The voluntary drive to breathe is not decreased in hypercapnic patients with severe COPD

Ventilatory neural drive in chronically hypercapnic patients with COPD: effects of sleep and nocturnal noninvasive ventilation

Sleep brings major challenges for the control of ventilation in humans, particularly the regulation of arterial carbon dioxide pressure (P _aCO2 ). In patients with COPD, chronic hypercapnia is associated with increased mortality. Therefore, nocturnal high-level noninvasive positive-pressure ventilation (NIV) is recommended with the intention to reduce P _aCO2 down to normocapnia. However, the long-term physiological consequences of P _aCO2 "correction" on the mechanics of breathing, gas exchange efficiency and resulting symptoms (i.e. dyspnoea) remain poorly understood. Investigating the influence of sleep on the neural drive to breathe and its translation to the mechanical act of breathing is of foremost relevance to create a solid rationale for the use of nocturnal NIV. In this review, we critically discuss the mechanisms by which sleep influences ventilatory neural drive and mechanical consequences in healthy subjects and hypercapnic patients with advanced COPD. We then discuss the available literature on the effects of nocturnal NIV on ventilatory neural drive and respiratory mechanics, highlighting open avenues for further investigation.

Hypercapnia in COPD: Causes, Consequences, and Therapy

Chronic obstructive pulmonary disease (COPD) is a progressive respiratory disorder that may lead to gas exchange abnormalities, including hypercapnia. Chronic hypercapnia is an independent risk factor of mortality in COPD, leading to epithelial dysfunction and impaired lung immunity. Moreover, chronic hypercapnia affects the cardiovascular physiology, increases the risk of cardiovascular morbidity and mortality, and promotes muscle wasting and musculoskeletal abnormalities. Noninvasive ventilation is a widely used technique to remove carbon dioxide, and several studies have investigated its role in COPD. In the present review, we aim to summarize the causes and effects of chronic hypercapnia in COPD. Furthermore, we discuss the use of domiciliary noninvasive ventilation as a treatment option for hypercapnia while highlighting the controversies within the evidence. Finally, we provide some insightful clinical recommendations and draw attention to possible future research areas.

Hypercapnia in Advanced Chronic Obstructive Pulmonary Disease: A Secondary Analysis of the National Emphysema Treatment Trial

RATIONALE

Hypercapnia develops in one third of patients with advanced chronic obstructive pulmonary disease (COPD) and is associated with increased morbidity and mortality. Multiple factors in COPD are thought to contribute to the development of hypercapnia including increased carbon dioxide (CO2) production, increased dead space ventilation, and the complex interactions of deranged respiratory system mechanics, inspiratory muscle overload and the ventilatory control center in the brainstem. However, these factors have not previously been systematically analyzed in a large, well-characterized population of severe COPD patients.

METHODS

This is a secondary analysis of the clinical, physiologic and imaging data from the National Emphysema Treatment Trial (NETT). All patients with complete baseline data for the key predictor variables were included. An inclusive list of 32 potential predictor variables were selected a priori based on consensus of the investigators and literature review. Stepwise variable selection yielded 10 statistically significant associations in multivariate regression.

RESULTS

A total of 1419 patients with severe COPD were included in the analysis; mean age 66.4 years (standard deviation 6.3), 38% females, and 422 (29.7%) had baseline hypercapnia. Key variables associated with hypercapnia were low resting partial pressure of oxygen in blood, low minute ventilation (Ve), high volume of exhaled carbon dioxide, low forced expiratory volume in 1 second, high residual volume, lower % emphysema on chest computed tomography, use of oxygen, low ventilatory reserve (high Ve/maximal voluntary ventilation), and not being at high altitude. Low diffusing capacity for carbon monoxide showed a positive association with hypercapnia in univariate analysis but a negative correlation in multivariate analysis. Measures of dyspnea and quality of life did not associate with degree of hypercapnia in multivariable analysis.

CONCLUSION

Hypercapnia in a well-characterized cohort with severe COPD and emphysema is chiefly related to poor lung mechanics, high CO2 production, and a reduced ventilatory capability. Hypercapnia is less impacted by gas exchange abnormalities or the presence of emphysema.

