMS can be used to generate sufficient inspiratory pressure, flow, and volume for activation of the inspiratory muscles.

Diaphragmatic dysfunction in patients with idiopathic inflammatory myopathies

Polymyositis, dermatopolymyositis, and inclusion body myositis imply chronic inflammation of skeletal muscles. Pulmonary complications include aspiration pneumonia, interstitial pneumonitis, or respiratory muscle myositis. This study aims at better describing their impact on respiratory muscle. Twenty-three consecutive patients (12 PM, 5 DM, 6 IBM) were studied (static inspiratory and expiratory pressures; diaphragm function in terms of the mouth and transdiaphragmatic pressure responses to bilateral phrenic stimulation). Pulmonary parenchymatous abnormalities were mild (6 cases) or absent. The mouth pressure produced by phrenic stimulation was 6.83+/-3.01 cm H2O, with 18 patients (78%) diagnosed with diaphragm weakness (<10 cm H2O) and lower values in DM (4.35+/-1.48 cm H2O) than in IBM and in PM (P<0.05). Diaphragm weakness is frequent and probably overlooked in inflammatory myopathies. Further studies are needed to delineate the clinical relevance of these results.

Task failure from inspiratory resistive loaded breathing: a role for inspiratory muscle fatigue?

The use of non-invasive resistive breathing to task failure to assess inspiratory muscle performance remains a matter of debate. CO2 retention rather than diaphragmatic fatigue was suggested to limit endurance during inspiratory resistive breathing. Cervical magnetic stimulation (CMS) allows discrimination between diaphragmatic and rib cage muscle fatigue. We tested a new protocol with respect to the extent and the partitioning of inspiratory muscle fatigue at task failure. Nine healthy subjects performed two runs of inspiratory resistive breathing at 67 (12)% of their maximal inspiratory mouth pressure, respiratory rate (fR), paced at 18 min(-1), with a 15-min pause between runs. Diaphragm and rib cage muscle contractility were assessed from CMS-induced esophageal (P(es,tw)), gastric (P(ga,tw)), and transdiaphragmatic (P(di,tw)) twitch pressures. Average endurance times of the first and second runs were similar [9.1 (6.7) and 8.4 (3.5) min]. P(di,tw) significantly decreased from 33.1 to 25.9 cmH2O in the first run, partially recovered (27.6 cmH2O), and decreased further in the second run (23.4 cmH2O). P(es,tw) also decreased significantly (-5.1 and -2.4 cmH2O), while P(ga,tw) did not change significantly (-2.0 and -1.9 cmH2O), indicating more pronounced rib cage rather than diaphragmatic fatigue. End-tidal partial pressure of CO2 ( PETCO2) rose from 37.2 to 44.0 and 45.3 mmHg, and arterial oxygen saturation (SaO2) decreased in both runs from 98% to 94%. Thus, task failure in mouth-pressure-targeted, inspiratory resistive breathing is associated with both diaphragmatic and rib cage muscle fatigue. Similar endurance times despite different degrees of muscle fatigue at the start of the runs indicate that other factors, e.g. increases in PETCO2, and/or decreases in SaO2, probably contributed to task-failure.

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.

Can diaphragmatic contractility be assessed by airway twitch pressure in mechanically ventilated patients?

BACKGROUND

In critically ill patients inspiratory muscle function may be assessed by measurements of maximal inspiratory airway pressure and the response of twitch transdiaphragmatic pressure (Pdi tw) to bilateral phrenic nerve stimulation. The first is limited by its total dependence on patient cooperation. Although the second approach is independent of patient volition, it is impractical because it requires oesophageal and gastric balloons. Because airway pressure is easily and non-invasively recorded in patients with artificial airways, we hypothesised that twitch airway pressure (Paw tw) reliably predicts Pdi tw and twitch oesophageal pressure (Poes tw) in mechanically ventilated patients.

METHODS

Thirteen mechanically ventilated patients recovering from an episode of acute respiratory failure received phrenic nerve stimulation at end exhalation. The rapid occlusion technique was used to record respiratory system mechanics.

RESULTS

Stimulations were well tolerated. Mean (SE) Paw tw at end exhalation was -8.2 (1.2) cm H(2)O and Poes tw and Pdi tw were -7.3 (1.1) and 10.4 (1.8) cm H(2)O, respectively. Stimulations produced a good correlation between Paw tw and Pdi tw (p<0.001), although the limits of agreement were wide. The results were similar for Poes tw. No relationship was found between the Paw tw/Poes tw ratio and respiratory system compliance or airway resistance. Paw tw reproducibility was excellent (mean coefficient of variation 6%, range 3-9%).

CONCLUSIONS

Despite a good correlation between Paw tw and Poes tw, Paw tw did not reliably predict Poes tw or Pdi tw in mechanically ventilated patients. Nevertheless, the excellent reproducibility of Paw tw suggests that it may be a useful means of monitoring inspiratory muscle contractility in the routine care of mechanically ventilated patients.