Assessment of Central Drive to the Diaphragm by Twitch Interpolation: Normal Values, Theoretical Considerations, and Future Directions

BACKGROUND

The twitch interpolation technique is a promising tool for assessing central drive to the diaphragm. It is used to quantify the degree of voluntary diaphragm activation during predefined breathing maneuvers.

OBJECTIVES

This study was designed to (a) determine reference values for the level of voluntary activation of the diaphragm using the twitch occlusion technique in healthy adults and (b) explore the association between central drive to the diaphragm and volitional tests of respiratory muscle strength.

METHODS

Twenty-seven healthy volunteers aged 26 ± 14 years (18 male) were enrolled. Twitch transdiaphragmatic pressure (Pdi) was determined at relaxed functional residual capacity in response to cervical magnetic stimulation (CMS) of the phrenic nerves. The subjects were then instructed to gradually increase voluntary activation of the diaphragm, and the effects of superimposed magnetic stimuli on voluntary Pdi were assessed.

RESULTS

The twitch Pdi amplitude following CMS linearly decreased with increasing inspiratory effort. The resulting diaphragm voluntary activation index (DVAI) during maximal voluntary contraction was 75 ± 15% irrespective of gender or age. Twitch duration, half relaxation time, and area under the curve of superimposed Pdi deflections did not show a linear but an exponential association with increasing voluntary activation of the diaphragm. More than 2/3 of the decrease in the above values was evident after 1/3 of voluntary diaphragm contraction. Forced vital capacity (FVC) was inversely correlated with the DVAI.

CONCLUSIONS

Twitch interpolation allows for assessment of central drive to the diaphragm. The maximum DVAI is independent of gender or age, and significantly related to FVC but not to maximum inspiratory pressure or Pdi as direct measures of diaphragm strength.

Absence of dynamic hyperinflation during exhaustive exercise in severe COPD reflects submaximal IC maneuvers rather than a nonhyperinflator phenotype

Approximately 20% of chronic obstructive pulmonary disease (COPD) patients have been considered to have a “nonhyperinflator phenotype.” However, this judgment depends on patients making a fully maximal inspiratory capacity (IC) maneuver at rest, since the IC during exercise is compared with this baseline measurement. We hypothesized that IC maneuvers at rest are sometimes submaximal and tested this hypothesis by measuring IC and associated neural respiratory drive at rest and during inhalation of CO2 and exercise in patients with COPD. Twenty-six COPD patients [age 66 ± 6 yr, mean forced expiratory volume in 1 s (FEV1) 40 ± 11% predicted] and 39 healthy subjects (age 39 ± 14 yr, FEV1 98 ± 12% predicted) were studied. IC and the diaphragm electromyogram (EMGdi) associated with it (EMGdi-IC) and forced inspiratory vital capacity (FIVC) and its corresponding EMGdi (EMGdi-FIVC) were measured during inhalation of 8% CO2 (8% CO2-92% O2) and room air. Incremental exhaustive cycle ergometer exercise was also performed in both patients with COPD and healthy subjects. IC, EMGdi-IC, FIVC, and EMGdi-FIVC during breathing 8% CO2 were significantly greater than those during breathing room air in both patients with COPD and healthy subjects (all P < 0.001). EMGdi-IC in patients with COPD constantly increased during exercise from 145 ± 40 µV at rest to 185 ± 52 µV at the end of exercise but change in IC was variable. Neural respiratory drive and its relevant IC increased during hypercapnia. Exercise-related hypercapnia in patients with COPD raises neural respiratory drives, which compensate for IC reduction, leading to underestimation of dynamic hyperinflation measured by IC at rest breathing room air.

NEW & NOTEWORTHY Inspiratory capacity measured during hypercapnia is higher than that during eucapnia. Thus total lung capacity is not always be achieved by a standard inspiratory capacity maneuver, leading to risk of underestimation of dynamic hyperinflation in patients with severe chronic obstructive pulmonary disease after exhaustive exercise.

Evaluating Stable Chronic Obstructive Pulmonary Disease by Ultrasound

Background and Aim

The purpose of the study was to evaluate the relationship between COPD severity and the diaphragmatic function measured by point-of-care US in patients with stable COPD.

Method

A total of 61 patients with COPD and 40 healthy subjects who had been admitted to Ufuk University Hospital between December 2018 and May 2019 were enrolled. Point-of-care US was performed, and lung silhouette and anterior, right, and left hemidiaphragm method in M-mode were used to evaluate the diaphragm.

Results

The point-of-care US measurements, lung silhouette method right (Lung Sil R), lung silhouette method left (Lung Sil L), right hemidiaphragm US method in B-mode (Ant B-Mode R), and right hemidiaphragm US method in M-mode (Ant M-Mode R), were significantly different among groups (P < 0.001 for each). FEV1 was strongly correlated with Lung Sil R, Lung Sil L, Ant B-Mode R, and Ant M-Mode R (r = 0.963, P < 0.001; r = 0.956, P < 0.001; r = 0.953, P < 0.001; and r = 0.917, and P < 0.001, respectively). Negative correlations were detected between the number of exacerbations per year and Lung Sil R and the number of exacerbations per year and Ant M-Mode R (r = −0.599, P < 0.001 and r = −0.587, and P < 0.001, respectively).

Conclusion

In this study, FEV1 and annual number of exacerbations turned out to be strongly correlated US findings. The use of US in COPD patients could help to support clinical decision, but further clinical studies are necessary to confirm those findings.

Acute hypercapnia does not alter voluntary drive to the diaphragm in healthy humans

Although systemic hypercapnia is a common outcome of pulmonary disease, the relationship between hypercapnia and voluntary diaphragmatic activation (VA_di) is unclear. To examine whether hypercapnia independent of ventilatory work contributes to reduced central motor drive to the diaphragm in healthy humans, 14 subjects spontaneously breathed room air (NN) or a hypercapnic gas mixture (HH; 7% CO₂ with air) while at rest. Thereafter, subjects volitionally hyperventilated room air (NH) matching the minute ventilation recorded during HH while maintained at eucapnic levels. Twitch interpolation with bilateral magnetic stimulation of phrenic nerves at functional residual capacity was used to assess VA_di during the three trials. Although P_ETCO₂ was elevated during HH compared with NN and NH (52 vs 36 mmHg), VA_di was not altered across the trials (HH = 93.3 ± 7.0%, NN = 94.4 ± 5.0%, NH = 94.9 ± 4.6%, p = 0.48). Our findings indicate that the magnitude of hypercapnia acutely imposed may not be effective in inhibiting voluntary neural drives to the diaphragm in normal resting individuals.

Diaphragm Thickness and Inspiratory Muscle Functions in Chronic Stroke Patients

Background

The aims of this study are to investigate the difference between the diaphragm thickness at end expiration and the thickness at total lung capacity (TLC), and to examine differences in inspiratory muscle function between stroke patients and healthy individuals.

Material/Methods

Forty-five stroke patients and 49 healthy volunteers were included in this study. Diaphragm thickness was measured at end expiration and at TLC by ultrasonography. The maximal inspiratory pressure (MIP), peak inspiratory flow (PIF), vital capacity (VC), and inspiratory muscle endurance (IME) were assess to evaluate inspiratory muscle function.

Results

In stroke patients, the diaphragm was significantly thinner on the affected side than the less affected side at end expiration and at TLC. The change between the thickness at end expiration and at TLC were also significant on both sides. Between groups, the difference in diaphragm thickness at end expiration was not significant, but at TLC, the diaphragms were significantly thicker in healthy individuals than on either side in stroke patients, and the change in diaphragm thickness was significantly greater for healthy individuals. Inspiratory muscle functions were also significantly greater in healthy individuals. MIP, PIF, and VC were positively correlated with the change in thickness in healthy individuals, and MIP was positively correlated with the change in thickness and IME in stroke patients.

Conclusions

Stroke patients showed decreases in the thickening ability of the diaphragm at TLC and in inspiratory muscle function. The change between the diaphragm thickness at end expiration and at TLC was positively correlated with MIP, PIF, and VC.