Validation of improved recording site to measure phrenic conduction from surface electrodes in humans

Phrenic nerve stimulation, electrical (ES) or from cervical magnetic stimulation (CMS), allows one to assess the diaphragm contractile properties and the conduction time of the phrenic nerve (PNCT) through recording of an electromyographic response, traditionally by using surface electrodes. Because of the coactivation of extradiaphragmatic muscles, signal contamination can jeopardize the determination of surface PNCTs. To address this, we compared PNCTs with ES and CMS from surface and needle diaphragm electrodes in five subjects (10 phrenic nerves). At a modified recording site, lower and more anterior than usual (lowest accessible intercostal space, costochondral junction) with electrodes 2 cm apart, surface and needle PNCTs were similar (CMS: 6.0 +/- 0.25 ms surface vs. 6.2 +/- 0.13 ms needle, not significant). Electrodes recording the activity of the most likely sources of signal contamination, i.e., the serratus anterior and pectoralis major, showed distinct responses from that of the diaphragm, their earlier occurrence strongly arguing against contamination. With ES and CMS, apparently uncontaminated signals could be consistently recorded from surface electrodes.

Bilateral anterolateral magnetic stimulation of the phrenic nerves can detect diaphragmatic fatigue

BACKGROUND AND STUDY OBJECTIVES

Measurement of twitch transdiaphragmatic pressure (TwPdi) during bilateral phrenic nerve stimulation is presently the best method to detect diaphragmatic fatigue in humans. The stimulation methods that are currently employed (ie, transcutaneous electrical stimulation [TES] and cervical magnetic stimulation [CMS]) have limitations. Bilateral anterolateral magnetic stimulation of the phrenic nerves (BAMPS) was recently described. The purpose of this study was to determine whether BAMPS can reliably detect diaphragmatic fatigue, and to compare the results with BAMPS with those obtained with the other stimulation techniques.

SUBJECTS

Twelve healthy subjects participated in the study.

METHODS

TwPdi was measured during TES, CMS, and BAMPS before and 10, 30, and 60 min after a potentially fatiguing task. Voluntary hyperpnea to task failure was used as the fatiguing task because this task has previously been shown to reliably produce contractile fatigue of the diaphragm. To determine the reproducibility of BAMPS, TwPdi was measured before and after a nonfatiguing task in 10 of the subjects.

RESULTS

TwPdi fell significantly after the hyperpneic task with all three stimulation techniques, and the amount by which TwPdi fell after hyperpnea was not significantly different for the different stimulation techniques. The percentage fall in TwPdi after hyperpnea was significantly correlated between stimulation techniques (CMS vs BAMPS, r = 0.72; TES vs BAMPS, r = 0.84; and TES vs CMS, r = 0.67). The mean (+/- SE) within-subject, between-trial coefficient of variation for TwPdi during BAMPS was 5.1 +/- 0.1%.

CONCLUSION

BAMPS is highly reproducible and at least as good at detecting diaphragmatic fatigue as the other stimulation techniques.

Diaphragmatic fatigue and high-intensity exercise in patients with chronic obstructive pulmonary disease

Patients with chronic obstructive pulmonary disease (COPD) are at a mechanical disadvantage and should be predisposed to the development of diaphragmatic fatigue when the ventilatory system is stressed by exercise. The purpose of this study was to determine whether patients with moderately severe COPD develop contractile fatigue of the diaphragm after cycle exercise to the limits of tolerance. Twelve male patients with COPD, age 61.4 +/- 3.0 yr, participated. Their forced expiratory volume in 1 s (FEV(1)) was 1.79 +/- 0.14 L, 49.6 +/- 3.4% of predicted. Patients cycled at 60-70% of their predetermined maximal work capacity until they had to stop because of intolerable symptoms. Twitch transdiaphragmatic pressure (Pdi,tw) was measured during cervical magnetic stimulation before and 10, 30, and 60 min after exercise. A persistent fall in Pdi,tw postexercise of >/= 10% was considered potentially indicative of contractile fatigue of the diaphragm. Patients cycled for 10.2 +/- 2.0 min at a workload of 59.9 +/- 4.3 W. Patients exercised maximally relative to their capacity reaching a peak oxygen consumption (V O(2)) of 108.1 +/- 2.8% of the peak V O(2) obtained during a preliminary maximal incremental exercise test. Pdi,tw was not significantly different from baseline at any time postexercise. Pdi,tw was 19.9 +/- 1.6 cm H(2)O at baseline, 19.6 +/- 2.0 cm H(2)O at 10 min postexercise, 18. 6 +/- 2.0 cm H(2)O at 30 min postexercise, and 19.5 +/- 1.7 cm H(2)O at 60 min postexercise. In the individual patients, two of the patients had a persistent >/= 10% fall in Pdi,tw postexercise, potentially indicative of contractile fatigue of the diaphragm. In conclusion, the majority of patients with moderately severe COPD do not develop contractile fatigue of the diaphragm after high-intensity constant workload cycle exercise to the limits of tolerance.