Functional connectivity and information flow of the respiratory neural network in chronic obstructive pulmonary disease

Breathing involves a complex interplay between the brainstem automatic network and cortical voluntary command. How these brain regions communicate at rest or during inspiratory loading is unknown. This issue is crucial for several reasons: (i) increased respiratory loading is a major feature of several respiratory diseases, (ii) failure of the voluntary motor and cortical sensory processing drives is among the mechanisms that precede acute respiratory failure, (iii) several cerebral structures involved in responding to inspiratory loading participate in the perception of dyspnea, a distressing symptom in many disease. We studied functional connectivity and Granger causality of the respiratory network in controls and patients with chronic obstructive pulmonary disease (COPD), at rest and during inspiratory loading. Compared with those of controls, the motor cortex area of patients exhibited decreased connectivity with their contralateral counterparts and no connectivity with the brainstem. In the patients, the information flow was reversed at rest with the source of the network shifted from the medulla towards the motor cortex. During inspiratory loading, the system was overwhelmed and the motor cortex became the sink of the network. This major finding may help to understand why some patients with COPD are prone to acute respiratory failure. Network connectivity and causality were related to lung function and illness severity. We validated our connectivity and causality results with a mathematical model of neural network. Our findings suggest a new therapeutic strategy involving the modulation of brain activity to increase motor cortex functional connectivity and improve respiratory muscles performance in patients. Hum Brain Mapp 37:2736–2754, 2016. © 2016 The Authors Human Brain Mapping Published by Wiley Periodicals, Inc.

B-mode ultrasound assessment of diaphragm structure and function in patients with COPD

BACKGROUND

Electromyographic evaluation of diaphragmatic neuromuscular disease in patients with COPD is technically difficult and potentially high risk. Defining standard values for diaphragm thickness and thickening ratio using B-mode ultrasound may provide a simpler, safer means of evaluating these patients.

METHODS

Fifty patients with a diagnosis of COPD and FEV₁ < 70% underwent B-mode ultrasound. Three images were captured both at end expiration (Tmin) and at maximal inspiration (Tmax). The thickening ratio was calculated as (Tmax/Tmin), and each set of values was averaged. Findings were compared with a database of 150 healthy control subjects.

RESULTS

There was no significant difference in diaphragm thickness or thickening ratio between sides within groups (control subjects or patients with COPD) or between groups, with the exception of the subgroup with severe air trapping (residual volume > 200%), in which the only difference was that the thickening ratio was higher on the left (P = .0045).

CONCLUSIONS

In patients with COPD presenting for evaluation of coexisting neuromuscular respiratory weakness, the same values established for healthy control subjects serve as the baseline for comparison. This knowledge expands the role of ultrasound in evaluating neuromuscular disease in patients with COPD.

Ventilatory failure, ventilator support, and ventilator weaning

The development of acute ventilatory failure represents an inability of the respiratory control system to maintain a level of respiratory motor output to cope with the metabolic demands of the body. The level of respiratory motor output is also the main determinant of the degree of respiratory distress experienced by such patients. As ventilatory failure progresses and patient distress increases, mechanical ventilation is instituted to help the respiratory muscles cope with the heightened workload. While a patient is connected to a ventilator, a physician's ability to align the rhythm of the machine with the rhythm of the patient's respiratory centers becomes the primary determinant of the level of rest accorded to the respiratory muscles. Problems of alignment are manifested as failure to trigger, double triggering, an inflationary gas-flow that fails to match inspiratory demands, and an inflation phase that persists after a patient's respiratory centers have switched to expiration. With recovery from disorders that precipitated the initial bout of acute ventilatory failure, attempts are made to discontinue the ventilator (weaning). About 20% of weaning attempts fail, ultimately, because the respiratory controller is unable to sustain ventilation and this failure is signaled by development of rapid shallow breathing. Substantial advances in the medical management of acute ventilatory failure that requires ventilator assistance are most likely to result from research yielding novel insights into the operation of the respiratory control system.

Patient machine interface for the control of mechanical ventilation devices

The potential of Brain Computer Interfaces (BCIs) to translate brain activity into commands to control external devices during mechanical ventilation (MV) remains largely unexplored. This is surprising since the amount of patients that might benefit from such assistance is considerably larger than the number of patients requiring BCI for motor control. Given the transient nature of MV (i.e., used mainly over night or during acute clinical conditions), precluding the use of invasive methods, and inspired by current research on BCIs, we argue that scalp recorded EEG (electroencephalography) signals can provide a non-invasive direct communication pathway between the brain and the ventilator. In this paper we propose a Patient Ventilator Interface (PVI) to control a ventilator during variable conscious states (i.e., wake, sleep, etc.). After a brief introduction on the neural control of breathing and the clinical conditions requiring the use of MV we discuss the conventional techniques used during MV. The schema of the PVI is presented followed by a description of the neural signals that can be used for the on-line control. To illustrate the full approach, we present data from a healthy subject, where the inspiration and expiration periods during voluntary breathing were discriminated with a 92% accuracy (10-fold cross-validation) from the scalp EEG data. The paper ends with a discussion on the advantages and obstacles that can be forecasted in this novel application of the concept of BCI.

The effect of acute non-invasive ventilation on corticospinal pathways to the respiratory muscles in chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease is associated with altered cortical excitability. The relevance of this to the need for non-invasive ventilation is not known. We assessed the diaphragm response to transcranial magnetic stimulation in terms of motor threshold and latency as well as assessing intracortical excitability using paired stimulation in eight long-term users and six non-users of home ventilation with COPD. Overall, intracortical facilitation was strongly correlated with inspiratory muscle strength (r² 0.72, p < 0.001) whereas intracortical inhibition was correlated with PaCO₂ (r² 0.51, p = 0.01). The two groups did not differ in motor evoked potential or latency, nor in the excitability of intracortical inhibitory or facilitatory circuits assessed using paired stimulation. The acute effect of isocapnic non-invasive ventilation was studied in six established ventilator users. Diaphragm motor evoked potential fell but there was no effect on intracortical facilitation or inhibition, implying an effect of neuromechanical feedback at brainstem or spinal level.

Respiratory muscle function and activation in chronic obstructive pulmonary disease

Inspiratory muscles are uniquely adapted for endurance, but their function is compromised in chronic obstructive pulmonary disease (COPD) due to increased loads, reduced mechanical advantage, and increased ventilatory requirements. The hyperinflation of COPD reduces the flow and pressure-generating capacity of the diaphragm. This is compensated by a threefold increase in neural drive, adaptations of the chest wall and diaphragm shape to accommodate the increased volume, and adaptations of muscle fibers to preserve strength and increase endurance. Paradoxical indrawing of the lower costal margin during inspiration in severe COPD (Hoover's sign) correlates with high inspiratory drive and severe airflow obstruction rather than contraction of radially oriented diaphragm fibers. The inspiratory muscles remain highly resistant to fatigue in patients with COPD, and the ultimate development of ventilatory failure is associated with insufficient central drive. Sleep is associated with reduced respiratory drive and impairments of lung and chest wall function, which are exaggerated in COPD patients. Profound hypoxemia and hypercapnia can occur in rapid eye movement sleep and contribute to the development of cor pulmonale. Inspiratory muscles adapt to chronic loading with an increased proportion of slow, fatigue-resistant fiber types, increased oxidative capacity, and reduced fiber cross-sectional area, but the capacity of the diaphragm to increase ventilation in exercise is compromised in COPD. In COPD, neural drive to the diaphragm increases to near maximal levels in exercise, but it does not develop peripheral muscle fatigue. The improvement in exercise capacity and dyspnea following lung volume reduction surgery is associated with a substantial reduction in neural drive to the inspiratory muscles.

Role of the respiratory muscles in acute respiratory failure of COPD: lessons from weaning failure

It is problematic to withhold therapy in a patient with chronic obstructive pulmonary disease (COPD) who presents with acute respiratory failure so that detailed physiological measurements can be obtained. Accordingly, most information on respiratory muscle activity in patients experiencing acute respiratory failure has been acquired by studying patients who fail a trial of weaning after a period of mechanical ventilation. Such patients experience marked increases in inspiratory muscle load consequent to increases in resistance, elastance, and intrinsic positive end-expiratory pressure. Inspiratory muscle strength is reduced secondary to hyperinflation and possibly direct muscle damage and the release of inflammatory mediators. Most patients recruit both their sternomastoid and expiratory muscles, even though airflow limitation prevents the expiratory muscles from lowering lung volume. Even when acute hypercapnia is present, patients do not exhibit respiratory center depression; indeed, voluntary activation of the diaphragm, in absolute terms, is greater in hypercapnic patients than in normocapnic patients. Instead, the major mechanism of acute hypercapnia is the development of rapid shallow breathing. Despite the marked increase in mechanical load and decreased force-generating capacity of the inspiratory muscles, patients do not develop long-lasting muscle fatigue, at least over the period of a failed weaning trial. Although the disease originates within the lung parenchyma, much of the distress faced by patients with COPD, especially during acute respiratory failure, is caused by the burdens imposed on the respiratory muscles.

Blunted ventilatory response to hypoxia/hypercapnia in mice with cigarette smoke-induced emphysema

It has been reported that the degree of emphysema induced by chronic cigarette smoke (CS) is greater in female C3H/HeN mice as compared to other mouse strains. We hypothesized that these mice would develop the similar major characteristics seen in hypercapnic patients with chronic obstructive pulmonary disease (COPD), including emphysema, pulmonary inflammation, hypercapnia/hypoxemia, rapid breathing, and attenuated ventilatory response (AVR). Mice were exposed either to CS or filtered air (FA) for 16 weeks. After exposure, arterial blood gases and minute ventilation were measured before and during chemical challenges in anesthetized and spontaneously breathing mice. We found that as compared to FA, CS exposure caused emphysema and pulmonary inflammation associated with: (1) hypercapnia and hypoxemia, (2) rapid breathing, and (3) AVR to 25 breaths of pure N(2), 5% CO(2) alone, and 5% CO(2) coupled with 10% O(2). The similarity of these pathophysiological characteristics between our mouse model and COPD patients suggests that this model could be effectively applied to study COPD pathophysiology, especially central mechanisms of the AVR genesis.

Anaerobic metabolism of inspiratory muscles in COPD

OBJECTIVE

The purpose of this study was to determine if the respiratory muscles of patients with COPD could be made to function anaerobically, as evidenced by an increase in arterial blood lactate concentration ([lactate](a)) during specific loading of the inspiratory muscles and, if so, the effect of a programme of high-intensity inspiratory muscle training on this function.

METHODS

In seven patients with COPD (FEV(1) = 33 +/- 14% of predicted), measurements of [lactate](a) were made each minute during progressive inspiratory threshold loading to voluntary exhaustion. These tests were performed before and after an 8-week programme of specific high-intensity inspiratory muscle training, combined with general whole-body exercise training.

RESULTS

During inspiratory muscle loading small increases in [lactate](a) (0.83 +/- 0.32 mM) were observed in two subjects before training, and in five subjects after training (0.69 +/- 0.57 mM). [Lactate](a) only increased when the inspiratory work rate exceeded 6.9 cm H(2)O L/min per kilogram of body weight, and when baseline maximum inspiratory pressure exceeded 65 cm H(2)O.

CONCLUSIONS

The results of this study demonstrated that it is possible for COPD patients to increase inspiratory muscle work rate to a level requiring a major energy contribution from anaerobic glycolytic metabolism. This was only seen when inspiratory muscle strength and endurance were sufficient to allow it. Some patients who failed to demonstrate an increase in [lactate](a) at baseline did so after a programme of high-intensity inspiratory muscle training.

Respiratory and Skeletal Muscles in Hypogonadal Men with Chronic Obstructive Pulmonary Disease

Hypogonadism, found in about one-third of patients with chronic obstructive pulmonary disease (COPD), has potential for decreasing muscle mass and muscle performance. Compared with eugonadal patients, we hypothesized that hypogonadal patients with COPD have decreased respiratory and skeletal muscle performance. Nineteen hypogonadal and 20 eugonadal men with COPD (FEV(1) 1.14 +/- 0.08 and 1.17 +/- 0.11 L [standard error], respectively) were studied. Diaphragmatic contractility, assessed as transdiaphragmatic twitch pressure generated by phrenic nerve stimulation, was similar in hypogonadal and eugonadal patients: 20.6 +/- 2.2 and 19.8 +/- 2.5 cm H(2)O, respectively. During progressive inspiratory threshold loading, hypogonadal and eugonadal patients had similar respiratory muscle endurance times (302 +/- 29 and 313 +/- 48 seconds, respectively) and airway pressure sustained during the last minute of loading (38.2 +/- 3.0 and 40.5 +/- 4.7 cm H(2)O, respectively) (similar to predicted values in healthy subjects). Hypogonadal and eugonadal patients had equivalent limb muscle strength and endurance. During cycle exercise to exhaustion, exercise performance, gas exchange, and respiratory muscle recruitment (estimated by esophageal and gastric pressure swings during tidal breathing) were similar in both groups. In conclusion, hypogonadism does not decrease respiratory or limb muscle performance and exercise capacity in men with moderate-to-severe COPD who, for the most part, are not underweight.

Corticospinal control of respiratory muscles in chronic obstructive pulmonary disease

Patients with chronic obstructive pulmonary disease (COPD) face an increased respiratory load and in consequence have an elevated respiratory drive. We used transcranial magnetic stimulation (TMS) to investigate associated changes in corticospinal excitability both at rest and during voluntary facilitation at different levels of inspiratory effort. Diaphragm and abdominal motor thresholds were significantly lower in COPD than healthy controls, but the quadriceps response was the same. In patients there was a significant increase in diaphragm response from rest during 20% inspiratory efforts but no further increase with greater efforts. In controls there was a further stepwise increase at 40% and 60% of inspiratory effort. The cortical silent period was significantly shorter in COPD. Using paired stimulation to study intracortical inhibitory and excitatory circuits we found significantly less excitability of intracortical facilitatory circuits in patients at long (>7 ms) interstimulus intervals. These results suggest that there is a ceiling effect in motor control output to the respiratory muscles of patients with COPD.

Assessing Respiratory Drive and Central Motor Pathway in Humans: Clinical Implications

Feedback from sensory elements as well as projection from higher Central Nervous System structures modify the level and pattern of motor outflow to the respiratory muscles and hence ventilation. In this review we describe the different methods to evaluate the degree to which higher centers determine the level and pattern of ventilation and coordinate use of the respiratory muscles in healthy humans and in patients with a number of respiratory disorders.

Disorders of the respiratory muscles

The act of breathing depends on coordinated activity of the respiratory muscles to generate subatmospheric pressure. This action is compromised by disease states affecting anatomical sites ranging from the cerebral cortex to the alveolar sac. Weakness of the respiratory muscles can dominate the clinical manifestations in the later stages of several primary neurologic and neuromuscular disorders in a manner unique to each disease state. Structural abnormalities of the thoracic cage, such as scoliosis or flail chest, interfere with the action of the respiratory muscles-again in a manner unique to each disease state. The hyperinflation that accompanies diseases of the airways interferes with the ability of the respiratory muscles to generate subatmospheric pressure and it increases the load on the respiratory muscles. Impaired respiratory muscle function is the most severe consequence of several newly described syndromes affecting critically ill patients. Research on the respiratory muscles embraces techniques of molecular biology, integrative physiology, and controlled clinical trials. A detailed understanding of disease states affecting the respiratory muscles is necessary for every physician who practices pulmonary medicine or critical care medicine.

Is weaning failure caused by low-frequency fatigue of the diaphragm?

Because patients who fail a trial of weaning from mechanical ventilation experience a marked increase in respiratory load, we hypothesized that these patients develop diaphragmatic fatigue. Accordingly, we measured twitch transdiaphragmatic pressure using phrenic nerve stimulation in 11 weaning failure and 8 weaning success patients. Measurements were made before and 30 minutes after spontaneous breathing trials that lasted up to 60 minutes. Twitch transdiaphragmatic pressure was 8.9 +/- 2.2 cm H2O before the trials and 9.4 +/- 2.4 cm H2O after their completion in the weaning failure patients (p = 0.17); the corresponding values in the weaning success patients were 10.3 +/- 1.5 and 11.2 +/- 1.8 cm H2O (p = 0.18). Despite greater load (p = 0.04) and diaphragmatic effort (p = 0.01), the weaning failure patients did not develop low-frequency fatigue probably because of greater recruitment of rib cage and expiratory muscles (p = 0.004) and because clinical signs of distress mandating the reinstitution of mechanical ventilation arose before the development of fatigue. Twitch pressure revealed considerable diaphragmatic weakness in many weaning failure patients. In conclusion, in contrast to our hypothesis, weaning failure was not accompanied by low-frequency fatigue of the diaphragm, although many weaning failure patients displayed diaphragmatic weakness.